Testing Transient Directional Ground Fault Determination

Practical Example of Use

2

Testing Transient Directional Ground Fault Determination

Version: Expl_GndFlt_Transient.ENU.1 - Year: 2014

© OMICRON electronics. All rights reserved.

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© OMICRON 2014 Page 3 of 23

Content

Preface ......................................................................................................................................................... 4

1 Application Example ............................................................................................................................ 5

2 Theoretical Introduction to Directional Ground Fault Determination ............................................. 7 2.1 Characteristic of the Ground Fault Current .................................................................................... 7 2.2 Determination of the Fault Direction .............................................................................................. 8

2.2.1 Physical Background of the Transient Ground Fault ......................................................................... 8 2.2.2 Directional Determination based on Transient Signals ..................................................................... 8

2.3 Voltage Measurement for Directional Ground Fault Determination .............................................. 9

3 Practical Introduction to Transient Directional Ground Fault Testing .......................................... 11 3.1 Defining the Test Object .............................................................................................................. 11

3.1.1 Device Settings ............................................................................................................................... 12 3.2 Global Hardware Configuration of the CMC Test Set ................................................................. 12

3.2.1 Example Output Configuration for the Directional Ground Fault Function ...................................... 13 3.2.2 Analog Outputs ............................................................................................................................... 14 3.2.3 Binary Inputs ................................................................................................................................... 14

3.3 Local Hardware Configuration for Transient Directional Ground Fault Testing ........................... 15 3.3.1 Analog Outputs ............................................................................................................................... 15 3.3.2 Binary Inputs ................................................................................................................................... 15

3.4 Defining the Test Configuration ................................................................................................... 16 3.4.1 General Approach ........................................................................................................................... 16 3.4.2 Ground Fault Test Module .............................................................................................................. 17 3.4.3 Directional Test ............................................................................................................................... 22

Support ....................................................................................................................................................... 23

© OMICRON 2014 Page 4 of 23

Preface

This paper describes how to test the directional ground fault determination function (based on transient values) for the localization of ground faults in meshed or radial isolated and compensated networks. It contains an application example that will be used throughout the paper. The theoretical background of the transient directional ground fault determination function will be explained. This paper also covers the definition of the necessary Test Object settings as well as the Hardware Configuration for testing this function. Finally the Ground Fault test module will be used to test the directional decision of the transient ground fault determination. Supplements: Sample Control Center file Example_GroundFault_Transient.occ (referred to in this

document). Requirements: Test Universe 3.00 or later; Ground Fault; Ramping and Control Center licenses. Note: Testing the steady-state ground fault protection is not described in this document. For further

information regarding this subject, see Example_GroundFault_SteadyState.pdf.

© OMICRON 2014 Page 5 of 23

1 Application Example

110kV

600/1

A

A

A

N

N

N

110 kV

a

a

a

n

n

n

da dn

dn

dn

da

da

VResidual

110000 V 100 V 100 V

33 3

BB A BB B

Feeder Protection Relay

(not a part of this document)

IN Transient Ground

Fault Relay

Figure 1: Feeder connection diagram of the application example

© OMICRON 2014 Page 6 of 23

Parameter Name Parameter Value Notes

Frequency 50 Hz

VT (primary / secondary phases / secondary open delta)

110000 V 100 V 100 V

33 3 VResidual (open delta winding) is connected

CT (primary/secondary) 600 A /1 A CT ratio of the current transformer

Transient Ground Fault Settings

90 ms Delay time to prevent an unwanted operation due to switching operations. 1)

2 s Delay time for continuous ground fault signaling. 2)

1) The value of this time depends on the relay type and is fixed (approximately 40 .... 90 ms). For isolated networks, this parameter typically is deactivated.

2) Depending on the relay type, this value might be fix. It is typically in the range of 2 … 4 s.

Table 1: Relay parameters for this example

Note: In this example, the transient ground fault function indicates the direction of the fault (forward or

reverse), but it does not trip.

© OMICRON 2014 Page 7 of 23

2 Theoretical Introduction to Directional Ground Fault Determination

2.1 Characteristic of the Ground Fault Current

A fault inception is a sudden change of state in the power system. Therefore, the power system will transition from one steady state to another. In isolated networks, the fault current during and after this transition is mainly caused by the phase-to-ground capacities of the network. In compensated networks, the Petersen coil (arc extinction coil) that is connected between the star point and ground also contributes to the fault current. Therefore, the time signal of the ground fault current consists of two basic components:

Figure 2: Characteristic of the ground fault current (with Petersen coil)

1. The transient ground fault current, which is caused by discharging and charging of the power system components. The peaks of this current can be several times higher than the nominal current of the power system. It has a frequency range between 500 Hz and 8 kHz for the discharge and 70 Hz and 4 kHz for the charge of the phase-to-ground capacities of the power system. After the fault inception, it lasts only for a few periods of the nominal frequency of the power system. Transient ground fault relays can use this current to determine the direction of a ground fault.

2. The steady state ground fault current, which is caused by the phase-to-ground capacity of the non-faulty phases and – for compensated networks only – by the Petersen coil. The magnitude of the ground fault current depends on the phase-to-ground capacities and on the network configuration (isolated or compensated) and therefore, can be much smaller than the magnitude of the nominal current. As it is a steady-state current, it has the nominal frequency of the network (50 Hz or 60 Hz). This ground fault current can be used by the directional ground fault determination function (sensitive, permanent or watt-metrical ground fault determination) to determine the direction of the ground fault.

Note: The following document will only focus on the directional ground fault determination based on

transient values.

1 2

© OMICRON 2014 Page 8 of 23

2.2 Determination of the Fault Direction

2.2.1 Physical Background of the Transient Ground Fault

The ground fault inception will cause the phase-to-ground voltage of the faulty phase to break down, whereas the phase-to-ground voltage of the non-faulty phases will rise. This will result in an oscillating ground fault current directly after the fault inception that consists of two components.

> Discharge of the phase-to-ground capacities of the faulty phase: All phase-to-ground capacities of the faulty phase within the galvanic connected network will be discharged. This process starts immediately after the fault inception. The time characteristic of this process depends on the electrical parameters of the overhead lines or cables. It has a frequency range between 500 Hz - 8 kHz.

> Recharge of the phase-to ground capacities of the non-faulty phases: The phase-to-ground capacities of the non-faulty phases will be recharged. The magnitude of this current depends on the ground fault location, the ground fault resistance, the electrical network parameters, and the actual phase-to-ground voltage as result of the residual voltage. It has a frequency range between 70 Hz – 4 kHz.

2.2.2 Directional Determination based on Transient Signals

a) b)

Figure 3: Transient ground fault forward (a) and reverse (b)

Figure 3 shows typical behavior of the residual voltage and the sum current directly at the fault inception of a ground fault in a compensated network.

1. In the faulty feeder, the signs of the residual voltage and the current sum are in opposition. Therefore, the transient ground fault relay will detect "ground fault forward".

2. In the non-faulty feeder, the sign of the residual voltage and the current sum are the same. Therefore, the transient ground fault relay will detect "ground fault reverse".

Note: This method applies to isolated, compensated, and high impedance grounded networks.

1 2

© OMICRON 2014 Page 9 of 23

2.3 Voltage Measurement for Directional Ground Fault Determination

For a correct ground fault direction determination and for the pick-up of the residual voltage, the direction and the magnitude of the measured voltages must be correct. Figure 4 shows the connection and the phasors of the different voltages.

A

A

A

N

N

N

a

a

a

n

n

n

da dn

dn

dn

da

da

A B C

AV

BV

CV

ResidualV

110,000 V

3

100 V

3

100 V

3

Calculated zero sequence voltage from phase voltages

Primary

0°

+90°

180°

-90°BV

CV

0V

N G 0°

+90°

180°

-90°BV

CV

0V

Secondary

Measured residual voltage at open delta winding

Primary Secondary

0°

+90°

180°

-90°BV

CV

ResidualV

0°

+90°

180°

-90°BV

CV

ResidualV

N G

Figure 4: Voltage measurement for directional ground fault protection

It is possible to either measure the residual voltage at an open delta winding or to calculate the zero sequence voltage out of the phase voltages. The measured residual voltage will be:

A B C

Residual

V V VV

3

The calculated zero sequence voltage will be:

A B C

0

V V VV

3

© OMICRON 2014 Page 10 of 23

Table 2 shows an example of how to calculate the respective voltages. Primary Voltages Secondary Voltages Residual Voltage Zero Sequence Voltage

A

B

C

V 0 kV 0°

V 110 kV -150°

110 kV +150°V

A

B

C

V 0 V 0°

V 100 V -150°

100 V +150°V

A B C

Residual

V V VV

3

100 V 180°

2

1 A

2

2 B

0 C

A B C

0

1 a aV V1

V 1 a a V3

1 1 1V V

V V VV

3

57.74 V 180°

Table 2: Example of the voltages during a ground fault

© OMICRON 2014 Page 11 of 23

3 Practical Introduction to Transient Directional Ground Fault Testing

There are different approaches on testing protection systems:

> Conventional testing: The test values are defined manually to test specific parameters or specific protection functions.

> Simulation-based testing: The test values are obtained from a transient simulation of the power system to test the function of the protection system as a whole.

The Ground Fault test module is designed for simulation-based testing of transient ground fault relays and directional ground fault relays based on steady-state values. It is able to simulate the power system and to directly output the simulation results with a CMC test device. This test module can be found on the start screen of the OMICRON Test Universe. It can also be inserted into an OCC file (Control Center document).

3.1 Defining the Test Object

Before testing can begin, the settings of the relay to be tested must be defined. In order to do that, the Test Object has to be opened by double clicking the Test Object in the OCC file or by clicking the Test Object button in the test module.

© OMICRON 2014 Page 12 of 23

3.1.1 Device Settings

General relay settings (e.g., relay type, relay ID, substation details, CT and VT parameters) are entered in the RIO function Device.

Note: The parameters V max and I max limit the output of the currents and voltages to prevent

damage to the device under test. These values must be adapted to the respective Hardware Configuration when connecting the outputs in parallel or when using an amplifier. The user should consult the manual of the device under test to make sure that its input rating will not be exceeded.

3.2 Global Hardware Configuration of the CMC Test Set

The global Hardware Configuration specifies the general input/output configuration of the CMC test set. It is valid for all subsequent test modules and, therefore, it has to be defined according to the relay’s connections. It can be opened by double-clicking the Hardware Configuration entry in the OCC file.

© OMICRON 2014 Page 13 of 23

3.2.1 Example Output Configuration for the Directional Ground Fault Function

IA IB IC N

VResidual

N

Figure 5: Wiring of the analog outputs of the CMC test set.

Note: Depending on the relay type, the three phase-to-ground voltages might be used instead of the

residual voltage.

© OMICRON 2014 Page 14 of 23

3.2.2 Analog Outputs

The analog outputs, binary inputs and outputs can all be activated individually in the local Hardware Configuration of the specific test module (see chapter 3.3 ).

3.2.3 Binary Inputs

1. The binary contacts of the directional indicators have to be connected to binary inputs. Any free binary input can be used.

2. For wet contacts, adapt the nominal voltages of the binary inputs to the voltage of the respective signal, or select Potential Free for dry contacts.

3. The binary outputs and analog inputs etc. will not be used for the following tests.

Forw

ard

Dire

ction

Reve

rse D

irectio

n

Figure 6: Wiring of the binary inputs of the CMC test set.

1

3

2

© OMICRON 2014 Page 15 of 23

3.3 Local Hardware Configuration for Transient Directional Ground Fault Testing

The local Hardware Configuration activates the outputs/inputs of the CMC test set for the selected test module. Therefore, it has to be defined for each test module separately. It can be opened by clicking the Hardware Configuration button in the test module.

3.3.1 Analog Outputs

3.3.2 Binary Inputs

© OMICRON 2014 Page 16 of 23

3.4 Defining the Test Configuration

3.4.1 General Approach

When testing the directional ground fault protection function, the following steps are recommended:

This test is performed with the Ground Fault test module. It verifies the directional decision of the transient ground fault relay by simulating the voltages and currents at a faulty and a non-faulty feeder.

IN

Transient Ground Fault Relay

Forward Dir.

(+)

(-)

Reverse Dir.

(+)

(-)

VResidual

Figure 7: Wiring of the CMC test set for testing the directional decision of transient ground fault relays

Note: Always check the manual of the relay to make sure that the test set is wired correctly.

© OMICRON 2014 Page 17 of 23

3.4.2 Ground Fault Test Module

The Ground Fault test module simulates ground faults in a power system that consists of:

1. Infeed: This is the source for the power system.

2. Transformer: It transforms the power from the infeed to the test network. It also provides the star point for possible Petersen coil or resistor connections.

3. Network: This is a radial network that consists of one busbar and multiple feeders that are connected to the busbar. Figure 8 shows the elements of the network. The ground fault will always be simulated in feeder A. Feeder B will always be a non-faulty feeder. (See below for how to simulate forward or reverse faults.) All remaining feeders are combined to one element, the "Remaining System".

Feeder A

Feeder B

Feeder C

Feeder D

Feeder E

Figure 8: Definition of the network elements of the test module Ground Fault

To test a ground fault relay, the parameters of these elements have to be set.

1 2 3

© OMICRON 2014 Page 18 of 23

Test tab

1. The nominal frequency of the network can be entered here. It is also possible to use the frequency from the test object.

2. With this option, the network type can be set. Choose Cable if the network to be simulated is a pure cable network or a mix between cables and overhead lines. Select Open line if a pure overhead line network should be simulated.

3. The phase in which the fault should be simulated can be selected here.

4. Select Permanent to test the directional ground fault function based on steady state values. Select Transient to test transient ground fault relays. Note: See Example_GroundFault_SteadyState.pdf for more information on testing directional ground fault relays based on steady state values.

5. The ground fault will always be simulated in feeder A. To define whether a faulty or a non-faulty feeder should be simulated, the relay location can be changed. Select Feeder A to simulate a faulty feeder or Feeder B to simulate a non-faulty feeder.

6. This option defines the direction of the CT star point. Select Dir. line if the star point of the secondary CT circuit is towards the line. If it is towards the busbar, select Dir. busbar. Figure 9 shows the connection of the CTs.

7. To simulate a ground fault correctly, the test module needs a ground fault resistance. This is the resistance between the faulty phase and ground. The default value can be used if this value is unknown.

Dir. line Dir. busbar

Protectedobject;e.g. line

Busbar Busbar

Protectedobject;e.g. line

Feeder Protection Relay

Transient Ground Fault

Relay

Feeder Protection Relay

Transient Ground Fault

Relay

Figure 9: CT connection of transient ground fault relays depending on the CT star point location

1

2

3

4

5

6

7

© OMICRON 2014 Page 19 of 23

Transformer tab

1. Enter the nominal voltages of the transformer here.

2. Define the grounding of the transformer star point with these options. Select Isolated if the simulated network should have no connection between star point and ground. Choose Compensated for compensated networks. The detuning must be set if Compensated is selected. The definition of the detuning is:

Cap Ind

Cap

I - Iv

I

. With ICap as capacitive ground fault current and IInd

as inductive current of the Petersen coil.

For compensated networks, it is also possible to add a grounding resistance in parallel to the Petersen coil. This increases the resistive part of the ground fault current. This setting is entered as primary value.

3. Here, the rated power of the transformer is defined.

4. Also the transformer impedance has to be set for the simulation. This parameter is equivalent to the relative short circuit voltage and can be found on the name plate of the transformer.

5. This value is the short circuit power of the infeed at the HV side of the transformer.

Network tab

6. For the simulation, it is also necessary to enter the line parameters of the faulty feeder. These values resemble the line or cable between the busbar and the fault. The resistance and the reactance have to be defined for the positive and the zero sequence as primary values.

7. These parameters define the primary capacitive ground fault current. Network: The combined ground fault current of the remaining system. Feeder A: The ground fault current that is caused by the phase-to-ground capacities of the faulty feeder. Feeder B: The ground fault current of the non-faulty feeder.

8. When testing with secondary current injection, the CT ratio of the respective CT must be entered here. It is also possible to use the CT ratio from the test object by selecting the checkbox.

1

2

3

4

5

6

7

8

© OMICRON 2014 Page 20 of 23

General tab

1. Here, the start condition of the test can be set. This defines the behavior of the test module after clicking the

Start/continue test button on the toolbar. When selecting Immediately, the test will begin as soon as Start/continue test is clicked. If On binary input: is activated, the test will only start when the selected binary input is triggered. When On time trigger is selected, the test will only start after a signal from a connected time source as specified in the Hardware Configuration.

2. The prefault time defines the time before the ground fault is simulated. This time only applies if Transient is selected at the Test tab.

3. With this option, the duration of the fault can be defined. Make sure the time is long enough to get the correct reaction from the relay. If a permanent ground fault indication is to be tested, this time has to be long enough to make sure that this indication can be triggered.

Measurement View

It is also possible to do time measurements of the connected binary triggers.

4. The name of the measurement can be entered here.

5. With this option, it is possible to ignore all changes of the binary inputs before the selected event. This ensures that the time measurement is not influenced by binary signals that occurred before the measurement.

6. For the time measurement, a start and a stop condition have to be defined. These conditions can be either events like the start of the voltage output or the fault inception or binary inputs.

7. To assess the time measurement, the expected time as well as the respective tolerances must be entered here. Consult the relay manual to define the time tolerances.

8. The measured time as well as the deviation to the expected time are displayed after the test. If the measured deviation is within the tolerances, the measurement will be assessed as passed. Otherwise it will be assessed as failed.

Right-click the measurement, and select Add to add more measurements if necessary.

1

2

4 5 6 7

3

8

© OMICRON 2014 Page 21 of 23

Time Signal View

The time signal view shows analog and binary signals of the test. It can be used to check whether the binary contacts of the relay reacted as expected. Also the cursors (1) can be used via dragging to measure times manually.

1

© OMICRON 2014 Page 22 of 23

3.4.3 Directional Test

The directional test is performed by simulating the same ground faults but changing the relay location. As shown in Figure 10, the first test is performed with the relay located in the faulty feeder (Feeder A). The second one is then performed with the relay in a non-faulty feeder (Feeder B). The remaining settings of the Test view are equal.

Figure 10: Definition of a faulty and a non-faulty feeder

The assessment of the test can be done via:

> Time Assessment: Define an automatic time measurement between the start of the ground fault output and the appearance of the forward direction indicator. With this measurement, it is possible to check whether the forward direction was detected correctly and to confirm the delay time of the directional decision.

> Time Signal View: Use the time signals of the analog outputs and the binary inputs to manually check whether the relay operated as expected.

The assessment of the test depends on the test step:

> Faulty Feeder: During this test, the relay must detect the forward direction of the fault correctly.

> Non-Faulty Feeder: During this test, the relay must detect the reverse direction of the fault correctly. Note: The test can always be assessed manually by selecting Manual Assessment on the Home

ribbon.

Feedback regarding this application is welcome via email to TU-feedback@omicron.at.

© OMICRON 2014 Page 23 of 23

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