plc based load sharing report

ProgrammingTerminal

CPUOutputmodule

Memory

Power Supply

InputModule

PLC Based Load Sharing

Introduction

Efficient load control of electrical generating plant is of critical importance to the operation of a DP Classed vessel. Although DP vessels have been operating for almost thirty years and Classification Society Rules governing the design and construction of such vessels have been in place, and undergoing continuous revision, since the nineteen eighties, experience has shown that a number of misconceptions remain where load sharing systems are involved.

This paper seeks to identify the most common load control and power management philosophies, their strengths and weaknesses in relation to DP classification and to assist the system designer in determining the most suitable implementation for a given project application.

Although a number of design solutions exist in the more complex implementations the basic topologies and philosophies are largely similar and will be treated generically in the paper. However, in less complex systems solutions may be unique to a particular manufacturer. In such instances the manufacturer will be specifically identified. This identification is not intended to indicate the author’s endorsement or preference for any one product in relation to another.

PLC HARDWARE

A programmable logic controller consists of the following components:

Electronics & communication Eng. Institute of technology, Nirma University 1


Figure 1 PLC Hardware Block Diagram

Central Processing Unit (CPU). Memory. Input modules. Output modules and

Power supply.

A PLC hardware block diagram is shown in Figure. The programming terminal in the diagram is not a part of the PLC, but it is essential to have a terminal for programming or monitoring a PLC. In the diagram, the arrows between blocks indicate the information and power flowing directions.

CPU

Like other computerized devices, there is a Central Processing Unit (CPU) in a PLC. The CPU, which is the “brain” of a PLC, does the following operations: Updating inputs and outputs. This function allows a PLC to read the status of its input terminals and energize or deenergize its output terminals. Performing logic and arithmetic operations. A CPU conducts all the mathematic and logic operations involved in a PLC. Communicating with memory. The PLC’s programs and data are stored in memory. When a PLC is operating, its CPU may read or change the contents of memory locations.

Scanning application programs. An application program, which is called a ladder logic program, is a set of instructions written by a PLC programmer. The scanning function allows the PLC to execute the application program as specified by the programmer. Communicating with a programming terminal. The CPU transfers program and data between itself and the programming terminal.A PLC’s CPU is controlled by operating system software. The operating


PLC Based Load Sharingsystem software is a group of supervisory programs that are loaded and stored permanently in the PLC’s memory by the PLC manufacturer.

Memory

Memory is the component that stores information, programs, and data in a PLC. The process of putting new information into a memory location is called writing. The process of retrieving information from a memory location is called reading. The common types of memory used in PLCs are Read Only Memory (ROM) and Random Access Memory (RAM). A ROM location can be read, but not written. ROM is used to store programs and data that should not be altered. For example, the PLC’s operating programs are stored in ROM.

A RAM location can be read or written. This means the information stored in a RAM location can be retrieved and/or altered. Ladder logic programs are stored in RAM. When a new ladder logic program is loaded into a PLC’s memory, the old program that was stored in the same locations is over-written and essentially erased.

The memory capacities of PLCs vary. Memory capacities are often expressed in terms of kilo-bytes (K). One byte is a group of 8 bits. One bit is a memory location that may store one binary number that has the value of either 1 or 0. (Binary numbers are addressed in Module 2). 1K memory means that there are 1024 bytes of RAM. 16K memory means there are 16 x 1024 =16384 bytes of RAM.

Input modules and output modules

A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energize outputs. The decisions are made based on the statuses of inputs and outputs and the ladder logic program that is being executed. The input devices used with a PLC include pushbuttons, limit switches, relay contacts, photo sensors, proximity switches, temperature sensors, and the like. These input devices can be AC (alternating current) or DC (direct current). The input voltages can be high or low. The input signals can be digital or analog. Differing inputs require different input modules. An input module provides an interface between input devices and a PLC’s CPU, which uses only a low DC voltage. The input module’s function is to convert the input signals to DC voltages that are acceptable to the CPU. Standard discrete input modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and transistor-transistor logic (TTL) level.

The devices controlled by a PLC include relays, alarms, solenoids, fans, lights, and motor starters. These devices may require different levels of AC or DC voltages. Since the signals processed in a PLC are low DC voltages, it is the function of the output module to convert PLC control signals to the voltages required by the controlled circuits or devices. Standard discrete output modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and TTL level. Electronics & communication Eng. Institute of technology, Nirma University 3

PLC Based Load SharingPower Supply

PLCs are powered by standard commercial AC power lines. However, many PLC components, such as the CPU and memory, utilize 5 volts or another level of DC power. The PLC power supply converts AC power into DC power to support those components of the PLC.

Programming Terminal

A PLC requires a programming terminal and programming software for operation. The programming terminal can be a dedicated terminal or a generic computer purchased anywhere. The programming terminal is used for programming the PLC and monitoring the PLC’s operation. It may also download a ladder logic program (the sending of a program from the programming terminal to the PLC) or upload a ladder logic program (the sending of a program from the PLC to the programming terminal). The terminal uses programming software for programming and “talking” to a PLC.

WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the I/P module. At the output of I/P module the field status converted into the voltage level required by the CPU is always available.

At the beginning of each cycle the CPU brings in all the field I/P signals from I/P module & stores into its internal memory called as “PII”, meaning process image input.

The programmable controller operates cyclically meaning when complete program has been scanned; it starts again at the beginning of the program.

Figure 2. Working Of PLC

A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and


FIELD

SIGNALS

INPUT

MODULE

CPU

……………………

:

PII : PIQ

:

PLC Based Load Sharingnot worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.

Figure 3. Process in PLC

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? Howabout the third... It records this data into its memory to be used during the next step.

Step 2-Execute Program-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.

Step 3-Update Output Status-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.

Process Control and Automation

Process Control

“The process of recognizing the state of the process at all times, analyze the information according to the set rules and guidelines and accordingly actuate the control elements is referred to as process control.”

RECOGNISING THE STATUS


PROCESSTHE

INFORMATION

ACTUATECONTROL ELEMENTS

RULESAND

GUIDELINES


Figure 4. Process Control

In control of process all these actions can be taken manually with human involvement or in a semiautomatic or fully automatic manner.

Automation

“Automation is basically the delegation of human control functions to technical equipment aimed towards achieving:

Higher productivity. Superior quality of end product. Efficient usage of energy and raw materials. Improved safety in working conditions etc.”

Methods adopted for Process Control and Automation

Manual control Hard wired logic control Electronics control PLC control

Hardwired Control

This was considered to be the first step towards automation. Here the contractor & relays together with timers & counters were used.

Electronics Control

With the advent of electronics, the logic gates started replacing the relays & auxiliary contractors in the control circuits & timers.

With changes, the benefits are:


RULESAND

GUIDELINES

PLC Based Load Sharing 1) Reduced space requirements

2) Energy saving 3) Less maintenance and hence greater reliability etc.

With electronics, the implementation of changes in the control logic as well as reducing the project lead-time was not possible.

Programmable Logic Controller

With microprocessor and associated peripherals chips, the process of control and automation went a radical change.

Instead of achieving the desired control or automation through physical wiring of control devices, in PLC it is through a program or software. Thus these controllers are referred to as “programmable logic controllers”.

The programmable controllers have experienced an unprecedented growth as universal element. It can be effectively used in applications ranging from simple control like replacing small number relays to complex automation problem.

Load Sharing: An Overview

All generating sets require some form of load control. This is true even for single generatoroperation. The central component in any load control scheme is the engine governor. Twoprincipal governor types are available:

1. Mechanical2. Electrical/Electronic

The basic operating characteristics and principles of mechanical governors have changed little since the invention of the Steam Engine Governor by James Watt in 1788. The governor works by means of heavy balls, which rotate on the end of linkages and move in or out because of centrifugal force according to the speed of rotation.


PLC Based Load SharingThe movement of the balls closes or opens the steam valve to the engine. When the engine speed increases too much, the balls fly out, and cause the steam valve to close, so the engine slows down. The opposite happens when the engine speed drops too much. Instead of controlling steam valves, the modern diesel mechanical governor controls the position of the fuel rack on the engine and regulates the flow of fuel into the cylinders

Figure 5. The Watt Steam Governor 1788

The Electronic Governor

The electronic governor, which may use analogue or digital means, seeks to replicate the actionsof its older counterpart using electronic sensors to detect flywheel speed and electrical load. Electrical load is determined by measuring the inter-phase voltage and current drawn from the generator, a proportional DC voltage is applied to a summing point where it is compared with thespeed signal derived from a magnetic pick up device on the engine flywheel. The resultant outputis applied to an amplifier which produces the drive signal for the engine fuel rack actuator.



Figure 6. Basic Electronic Control with Droop Function

Droop is a means by which the engine may be regulated such that its speed is proportional to theload taken. By fixing such a value several generators may be connected in parallel and broadly share the total load. This is the purpose of the resistor bridge network shown in Figure 2.

Overview about several modes

Droop Mode :Droop mode of genset operation provides the highest level of failsafe operation in a power distribution network. However, since no two engines have identical reaction properties all droopcontrolled networks must be constantly monitored to ensure equal load distribution and to prevent accidental loss of any one unit due to over or under load conditions.

Isochronous modeIsochronous mode ensures equal distribution of load across all connected units and permits individual load control of any one unit to permit temporary maximum loading conditions. Thesetwo factors permit a maximization of load and maintenance efficiency which cannot be achievedin the droop mode of operation.

Analogue Isochronous Control

Pure analogue isochronous control contains inherent design flaws which preclude its use in DPClass 2 operations unless the implementation is specifically designed for split bus operation. Split bus mode of operation in isochronous mode tends to negate the basic advantages of thisdesign.

Digital Isochronous ModeImplementation of digital solutions to load share and synchronisation has the potential of providing a cost effective solution to the issues involved in the design and operation of a modern


PLC Based Load Sharingdiesel electric DP vessel. However, no design product range known to the authors has the necessary built in redundancy to permit direct implementation in a DP Class 2 load sharing andpower management system without the inclusion of additional monitoring and control equipmentin the system.

It is considered that, as long as the DP market remains a small percentage of the marine powermanagement total market that little change will be seen in this field.Of all the implementations discussed only two carry full Class Type Approval. Use of thealternate methods requires individual application for acceptance to the Class Society, this carriesadditional cost/time burdens to the project.

Isochronous Control

Isochronous control of a number of generators may be accomplished by disconnecting the DroopAdjust resistor and connecting the voltage developed across this point to the same point in the other generators connected to the bus. In this way the load shared may be directly proportional to


PLC Based Load Sharingthe relative capacities of the connected generators. This is illustrated in Figure 3.

Figure 7.Basic Electronic Governor with Isochronous Control

This circuit is the basis of all analogue load sharing devices on the market today. It can readily be seen that a short circuit across the load share point in the bridge circuit would result in all connected generators being commanded to reduce speed to minimum and shed all load.

Distributed Load Sharing


PLC Based Load SharingIn distributed load sharing topologies the load information is passed on some form of serial datacommunications network between either individual governors or a centralised power management system. On large installations, such as current generation drilling vessels, distributed load sharing is often performed as part of a distributed vessel management system. Such a system is illustrated in Figure 4.

Figure 8. Distributed Load Management System

In this format there are several layers of redundancy, the power management controllers, thecommunications network, the PLC controllers and, finally, the governor.

In such installations it is common to operate the governor in ‘droop’ mode with external controlof speed, and therefore load, being exercised by the power management network. It can be seenthat, two faults are required in any part of the control system before the governor is forced intodroop mode.

Networks of this type have been successfully implemented by ABB, Kongsberg, Alstom and Nautronix and may be adapted to reflect the main bus configuration with any one PLC controlling more than one individual generator set.Installation of a distributed vessel management system with such high levels of redundancy and complexity require considerable

capital expenditure and for this reason the designers of smaller vessels, must seek more cost effective solutions to the issues of load control.



The Woodward Digital Load sharing Solution

The Woodward Governor Co. has an extensive range of intelligent digital load control equipment. Products include electronic governors and automatic synchonising modules. Much of the product range uses the Echelon ™ communications bus protocol for data communication. A typical example of an automatic bus control network is shown below in Figure 5.

Figure 9. Standard Digital Load Share Network

Although the Echelon ™ bus is highly reliable and used in a number of industrial applications itis apparent that no bus redundancy is available in the event of a short circuit across the communication net. The internal software for the products does not monitor bus activity and thedefault condition of the controllers would be last speed commanded in the event of total communication failure. It is clear that this condition does not meet the requirements of DP Class2 redundancy.

PLC Based Load Monitoring

This solution may be used in either analogue or digital load-sharing systems and in some aspectsresembles a ‘cut-down’ version of the distributed VMS system described above in Section 4A dedicated PLC may be programmed to monitor the loads carried by each geneset. In the eventthat the load seen on any on unit exceeds a given window the PLC may command all units toenter droop mode. A failsafe watchdog unit monitors the operation of the PLC and can be used toalert the vessel staff in the event of PLC failure. In devising such a system considerable attention must be made to programming of the tolerance window and load averaging. Speed of response to anomalous situations is also a critical factor.



Figure 10. PLC Based Load Share System

An Economic Distributed Solution

OEM of Houston have developed a similarly cost effective solution to the issue. This approach was taken for the need for the provision of an economic load sharing scheme for offshore jack-upPlatforms, although it was recognized that the solution could equally be applied to marine Propulsion and DP power plants.The basic topology of the OEM solution appears similar to the distributed control networks Described above and although it may be used a part of a fully distributed network it has a number of advantages in designs where budget is a major consideration.

OEM have developed a Woodward 2301 series electronic governor engine which can be inserted in a standard Allen Bradley PLC5 series controller rack.. The 2301 electronic controller is basically similar to the analogue controllers described above.

The PLC5 system may be configured with ‘hot’ back-up controllers and a redundant stable Communications network thus giving a high level of redundancy compatible with the requirements of DP Class 2 systems. Theoretically, the worst case failure is the loss of either PLC backplane or failure of the 2301 governor engine card, resulting in loss of no more than oneGenerator to the distribution network.



Figure 11. Distributed Load Management

Hybrid Control

Legacy Automation Power and Design of Houston are currently developing a hybrid system which makes use of both analogue and digital techniques.The system combines governor, synchronization, and load sharing capabilities as well as externalinterfaces for use in a general power management scheme. The system shown in Figure 11 is for a single bus system but can easily be expanded for multiple bus operation.

The load share line is arranged in a ring loop and is, therefore, resistant to any single open circuitWithin the loop. A short circuit on the loop will be recognized by the load share voltage level Dropping to zero, the PLC controllers will respond to this state by entering droop mode control. A bus dedicated load monitor and control unit takes in raw power data from all connected controlPLCs and has the capability of reducing power consumed on all suitably equipped major consumers. This provision serves to enhance anti-blackout control in the main PMS.

Figure 12. Hybrid System, Legacy Automation



Conclusions

At the current time, a number of solutions to the issues discussed are available to the designer and these should be considered on a case by case basis.It is incumbent upon the designer to be aware of the burdens placed by the necessity to meet therequirements of DP Class and the inherent advantages and disadvantages of the various maintropologies available.

References

Design Considerations for Electric Propulsion of Specialist Offshore Vessels,Taylor and Williams, Institute of Marine Engineers, October 1984 Governing Fundamentals, Woodward Governor Company, Product Manual 25195 Barber Coleman Company, Basic Governing Information In addition, the Author would also like to note the assistance of the following in the

preparation of this paper: Alstom USA, Houston Kongsberg-Simrad USA, Houston Gerhardts Inc. New Orleans OEM Electric, Houston Legacy Automation Power and Design, Houston


plc based load sharing report

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