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PLC Basics and Voltage Sag Susceptibilities - Part 1The electrical upgrade of the green carbon facility at the aluminum plant was complete. A myriad of programmable logic controllers (PLCs) and PC-based graphic user interface (GUI) systems replaced the hardwired system installed in the 1960s. The plant manager proudly watched the GUI screens as the fully automated system performed batching, mixing, and conveying operations faster than ever - with little operator intervention.

The electrical upgrade of the green carbon facility at the aluminum plant was complete. A myriad of programmable logic controllers (PLCs) and PC-based graphic user interface (GUI) systems replaced the hardwired system installed in the 1960s. The plant manager proudly watched the GUI screens as the fully automated system performed batching, mixing, and conveying operations faster than ever - with little operator intervention.

After spending $10 million on the cutover from hardwired relay control to the new PLC-based system, the change was finally complete. It was a new day for the plant. Then it happened. The lights blinked in the facility - an event that typically occurred at least twice a month. A minor voltage sag (to 75% of nominal), lasting only 5 cycles, occurred on the utility grid. Although it rarely affected the old hardwired system, the humming of the conveyors, crushers, mixers, and batching system stopped.

As the operators began to scramble to restart the automatic operations, confusion reigned as the batch-weighing system couldn't decipher how much of each ingredient was in the hopper and how much more needed to be added. Stopping at different points in its respective mix sequences, the PLC no longer knew how long the 10 batch mixers needed to continue operations to complete its cycles. The plant manager's face reddened with anger as he contemplated the cost of the new system and the reliability of the old scheme. The operators' confusion turned to frustration as they realized the batch in the weigh hopper would have to be scrapped because the system had lost track of the contents.

Events like this happen in many facilities that employ PLC systems in control schemes. Untold millions of dollars are lost when PLC-based control systems are upset by voltage sag events. However, with proper electrical and software design techniques, these systems can be made much less susceptible to voltage sag phenomenon.

The Voltage Sag To understand why PLCs are susceptible to voltage sags, it is important to understand the voltage sag itself. Industrial manufacturers almost always incorrectly assume all events that affect electrical equipment are "power surges," since the shutdown may have occurred during a lightning event. Although overvoltage conditions (known as voltage swells and surges) can occur, EPRI research has confirmed short duration reductions in voltage (voltage sags) lead to the most frequent complaints from industrial customers. These events typically occur when weather, trees, or animals instigate a line-to-ground fault on the utility grid.The depth of the event seen by the industrial customer is determined by the magnitude of the fault current, stiffness of the grid, and the closeness of the customer's facility to the location of the fault. The duration of the event relates to the breaker-clearing time on the utility system. Typically described in terms of magnitude and duration (Fig. 1), voltage sag events can affect the operation of sensitive production equipment leading to shutdown, malfunction, lost product, and diminished revenue. When a voltage sag results in equipment shutdown or malfunction during normal power system operation, the equipment becomes incompatible with its electrical environment or develops poor system compatibility.Typical voltage sag durations range from four to 30 cycles, depending on whether the facility is fed from the utility's transmission system (e.g., 69kV or 161kV), which is somewhat stiff, or a utility's distribution system (e.g., 13.8kV or 34.5kV), which typically cannot supply as much fault current.

PLC Basics and Voltage Sag Susceptibilities Fig. 2 shows a typical PLC I/O rack with a power supply, CPU, discrete input and output modules, and analog input and output modules. The following sections outline the function of each of these components and discuss related power quality considerations for each.

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PLC I/O rack power supply. Utilizing the typical switch-mode power supply topology, the PLC power supply can be either a pillar of power quality robustness or an Achilles heel. Although available for both AC and DC input power sources, the most commonly procured units use an AC-input source of 120/230VAC.

Small in relative power output, the PLC power supply usually produces from 40W to 80W DC, for use across the I/O rack back plane. The purpose of the unit is to supply DC power to all devices physically mounted in the PLC rack. These devices may include the CPU and communications module(s), as well as discrete and analog I/O modules.

Typically, the PLC power supply does not provide power to field devices such as sensors, transmitters, motor starters, and solenoids. Some PLC manufacturers may use the power supply to provide an analog output signal for control valve and drive interfacing. Others require an additional external power supply for these functions.

Most PLC power supplies also perform continuous diagnostics for line voltages that are outside the tolerance envelope. If the power supply detects a serious problem, it will notify the CPU to halt program execution and shut down process operations. (Fig. 3 displays the general topology of a PLC power supply.)

PLCs monitor either the level of the incoming AC plant line voltage or the level of the power supply DC output to decide when to shutdown during voltage sag events. The leading U.S. PLC manufacturer's power supply literature states the following voltage sag shutdown philosophy."Each AC-input power supply generates a shutdown signal on the back plane whenever the AC line voltage drops below its lower voltage limit, and removes the shutdown signal when the line voltage comes back up to the lower voltage limit. This shutdown signal is necessary to ensure that only valid data is stored in memory."

With this philosophy, the manufacturer's most common PLC product will react to voltage sags as short as one cycle in duration. It is also interesting to note that other product lines from the same manufacturer base shut down on the PLC power supply's DC output. Since a DC power supply can inherently store energy in the power supply capacitors, sensing the DC level rather than the incoming AC line voltage can lead to improved system compatibility.

PLC I/O power quality issues. PLC inputs and outputs (I/O) can be grouped into four main categories: discrete inputs (DI), discrete outputs (DO), analog inputs (AI), and analog outputs (AO).Discrete input (DI) modules are available for AC or DC sensor types. DIs include "on/off" status signals from push buttons, selector switches, motor starter auxiliary contacts, relay contacts, and process sensor inputs such as pressure, flow, proximity, or zero speed. Typical wiring examples for AC and DC discrete input modules as well as typical field devices such as a proximity switch and push-button station are shown in Fig. 4, on page 45.

The susceptibility of the DI module to voltage sags is only relevant if the PLC power supply has not already led to a system shutdown. Since DI modules are designed to react quickly to detect an input status change, they also can react quickly when a voltage sag event occurs. The common response time for AC inputs to detect a transition from "on" to "off" can be as short as 11 ms, which is less than one cycle. For DC inputs, response time for input status changes can be even shorter - 4 ms (1/4 cycle) is possible.

Once the DI module senses a real or perceived change in the status of the input, the PLC program will react. Since the effects of the voltage sag may translate directly to lower voltages at the input terminals of the module, the control system may misinterpret an "on" condition to actually be an "off" condition. Such false negative conditions from a process sensor can lead to the malfunction or immediate shutdown of the process.In the case of AC input modules, the voltage sag immediately passes to the input terminals of the module. In the case of DC modules, the external DC power supply acts to filter a voltage sag so that the output power to the sensors may not be affected. The ability of the DC power supply to provide this "embedded"

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mitigation to voltage sags is dependent on the topology, sizing, load, and input voltage. If the DC power supply is unregulated, virtually no stored energy will be present to mitigate the voltage sag. However, a robust power supply means the input sensor signals also will remain robust to voltage sags.

Available in AC or DC types, discrete output (DO) modules act to switch the "on/off" voltage signal to field devices such as motor starters, relays, solenoids, and pilot lights. The susceptibility of the DO module is directly related to the PLC power supply shutdown signal as well as the susceptibility of the individual loads connected to the module. Since the module simply acts as a switch to the individual loads, it has little ability to affect the voltage sag response of the system. If the PLC decides to shut down as a result of a voltage sag, all DO signals typically will drop unless it is specially configured on the I/O rack. Most end users do not opt to allow the outputs to stay powered in this state because such a condition may lead to safety and machine damage issues. A typical AC module and motor starter is shown in Fig. 5.In the case where the PLC power supply is robust to voltage sags, and field devices (such as motor starters and relays) are susceptible to voltage sags, the process may still malfunction or shut down. To ensure all outputs are robust, the most comprehensive approach is to ensure that the control power voltage source is robust. The system integrator can do this by conditioning the power source in an AC system or by using a robust DC power supply and DC output module, which, in turn, would require the use of DC- powered field devices such as motor starters, relays, and solenoids.Utilizing DC signal ranges such as 4mA to 20mA, 1V to 5V, or 0V to 10V, analog input (AI) modules receive a continuous DC current or voltage signal from process transmitters. DC power supplies are required to source the voltage or current loops for the AI signals. Therefore, the voltage sag susceptibility of this module and the process transmitters is related to the ability of the external DC power supply to ride through the voltage sag.

Two basic configurations for process transmitters are known as "2-wire" and "4-wire," each of which leads to differing power quality considerations. An external DC power supply runs a 2-wire process transmitter (Fig. 6). This same supply may provide DC power for all transmitters in the system or control cabinet. With a single source of DC power, the AI signals can be made robust to voltage sags if the DC supply is robust.

With the 4-wire transmitter topology (Fig. 7), an external AC voltage source must power the transmitter. In this configuration, the transmitter provides the continuous DC signal to the individual channel on the AI card. The required DC power supply is located within the transmitter itself. For these reasons, you must consider the voltage sag robustness of the AC power source for each of the 4-wire transmitters in the process.

Analog output (AO) modules provide a continuous DC voltage or current signal to field devices. Examples of AO control loops include position control of a proportional valve or the speed control of a motor through an AC or DC adjustable-speed drive (ASD). Depending on the manufacturer and module type, these signals can be sourced by the PLC power supply through the I/O rack back plane or by an external DC supply. Therefore, the stability of the output signal to the field device is dependent on the robustness of the DC voltage source.

When sourced by the PLC power supply, tests indicate the PLC normally will shut down before the DC output voltage and integrity of the control signal is affected. When the PLC shutdown occurs, the analog signal is removed from the field device, which will directly affect the position of a valve, or the speed of a motor. In the case where an external DC power supply is required to source the AO current loop, the robustness of the power supply to voltage sag may directly affect the control of the process.PLC CPU Module and Programming Considerations The Central Processing Unit (CPU) module. This device is the brains of the PLC. Usually occupying a single slot in the PLC rack, the CPU module (also referred to as the processor) holds the control program in random access memory (RAM). The CPU module receives operating power through the I/O rack back plane via the rack's power supply.

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The I/O rack back plane also contains a data bus for communications between the PLC and rack I/O. A lithium battery and/or electrically erasable programmable read only memory (EEPROM) chip is typically used to maintain the PLC program in the event of lost power.The CPU reads the input data table information, solves the control program, and updates the output data table. The PLC performs housekeeping to check it and other hardware components for faults and errors. You can use a secondary microprocessor to transfer data from the system inputs into the data table and from the data table to the system outputs.

The time required for the PLC to read the inputs, solve the control program, and update the output table is known as the "scan rate." This time varies greatly depending on the CPU model, size of the control program, and architecture of the system. A more definitive measurement of PLC response is known as "throughput."

Throughput refers to the amount of time required to detect an input from the field device, solve the control program, and manipulate an output field device. The throughput time includes the scan time plus the amount of time it takes for the actual PLC module's electronics to detect, input, and switch the state of an output.

Throughput measurements for PLCs can be as short as 17 ms (approximately one 60 Hz cycle) to several hundred milliseconds depending on the size of the control program and the number and speed of the I/O modules. With the ability to sense a state change and switch an output signal in such a short time, it is easy to understand why process upsets and shutdowns occur as a result of voltage sags.Control programming techniques. The PLC control program may take various forms. The most basic and common control program format is ladder logic. This control program format was created to model hardwired electrical relay logic and is subsequently very user friendly for maintenance electricians. A ladder logic PLC program uses conventional seal-in techniques that have been used in past relays. Other program languages commonly used today include sequential function chart (SFC), BASIC, and C++.The method or technique the PLC programmer uses to control process equipment is a potential cause for PLC system PQ immunity problems. For example, in process applications, the process step of a batch may be held in the PLC's memory by using a conventional seal-in technique in the ladder logic. If the PLC experiences a shutdown and restart as a result of voltage sag, the process state of the batch will likely be cleared since the seal-in will be lost. As stated earlier, this may lead to the loss of the batch.

A better technique is to write process step information into nonvolatile memory areas that are not cleared when the PLC shuts down and restarts. By placing a process step number into a nonvolatile memory location, the PLC can then know where to resume process operations. This approach, which is known as the "state-machine method," can be a powerful ally in helping to restart a control system when a voltage sag or outage-related upset occurs in your facility.

Overview PLCs can react quickly when voltage sags occur, shutting automation systems down for events as short as one cycle. Susceptibility of the power supply or misinterpretation of I/O signals usually causes the shutdown. The use of power supplies that monitor the DC output rather than the AC input when deciding to halt operations can lead to improved ride-through.In the PLC I/O, voltage sags can lead to the detection of false negative conditions from sensors. Such occurrences can bring the process to a halt or cause a malfunction. In instances were the PLC is upset as a result of a voltage sag or outage, the programming technique used by the system integrator can lead either to a slow and costly recovery or a speedy resumption of automated operations.Note: In the second part of this article, we will introduce voltage sag tests and practical guidelines to help integrators and users of PLC-based systems make proactive design changes to improve voltage sag response.Acknowledgments: This two-part series of articles is based on recent work conducted by EPRI and funded by the California Energy Commission. The information presented would not have been possible without the diligent efforts of Brian Laan and Promad Kulkarni of the commission. For more information about EPRI PEAC Corp., visitwww.epri-peac.com.

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PLC Basics and Voltage Sag Susceptibilities Part 2

In Part 1 of this two-part series, we explored PLC basics and voltage sag susceptibilities. In this segment, we present PLC voltage sag test results along with guidelines for making PLC systems more robust to voltage sags. The tests, conducted by EPRI and funded by the California Energy Commission (CEC), helped establish PLC baseline performance guidelines for improving system compatibility. These results are complementary to similar tests conducted by EPRI in 1995.

In Part 1 of this two-part series, we explored PLC basics and voltage sag susceptibilities. In this segment, we present PLC voltage sag test results along with guidelines for making PLC systems more robust to voltage sags. The tests, conducted by EPRI and funded by the California Energy Commission (CEC), helped establish PLC baseline performance guidelines for improving system compatibility. These results are complementary to similar tests conducted by EPRI in 1995.Voltage Sag Test Results

EPRI conducted voltage sag tests on five common PLCs. The PLCs (referred to as A, B, C, D, and E) were subjected to voltage sags in a test setup using a portable sag generator with and without power conditioning. The setup included additional relays, power supplies, and motor starters to make the load of the system reach a target of 2A.

To characterize the operation of the PLCs during a power quality disturbance, a PQ test algorithm was programmed into each PLC. The program monitored the status of various AC input module channels and controlled the status of various AC output channels. A general-purpose control relay was wired to an AC output module channel, and an AC input module channel monitored relay status. The written program provided the latching logic to hold the output relay “on” after receiving the appropriate input.If the relay momentarily opened as a result of the voltage sag, and the input channel detected the opening of the relay, the program latch would drop the output module signal. The test device recorded this condition as a system failure since such an event could easily upset an automated process. But if the PLC power caused a shutdown as a result of the sag, the device noted the condition as a CPU failure.Without Power Conditioning

Excluding PLC D, the responses of the remaining PLC power supplies are more robust than the overall system response (denoted as system failure). These units appear to ride through the voltage sag based on the output voltage of the DC power supply rather than the AC input voltage. This result means the PLC CPU may continue to operate even if the voltage sag has disturbed the I/O and field devices. Such an event in a process control system can lead to a possible malfunction or process shutdown.Unlike the remaining units, PLC D forced a shutdown when a 2-cycle, 78%-of-nominal (or less) voltage sag occurred. This PLC shuts down based on the AC input voltage. Denoted as CPU failure, this response ensures the PLC will shut down before the control system malfunctions.In comparison to earlier EPRI tests, the I/O racks contained fewer I/O modules (six I/O modules in earlier tests versus two modules in this series of tests). Therefore, the PLC DC power supply modules in these tests were more lightly loaded.With Power Conditioning

EPRI repeated voltage sag tests on the five PLCs when the power source underwent conditioning. Test conductors used a constant voltage transformer (CVT), two offline UPS units, and one online UPS to mitigate voltage sags.

CVT test results. All PLCs exhibited superior voltage sag ride-through with the 500VA CVT power conditioner installed. On average, induced shutdown levels on the five PLCs' power supplies improved from an average of 62.6% of nominal without power conditioning down to 23.2% of nominal voltage with the CVT. Furthermore, the system failure shutdown level dropped from an average of 73.6% without power conditioning to an average of 33.4% of nominal with the CVT in place.

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UPS test results. The results of the offline and online UPS tests were very different. During the voltage sag, the two offline UPS units used in the project produced a “simulated” sine-wave output. Furthermore, the offline units required about 4 ms (¼ cycle) to switch over to the battery source. Typically, this duration is not critical since most control equipment can withstand voltage sags of such short duration.Although the two square-wave output UPS units kept the PLCs powered and averted a CPU failure, some of the PLC 120VAC input cards could not resolve the square wave. This led to the PLC detecting logic level “0” instead of logic level “1.” As a result, the output relay was dropped when the UPS kicked on and the inputs could not be resolved. In all, three out of the five PLCs tested could not resolve the square wave on the AC input card and experienced system failures as soon as the UPS transferred. On the other hand, the tests found the line-interactive UPS to be compatible with all PLCs tested, allowing continued operation even in a complete power outage.ConclusionVoltage sags as short as one cycle can affect PLCs, leading to the shutdown of automated control systems and the loss of production time and money. However, proper hardware and software integration leads to vast improvements in the response of these systems to power disturbances.When attacking voltage sag-related problems, it is important to ensure that the PLC and the I/O control power are robust. Techniques such as using power conditioners for the AC source voltage or employing a robust DC control power source provide effective mitigation. In addition, you must carefully select power conditioning devices to ensure the entire system will not be made less compatible as a result. Power conditioning devices that produce square-wave outputs are not compatible with all PLC systems. Square-wave outputs are available on many battery-based UPS systems as well as on newer technology capacitor-based power conditioners.Top Ten Guidelines for Improving PLC Voltage Sag Performance

1. Avoid mismatched control power voltages. If the actual PLC system nominal voltage is lower than the expected nominal input voltage, the control system will be more susceptible to sags. Such mismatches can occur when control power transformers are tapped low or a 230VAC input PLC power supply is connected to a 208VAC source. For relays and contactors, a mismatch of 10% of voltage equates to an increase in susceptibility by 10%. But in DC power supplies, the energy stored in the internal capacitors can be as much as 18% lower when the input voltage is mismatched by as little as 10%—directly equating to a reduction in ride-through time.

2. Provide a robust power source for PLC power supply and I/O control power.Ensuring the PLC power supply response to voltage sags will be robust without considering the I/O control power is only a partial fix. Although the PLC CPU may survive voltage sags, the system is still likely to suffer process upsets. Therefore, you must consider the control power and I/O power.

3. Consider using DC to power the PLC and I/O. EPRI tests have confirmed that using a DC power scheme for the PLC power supply and I/O control power is an ideal embedded solution for solving voltage sag-related shutdowns. The best way to design the system with this approach is by the integrator, since the PLC power supply must be specified as a DC input type. However, you must specify the I/O modules, sensors, relays, solenoids, and motor starters for DC control voltages. In systems were the I/O control voltage is already DC, the solution is as easy as replacing the AC input power supply module with the comparable DC input power supply module. It is important to ensure the DC power source, typically 24VDC, is robust as well.

4. Use universal input switching power supplies, wired phase-to-phase. Typically, the universal input-type power supply has a voltage range of 85VAC to 264VAC. When connected phase-to-phase in a 208VAC system, the power supply can continue to operate down to 41% of nominal. Specify this type of supply for DC-powered instrumentation, I/O control voltage, and external DC PLC power supplies.

5. Do not overload DC power supplies. Since the amount of voltage sag ride-through time available from a typical linear or switch-mode DC power supply is directly related to the loading, DC power supplies should not be running at maximum capacity. Oversizing by at least two times the expected load will help the

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power supply ride through voltage sags. This is only critical for systems that do not use a universal input power supply wired in a phase-to-phase configuration.

6. Use a robust control relay for the master control relay (MCR). The importance of selecting robust control components for the MCR circuit cannot be understated.When used in the safety circuits or as subsystem power contactors, the selection of the MCR can make a large difference in the control system's ability to survive voltage sags. Before installing a relay or contactor into the design scheme, the integrator should bench test the units for voltage sag immunity. One way to improve the MCR circuit is to avoid general-purpose “ice cube” relays as the MCR because they are too sensitive to voltage sags—use a small 4-pole contactor instead. In general, a small contactor can survive voltage sags as low as 40% to 55% of nominal.

7. Properly maintain PLC battery. Many PLCs utilize lithium-ion batteries to maintain their control programs and nonvolatile memory data in the event of a power loss or voltage sag-induced shutdown. Such a loss of the PLC program may result in extended downtime because you'll need to locate the latest backup, then reload and restart the process. Since the active process data will be lost in this situation, scrapping of product may be inevitable.

8. Use a state-machine programming method and/or nonvolatile PLC memory. When properly coded, this type of programming ensures the system will not lose its place in the event of a voltage sag/outage and will result in quicker restart times.

9. Consider the power source for analog input signals. For analog signals, ensure the source is robust. If you use 2-wire transmitters, the DC power supply should be lightly loaded or naturally robust, as noted in guidelines 4 and 5. If you use 4-wire transmitters, consider providing power conditioning for the AC voltage source.

10. Only use compatible power conditioners. A properly sized CVT or line-interactive UPS greatly enhances a PLC system's ability to ride through voltage sags. Avoid power conditioners with square-wave outputs since the AC input module channels on the PLC may not be able to resolve the square-wave signal. Only use square-wave output power conditioners with the PLC manufacturer's assurance that the PLC power supply and I/O cards are compatible.

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Coordination Guidelines for Susceptible Electronic Loads — Part 1Ensuring uptime and protecting against power quality problems step by stepWhen it comes to providing suitable power, grounding, communications, and environmental control for a facility that has significant amounts of susceptible electronic equipment, there are obviously many factors to keep in mind. Although each installation has its own unique requirements, there are many characteristics common to all projects.To help you or your client make more informed decisions on designing, installing, or retrofitting a distribution system that powers susceptible electronic equipment, check out the following guidelines, derived from practical working checklists used by specialists in power quality auditing and compiled in the book “Quality Power for Sensitive Electronic Equipment,” written by the editors of EC&M and edited by the late Ray Waggoner.Continuity of processing operations. Before purchasing equipment for electronic data processing, process control, communications, instrumentation, or similar applications, you should first study the operations planned for the facility. This will help you develop answers to the following questions:

What length of downtime can the facility tolerate? Many operations can withstand 5 to 20 milliseconds of downtime by their internal ride-through capacity.

How frequently can such downtime occur? Some locations tolerate one to three power outages per year, but require some additional protection if occurrences total several times a month (20 to 25 times a year).

Are momentary interruptions acceptable? Some hardware and software systems provide for “hold and store” methods of preserving data in case of momentary interruptions.

What is the cost in revenue of interruptions and downtime in terms of lost data, “reboot” time, damaged hardware or software, corrupted intercommunications, and lost processing throughput? The cost of interruption may justify investment in power and grounding improvements.

What power quality-related events will interrupt data input, processing, and useful output? For example, even with continuous power to processing hardware, interruption of a separately powered process cooling plant could halt operations. While water-cooled processors may be able to withstand a few degrees of temperature change, they cannot stand loss of circulation.

Are operations dependent on communication lines that also require continuity of electrical power? In other words, you must decide whether or not a telephone system should be on conditioned power.

Is the backup or alternate site operated from the same power source, and are its communications also dependent on electrical power? You should avoid dependency on common facilities (the same power poles, side of the street, or underground trench) whenever possible.Facility location considerations. Although some factors are out of your control, you must think about how they may affect operation of your facility. To help sort things out, consider the following:

Will the site be subject to natural hazards? Storm damage from wind, rain, snow, sleet, ice, lightning, flooding, tidal waves, moving ice, fire, earthquakes, and mud slides are some items that could affect operations.

Will the site be subject to man-made hazards? Vandalism, sabotage, malicious mischief, arson, vehicle collisions, riots, insurrections, or even war (in extreme circumstances) are all possible causes of interruptions.

Will the facility be subject to offsite damage to support services? Power and communications as well as gas, water, and sewer lines may be vulnerable.

Will the site be subject to lightning and excavation damage? Even if installed underground, services damage may occur. If there is more than one power feeder and communications cable to the building, each should be placed in a separate trench.

Are the local electrical safety and other building codes overly restrictive in permitted construction materials and assembly methods? Unnecessary restrictions can dictate the use of expensive labor-intensive construction materials and assembly methods, which can increase initial installation costs and continuing maintenance expenses.

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Commercial power considerations. Facilities having high percentages of susceptible electronic equipment are extremely dependent on a source of dependable electric power. As such, you should analyze the service record of the utility serving the existing or proposed site. Here are some questions you should pose to your local utility:

What are its recorded power outage and disturbance records? Be careful here: Some utilities record only “outages,” which can be defined as an interruption of power lasting more than 5 minutes. Intermittent breaker operations of 5 to 20 Hz might not be normally recorded. Therefore, make sure you ask for the number of breaker operations counted on the facility's feeder per quarter per year.

How does its power quality history compare with other utilities serving other sites? Use the site analysis surveys offered by site preparation and coordination companies. These studies indicate what items are typically measured and reported.

What are the power quality records and system operating performances of other end-users in the area? Make sure you compare each history with the different types of power conditioning devices in use.

What support does the local electric utility provide? Many utilities offer customers a variety of programs for analysis, design, and equipment selection to help enhance power and grounding infrastructure. This may even include equipment leasing or provision of premium quality power supplied directly from a utility-owned power-conditioning substation.

What are the available utility options for normal and backup feeders? Depending on the facility location, the utility can provide a dual-feeder service, which can eliminate expensive long-term power support equipment such as UPS systems, batteries, and engine generators.

Can the utility provide separate distribution lines fed from separate substations? This allows for utility service maintenance and accidental outage on one feeder without sacrificing continuous power. You should use the same strategy for telephone and data communications cables.

Is the cost of electricity excessively high at the facility site? Here, you should compare the electricity costs at different locations. Find out if there are demand charges and/or low power factor penalties. In other words, find out what effect will system efficiency have on actual operating dollars.Coordination and planning with CPU vendor.. The characteristics of solid-state electronic equipment vary per manufacturer. Therefore, it's important that you have specific performance data on the proposed equipment. The following is a list of things you should do with the selected or prospective CPU vendor:

1. Obtain general specifications for overall power quality, grounding, and communication installation.2. Obtain an equipment list that contains specifications on line frequency (with tolerance limits and maximum

rate-of-change); line voltage(s), phases, nominal value(s) with upper and lower limits; load characteristics for each EDP unit, including kVA, kW, phases, and amperes on each phase for both normal and starting; combined system kVA, kW, power factor, amperes (both rms and peak), and heat output in BTU/hr or equivalent metric units; maximum starting inrush currents for units having the 10 largest values; and site preparation and installation planning manuals.

3. Discuss the proposed system to determine the CPU characteristics. How well will it operate, and how will it fail? Concentrate on the general specifications in determining specific areas of power and grounding enhancement or failure protection, in accordance with IEEE 1100 (the Emerald Book).

4. Determine the existing or anticipated conditions to be encountered at the facility and obtain an agreement from the vendor on the protection that will provide the best operating integrity for the system. Make sure you discuss the following items:

o Power system quality: Providing a constant source of relatively undisturbed electrical power of adequate

load capability, suited for the loads, and maintained within limits set for nominal values of line voltage, frequency, wave shape distortion (harmonics), voltage sags, impulse surges, transients, electrical noise, and other attributes of the power source.

o Solid-state electronic equipment susceptibility: Equipment should withstand the disturbances and

aberrations of electrical environments in which it is expected to operate, especially the power source.

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o Compatibility of load equipment and power sources: This includes harmonizing their respective

characteristics, including interactions between electronic equipment and their power sources. Where source and load both consist of high, dynamic, internal impedances, the risk of distorting the electrical wave shape is high. These distortions can easily lead to power conditioning applications causing more interference than they are supposed to remove.

o Grounding, referencing, and shielding: Provide equipotential signal referencing and shielding that

is compatible with grounding requirements for safety.o Power distribution: Avoid unwanted coupling by employing isolation and coordination of power circuits

and their respective grounds.o Conductor sizing and over current protection: Protection from faults and overloads, with special attention

to the effects of harmonic load currents.o Lightning and surge suppression/protection: Providing lightning, power switching, and electrostatic

discharge (ESD) protection system.5. Discuss with the vendor any conditions that will make the system fail. The definition of “power

interruption” involves time duration. The fact that single-phase AC line voltage varies sinusoidally and passes through zero twice each cycle does not mean there is a power failure every 1/120 th second. Electronic equipment is typically designed to store enough energy in its DC filtering elements to ride through each AC zero crossing plus some additional time. Most electronic equipment can withstand a one-half cycle interruption without disturbing the filtered and regulated DC power supplied to logic circuits.

6. Make sure those responsible for electrical wiring and apparatus installation are reasonably responsive to the solid-state vendor and end-user's needs. They should be adequately trained and have the necessary understanding to work cooperatively with the end-user to solve power quality-related problems. In addition, they must also be able to merge noise-control procedures with their construction practices.Remember, the most important needs of susceptible electronic equipment are a dependable source of electrical energy, a grounding system that provides a stable platform for consistent operation, and protection against transient surges. You must address these factors as well as others at several different levels if your facility operation efforts are to be truly successful.Next month, we'll cover design of the power system that will serve the susceptible electronic equipment, matching system power requirements to power conditioning alternatives, grounding for consistent noise suppression, redundancy requirements, data communications protection, and lightning protection.

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Coordination Guidelines for Susceptible Electronic Loads — Part 2Ensuring uptime and protecting against power quality problems step by stepAs noted in Part 1 of this article, starting on page 22 of the September 2006 issue, there are many factors to consider before deciding how to provide suitable power, grounding, communications, and environmental control for a facility with significant amounts of susceptible electronic equipment. In the last installment, we discussed continuity of processing operations, facility location considerations.As noted in Part 1 of this article, starting on page 22 of the September 2006 issue, there are many factors to consider before deciding how to provide suitable power, grounding, communications, and environmental control for a facility with significant amounts of susceptible electronic equipment. In the last installment, we discussed continuity of processing operations, facility location considerations, commercial power issues, and coordination and planning with the CPU vendor. Now let's get into the actual design parameters.Designing/upgrading the power system Having gathered as much of the required background information on the facility and equipment as possible, you can begin the actual planning and engineering of the new or upgraded power system that will serve susceptible electronic equipment in your facility or your client's. Just follow the steps listed below.Step 1: Compare power requirements with available power. Here's where you should compare the requirements of any susceptible electronic equipment with the existing or available power source. Remember, you can use a shielded transformer at the point of use for both voltage transformation and common-mode voltage isolation. You do the latter by referring the secondary voltage to a central grounding point.Typically, the feeder voltage to the IT room or area is 480V, 3-phase, while the utilization voltages at most of the solid-state devices is 208V, single-phase or 120V, single-phase.Step 2: Determine the supply voltage to the PDU. If the equipment in the IT room or area is to be fed by a power distribution unit (PDU) containing an isolation transformer and output circuit breakers, you must verify what input voltage is best and most efficient for the application. The voltage can be 208V, 3-phase, 480V, 3-phase, or 600V, 3-phase. The higher voltages are more efficient, have a lower percentage line voltage drop, and generally cost less to install for a given kVA rating.In this type of installation, the secondary output circuit breakers and conductors feeding individual load units are not called branch circuits because they are system interconnections rather than part of the building wiring. As such, the power peripherals and their interconnecting cables are subject to UL 60950, “Safety of Information Technology Equipment,” examination and listing rather than inspection under the NEC or other applicable electrical codes for building wiring.Step 3: Itemize loads and prepare wiring connection schedule. First, place each susceptible electronic load that draws more than 5A on a separate power peripheral circuit, with its own circuit breaker and interconnecting power cable.Second, arrange all single-phase loads so that they will be evenly distributed over the three phases and neutral. In other words, balance these loads as best as possible.Third, decide which loads must be fed from the PDU. Remember, not all electronic loads need to be fed from the same power source to avoid ground potential differences. By using separate shielded isolation transformers, you can feed specified loads from the UPS in the PDU. Other less critical loads, such as printers, for example, can operate on “commercial” power.Fourth, use separate shielded isolation transformers to feed easily disturbed loads, such as memory, for example.Fifth, check for loads having a DC component of current. These may include devices with half-wave or unsymmetrical rectification or SCR control.Finally, check for loads having a very high harmonic content in their load currents. These can create operating problems due to saturation or overheating the first upstream transformer or motor-generator

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set. Besides high temperature rise, you may see excessive operating costs, poor efficiency, and distorted output voltage and input current waveforms. Such effects can render UPS systems inoperative and can cause very high peak magnetization current pulses at the inputs to some distribution transformers. You should refer such issues to the device manufacturer to verify what can be done to eliminate or reduce the problem. Otherwise, you may need to install special line conditioning equipment such as tuned filters.Step 4: Examine site layout proposals for environmental compatibility. Here, you should be asking the following questions:

Has the plant manager or engineer placed and sized processing cooling locations according to heat-producing areas on the floor?

Has the IT manager addressed present and future under floor space needs? What flexibility has the IT manager planned for expansion of the power, grounding, and communications

protection systems? Has the IT manager planned for a central operator station for environmental control or access? Has the IT manager planned the routing, type, and physical location of data communications for both

present and future compatibility with the site?Step 5: Coordinate vendor-provided equipment power conditioning with overall power quality goals. Individual vendor-provided power conditioning techniques may interact with each other to create unfavorable conditions, resulting in disrupted data processing. Make sure you examine each vendor device for its internal impedance so that you can coordinate all equipment having high and low dynamic impedance. By doing so, you'll soften and possibly reduce the distortions between individual solid-state power supplies.Step 6: Incorporate unit cabling restrictions with room layout plans. Make sure you locate each unit so that it will not interact physically with any surrounding sensitive units, such as locating printers near open disk drives, allowing print dust to infiltrate the drives.Matching system power requirements to power conditioning alternatives Part of your design process should include finding answers to the following questions.How much capacity (kVA, kW) is needed? While the sum total of power in a specific list is the total “connected” load, the actual measured power usage may be less because each unit may not have all the options installed and may not draw the maximum connected load continuously or simultaneously.First, make sure you include allowances for future system growth. Power requirements for memory and controllers, multiplexers, and exchanges needed to address and share large memory may grow faster than the rest of the system.Second, divide the total power (W) by the floor space devoted to data processing use. Typical large systems use 50W/ft2 to 60W/ft2. So, a 50-ft × 100-ft room might involve 250kVA of power capacity. If your proposed installation's power capacity is substantially more or significantly less than this, you should verify the reasons why.Can the power source handle short-term demands? In other words, verify if the power source's internal impedances and momentary overload characteristics are adequate to handle short-term demands without installing more capacity than needed for the steady-state load. Basically, the internal impedances should be low enough so that transient currents will not create excessive load-induced line voltage disturbances.Have you verified the operating efficiency under partial load conditions? The cost of additional electric energy for a drop of 15% in efficiency (say from 88% down to 73%) can be in excess of $10,000 per year, for every 100kW of load.You should compare the estimated total load with the expected steady-state actual running load of the system, and then use the result in determining the “partial load” ratio as a percentage of the nameplate kVA/kW of the power conditioning device. Rarely do installed systems run higher than 50% to 60% of the nameplate capacity of the power source.

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To ensure optimum efficiency, make sure you request, in writing, specific equipment manufacturers' guarantees at 50% load operation, with the guarantee containing wording noting compensation to the end-user for operations not meeting this standard.What are necessary power conditioning characteristics? Without supplemental energy storage devices, many units will ride through a 5 msec to 20 msec interruption of power without malfunction, provided that noise impulses associated with the interruptions do not reach and corrupt digital signals by other paths.Ferroresonant and synthesizer transformers may slightly enhance ride-through capability in some cases, provided the loads are not sensitive to phase shifts during their correction of line voltage variation, and that the loads can be limited to 75% of the device rating.Motor-generators can extend ride-through to as long as 20 seconds. Make sure you check the manufacturer of the equipment to be protected to determine whether this equipment needs synchronous 60-Hz power or whether it can tolerate induction motor drives with lower (varying) frequency of output.UPS systems can typically extend ride-through from 5 minutes to 30 minutes or longer. UPS installations with emergency standby diesel engine-generators can extend ride-through to almost indefinite lengths of time.Examine the power conditioner or independent power source for its limits on load current, kVA, and kW output. This will vary with the power factor of the load. Some devices, such as static UPSs, have very little overload capability and depend upon a stiff, low-impedance bypass power source (usually the serving public utility) to supply large starting loads. In addition, verify that the power source and any bypass will supply all inrush needs and still provide the needed power quality.If you intend on using voltage regulators, make sure you verify that their response times are fast enough to follow line voltage changes, yet not interact with regulators in sensitive loads.Finally, determine if a step-by-step approach to power conditioning will benefit your installation. For optimum lowest dollar outlay, first consider a power conditioner with provisions for in-the-field conversion back to battery back-up support at a later date, without loss of initial investment.What are the predicted line voltage sags, swells, and impulse transients? You can expect ordinary switching of loads to create momentary impulse voltages as high as the peak value of the sine wave.What are the significant sources of load-induced transients? You should consider the following ideas to minimize the effects of these transients:

Specify and order “soft start” system operations. Put units with high inrush on separate circuits. Put sensitive units and other units that create disturbances on separate shielded isolation transformers.

Grounding for consistent noise suppression One of the most important items you should consider in any design is the required signal and safety grounding. During the design process, determine the following.Are grounding requirements of solid-state equipment manufacturers consistent with the IEEE Emerald Book? If not, you should discuss any differences and evaluate any underlying rationale. In fact, the manufacturer may have good reasons for its differing requirements.Is a shielded isolation transformer required or recommended? This affects the point where logic ground conductors and power source neutral grounding points will come together at a common point.How will all conductors be brought to the point of delivery to your system? Verify that all power, communications, and grounding conductors will come through one very close-coupled “entrance.” Scattering the entrances and exits for the wiring increases the risk of noise voltages and transient impulses circulating through the system.Where will the system's central grounding point be located? If a modular power center, such as a PDU, is used, the central grounding point may be located within this equipment.Is the IT room raised floor support structure suitable for use as part of the SRG?  In other words, does the raised flooring's structure come with interconnected bolted horizontal struts that are suitable for use in a

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signal reference grid. This could save much money compared with construction using copper conductors or straps, and could also enhance performance.Is there separation between ground conductors for different equipment? Grounding conductors for susceptible electronic equipment should be separated from those for nonsensitive equipment, except at some upstream common connection, which typically is at the building service entrance or other common separately derived power source.Will the communications and power grounding systems be bonded together? This should be done at an appropriate upstream point. It's needed for safety and to minimize noise voltage differences without providing conducting paths through the grounding conductors of the susceptible systems.After answering these questions, you should verify all of the following items:

All grounding conductors and conducting pipes penetrating the sensitive IT area are bonded together by a short, robust connection outside the area.

All susceptible units and their accessories are listed or approved by UL or other acceptable agencies that are recognized by the municipality in which the units are to be installed.

The premises wiring meets the local and NEC requirements. The installation complies with the listing by the manufacturer.

Redundancy requirements Now that you've designed the basic power system for the susceptible electronic equipment, you need to explore the necessity for redundancy as part of further protection by following these steps.Step 1: Perform a simple failure analysis. Here you would assume that each piece of equipment, its power source, wiring, and specific wiring device could fail or must be de-energized for maintenance or service. First, determine if the electronic systems will continue to function and recover in such instances. Then, determine if redundant bypass paths in the power sources around conditioning equipment and major pieces of electrical apparatus will provide the ability to continue in the event of failure, service, or replacement. Keep in mind probable failure rates and frequency of necessary maintenance.Step 2: Determine how future growth relates to redundancy. In other words, should you include a margin for future growth in the form of smaller distribution units rather than initially selecting a single unit that is adequate for current and all future growth?Step 3: Determine if floor space and HVAC capacity are sufficient. These are important considerations in that they affect the ability to install new systems and get them operational before dismantling the existing ones. You may not need power conditioning for this purpose because of reserve capacity, but you may face the problem of transferring the power source, with a minimum of service interruption, to the new system.Lightning protection Protection from transient voltages caused by lightning can be a major factor in areas having a high incidence of thunderstorms. Following are important items you should address in providing this protection.First, ensure the new or existing facility structure has a lightning protection system installed by a Master Label contractor in accordance with NFPA 780, “Standard for Installation of Lightning Protection Systems.” As a point of reference, buildings in which structural steel is bonded together by welding (as opposed to reinforcing steel in concrete, which can be electrically discontinuous or merely touching) offer better lightning protection of circuits.Second, make sure that conductive parts from roof-mounted equipment (lightning rods, etc.) do not create a path to ground via susceptible circuits. Air conditioning cooling pipes from the roof to air handlers should not become a direct path for lightning to reach critical circuits or central grounding points. Also, verify that all metallic conduit and/or pipe runs enter and exit at a single access point and that all parts are bonded together and to building steel.Third, make sure that lightning protection down conductors are separated from power and communication circuits by at least 6 feet to protect against induced noise. Also, these conductors should be taken to ground via a path separate from the equipment grounding system.

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Fourth, protect all incoming power and communications equipment conductors with surge protection devices having shunt overvoltage paths to ground and series impedances to limit surge currents. The place for the secondary lightning protection is at the building service entrance. You should also place supplementary protection at the input and output of load devices such as rectifier/chargers for UPS installations, motor-generators, and voltage regulators, as well as the load devices themselves and their communication ports.The location in which susceptible electronic equipment is installed requires a special electrical infrastructure that will reduce and even eliminate power quality problems and ensure reliable equipment operation. Basically, your electrical infrastructure design should include a dependable source of power, a grounding system that provides a stable platform for consistent equipment operation, and protection against transients.

Dealing with Voltage Sags in Your FacilityUnderstanding the sensitivity of all parts of your system processes is critical in avoiding unwanted costs associated with equipment downtimeIf you're in charge of power quality at your facility, the very thought of a power outage not to mention the slew of problems that accompany one should make you shudder. But don't underestimate the damage voltage sags can impose on your systems. Typically affecting equipment and process operation much more frequently than actual outages, these disturbances occur whenever there is a fault on the power.If you're in charge of power quality at your facility, the very thought of a power outage — not to mention the slew of problems that accompany one — should make you shudder. But don't underestimate the damage voltage sags can impose on your systems. Typically affecting equipment and process operation much more frequently than actual outages, these disturbances occur whenever there is a fault on the power supply system, regardless of whether or not the fault actually causes an outage.For instance, a short circuit in a transmission line rarely causes an outage (due to the network nature of the transmission system), but it can cause many customers to experience a short-duration voltage sag (typically 3 cycles to 12 cycles) that may affect many processes. This makes it even more important for you to understand the sensitivity of your equipment to voltage sags — and the options available for protecting it.How many voltage sags can you expect to see? The number of voltage sags that can occur at your facility depends on where you're located, the characteristics of your utility's distribution system (underground vs. overhead, lengths of the distribution feeder circuits, and number of feeders), lightning level in the area, number of trees adjacent to the power lines, and several other factors.From 1993 to 1995, EPRI conducted a major benchmarking project for utility distribution systems across the United States (An Assessment of Distribution System Power Quality, Volumes 1-3, TR-106926, V1-V3, EPRI, Palo Alto, Calif., 1996). Let's call this study DPQ1. Based on the results of this project, distribution utility customers could expect an average of about 18 events per year in which the minimum voltage would go below 70% (referred to as SARFI-70). This is much higher than the average number of outages a customer could expect, which is about 1.3.

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EPRI performed a follow-up study (Distribution System Power Quality Assessment: Phase II, TR-1001678, EPRI, Palo Alto, CA, 2003) using results from the permanent monitoring systems that many utilities have in place. Let's call this study DPQ2. The particular focus of this study was on understanding voltage sag performance and the effect of various system factors on this performance. The results of this second study were very consistent with those from the EPRI DPQ1 study.How sensitive is your equipment? The main factor that determines whether voltage sags will affect your facility is the sensitivity of the equipment in your process. The sensitivity curve developed by the Information Technology Institute Council (ITIC) shows that computer equipment should be able to ride through short-duration voltage sags, if the voltage doesn't go below 70% (Fig. 1). For sags of longer duration, voltages below 80% could affect the equipment.

The semiconductor industry developed a recommended equipment voltage sag tolerance curve that specifies improved ride-through for the first 200 milliseconds, which provides substantial benefit for many voltage sags. Figure 2 shows example voltage sags plotted along with the ITIC and SEMI F47 tolerance curves. (SEMI F47 refers to the semiconductor industry standard, Semiconductor Equipment and Materials International, 1999.). The Table summarizes the expected voltage sag performance for different types of systems, comparing the performance you can expect if your equipment has ride-through characteristics specified by either ITIC or SEMI F47.

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Unfortunately, you often don't know the actual sensitivity of the equipment within a facility; therefore, you don't know whether or not voltage sags will affect them. However, there is reference information you can use to judge equipment susceptibility to voltage sags. According to the book Electrical Power Systems Quality (ISBN 0-07-138622-X), there are three categories of equipment sensitivity:

1. Equipment sensitive only to the magnitude of a voltage sag. Here, the important characteristic is the sensitivity to the minimum (or maximum) voltage magnitude experienced during a sag (or swell), with the duration of the disturbance usually being of secondary importance. Devices in this category include under voltage relays, process controls, motor drive controls, and many types of automated machines, such as semiconductor manufacturing equipment.

2. Equipment sensitive to both magnitude and duration of voltage sag. The important characteristic of this group is the sensitivity to the duration of which the rms voltage is below a specified threshold where the equipment trips. This group includes almost all equipment using electronic power supplies.

3. Equipment sensitive to characteristics other than magnitude and duration. Some devices are affected by other sag characteristics, such as phase unbalance during a sag event, the point-in-the-wave at which the sag is initiated, or any transient oscillations occurring during the disturbance.These characteristics are more subtle than magnitude and duration, and their impacts are much more difficult to generalize.Where's the best place to protect equipment? There are many options for protection of equipment sensitive to voltage sags. You can apply protection at levels ranging from the entire plant all the way down to individual components of a specific tool. If a significant portion of the equipment in the plant is sensitive, it might be worthwhile to consider protection at the plant level. However, it's unusual for all the equipment in your facility to require support during voltage sags. In fact, many types of equipment can ride through these short-duration events without a problem.Adjustable-speed drive (ASD) manufacturers offer options for riding through voltage sags. Most motor loads can ride through voltage sags without affecting the process due to the inertia of the motor and the load — unless the motors drop out because of the sensitivity of contactors or relays protecting the motor.Before you spend a lot of money on power conditioning equipment to protect your entire process, make sure the process controls themselves aren't causing the whole process to shut down during voltage sags. Programmable logic controllers (PLCs), relays, and contactors are often the most sensitive equipment. Contactor holding coils and relays can drop out during a voltage sag, resulting in the entire process shutting down. In these cases, it may be possible to improve the performance of the whole by protecting the circuit.There are many options for protecting individual loads or groups of equipment within a facility. Although you can use traditional UPS systems, this may not be the best solution. If most of the events affecting the process are voltage sags and not interruptions, there are probably many more economical alternatives. Even if you need protection for short-duration interruptions, new options with flywheels, superconducting magnets, or capacitors for energy storage might be preferable because they're much smaller and don't require battery maintenance.Other devices you can use at the equipment level include:

Constant voltage transformers (ferroresonant transformers), which are 1:1 single-phase devices that are excited at the high point on their saturation curves, provide an output voltage that is not significantly affected by input voltage variations.

Magnetic synthesizers, which are 3-phase devices that take advantage of their 3-phase magnetics to provide improved voltage sag support and regulation.

Active series compensators (1kVA to 5kVA, single-phase), which boost voltage by injecting a voltage in series with the remaining voltage during a voltage sag. These devices are also referred to as dynamic voltage restorers (DVRs), dynamic sag correctors (DySCs), or automatic voltage conditioners (AVCs).For ASDs, you may be able to purchase an option for ride-through from the manufacturer. If not, one vendor offers a device to support the voltage in the DC link of the drive during voltage sag conditions,

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allowing the inverter to continue operating and supplying voltage to the motor until the input voltage returns to normal.Of course, adopting equipment specifications that specify voltage sag ride-through levels, such as SEMI F47 (ride-through capability during short-duration voltage sags of 50% of nominal), can help assure a minimum capability at the equipment level. (See Sidebar below)For most industrial facilities, you can use medium-voltage-rated active series compensators to boost the voltage during voltage sags. This type of protection can be much more economical and require less maintenance than UPS systems because little or no energy storage is needed.It's also possible to improve performance through supply system modifications, such as static transfer switches and fast transfer switches. Static transfer switches, available in medium-voltage ratings, use power electronic switches to make the transfer within about a quarter of an electrical cycle. Fast transfer switches that use vacuum breaker technology can transfer in about two electrical cycles, which may be fast enough to protect many sensitive loads.Finding the best option. Finding the optimum investment in technologies to improve voltage sag compatibility depends on the number of voltage sags you can expect, the cost of disruptions to your process, and the characteristics of your equipment.Deciding on the best alternative for improving voltage sag ride-through performance at your facility is a problem that comes down to simple economics. First, you have to understand the sensitivity of the equipment and how much it costs every time voltage sag negatively affects the equipment. Then you need information from your electric utility so you can estimate the number of voltage sags on your system per year. With this information in hand, you can then determine your costs associated with voltage sags. The optimum solution will minimize the combined costs of the ride-through solution and the resulting losses from the events not solved by the specific solution — the cost of the solution plus the cost of the disturbances.The solution costs are lower as you focus on the particular equipment and controls that are sensitive. However, this approach may have additional costs associated with characterizing the sensitivity of the process components and installation. Understanding the sensitivity of all parts of your process is usually very worthwhile in coming up with the best solution.

Sidebar: Improving Equipment Ride-Through CharacteristicsUltimately, better equipment design is the best long-term solution for voltage sag problems. If manufacturers offered options for improved ride-through, it might be more economical to purchase these options than to install external devices for protection.The semiconductor industry, in cooperation with EPRI and electric utilities, recognized this and developed a set of standards to provide better compatibility between the equipment characteristics and the characteristics of the utility supply. The SEMI F47 standard specifies an improved voltage sag ride-through for process tools. It requires a ride-through down to 50% voltage for 200 milliseconds, which will significantly reduce the number of voltage sags that may cause process disruptions in semiconductor plants.Many other industries can use this as a model to improve compatibility. In fact, EPRI Solutions has been working with utilities and industries such as the automotive industry and the food processing industry to help develop more widespread guidelines for equipment voltage sag performance.

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Coil Hold-In Device Solves One Plant's Sag ProblemVoltage sags and momentary interruptions plague power quality professionals worldwide. No

business can afford the costly production losses that sometimes occur when small control relays de-energize during these events. This was the case for one facility in Marietta, Ohio. The plant, which produces polyvinyl styrene pellets for end-use processing at other facilities, found the control power circuits for its cooling water systems were particularly susceptible to voltage variations. Plant personnel wanted a cost-effective solution that would fit inside their existing control center and avoid the need for an extensive motor control redesign.

Voltage sags and momentary interruptions plague power quality professionals worldwide. No business can afford the costly production losses that sometimes occur when small control relays de-energize during these events. This was the case for one facility in Marietta, Ohio. The plant, which produces polyvinyl styrene pellets for end-use processing at other facilities, found the control power circuits for its cooling water systems were particularly susceptible to voltage variations. Plant personnel wanted a cost-effective solution that would fit inside their existing control center and avoid the need for an extensive motor control redesign.

The pellet plant houses 3-phase AC induction motors for pumps, compressors, agitators, extruders, vacuum systems, and cooling water systems. Adjustable-speed drives (ASDs) control some of the critical motors, and an emergency generator is available in case of a power failure.

During deep voltage sags (like the one shown in Fig. 1) and momentary interruptions, nearly all of the plant's process equipment would trip offline. In addition, plant personnel noticed that, during minor voltage sags, the cooling water controls were more sensitive than the rest of the process controls. In fact, they would trip offline when everything else stayed online. If the cooling process was unavailable, then the manufacturing process would have to be stopped, resulting in out-of-spec product and lost revenue.Motor Control Problem

The cooling process at the plant operates by a series of motor-driven pumps, fans, and cooling-tower fans with various controls for temperature and flow rate. It may implement ASDs and almost always uses standard motor control circuitry. The process control is the same for the plant's pumps and fans.

To understand how voltage variations impact motor control circuits, take a look at Fig. 2. It shows a typical start/stop circuit where the start button closes the loop to allow the coil of the control relay (CR) to energize and close the CR contacts.

When the contacts close, coil M energizes and the M contacts close, completing the path required to bypass the start button. The M contacts causing the 3-phase, motor-driven pump process to start are not shown. At this point, they are closed and the pump starts.

Everything works fine until someone presses the stop button and de-energizes the CR coil. When the CR coil de-energizes, the CR contacts open. Then coil M opens the contacts going to the pump, thereby stopping the process and shutting everything off until the start button is reenergized either manually or remotely. There are a multitude of variations on this theme, depending on safety philosophy, preferred control strategy, and other considerations. The basic start/stop concept, however, is similar.If most start/stop controls are designed this way, then what's the big deal? Let's assume that everything is running smoothly and the process is delivering high-quality product. If a squirrel runs across a transformer in the substation ten miles from the plant, the control circuit may experience a voltage sag. Although no one pushed the stop button, the sag causes coil CR to de-energize and the cooling water pumps to shut down. This leads to a chain reaction of safety shutdowns, all because of a disturbance that lasted less than 1/15th of a second.

For this case, engineers decided to focus on the plant process's weak link — the motor control circuitry. To prevent these process elements from tripping out, they decided to use power conditioning at the control-circuit level to momentarily hold the contacts and starters closed during voltage sags.

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Holding “In” for a SolutionTo achieve these ends, engineers chose a coil hold-in device (see the photo). The coil hold-in

device improves the voltage-sag and interruption tolerance of relays, contactors, and motor starters. You connect it between the AC source voltage and the coil of the relay or starter you want to protect. During voltage sags, the device maintains a current flow through the selected relays, contactor coils, or motor starter coils — with sufficient coil energy to hold in the 3-phase, power-circuit electromechanical device's contacts.

Reliable hold-in devices provide a controllable current for the coils of sensitive relays and starters, and they enable coils to be virtually unaffected by voltage sags. If electric power fails, or if someone pushes the emergency stop button, the coil hold-in device immediately disengages the selected coils. In addition to the sag ride-through benefit, preferred hold-in devices also provide surge protection that prevents damage to the coils from high-energy transients. Fig. 3 compares the coil performance with and without a hold-in device installed.Conclusion

Use of a coil hold-in device at the polystyrene processing facility significantly reduced cooling-system dropouts. Plant personnel had previously estimated production losses at $5,000 per dropout. They spent approximately $14,000 to improve the control ride-through for all of the pumps' cooling tower fans and the instrument air controls. The majority of this dollar figure went to engineering costs for the development of new control-circuit drawings. With an average dropout rate of 13 per year, plant personnel should see payback on their investment in less than four months.

Coil hold-in devices provide a cost-effective solution for applications at the control-circuit level. The key to these devices, or any other common control-level solutions, is to know the exact causes of the nuisance-tripping problems and their affected components. Once you've gained a clear understanding of the problem, you must follow through to ensure personnel safety and equipment protection.