unrestrained low-impedance bus differential – should...

Unrestrained Low‐Impedance Bus Differential – Should I Use It?

Unrestrained Low-Impedance Bus Differential – Should I Use It?

Thomas Ernst – GE Grid Solutions Ernest Poggi – Xcel Energy, Colorado

Mohamed M. Omer – Xcel Energy, Colorado

Contact author:

Thomas Ernst GE Grid Solutions 4523 Jeremiah Rd Cookeville, TN 38506 715-718-1384 [email protected] 1.0: Abstract The work horse of low‐impedance bus differential protection is the restrained percent differential element but many modern microprocessor relays also include an unrestrained (AKA: high‐set or instantaneous) element which, when not properly applied, can cause false trips. In this paper, we explore the advantages and disadvantages of the use of unrestrained low‐impedance bus differential protection including a look back at the history of bus differential and the evolution of the unrestrained element. At the end the authors draw conclusions about the applicability of the element and cautions associated with using it. 2.0: Introduction The fundamental function of low‐impedance bus differential protection is the restrained percent differential element. Modern microprocessor based relays have increased the popularity of restrained low‐impedance bus differential elements since they are typically faster and offer more functionality than microprocessor based high impedance bus differential elements. These modern microprocessor low‐impedance differential relays are typically multi‐function relays, including over‐current, directional control, metering and unrestrained (AKA: high‐set or instantaneous) differential elements. While this multi‐functionality offers flexibility and versatility the unrestrained differential element can cause false trips if not properly applied. In this paper, the authors explore the advantages and disadvantages of the use of unrestrained low‐impedance bus differential protection including a look back at the history of low‐impedance bus differential and the evolution of the unrestrained element. A case study of an actual unrestrained bus differential element mis‐operation is used to illustrate the application concerns and support the conclusions. At the end the authors draw conclusions about the applicability of the element and cautions associated with using it.


3.0: History of low impedance bus differential and the evolution of the unrestrained element The earliest differential relays were differentially connected induction disc time over‐current devices. The time delay was needed to handle inrush current on transformers and CT errors including saturation. These were essentially low‐impedance unrestrained differential elements. For buses, the inverse time characteristic appeared to be a good match since the larger faults operated faster. While this may be a good strategy for internal faults, security against CT saturation on external faults required the time delay be set for slow clearing, even on large faults. Since the relay had no way of distinguishing between internal faults and external faults with CT saturation, the required tripping times were too long for good bus protection.

Figure 1: One‐line diagram for the connection of a low‐impedance unrestrained bus differential using an inverse‐time over‐current relay.

In the 1930’s it was seen that bus differential zones were prone to mis‐operations for external faults using a simple overcurrent relay differentially connected the CTs. Prior to the establishment of standards for instrument transformer performance, it was difficult to accurately predict the CTs’ operation during short circuits. Eventually it was observed that using a toroid design core with evenly distributing windings on all the secondary winding’s taps allowed the user to neglect the CT’s series reactance. Following this realization, there were several attempts to design relays that handled the deficiencies in performance for transient behavior. One early attempt at applying a straight overcurrent to the scheme utilized an evaluation of the time constant of the primary circuit, the available short circuit current on the secondary and the standard error (< 10%) for a current generating 20 times secondary rated current (20 * 5 or 100 amps secondary rating). [ref 1] This is still a criterion that we hear about today and, at locations electrically remote from generating stations, this can be applied with a margin for satisfactory results. It is well advised to look at the circuits that are connected to the bus and the accuracy expected of the impedance modeling of the connected machinery and lines to assure that the fault current X/R ratio is low.


The next evolution in bus protection was the electro‐mechanical high impedance bus differential relay (PVD, KAB, etc.). The high impedance relay scheme utilizes a voltage coil to measure error voltage and avoid circulating error currents in its operating winding created by a saturated CT on an external fault. Current is circulated in the secondary of the saturated CT from the current contributions from the other CTs. The saturated CT does not replicate its primary current but is assumed to have a shorted magnetizing branch whereby the primary is shunted and the secondary current from other CTs is also shunted. The voltage drop from the CT summation point to the saturated CT is the error voltage that is considered and a margin is selected and a voltage threshold is set to establish a trip voltage. To protect the operate circuit from transient overvoltage a nonlinear resistor is installed. In many designs an over‐current element is placed in series with the nonlinear resistor to measure leakage current. A current threshold can be set for the leakage current that corresponds to the trip voltage plus a margin derived from the nonlinear resistor’s characteristic plot. [ref 2]

Figure 2: Connection diagram for the high impedance bus differential relay

The electro‐mechanical low impedance restrained bus differential was developed to improve security during external faults (CA, IFD, NBD, etc.). Each winding current is passed through restraint coils on the induction unit (IU) which produce a restraining torque. The differential current is passed through an operate coil on the IU to produce an operating torque. If the operating torque exceeded the restraining torque the relay operates. These relays operate with no intentional delay. However, even though the IU only needs to rotate a few degrees, the operating time is highly variable as a function of the restraining torque with operating times from 0.07 – 0.7 seconds (see Figure 3: Operating time of typical electro‐mechanical low impedance bus differential relay).


Figure 3: Operating time of typical electro‐mechanical low impedance bus differential relay

Electro‐mechanical low impedance bus differential relays did not include an instantaneous unrestrained differential element operated by the differential current only. The IFD relay does have an instantaneous unit operated by the differential current but it is used as a fault detector check‐zone for the restrained element and does not trip the bus directly. During the brief reign of analog electronic relays, several analog electronic high impedance bus differential relays were offered (BE1‐87B, SBD, RADSB) but the authors have only found one example of a restrained analog electronic bus differential relay (RADSS). This is an electronic relay that makes use of a hybrid type of circuit that allows for CTs of different ratios and classes to be utilized in a differential scheme. This is a medium impedance relay rather than a low impedance relay. All CTs are brought to the relay and auxiliary windings are provided for the CT ratios to be taken to the comparator. An analog electronic comparator evaluating operate vs. restraint is made in similar fashion to through current restraint relays. No unrestrained element was provided.


With the introduction of microprocessor based relays the restrained low impedance bus differential relay came into popular use. Microprocessor based restrained low impedance bus differential elements have much faster and more consistent operating times than their electro‐mechanical predecessors and are typically faster than microprocessor based high impedance bus differential elements. Considering the ability to capture all the CT waveforms and to run security check zones such as directional checks and CT saturation detection, the microprocessor based low impedance restrained bus differential is fast becoming the preferred method of protecting HV and MV buses. 4.0: Modern Unrestrained Low Impedance Bus Differential As with most microprocessor based relays, the modern low impedance restrained bus differential is a multi‐function device including such things as over‐current with directional control and metering. Most manufacturers also offer an unrestrained bus differential element. As discussed earlier, there was no electro‐mechanical counterpart to this unrestrained element other than the use of a simple inverse time over‐current relay connected differentially. While the authors found no clear information on the reason behind offering an unrestrained bus differential element, the idea might have come from the transformer differential relay where the unrestrained or instantaneous element has long been used to trip a faulted transformer during in‐rush when the restrained element is severely restrained or blocked. Of course, this issue does not directly apply to buses since they do not exhibit notable in‐rush when energized. There is a sensitivity concern with any restrained differential during external faults when the restraint current is very high causing a corresponding high differential current tripping threshold. Considering that transformers often fail during external faults; this desensitization can cause delayed tripping on such transformer failures. This is not a serious problem for restrained bus differential since external faults do not typically cause internal bus faults. The one exception would be an internal fault in a bus boundary breaker that trips to clear an external fault. In this case, the external fault essentially evolves into an internal fault and there will be no significant additional operating time as the restrained element moves between the restraint and trip regions. Regardless of the origins of the modern unrestrained bus differential element, the use of it cannot be justified by the fact that the manufacturer provides it. Nor can the user assume that the element will be useful and secure for the user’s application if the manufacturer’s setting examples are followed. Like all elements in multi‐function relays, the user takes ultimate responsibility for the decision to use the element and to be sure that the settings selected are applicable to the installation and that the application is secure. As we mentioned earlier, the restrained low impedance bus differential characteristic was developed to deal with CT errors including saturation. With high region 2 slope settings, this element is quite secure against mis‐operations by itself. Add directional supervision and CT saturation detection and this modern restrained element is extremely secure against mis‐operations during external faults. So why would a user want to apply an unrestrained bus differential element? One advantage of the unrestrained element is that it is slightly faster than the restrained element for high level internal faults. In the electro‐mechanical world, this would have been a significant advantage since the restrained element’s operating time was highly variable. However, the speed of the restrained element in the modern microprocessor relay is faster and more consistent. Since both the restrained and unrestrained elements operate on the fundamentally filtered waveforms, the operating time of the unrestrained is typically on the order of 2 – 4 milliseconds faster than the restrained element. Considering that the operating time of the restrained element is typically in the range of 12 – 20 milliseconds, the increased speed of the unrestrained element is not significant in most applications.


The unrestrained element has several application concerns. The user needs to clearly define what problem they are looking to solve before they enable the unrestrained element. Unlike transformer restrained differential elements, the restrained bus differential element will operate for any fault where the unrestrained element will operate so there are no “holes” which we need the unrestrained element to fill. Consequently, there is not a compelling reason to enable the unrestrained element if the speed of the restrained element is adequate for system stability. The biggest concern with any differential element is CT saturation. CT saturation can occur because of very high fault current magnitudes. Generally, this is predictable and the pickup of the unrestrained element can be set higher than the expected differential current caused by CT saturation. However, CT saturation can also occur due to DC off‐set and remnant flux. The remnant flux problem is especially concerning as it is difficult to predict and can cause unexpected high differential current during an external fault. This high differential current can exceed the unrestrained element pick‐up, causing a false trip. The security of the unrestrained differential element can be improved with supervision by a directional element but this will negate speed advantage that the unrestrained element has over the restrained element.

5.0: Case Study: Unexpected Operation of a 115 kV Unrestrained Bus Differential Element during an external Fault. The substation where the fault occurred includes two 115 kV yards owned by different utilities. The service one‐line diagram is shown in Figure 4. The first 115 kV yard owned by the first utility is a double‐breaker configuration with five bays and two main busses: East and West. The second 115 kV yard owned by the second utility is a main‐and‐transfer bus configuration with four bays. The main 115 kV bus of the second yard is tied to 115 kV East bus of the first yard via a normally closed tie breaker, 9999. Further, a normally open transfer breaker, 9990, on the 115 kV East bus in the first yard can be used to carry load on any of the elements in the second 115 kV yard via the transfer bus.

Each of the 115 kV busses in the first 115 kV yard are protected with dual bus differential. The metering and relaying (M&R) one‐line diagrams for the 115 kV West bus primary and secondary relays are shown in Figure 5 and 6, respectively. The primary protection for each bus is a high‐impedance high‐speed bus differential scheme using a solid‐state technology based relay. The secondary protection for each bus is a low‐impedance high‐speed bus differential scheme using a microprocessor based relay. In the secondary low‐impedance protective scheme, both the restrained and unrestrained bus differential elements were applied. All of the breakers in this first 115 kV yard are 3000 ampere type with multi‐ratio bushing current transformers (CTs) that are C800 class with a full tap of 1200/5. Each bus protection scheme uses dedicated CTs set at the full tap of 1200/5.


Figure 4: Service One‐line Diagram of the Substation

Figure 5: M&R One‐line Diagram for the 115 kV West Bus Primary Differential Relay


Figure 6: M&R One‐line Diagram for the 115 kV West Bus Secondary Differential Relay

The 36/48/60 MVA 115/44/7.2 kV autotransformer bank #2 is protected with two 115 kV high‐side breakers (9922 and 9992) and single 44 kV low‐side breaker (4651). This autotransformer bank is protected with dual transformer differential schemes. Both the primary and secondary protection for this autotransformer bank employ dual‐slope percentage transformer differential schemes using microprocessor based technology from different manufacturers. In addition to the transformer differential scheme, the secondary protection also includes high‐side backup time‐delay phase distance and tertiary overcurrent schemes. The M&R one‐line diagrams in Figure 5 and 6 show the CTs used for autotransformer bank primary and secondary differential relays. Phase and ground overcurrent protection is applied at the 44 kV low‐side using electromechanical relays.

As mentioned above, the dedicated bushing CTs used for the 115 kV West secondary low‐impedance bus differential relay are class C800 and set at the full tap of 1200/5. The saturation voltage (VS) for these CTs is approximately 440 V. The burden (RS) on the CTs used for this relay from breaker 9922 is approximately is 2.88Ω. The maximum unsaturated symmetrical fault current is as follows:

_4402.88Ω

152.78

The maximum symmetrical fault current via breaker 9922 external to the 115 kV West bus zone of protection is approximately 20,000 primary amperes, and it occurs for a 3‐phase fault at the 115 kV high‐side bushings of the autotransformer bank #2.

20000240

83.33


Since this maximum symmetrical fault current is below the maximum unsaturated current calculated above, the CTs should adequately reproduce the maximum fault current. That is, the CT will be in the linear region for this maximum and the exciting current or magnetizing will be minimal, below 1 secondary amperes. However, other factors such the DC offset component of the primary fault current and the remnant flux in the core may limit the linear operation of the CT significantly. Because of such factors, the CT may saturate for AC currents below the maximum unsaturated value calculated. The unrestrained element pickup on the 115kV West bus secondary low‐impedance differential was securely set based on the manufacturer recommendation and taking into account checks and margins for errors from AC saturation, DC offset, and DC saturation. The magnetizing current of the saturated CT will appear as a differential current. Assuming that 50% the maximum fault current calculated above would show as magnetizing and therefore error differential current, the unrestrained element pickup should set above the following value.

_ 283.33

241.67

Using a 20% security margin, the unrestrained differential element pickup was set as follows:

1.2 ∗ 41.67 50.00

A phase‐to‐ground (B‐G) fault took place on a pot head at the 44 kV low‐side of the autotransformer bank #2. This fault was cleared by the low‐side overcurrent protection; the low‐side breaker, 4651, tripped and auto‐reclosed. Twenty minutes later, a second phase‐to‐ground (B‐G) fault at the 44 kV low‐side of the autotransformer took place and then evolved into 115 kV high‐side phase‐to‐phase to ground (B‐C‐G) fault within the autotransformer tank. The location of the high‐side B‐C‐G fault is shown in Figure 7. All of the 115 kV breakers on the 115 kV West bus in addition to the 44 kV breaker, 4651, tripped. It must be noted that the 115 kV breaker, 9992, was out of service at the time of this event. Event records and relay targets indicated the following relays operated for this fault:

1. The autotransformer bank 2 primary differential relay 2. The autotransformer bank 2 secondary differential relay 3. The 115 kV West bus secondary low‐impedance differential relay


Figure 7: Location of the Evolving B‐C‐G 115kV High‐Side Fault

A review of the event records from the transformer bank 2 relays indicated that both relays operated for the phase‐to‐phase to ground (B‐C‐G) fault internal to the zone of the protection. The magnitude of the fault current was approximately 19,000 A at the 115 kV level. The fault was external to the zone of protection of the 115 kV West bus secondary relay but the unrestrained element on the 115 kV West bus secondary low‐impedance differential relay operated incorrectly. The restrained element on this secondary relay did not operate due to the saturation detector and directional check algorithms that supervise it. In addition, it must be noted that the 115 kV West bus primary high‐impedance relay did not operate for this external fault.

Figure 8 shows the unfiltered waveforms for the restraint inputs to the autotransformer bank 2 differential relay from the 115 kV high‐side, breaker 9922, and the 44 kV low‐side, breaker 4651, in addition to the digital elements. It is easily noted from the waveforms in Figure 8 that all the B and C phase CTs were saturated to some degree. It was also evident from the oscillography in Figure 8 that the bank 2 transformer differential operated on the unrestrained element; the restrained element did not operate due to blocking caused by significant second harmonic content in the differential current apparently created by the saturated CT secondary waveforms.


Figure 8: Unfiltered Oscillography from Bank 2 Transformer Differential Relay

Figure 9 shows the waveforms from all the restraint inputs to the 115 kV West bus secondary low‐impedance relay on B and C phases in addition to the digital elements within the relay. Figure 10 shows the differential and restraint current magnitudes for the B and C phases for the same relay. It is noted that waveforms from the B and C phases on breaker 9922 show significant signs of distortion due to saturation. The 115 kV West bus secondary low‐impedance relay incorrectly operated on the unrestrained differential element on the B‐phase differential zone; the same element on the C‐phase differential zone did not operate. Further, the restrained elements on both phases did not operate due to blocking from the saturation detection and directional check algorithms that supervise them.


Figure 9: 115 kV West Bus Secondary Differential Relay B and C Phase Current Waveforms


Figure 10: 115 kV West Bus Secondary Differential Relay B and C Differential and Restraint Magnitudes

Figure 11 includes the waveforms for the B and C phase restraint inputs from breaker 9922 wired to the 115 kV West bus secondary low‐impedance differential relay. It was obvious that both CTs showed some degree of saturation. The saturation of the B‐phase CT is much worse than C‐phase although the peak current magnitude is 20 kA on C‐phase versus 15 kA on B‐phase. Figure 12 shows the phasor diagrams for the restraint inputs for the B‐phase differential zone before and after the initiation of the 115 kV B‐C‐G fault. Note that the phasor diagram after the initiation shows significant change in the phase angle of the B‐phase current from breaker 9922 (shown as L5 in Figure 12), which likely represents another sign of acute saturation of the CT. It is important to mention here that the CTs wired from breaker 9922 to the 115 kV West bus differential relay are identical and have similar secondary wiring; therefore, one would expect similar response.


Figure 11: 115 kV West Bus Secondary Differential Relay waveforms from BKR 9922 B and C CTs

Figure 12: 115 kV West Bus Secondary Differential Relay B‐Phase Phasors


There was extensive testing done by both the owner and the relay manufacturer to confirm the initial conjecture of saturation resulting in a major bus trip and outage. The oscillography of the relays, while very informative, is subject to some filtering response and some gaps. There are indications of significant waveform distortions. Information lacking on the oscillography and needing confirmation included: CT polarity, ratio, remnant flux. The tests performed were the routine tests as those outlined in IEEE 57.13, specifically the excitation test for turn‐turn insulation and core health, primary to secondary ratio tests, and polarity tests. Further, secondary conductor insulation testing as well as single point grounding was performed. The results of all of these tests confirmed there were no latent defects in the circuits. All plots for the CTs on the 9922 breaker were confirmed to have similar knee point voltage and were consistent with the C‐800 classification. Tests were also performed on the relay with secondary currents of 90 amperes were injected into the relay elements and were seen to remain stable for external fault simulation. This test was made in addition to the normal pickup and through current restraint tests. The relay was returned to the manufacturer where it was subjected to the full set of acceptance tests used to confirm that the A/D circuits performed correctly and that the waveforms captured in the oscillography were accurate representations of the actual waveforms experienced. The results of all the testing solidified the initial conclusion that a long time constant transient as a consequence of point on wave for fault inception in a high X/R network source likely occurred and fooled the unrestrained differential element to trip.

General observation about this fault condition:

1. CTs on breaker 9922 wired to the 115 kV West bus secondary low‐impedance differential relay, the B and C phases, were heavily saturated during this fault. The fault magnitude was under maximum symmetrical current that can be reproduced without saturation based on the CTs’ class and manufacturer provided data. Extensive testing process proved the CTs were healthy, had the correct secondary wiring, and should have operated within the provided specification. Therefore, it is concluded that the CTs were heavily saturated due to remnant magnetization caused by the sequential faults. That is, immediately prior to the erroneous trip of the 115 kV West bus secondary low‐impedance differential relay was a transformer through‐fault event, and the interruption for that through‐fault left remnant flux on the CTs. The remnant magnetization coupled with high offset and long decay time for the subsequent fault at the 115 kV high‐side of the transformer bank 2 created an even greater excursion into saturation beyond the contemplated margin that was part of the original setting calculation.

2. On the 115 kV West bus low‐impedance differential relay, the restrained differential element did not erroneously operate under the same heavy saturation conditions for which the unrestrained element erroneously tripped. The restrained element did not over‐trip because of the supervision provided by the saturation detection and directional check algorithms, which both worked as expected for this heavy saturation external fault conditions.

3. The unrestrained differential element on the 115 kV West bus differential relay, as with other manufacturers, is only 2 – 4 milliseconds faster as compared to the restrained element. However, this extremely minor gain in speed comes at greater cost of reduced security. Such security risks were found to be unjustified. Therefore, it was decided to disable the unrestrained differential element in the low‐impedance bus differential at this location and all other location where low‐impedance differential elements.


4. While we rely on the good engineering practices of manufacturers, we also accept the real‐life issues with analog‐digital conversion and the filtering. Filtering is time dependent and the sampling rate rejects higher order harmonics. There are response characteristics in magnitude and phase. The input circuits are analog and the auxiliary transformers and hardware based filters are designed to be simple pass through functions that should not contribute to error. Phase shift is possible but not expected. However, in this type of fault, the phase to ground fault evolving to phase to phase to ground in a partial cycle following a low side‐transformer fault, it is reasonable to expect that the unrestrained high set element is subject to significant error from the severely saturated CT that produces the current. The simple design of the unrestrained differential element allows it to be fast but appears to be insufficient to discriminate for this fault. While all networks produce a response that will have some error in phase and magnitude and we expect that the relay is designed correctly to provide for most situations without tripping on external faults, it is clear that the standard setting calculation method identified in the manual does not contain adequate margins for this unique type of evolving fault.

5. In spite of the heavily saturated CTs and distorted waveforms, the unrestrained transformer differential elements on the relays protecting bank 2 operated correctly as backup functions for the restrained transformer differential elements, which were inhibited by second harmonic blocking. In contrast, the unrestrained differential element in the 115 kV West bus secondary differential relay incorrectly operated for this condition. That is, while the unrestrained differential element provides indispensable backup function and is well suited for the transformer protection applications, the unrestrained differential element serves no necessary function and carries serious security risks in the bus protection applications.

6.0: Conclusions The fundamental function of low‐impedance bus differential protection is the restrained percent differential element. Modern microprocessor based relays have increased the popularity of restrained low‐impedance bus differential elements since they are typically faster and offer more functionality than microprocessor based high impedance bus differential elements. These modern microprocessor low‐impedance differential relays are typically multi‐function relays, including over‐current, directional control, metering and unrestrained (AKA: high‐set or instantaneous) differential elements. While this multi‐functionality offers flexibility and versatility the unrestrained differential element can cause false trips if not properly applied. The unrestrained differential element in transformer protection is clearly needed as a means of tripping a failed transformer when the restrained element is severely restrained or blocked during energization. There is no similar concern with bus differential since the restrained element is expected to operate for all internal faults where the differential current exceeds the minimum pick‐up. The only clear advantage that the authors have found of the unrestrained bus differential element is that it is typically 2 – 4 milliseconds faster than the restrained element. The user must seriously consider weather this slight speed advantage justifies the security risks associated with the unrestrained bus differential element.


7.0: References [1] "Current Transformer Concepts", Zocholl and D. W. Smaha, 46th Annual Georgia Tech Protective Relay Conference, Atlanta, GA, April 1992 [2] “Instantaneous Bus‐Differential Protection Using Bushing Current Transformers”, Seeley, Von Roeschlaub, AIEE Vol 67 PP 1709‐ 1718, 19486+9 [3] “Considerations in Applying Ratio Differential Relays for Bus Protection”, R. M. Smith, W. K. Sonnemann, G. B. Dodds, AIEE Transactions Vol 58, June 1939, PP 243 – 306. [4] “Relaying Current Transformer Applications”, Gabriel Benmouyal, Jeff Roberts and Stanley Zocholl, WECC Relay Working Group, June 10, 2014 [5] “Selecting CTs to Optimize Relay Performance”, Gabriel Benmouyal, Jeff Roberts and Stanley Zocholl, 1996 Pennsylvania Electric Association Relay Committee Fall Meeting, September 1996. 8.0: Biographies Ernest Poggi: Ernest Poggi received his BSEE from the University of Lowell (Lowell Technological Institute) in 1976. He has been involved in the design and operation of power system substations and system protection equipment and study work for 41 years including time with Florida Power & Light, Tri State G&T and Xcel Energy in Colorado, where he is a registered Professional Engineer in the State of Colorado. He was the Lead Engineer on the Lamar HVDC Converter Station. He has co‐authored papers delivered to Western Protective Relay Conference and CIGRE. Mohamed M. Omer: Mohamed M. Omer is a Senior System Protection Engineer at Xcel Energy. He has been with Xcel Energy in the system protection engineer since 2010. Prior to joining Xcel Energy, Mohamed has been with PPL Electric Utilities as Relaying and System Analysis Engineer. He received his Bachelor of Science (2006) and Master of Engineering (2008) in Electrical Engineering from Pennsylvania State University. He is a registered Professional Engineer in the State of Colorado and a member of the IEEE Power and Energy Society. Thomas Ernst: Tom Ernst is a P&C Technical Application Engineer for the GE Grid Solutions North American Commercial team. He has been with GE since 2011 supporting the Grid Automation Protection and Control Portfolio. Prior to joining GE, Tom has been with Minnesota Power as a Supervising Engineer, Delta Engineering International as a Manager of Electrical Engineering, HDR Engineering as a Manager of Electrical Engineering and Northern States Power as a Supervising Engineer. He received his Bachelor of Science in Electrical Engineering from the University of Minnesota in 1978 and his Master of Science in Power Systems from Michigan Technological University in 2008. He is a registered Professional Engineer in the State of Minnesota and a long-time member of the IEEE Power and Energy Society and Professional Communications Society.

unrestrained low-impedance bus differential – should...

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