Restricted Earth Fault Application Guide

REYROLLE

TECHNICAL REPORT

APPLICATION GUIDE TITLE:Restricted Earth Fault

REPORT NO:-

990/TIR/06

DATE :-

08 Feb. 2005

Application Guide - Restricted Earth Fault

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REYROLLE 1. INTRODUCTION A low impedance earth fault overcurrent relay may, with the addition of an external series resistor, and a nonlinear 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 Vstab Vfs Vpk If Ifs Rct RL N Vk Imag Inlr Rs Is Pcon Phalf

,relay circuit setting voltage ,min voltage required to ensure stability ,rms value of relay circuit voltage not withstanding CT saturation ,peak voltage produced across relay circuit during internal fault ,maximum in-zone fault current ,max through fault current ,CT secondary winding resistance ,CT lead resistance (loop) ,CT turns ratio ,CT knee point voltage ,CT magnetisation current ,non-linear resistor current ,setting resistance ,relay setting current ,continuous power rating of resistor ,0.5 second power rating of resistor

conditions

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.

Application Guide - Restricted Earth Fault

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REYROLLE 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 nonlinear 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

b) Its non-linear characteristic i.e. V = CI ; 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. 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. Thus

Vkmin = 2[10.5(3+3)] = 126V

Application Guide - Restricted Earth Fault

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REYROLLE 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. The value of magnetising current, Imag, at Vs is 0.011A.

Application Guide - Restricted Earth Fault

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REYROLLE 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 √3 x 132

x1 200

(Secondary Amps)

= 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 establishing the relay setting current required to achieve an appropriate primary operating current value.

Application Guide - Restricted Earth Fault

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REYROLLE 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)

¼

x 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.

Application Guide - Restricted Earth Fault

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REYROLLE 4.9. N = 200

Primary fault setting, or primary operating current (p.o.c.) = N (Imag + Is) 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

Application Guide - Restricted Earth Fault

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REYROLLE Figure 1 – Typical Application, Protection of Delta Winding

R1

R1

R1 Line Current Transformers

Vs

S2

S2

S2

A

B

C

S1

S1

S1

Non-Linear Resistor

R4

R2

P2

l >

P1 R3 HV LV

Application Guide - Restricted Earth Fault

R1 = C.T. Secondary Winding Resistance. R2 + R3 = Pilot Loop Resistance, RL. R4 = Setting Resistance. Vs = Relay Circuit Setting Voltage, or Stability Voltage.

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REYROLLE Figure 2 – Typical Schematic Diagram for Application of 4-Pole Argus 1 Relay to Star Winding C u r r e n t t r a n s fo r m e r s C la s s 5 P

C la s s P X A

S1 HV

A

S2

LV P1

B

S1

B

S2 C

S1

IM P O R T A N T N O T E . W h e re o n e s e t o f c u rre 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 P1

P2

S2

C N o n -lin e a r r e s is to r

R1 53 l >

49 l >

54

45 l >

50

25

A rg u s 1

l > 46

26

S1

S2 P2

Application Guide - Restricted Earth Fault

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

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REYROLLE Figure 3 – Typical Schematic Diagram for Application of 4-Pole Argus 1 Relay to Delta Winding A S2 B

S1

P1

S1

P2

S2 C

S1 53 l>

49 l>

54

45 l>

50

Argus relay (3OC + REF)

S2 25 l>

46

26 Non - linear resistor

R1

Application Guide - Restricted Earth Fault

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REYROLLE Figure 4 – Typical Current Transformer Magnetising Characteristic Class PX to IEC60044 Secondary winding resistance = 3 ohms at 75oC

R.M.S Volts (min) multiply by 30

B

A

10

Vk

a) Vk = 9.0 x 30 = 270V Iexc = 0.165 x 0.3

X

b) 1.1Vk = 9.9 x 30 Iexc = approx 1.5 x (a)

8

6

4

2

curve "A" curve "B"

0

.02

.06

.1 1.0

.14

.18 2.0

.22

.26

.3 3.0

Secondary Excitation Current Multiply by 0.3 R.M.S Amps (max)

Application Guide - Restricted Earth Fault

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