Calculation of relay settings for transmission lines - Distance protection

Introduction: Electricity is transferred on higher voltage for long distances. Transmission lines pass through forests, hills, fields etc befor reaching destination. Being exposed to uncontrolled atmosphere, faults on transmission lines are as high as 85% of the total faults in power system. These lines are protected by distance relays working on impedance function. Now with the advancement in optical fibre technology, line differential relays are also being used. Line differential relays are also having distance protection function which comes in to action whenever there is optical communication failure. In addition to protection these relays also work as fault locator which is also based on impedance measrement principle.

Line Parameters: Transmission line is a long conductor having resistance, inductance and capacitance distributed uniformly throughout the length. Following line constants are provided by designer based on calculations, which are used for relay settings:

Sr	Parameter		Unit
1	Positive sequence reactance	X1	ohm/km
2	Positive sequence resistance	R1	ohm/km
3	Zero sequence reactance	X0	ohm/km
4	Zero sequence resistance	R0	ohm/km

Conversion to Secondary value from Primary value: Above parameters are given for primary equipment. The protection relays are connected to primary equipment through Current transformer and Voltage transformer (CT and VT). Relay reads the current and voltage on secondary side of CT and VT. Therefore the parameters needs to be converted to secondary side as per CT and VT ratio.

Z secondary = Z primary x (CT Ratio / VT Ratio)

Setting calculation: We will drive settings for Station-A end relay of a 220kV line to station-B. Actual relay setting calculation will depend on many factors like relay make and model, network size etc. Here we are showing a simple example to get an idea of basics for relay setting calculation. 

VT ratio: 220kV/110V 
CT ratio: 800/1A

Primary side line parameters are:
X1 : 0.398 ohm/km
R1 : 0.069 ohm/km
X0 : 1.290 ohm/km
R0 : 0.281 ohm/km
Line length LL: 100 km
Next Longest line: 80 km

CT Ratio: 800/1 = 800
VT Ratio: 220kV/110V = 2000 
As shown in Fig-2:
Positive sequence impedance Z1 = Sqrt (R1^2 + X1^2) = 0.404 ohm/km
Line Angle = ArcTan (X1/R1) = 80.16 deg
Zero sequence impedance Z0 = Sqrt (R0^2 + X0^2) = 1.320 ohm/km

Line Angle = ArcTan (X0/R0) = 77.71 deg

Total line positive sequence impedance ZL = LL x Z1 = 40.4 ohm

Zone settings are shown in Fig-3 for a four zone protection relay. Zone-1, 2 & 3 are in forward direction and Zone-4 is in reverse direction. Typical zone settings are as below:

Zone-1: 80% of protected line = 40.04 x 0.8 = 32.320 ohm
Zone-2: 120% of protected line = 40.04 x 1.2 = 48.048 ohm
Zone-3: 100% of protected line + 120% of next longest line = 40.04 + (1.2 x 80 x 0.404) =  78.824 ohm
Zone-4: 10% of protected line = 40.04 x 0.1 = 4.04 ohm

Relay setting to be entered in relay (Secondary values):
Zone-1: Primary value x CTP/PTR = 32.320 x 800/2000 = 12.928 ohm
Zone-2: Primary value x CTP/PTR = 48.048 x 800/2000 = 19.219 ohm
Zone-3: Primary value x CTP/PTR = 78.824 x 800/2000 = 31.596 ohm
Zone-4: Primary value x CTP/PTR = 4.040 x 800/2000 = 1.616 ohm

Neutral compensation factor KZN = (Z0-Z1) / 3Z1 = 0.757
KZN Angle = ArcTan [(X0-X1)/(R0-R1)] - ArcTan (X1-R1) = -3.5 deg

Typical time settings for Zone is given below, however these are co-ordinated with relay settings of other elements of the network:

Zone-1 time delay: 0.0 sec
Zone-2 time delay: 0.5 sec
Zone-3 time delay: 1.0 sec
Zone-4 time delay: 0.5 sec

Calculating fault location from COMTRADE file through Wavewin

Introduction: Basic information regarding COMTRADE files (CFG and DAT) was given in earlier post here: Understanding disturbance record files. It is recommended to read this first.

Fault location is and important information, which is provided by fault locator installed for feeders. Normally it is inbuilt feature distance protection. Fault locator has record for latest faults locations (No. of records vary depending on Make/Model of device). Or there may be cases where we have COMTRADE files but access to device records is not available. In these cases we may calculate location from COMTRADE file.

Procedure: 

Open the file with Wavewin software. For fault location calculation, Clicke on Data > Fault Detector, A new window is opened as below

In the DR file, it can be seen that although Secondary is shown on the title bar, current and voltage values are in primary values (Voltage =241646V and Current =219A). This is because CT / VT ratio data is not available in this CFG / DAT file. It is taken as 1:1. It can be checked as below:

The information whether the CT and VT ratio is available or not, is an important information. We have to enter line parameters in Primary or Secondary based on it. In this example Secondary values are equal to Primary values of voltage and current (Due to 1:1 CT and VT ratio). Therefore we will enter primary side line parameters as per relay setting data: 

Positive sequence impedance Z1 = 0.301 ohm/km
Line angle = 80 deg
Zeo sequence compensation factor kZn = 0.750 
kZn angle = -10 deg
Line Length = 100km

In case of actual CT and VT ratio is available in disturbance record files, secondary parameters are to be entered.

Click here for more detail on line parameters

We can specify faulted phase in Advanced settings, which we have not used. Software has automatically sensed faulty phase and shown fault location.