High Impedance Restricted Earth Fault Implementation

High Impedance Restricted Earth Fault Implementation

Restricted earth fault protection in effective way to protect wye connected power transformer windings or generator stator windings from internal earth faults. Usually normal biased differential protection is not sensitive enough to cover faults near winding star point. Generally high impedance restricted earth fault can be implemented to any AQ protection relay that has Io> protection stage available. High impedance restricted earth fault however requires external components and sets certain requirements for wiring and CT selection. The main principle in high impedance differential protection is to wire the CTs in a way that the currents will cancel each other when fault is outside the protected area but in case of fault inside the protected area the current is forced to flow through the relays current measurement input and that way to trigger the protection stage selected for this use.

Picture 1: Connection to AQ protection relay. All current transformers need to have same ratio and nominal
secondary currents in this application. Rs is stabilizing resistor used to prevent nuisance tripping in case of
external fault and CT saturation. VDR is to limit high voltage peaks in case for internal faults.

CURRENT FLOW IN DIFFERENT FAULT TYPES

FAULT INSIDE PROTECTED AREA

When fault is inside of the protected area the currents flow against each other and are forced to flow through the relays current measuring input. The protection element will trip when current (I) is greater than the set value (Is). Formula for current is I=Vs/Rs. Voltage dependant resistor (VDR) is used to protect the CTs, wiring and relays current measuring input from peak voltage generated in heavy duty internal faults. Typical value for insulation withstand is 3kV peak meaning that if calculated maximum Vs voltage stays under 3kV the VDR is not needed. (For additional safety 2kV peak limitation is recommended.)

Picture 2: Fault inside of the protected area

FAULT OUTSIDE PROTECTED AREA

When the fault is outside of the protected area the currents flow cancelling each other in a way that no current goes (in ideal situation) to the relays current measuring input. However high fault current can flow through the protected area causing CT saturation which may lead to nuisance tripping if no stabilizing resistor (Rs) is installed.
Picture 3: Fault outside of the protected area

CALCULATIONS NEEDED FOR REF IMPLEMENTATION

CALCULATING THE VALUE FOR STABILIZING RESISTOR (RS)

Properly designed high impedance differential scheme contains correctly rated stabilizing resistor for maintaining protection stability in maximum through fault situation. Resistances in different sides of measurement circuit should be similar which means that protection relay is located in the middle of the measuring circuit.

EQUATION 1: FINDING OUT THE VALUE OF VS IN MAXIMUM TROUGH FAULT


Imaxt=Maximum through fault current not to cause the REF to trip
CTsec=Nominal secondary current of the CT
CTpri=Nominal primary current of the CT
Rct=Resistance of the CT secondary
Rw=Total resistance of wiring
Rr= Resistance of the relay current measuring input

EQUATION 2: FINDING OUT THE MINIMUM VALUE NEEDED FOR RS



Vs= Minimum setting voltage (Equation 1)
Iset= Setting value of the relay as secondary value

EQUATION 3: ESTIMATING THE KNEE POINT VOLTAGE OF THE CT (REF 1)

To assure the functionality in the internal fault condition the CTs knee point voltage should be twice the amount of Vs.


Vkp= Knee point voltage of the CT
Sn= Nominal power of the CT
kAlf=Accuracy limit factor of the CT
B=1.6 for 5Pxx and 1.9 for 10Pxx

CALCULATING THE REQUIRED NON-LINEAR RESISTOR (VDR)

As mentioned above, during an internal fault, high voltages may be generated to the measurement circuit. Therefore, calculations for this voltage rise must be done for verifying the insulation integrity in internal fault situations.

EQUATION 4: CALCULATING THE PEAK VOLTAGE ACCORDING TO A LINEAR CT
MODEL


Imaxf=Maximum fault current when the fault is inside the protected zone
CTsec=Nominal secondary current of the CT
CTpri=Nominal primary current of the CT
Rct=Resistance of the CT secondary
Rw=Total resistance of of wiring
Rs=Stabilizing resistor according (Equation 2)

EQUATION 5: CALCULATING THE PEAK VOLTAGE ACCORDING TO A NONLINEARCT MODEL OF SATURATED CT (REF 2)


Vkp=Knee point voltage of the CT (Equation 3)
The secondary voltage at which a 50% increase of primary current is needed to increase the
secondary voltage by 10%
Vp=Peak voltage according to a linear model of a CT (Equation 4)

CALCULATING THE THERMAL RATINGS FOR STABILIZING RESISTOR AND VARISTOR

Verifying the thermal rating of external components is important for application safety.

EQUATION 6: CONTINUOUS POWER RATING FOR THE STABILIZING RESISTOR


Pcont=Continuous power rating for stabilizing resistor
Vs=Relay setting voltage
Rs=Stabilizing resistor value

EQUATION 7 & 8: SHORT TIME POWER RATING FOR THE STABILIZING RESISTOR


Urms=RMS voltage over the stabilizing resistor in maximum internal fault
Vkp=Knee point voltage of the CT
Rs=Stabilizing resistor value
Pshort=Needed short time power rating for the stabilizing resistor

EQUATION 9: NECESSARY POWER RATING NEEDED FOR VDR


Pvar=Needed power rating for the varistor

EQUATION 10: CALCULATED ENERGY ABSORPTION CAPABILITY FOR VDR


t=maximum fault duration
Wvar= Needed energy absorption capability of VDR depending from maximum fault duration

CT SELECTION

When high impedance differential scheme is applied the CTs must always have the same ratio, the
same magnetizing curve and the same internal resistance. The magnetizing curve is linked to the
knee point voltage and that makes the knee point voltage the base for CT selection.
IEC 61869-2:2012 Instrument transformers – Part 2: Additional requirements for current
transformers defines PX class CTs. These CTs have current ratio error limitation and are of lowleakage
type which makes PX class best option for high-impedance differential schemes.
However, in REF applications class 5P is generally accepted.

EXAMPLE CASE

Data provided by customer:
CTpri 6000 A Current transformer primary current
CTsec 5 A Current transformer secondary current
CT Sn 5 VA Current transformer nominal power
CT alf 10 Current transformer accuracy limit factor
Rct 3.128 Ω Current transformer secondary resistance
Rw 0.067 Ω Total resistance of the wiring
Rr 0.005 Ω Protection relay input resistance
Imaxt 24000 A Maximum fault current in through fault
Imaxf 26100 A Maximum fault current in internal fault
Iset 5 % Desired setting sensitivity

Equation 1: Minimum setting voltage


Equation 2: Minimum stabilizing resistor at desired tripping level


Equation 3: CT knee point voltage, must be 2x minimum setting voltage Vs (ref 3)


Equation 4: Peak voltage according to a linear CT model


Equation 5: Peak voltage of saturated CT


-> Varistor recommended because Vsp>2kV

Equation 6: Continuous power rating for stabilizing resistor


Equation 7 & 8: Short time power rating for stabilizing resistor


Equation 9: Power rating needed for varistor:


Equation 10: Energy absorption capability needed for VDR with 0.5s fault clearing time



Selection proposal when using Metrosil VDRs and Ohmite resistors:

Ohmite 210 series D100J300E
-100W with overload capacity of 10 times rated wattage for 5 s which means that it can handle the
865W of short time power withstand requirement. Dielectric voltage withstand is 1000 Vac.
-If varistor is left out then D175J300E model is needed because it has higher dielectric voltage
withstand 3000 Vac.

Metrosil 600-A/S1/1213
-Maximum relay setting voltage 200Vrms with typical leakage current of 35mA and short time
current rating of 50A 1s.


Ref 1: "Extracted from Schneider Electric "Cahier Technique" no. 195"
Ref 2: "Extracted from Schneider Electric "Cahier Technique" no. 194"
Ref 3: “The International Journal of Engineering and Science (IJES) || Volume || 2 || Issue || 12 ||
Pages || 14-18 || 2013 ||”

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