Restricted Earth Fault Application Guide

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Application Guide - Restricted Earth Fault Page 1 of 12

TECHNICAL REPORT

APPLICATION GUIDE

TITLE: Restricted Earth Fault

REPORT

NO:- 990/TIR/06

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1. INTRODUCTION

A low impedance earth fault overcurrent relay may, with the addition of an external series resistor, and a non-linear resistor be connected as a high impedance restricted earth fault relay for the protection of transformer windings or the stator windings of large machines. This document provides guidelines and a worked example in establishing the relay settings and design parameters for external resistors and, where required, a non-linear resistor, for Restricted Earth Fault (REF) protection.

2. ABBREVIATIONS

Vs ,relay circuit setting voltage

Vstab ,min voltage required to ensure stability

Vfs ,rms value of relay circuit voltage not withstanding CT saturation

Vpk ,peak voltage produced across relay circuit during internal fault conditions

If ,maximum in-zone fault current

Ifs ,max through fault current

Rct ,CT secondary winding resistance

RL ,CT lead resistance (loop)

N ,CT turns ratio

Vk ,CT knee point voltage

Imag ,CT magnetisation current

Inlr ,non-linear resistor current

Rs ,setting resistance

Is ,relay setting current

Pcon ,continuous power rating of resistor

Phalf ,0.5 second power rating of resistor 3. THEORY OF REF SCHEMES

3.1. Determination of Stability

The stability of a REF scheme using a high impedance relay circuit depends upon the relay circuit setting voltage being greater than the maximum voltage which can appear across the relay circuit under a given through fault condition (i.e. external fault). This voltage can be determined by means of a simple calculation which makes the following assumptions:

a) One CT is fully saturated making its excitation impedance negligible.

b) The resistance of the secondary winding of the saturated CT together with the leads connecting it to the relay circuit terminals constitute the only burden in parallel with the relay.

c) The remaining CTs maintain their ratio. Thus the minimum stability voltage is given by: Vstab = Ifs (Rct + RL)

For stability, the relay circuit voltage setting should be made equal to or exceed this calculated value. No factor of safety is necessary because this is built into the assumptions made above.

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3.2. Current Transformer Requirements

The CTs used in this type of scheme should be of the high accuracy and low leakage reactance type and the minimum CT knee voltage should be greater than twice the minimum stability voltage setting calculated for the relay circuit.

3.3. Setting Resistor

If the relay used in the scheme has a low burden, then a series setting resistor will be required to provide the relay circuit setting voltage for stability. Assuming the relay burden is very small and the CTs do not have very low knee point voltages (less than 25V), the relay burden can be neglected and the setting resistor value is then given by:

Rs = Vs / Is

3.4. Non Linear Resistor

The maximum internal primary fault current in the protected zone will be the same as that for the stability condition when the primary network circuit is solidly earthed. This current may cause high voltage spikes across the relay at instants of zero flux since a practical CT core enters saturation on each half-cycle for voltages of this magnitude.

A formula in common use, which gives a reasonable approximation to the peak voltage produced under internal fault conditions, is expressed as:

Vpk = 2√ [2Vk(Vfs-Vk)]

To protect the CTs, the secondary wiring, and the relay from damage due to excessively high voltages, a non-linear resistor is connected in parallel with the relay circuit if the peak voltage would exceed 3kV. If the calculated peak voltage is less than 3kV it is not necessary to employ a non-linear resistor.

The type of non-linear resistor required is chosen by:- a) Its thermal rating as defined by the empirical formula:

P = 4/π x Ifs x Vk

b) Its non-linear characteristic i.e. V = CIB; where C and B are constants.

A resistor with C and B values is selected which ensures the peak voltage cannot exceed 3kV and, in the region of the relay circuit setting voltage, the current shunted by the non-linear resistor is very small (e.g. <10mA).

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4. WORKED EXAMPLE – PROTECTION OF POWER TRANSFORMER HV DELTA WINDING (FIGURE 1) USING THE REF ELEMENT OF AN ARGUS RELAY

4.1. Data required

(With values inserted from a typical example) 4.1.1. CT secondary winding resistance 3Ω 4.1.2. Lead resistance (loop) 3Ω

4.1.3. Magnetising characteristic of CT see Fig. 4 [ Vk > 270 ] 4.1.4. CT turns ratio 1/200

4.1.5. Nameplate rating of power transformer 30MVA 4.1.6. Voltage ratio of power transformer 132/11kV 4.1.7. Required primary fault setting 10% to 60% 4.1.8. Power transformer impedance 9.5% 4.1.9. System earthing solid

4.1.10. Maximum system fault level 3500MVA 4.1.11. Relay data, Argus 1 relay (REF/SEF version)

REF setting range 0.5% to 96% of In in 0.5% steps

AC burden, 5A tap ≤0.4VA

1A tap ≤0.2VA

4.2. A UK standard in use for some years now, EATS 48-3, recommends that the figure used for Ifs should

be 16 times the rated current of the protected winding. This is a typical figure based on infeeds to an external earth fault from the transformer under consideration, which is in parallel with the remainder of the system up to the point of connection of the transformer. Note that if this value results in an impractically high voltage setting requirement, it is usually acceptable to assume that the transformer impedance limits the through earth fault current to the through 3 phase fault current.

Ifs (primary) = 16 x transformer rating

√3 x system voltage Ifs (primary) = 16 x 30MVA = 2.1kA

√3 x 132kV

Ifs (secondary) = 2100/200 = 10.5A

The minimum CT knee point voltage should be greater than 2Ifs (RCT+RL) volts.

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Minimum stability voltage to ensure stability during maximum through fault is: Vstab > Ifs (RCT+RL)

> 10.5 (3+3) > 63V

Although the relay circuit voltage setting does not need to be more than this calculated value, since the CT Vk is more than 270V, a setting of 90V is proposed.

4.3. EATS 48-3 recommends that the primary fault setting should be in the range of 10% to 60% of the rated current of the protected winding (when the protected winding is connected to a solidly earthed power system).

The acceptable limits for the primary fault setting are: 3MVA to 18MVA = 13.1A to 78.7A

√3 x 132kV (i.e. for 30MVA transformer) Therefore the relay operating current limits are: 13.1 to 78.7 = 65mA to 400mA

200

4.4. The Argus relay REF element has a setting range from 0.005 to 0.96A in 5mA steps. An initial setting of 0.18A is chosen. However the shunt connection of all other paths must be subtracted from this to allow the actual fault setting to be determined. The fault setting is the actual current (primary amps) at which the relay operates. Shunt paths = number of CTs x their magnetising current + non-linear resistor (if required).

Thus, actual setting = 0.18 - 3Imag - Inlr*

* In restricted earth fault applications where the relay setting voltage is considerably lower than the non-linear resistor ‘C’ value, Inlr can be ignored. The magnetising current of all parallel CTs must be taken into account at

the relay setting voltage, Vs, which is now calculated.

In reference to figure 4, the magnetisation curve shows a knee point voltage of 270V. A stability voltage within the range Vk/4 to Vk/2 is normal unless a customer has special requirements, therefore a value of say 90V for Vs can be chosen. This is more than the minimum value of Vstab calculated at 63V (see section 4.2 above) and is less than Vk/2.

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Reverting to the calculation of current setting this can now be completed. Setting, in secondary amps,

Is = 0.18 - 3(0.011) = 0.147A

Say 0.15A (nearest setting for Argus 1 relay)

4.5. Based on a relay circuit setting voltage of 90V, the series stabilising resistor can now be calculated by the following formula,

Rs = (Vs - Vrelay)/Is

The Argus relay burden is very small and can be neglected. Rs = 90/0.150 = 600Ω

The resistor value of 600Ω can be obtained, with standard tolerance band e.g. +/- 5%. Thus the relay circuit setting voltage becomes,

Vs = 0.15 x 600 = 90V

4.6. To check whether a voltage limiting device is required to protect the relay circuit, calculate Vpk.

Vpk = 2√ [2Vk(Vfs-Vk)]> 3000 Where, Vk = 270V (from fig.4) Vfs = If (Rs + Rrelay) If = 3500 x 1 (Secondary Amps) √3 x 132 200 = 77 Amps Vfs = 77 x 600 = 46.2kV Thus, Vpk = 9.96kV

Therefore it is recommended that a voltage limiting device is connected into the circuit. If the shunt current at Vs is significant, relative to the relay setting, this must be taken into account in the calculation, item 4.4, in

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4.7. The resistors incorporated in the scheme must be capable of withstanding the associated thermal conditions.

Continuous power rating of the setting resistor = Pcon = (Icon)² x Rs

where Icon = continuous resistor current, normally taken as being the current at circuit

setting voltage

Pcon = 0.15² x 600 = 13.5Watt

The short time rating of the resistor is taken to be 0.5 seconds. This is considered so as to ensure that the relay circuit components are not damaged in the event of a circuit failure causing a fault to be cleared by back-up or CB fail protection.

The 0.5 second rating of the setting resistor, Phalf = Ir² x Rs, where Ir = Vf/Rs.

The rms voltage, Vf, developed across Rs under internal fault conditions is defined from the empirical formula as follows:

Vf = (Vk³ x Rs x If) ¼x 1.3

where If = rms value of secondary fault current for maximum system fault level, calculated above in 4.6 (If = 77

Amps).

Vf = (270³ x 600 x 77) ¼

x 1.3 = 1270V Therefore,

Phalf = Vf²/Rs = 1270²/600 = 2.69kW for 0.5 sec

4.8. The required thermal rating of the non-linear resistor can be calculated by the formula, P = 4/π x If x Vk

P = 4/π x 77 x 270 = 26.5kW

This is above the declared withstand of a standard 3” diameter type Metrosil non-linear resistor and a 6” diameter type would be required.

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4.9. Primary fault setting, or primary operating current (p.o.c.) = N (Imag + Is) N = 200 Imag at 90V is approximately 15mA Is = 0.15A

P.O.C. = 200(3 x 0.015 + 0.15) = 39A

Full load current at 30MVA = 131A Therefore P.O.C. = 30% of rating.

This ignores any current passed through the Metrosil at the setting voltage. With typical standard values for the Metrosil characteristic for B and C, the current at setting voltage would be relatively very low, e.g. < 1mA.

4.10. Recommended Settings and Components

>Argus REF setting = 0.15A (Resultant fault setting, e.g. the p.o.c. = 39A) >External setting resistor = 600Ω (Vs = 90V)

>Non-linear resistor = 3 inch type with ‘C’ and ‘B’ values chosen to suit, i.e. to limit the maximum peak voltage to 3kV (at maximum in zone fault = 77amp in this example), and to ensure that the current drawn by the Metrosil at the relay circuit setting voltage, is not significant (e.g. less than 1.0mA). If this current is significant it needs to be included in the expression for the p.o.c. in item 4.9 above

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Figure 1 – Typical Application, Protection of Delta Winding

R1 = C.T. Secondary Winding Resistance. R2 + R3 = Pilot Loop Resistance, RL. R4 = Setting Resistance.

V_s = Relay Circuit Setting Voltage, or Stability Voltage.

R4 V_s R1 R1 R1 R3 R2 l > Non-Linear Resistor Line Current Transformers S2 S1 A S1 S2 S2 S1 B C P1 P2 HV LV

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C u r r e n t t r a n s fo r m e r s l > l > l > l > R 1 5 3 4 9 4 5 5 4 5 0 4 6 2 5 2 6 S 1 S 2 H V L V P 1 P 2 P 1 S 1 S 2 S 1 S 2 S 1 S 2 A r g u s 1 A r g u s 1 h a v in g 2 O C + 1 E F + R E F R 1 = s e tt in g r e s is ta n c e IM P O R T A N T N O T E . W h e r e o n e s e t o f c u r r e n t t r a n s f o r m e r s a r e u s e d to p r o v id e b o t h ID M T a n d R E F p r o te c tio n s , t h e n th e y m u s t m e e t th e p e r fo r m a n c e r e q u ir e m e n t s fo r b o th p r o t e c t io n s y s t e m s P 2 A B C A B C N o n -lin e a r r e s is to r C la s s 5 P C la s s P X

Figure 2 – Typical Schematic Diagram for Application of 4-Pole Argus 1 Relay to Star Winding

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l > l > l > l > R1 Non - linear resistor Argus relay (3OC + REF) 53 54 49 50 45 46 25 26 P1 P2 A B C S1 S1 S1 S2 S2 S2

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Secondary Excitation Current Multiply by 0.3 R.M.S Amps (max) 10 B A 6 R.M.S V o lt s (min ) mu ltip ly b y 30 .02 .06 .1 .14 .18 .22 .26 curve "A" curve "B" 1.0 2.0 .3 3.0

Secondary winding resistance = 3 ohms at 75o_C

0 4 2 8 a) Vk = 9.0 x 30 = 270V Iexc = 0.165 x 0.3 b) 1.1Vk = 9.9 x 30 Iexc = approx 1.5 x (a)

V_k

X

Figure 4 – Typical Current Transformer Magnetising Characteristic Class PX to IEC60044