MONITORING FOR GEOMAGNETIC INDUCED CURRENT FLOW EFFECTS USING EXISTING EMS TELEMETERING

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Joseph N. Wrubel, Senior Memker, IEEE

Public Service E l e c t r i c and G a s Company

Newark, N J

Abstract

During the Solar Magnetic Disturbance (SMD) of March 13, 1989, which resulted in large magnitude Geomagnetic Induced Currents (GIC) in North America, it is believed that two phases of one of the generator step-up transformers at the Public Service Electric and Gas Company (PSE&G) Salem Generating Station were damaged. Subsequent to this event, a new application program was written on PSE&G's control center energy management system (EMS) to monitor the transformers at Salem and other locations in its bulk power system for the effect of GIC flows. This Monitor has successfully and accurately detected subsequent Solar Magnetic Disturbance (SMD) caused GIC effects in the transformers at both the Salem and Hope Creek Generating Stations. In turn, these measurements are used to alert the control center and generating station personnel of the hazard so that corrective action may be initiated.

Kev words: SMD, GIC, half-cycle saturation, EMS, MVAR consumption, operating instructions, excess MVAR, telemetering

Introduction On March 10, 1989, a gigantic solar flare erupted from a large sunspot area on the sun [l] . On March 13, 1989, the consequences of this event caused high magnitude GIC flows in North America. The most commonly known result of this solar storm was the Hydro-Quebec system collapse [2]. Less commonly known is that there was damage to two of the three phases of the Salem No.1 unit's generator step-up transformer ( G S U ) . This damage is suspected of having a GIC flow causal relationship. Immediately after the March 13, 1989 incident, the generation of combustible gases increased dramatically in these transformers phases. Two days later, the generating unit was taken out of service to inspect the transformers. The inspection revealed cumulative damage to two phases which were then condemned. All three phases of the GSU were replaced during the spring of 1989.

At the IEEE 1989 Summer Power Meeting at Long Beach, CA, a panel session was held to discuss the causes and consequences of solar distur- bances on electric bulk power systems. One of the concepts presented was that, in real-time, one could measure the effects of GIC flows on a transformer by summing the MVAR flow around the transformer. Based on this observation, PSE&G set about to implement such a Monitor on its EMS.

The Monitor The Monitor program is based on summing the MVAR around a transformer and subtracting from that result the W A R which theory predicts is being consumed by the transformer. It was felt necessary to subtract these known transformer MVAR requirements since, at full load, a Salem GSU requires up to 170 WAR, a number, which if ignored, would swamp out the effect for which the calculation is being performed. The conventional expression for the expected MVAR consumption is I * I * X where I is the current in each phase of the transformer and X is the positive sequence reactance of the transformer. If the metering were perfect and the reactance of the transformer was accurately known, the result of the Monitor calculation would be zero. In reality, however, the metering is not perfect and the reactance can change slightly due to internal transformer temperature changes associated with load level and ambient temperature.

In the specific case of Salem, insufficient real-time metering existed to isolate the MVAR on each side of the transformer to calculate the desired difference. The available metering consisted only of the two generator MVAR output meters and the three 500-Kv line flow W A R meters using single-phase, relay- quality primary instruments. See Figure 1 for the Salem switchyard simplified one-line diagram showing the location of these meters. No metering exists in the EMS on any of the station power transformers or the auxiliary power transformers at Salem.

In addition to setting up the algorithm for the sum of the residuals of the two Salem GSUs, algorithms were set up to calculate the residuals for the Hope Creek GSU, and for six other locations on the PSE&G system. These locations included three 500/230-Kv autotransformer stations, one 345/230-KV autotransformer station, one 230-138-Kv transformer station and one generating station with three GSUs.

The calculations are performed once every 3 0 seconds and, if any of the residuals calculated exceeds a preset limit, an alarm is created on the EMS. At the end of each eight hour shift, if any of the residuals has

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500 kv 500 kv

exceeded its alarm point in the prev shift, a hard copy report is created.

Basis of -ration When GIC flows through the upper layers of earth's crust, it encounter var resistances depending on the geology of material through which it is passing. In

Figure 1

ous

the

the ous

the case of the oceans, there is very little resistance and the current flow is unimpeded. On land, however, large deviations of subsurface resistances are encountered. In particular, igneous rock presents a high resistance to GIC. When such a high resistances is encountered, the current is diverted to any parallel path which presents a lower resistance to the current. If there are power lines which cross the igneous rock formation, and connections are available for the current to gain access to the power lines, then the currents will appear on the power lines. The connections which allow this diversion of currents can be delta-wye transformers with the wye solidly grounded on the high voltage side of the transformers. This is exactly the case at Salem.

Once the GIC is flowing through the transformer, it can cause half-cycle saturation of the transformer [3]. The consequence of this saturation is the severe distortion of the current wave form which can be looked at as the existence of many of the higher harmonics of the fundamental frequency currents. In the case of such GSUs, one can expect to find the 2,4,5,7,8,10,11 and 13th harmonics in fairly high magnitudes, depending on the magnitude of the GIC and the physical

Mvar me ttr %

construction characteristics of the transformer.

Looking at the calculation used in the Monitor, it is appropriate to examine the transformer reactance, X, used in the calculation. While most power engineers consider this number to be a constant for the transformer under steady state conditions, we tend to forget that it is very much frequency dependent and is derived as

X = 2*(pi)*f*L

where L is the inductance of the transformer and f is the fundamental frequency (usually 60 Hz in North America). When harmonics are flowing in the transformer, the apparent X is a composite number depending on the harmonic composition of the current wave form. Now it can be seen that the Monitor is actually measuring the effective change in reactance of the transformer caused by the distortion of the current wave form which in turn is caused by the half-cycle saturation of the trans- former due to the GIC flows.

A correlation has been demonstrated between the excess number of W A R consumed by a transformer due to saturation and the possible damage being caused to the transformer. In the case of PSE&G, a consultant was hired to provide this correlation for the specific GSUs installed at Salem and Hope Creek. Given this correlation, the Monitor can be used to predict when damage is occurring and the appropriateness of taking corrective action.

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Results On September 19, 1989, another SMD occurred which caused significant GIC flows in North America. The Salem Monitor created an alarm on the EMS. The residual or excess MVAR detected by the Monitor are shown in Figure 2. Also shown in Figures 3 , 4 and 5 are traces from the MVAR strip-charts for Salem No. 1, Salem No. 2 and PSE&G's Hudson Generating Station, 225 km (140 miles) from Salem. The correlation between the results of Monitor calculation and the MVAR recorded on the strip-charts is consistent in time, magnitude and direction.

This event is believed to have caused minor additional damage to one phase of Salem No.2'~ GSU. During the subsequent refueling of Salem No. 2, in November of 1989, it also was replaced. As a result of this event, an operating instruction was put in place at the PSE&G Electric System Operations Center (ESOC), Salem Generating Station and Hope Creek Generating Station which calls for the reduction of power output on the Salem and/or Hope Creek units if:

. a SMD of intensity K-5 or greater is

. the Monitor detects excess W A R requirements in one or more of the GSUs above its alarm point

known or forecast to be in effect

- the ESOC strip charts show the characteristic signature of GIC flows.

The outputs of the units have been reduced a small number of times since the September 1989 event in conjunction with this operating instruction and no further damage has been detected to the GSUs. In addition, another W A R meter has been installed at Salem on the 500-Kv side of the No. 2 GSU so that the calculated excess MVAR requirements of each of the two GSUs can be separated.

ImDlementation Considerations The Monitor was installed on a Honeywell 4010 purchased in 1972! This machine is in the twilight of its existence and will be retired as soon as PSE&G's new EMS is in service. [4] Since this old machine had almost no free time remaining, PSE&G elected to run the Monitor program only once every 30 seconds.

The code for the Monitor was written in assembly language. To conserve execution time, the problem was broken up into two pieces. First a loop was executed which, €or each transformer to be monitored, calculates its normal LossMVAR by accessing its Mw flow (P), its MVAR flow (Q) and its fundamental frequency, positive sequence reactance (X) - The loop proceeds to approximate the I * I term (which needs to be in per unit) by:

I * I = ( P * P 1 10000 + Q * Q 10000 )

Although it is possible, it was felt that the accuracy required did not justify the effort to correct this calculated current squared for the voltage which was known to be controlled between .95 and 1.05 for normal system operation.

The calculated LOSSMVAR for each transformer

is then stored in temporary locations within the program environment for later use.

Next, for each location where the Monitor is to function, a separate algorithm was set up incorporating the knowledge of the sign convention for the real-time metering, its location in the EMS data base, and the previously calculated transformer LossMVAR. In the case of Salem, the algorithm is as follows:

Excess W A R = MVAR(5021) + MVAR(5024) + MVAR(5037) + MVAR(Unit 1) + MVAR(Unit 2) - LossWAR(Unit 1 GSU) - LossMVAR(Unit 2 GSU)

A separate limit was provided for each location being analyzed by the Monitor and once the excess MVAR is calculated, it is compared with the limit. A value greater than the limit causes the alarm bit to be set for the alarm processor which then proceeds to annunciate the condition using the standard EMS alarm process. Once the excess W A R decreases below the limit by more than the deadband (initially set to 10 MVAR), the alarm bit is turned off and the EMS clears the alarm.

Further DeveloDment Once experience was gained with the Monitor, certain improvements became desirable. First, we wanted to separate the excess MVAR requirements of the two GSUs at Salem since there was a suspicion that only one of them was in trouble but we were reducing the output of both units to protect the GSUs. To this end, another MVAR meter was installed and its readings are now telemetered to the EMS. The Monitor was then modified to provide separate summations around each of the transformers.

Future Enhancements At certain locations being analyzed by the Monitor, the real-time meters do not add to zero in the absence of GIC, even taking transformer loss MVAR requirements into account. A modification of the Monitor would place a 'tuning constant' into each calculation to eliminate the effect of this error (it appears to be almost constant). Once this is accomplished, any indicated excess MVAR would indicate the effects of GIC flows.

The Monitor will be installed on the PSE&G's new EMS and will run at analog scan rate, about once every three seconds.

Conclusions For a relatively modest effort, any company or power pool operating an EMS can install software which can detect the occurrence of GIC flow in power system transformers. The computational efforts are modest, but the potential consequences of not protecting the transformers can be major, long-duration outages.

In addition, we have concluded once again that the telemetry brought to the EMS from remote locations using only single-phase, relay- quality primary sensing instruments (current transformers and coupling capacitors) is adequate for most EMS applications.

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References [l] J. G. Kappenman, V. D. Albertson,

"Bracing for the geomagnetic stormsf1, IEEE SPECTRUM, vol. 27, no. 3, pp. 27-33, March 1990.

[ 21 "March 13, 1989 Geomagnetic Disturbance", NERC 1989 SYSTEM DISTURBANCES, August 1990, pp. 36-60.

[3] R. J. Ringlee, J. R. Stewart, "Geomagnetic Effects on Power Systems", IEEE Power Engineering REVIEW, vol. 9, No. 7, pp. 6-9, July 1989.

[4] J. N. Wrubel, R. Hoffman, "The New Energy Management System at PSE&G" , IEEE Computer Applications in Power, vol. 1, no. 3, pp. 12-15, July 1988.

BioqraDhy Joseph N. Wrubel (M63, SM71) received the B.S. from Worcester Polytechnic Institute and the M.S. from the New Jersey Institute of Technology in 1961 and 1964 respectively, both in electrical engineering.

He joined PSE&G in 1961 and, since completing the cadet engineering program, has spent 8 years in transmission planning and 20 years in power system operations.

His present duties include the acquisition, development and maintenance of the advanced applications programs on PSE&GIs EMS.

Mr. Wrubel is a member of the IEEE Power Engineering Society and a Licensed Professional Engineer in the state of New Jersey.

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