some that solid-state digital relays are not subject to these problems. ... digital ground differential protection over a range of power transformer and CT sizes.
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Current Transformer Application With Digital Ground Differential Protection Relays Peter E. Sutherland (SM) Power Systems Energy Consulting GE Power Systems Schenectady, NY 12345 USA
Abstract Ground Differential Protection Relays (device 87G) have been used to protect the windings of resistance grounded power transformers. There have been problems of relay mis-operation and non-operation with electromechanical 87G relays due to current transformer (CT) burden and saturation. Due to their low burden, it has been assumed by some that solid-state digital relays are not subject to these problems. The purpose of this paper is to evaluate the truth of this assumption. Under what circumstances, and with what types of CTs, might problems occur? An analysis is performed of digital ground differential protection over a range of power transformer and CT sizes. Guidelines are developed for CT accuracy ratings suitable for this application. The effect of different relay algorithms on scheme performance relative to CTs is also discussed. Index Terms Differential Relaying, Ground Fault Protection, Transformer Protection
I. INTRODUCTION Industrial power distribution system substation transformers often utilize resistance grounded wye secondary windings for medium voltage power distribution. The purpose of this is to limit damage due to ground fault currents, while providing sufficient fault current for the operation of ground fault relaying. The relaying utilized for the protection against ground faults in the system may not provide sufficient protection of the transformer winding against internal faults because the back-up ground overcurrent relay in the transformer neutral to ground connection must be set to coordinate with downstream relays. In order to protect the winding itself, special relays are utilized. [5]
11. SINGLE FUNCTION MICROPROCESSORRELAY The schemes discussed here have been implemented with component-type relays, where one relay performs each function. A. Ground Differential Protection with a Time-Overcurrent Relay: The simplest method of ground differential relaying is to connect a time-overcurrent relay between the residual point of the phase CT's and a neutral-ground CT (Figure 1). Because the CT ratios are usually not equal, an auxiliary matching CT is required. When using an electromechanical relay, this application has required a neutral-ground CT with a high saturation voltage. The sensitivity required depends upon the portion of the winding 'to be protected. Assuming that the voltage is induced uniformly across the windings, a relay which is set at 5% of the maximum ground fault current will protect 95% of the winding. This would require a sensitivity of 20A for a 400A grounding resistor. The design issue is to select a relay-CT combination that will be sufficiently sensitive to cover the winding yet be insensitive to external faults. An external fault, in the worst case, will be the full 400A, and may cause saturation of the CT on the resistor. CT saturation 1200/5
Ground differential protection can be provided by digital overcurrent relays in conjunction with auxiliary ratio matching transformers. Ground differential protection can also be provided in multifunction digital relays. Transformer protection relays may include this feature using one of the schemes used with component relays. If a feeder protection relay is used on the secondary, in some cases this may have a ground directional feature that can be utilized for ground differential protection.
fig. 1. Time overcurrent relay connected as a differential ground relay.
0-7803-5843-0/00/$10.0002000 IEEE
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will occur when the voltage across the CT magnetizing branch exceeds the "knee point" of the saturation curve. The size of the burden impedance and the shape of the saturation curve will determine when saturation occurs. The ANSI class rating [8] of the CT specifies the voltage at which a 10% error in current is reached, and thus provides a good estimate of the saturation voltage without inspecting a saturation curve.
PERFORMANCEOF 810 WITH VARIOUS RELAY-CT COMBINATIONS
(&AD BURDEN 0.03 a) A. SEPARATELY POWERED RELAY
The burden on the line-side CTs is the relay burden times the square of the auxiliary CT ratio. Table 1 shows the calculation of the required minimum CT voltages where the leakage current is less than the relay tap setting. CT saturation calculation methods are given in [ 13 and [7]. With standard relays, a pickup setting of 0.5 or 1.OA would be used. A separately powered solid-state relay (Table l(a)) used in this application may have a typical burden of less than 0.lQ regardless of setting. Thus, there would be no potential problems of false operation on external faults due to CT saturation.
B. SELF POWERED RELAY
The use of a self-powered relay is illustrated in Table l(b). The relay burdens are given for a 0.5 Ampere relay for the 100/5 and 200/5 CTs. The relay burden for a 0.1 Ampere relay is given for the 400/5CT. The much higher burdens for the self-powered relay result in possible CT saturation in this application. Typical knee-point voltages are 17 Volts for a 100/5 CT, 30 and 60 Volts for the 200/5 and 400/5CTs. In this example, only the 200/5 CT is worth further examination for the self-powered relay. The last line of each table shows the effect of the auxiliary CT and relay impedance on the phase CTs. A typical knee point for a 1200/5 CT is about 200 Volts. In this example, only the 200/5 CT is worth further examination for the self-powered relay. A separately powered relay is recommended in this application.
111. MULTIFUNCTION MICROPROCESSOR RELAYS A. Ground Differential Protection with a Percentage Differential Rehy Relay Function New transformer protection schemes are usually built using multifunction microprocessor relays. Some relays of this type incorporate a ground differential function. This may be implemented as a percentage differential relay with inputs from each of the phase CTs and the ground CT. The relay connections are as shown in Figure 2. The block diagram of the protective function is shown in Figure 3.
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For a 3750 kVA, 4160V transformer, the relay settings are as follows. The current to be detected for 95% coverage of the winding with a 400A resistor is:
100%- %coverage 100% = 20A
I=
fault
1200/5
These equations can be solved for I1 : 11 =- If IR
IR -If
-
With IR = 400A and If = 20A, I2 = 1A. And I2 is 310. Then the differential current is:
The relay will be set to pickup at this value. The slope is set so that the relay will trip for the minimum differential current at the maximum line current (full load current + 310):
I
I Fig.
Transformer Protection Relay
2. Multifunction microprocessor transformer protection relay.
The full load phase current is 520A. The ground differential current, Igd, is the difference between the ground current, Ig, and the residual current, 310:
19 = -~100% = 3.6% 52 1 If the 1 Ampere correction had been neglected, the result would have been essentially the same.
The residual current 310 can be found by looking at the increase in per unit primary current, Ip, due to the fault. If the relay is to protect 95% of the winding, this is a small effect, which can usually be ignored. For example, if there is 20A flowing into an internal ground fault, the secondary load current in that phase is not decreased by 20A. In fact, it is increased by 1A. This can be seen from Figure 4, where the secondary winding is considered as an autotransformer. Let 11 be the current in the faulted section, If the fault current, and I2 the current in the rest of the winding due to the fault. Then: I2 = I, -I,
(3)
by Kirchkoff s law. And because of the turns ratio, If can be compared to the full-voltage resistor current IR:
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The effect of CT error can be evaluated for this relay. Assuming that saturation effects would occur only on the resistor side CT, and that the relay burden is 0.2 VA, resistive, CT leakage currents taken from a typical CT saturation curve are shown in Table 2. For the example above, the calculated slope setting would only be affected by a fraction of a percentage:
Slope =
I g d +Ileakuge Iline
x 100%
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1
Calculate 310
Calculate
TABLE 2 PERFORMANCE OF8 7 w~m VARIOUS ~ RELAY-0-COMBINATIONS (mAND LEAD BURDENAS PERTABLE1) TRANSFORMER F'ROTECllON RELAY
In order to prevent false tripping due to CT errors, the minimum slope setting should be about 4%. This will decrease the sensitivity slightly under full load conditions. Because the slope is calculated based upon the maximum phase current rather than the current into the differential circuit, the percentage values appear to be lower than with the traditional differential relay, although the effect is similar. The final setting is for time delay. This will be set to allow for instantaneous relays downstream to clear high current faults.
B. Ground Differential Protection with a Specialized Differential Relay Function
This approach is a digital implementation of the product-
type ground differential relay [ 6 ] , Figure 5. The standard product-type relay design is extended to include cases where the operating and polarizing currents are out of phase due to CT saturation. The phase angle saturation curves of typical CTs (Figure 6) show that the secondary voltage of a CT leads the input current by an angle of 70-80" in an unsaturated condition. This angle decreases to approximately 30" at the knee point, and then returns to 80" as the level of saturation increases. When operating into a primarily resistive burden, the secondary current will be in phase with the secondary voltage. Typical phase angle relationships for a saturated CT are shown in Figure 7. Referring to Figure 5, the residual current from the phase CTs is the reference, and the sum of the residual and neutral currents is the operating current. Considering an internal fault, a phase difference of less than 90" between the neutral and residual currents (Figure 8) due to neutral CT saturation will not be able to reverse the direction sensed by the relay. On the other hand, if one or two of the phase CTs saturates, the direction sensed by the relay could change. If the output of one CT is severely reduced (Figure 9), the residual current would be the sum of the other two phase currents, which could result in a fundamental frequency phase angle of approximately 180" between the residual and neutral CT currents. Thus, if saturation is approached on one or more phases, the relay could be fooled into thinking that an external fault had occurred. With a low burden digital relay, this is not likely to happen in a resistance grounded system, as can be seen for the 120015 CT in Table 2. In a solidly grounded system, as shown in Table 3, where the available fault current is much higher, this could only happen if the CT and lead burden were excessively high. The added burden can only exist if an auxiliary ratiomatching transformer is present. It is therefore recommended that numerical relays utilize digital ratio matching algorithm, rather than external auxiliary transformers.
I
F'igure 4. Transformer secondary winding ground fault currents.
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Case
I
w/o
I
AuxCT Max. burden W/O sat.
w/
CT Ratio 1200/5 1200/5 1200/5 1200/5 1200/5 1200/5 1200/5
I
I
I
12: 1, 6:1 or 3:l Aux
W) 10 20 50 10 20 50
CT
0.2VA Relay Burden
Cum.
I I
10
+
AuxCT
Avail. Fault Cum.
(A) 42 83 208 42 83 208 41.7
I
Min. CTVolt (V)
0.37 0.37 4.8 2.4 0.96
16 31 78 200 200200
I (Q) I (a) I I ~ 0 . 0 0 1 I 0.37 I ~0.001
