Laboratory 4 Test Manual

L4-1 ELEC4617 Power System Protection: Laboratory 4

School of Electrical Engineering and Telecommunication, University of New South Wales

ELEC4617 Power System Protection

Laboratory 4: Overcurrent and Over/Under frequency protection using

SEL-751A

Overcurrent protection is a typical method for protecting power systems. It senses the currents

flowing in the power systems using current transformers. When the currents are above the

normal operating current by pre-set margin, relay trips the breakers to isolate the fault.

With the wide spread applications of microgrids, their islanding detection has become an integral

part of system protection. Grid-tied inverters are basic forming components in microgrids.

Frequency-drift active islanding detection method can be incorporated into the inverter control

algorithm. When islanding happens, inverter frequency will drift quickly from its normal value.

By using over/under frequency protection, the happening of islanding can be detected

effectively.

In this laboratory session, both overcurrent protection and over/under frequency protection will

be conducted.

Objectives 1. Examine the basic functions of SEL-751A digital relay from Schweitzer. 2. Use Omicron software platform Test Universe to test SEL-751A relay operation

characteristics.

3. Use Schweitzer software AcSELerator QuickSet to configure the settings of SEL-751A; to monitor currents at secondary side of current transformers and status of relay logic and

trip signal.

4. Conduct overcurrent protection using the setup with pre-selected time-against-multiple-of-pick-up-current curve for SEL-751A.

5. Conduct over/under frequency protection using the setup with pre-set frequency boundaries.

Caution The rated current of SEL-751A current sensing windings is 5A. Whenever possible, one should

avoid from injecting large currents into the windings for too long. One also should not change

the upper-limit of output current from Omicron CMC356.

Introduction to the system The overall test system is shown in Fig. 1. The major forming components are Omicron

CMC356, SEL-751A, PC and network switch. Two control softwares are installed in PC, one

being Omicron Test Universe and the other being Schweitzer relay-configuring software

AcSELerator QuickSet.


Network Cable

Network Cable

Network Cable

Network Switch

Connection wireCMC356

SEL-751A

Figure 1 Overall test system

Software 1. Test Universe Omicron software platform for digital relay testing. It controls CMC356 to

replicate currents and voltages at the secondary sides of current transformers and voltage

transformers. It can detect the trip signal and other logical signal from SEL-751A or other

relays.

2. AcSELerator QuickSet Schweitzer software platform to control and configure SEL relays including SEL-751A.

Hardware 1. Omicron CMC356 for testing digital relays; it can replicate currents and voltages at

secondary sides of current transformers and voltage transformers.

2. SEL-751A digital relay for feeder protection. 3. Network switch for communication between computer and SEL-751A and CMC356. 4. Computer with the software installed.

Connections used in the experiment Typical connection of SEL-751A for overcurrent protection in industry is shown below.

Figure 2 Typical connections for overcurrent protection

In Fig. 2, Z01 Z08 are the input connections at the back-panel of SEL-751A relay. IA1, IB1, IC1 and Ia1, Ib1, Ic1 are the currents at the primary side and secondary side of the current

transformers installed in the power system under protection.


Between Z01 and Z02, Z03 and Z04, Z05 and Z06, Z07 and Z08, each has a current sensor to

measure the corresponding current.

In this test, the feeder under protection is assumed to have the following ratings and CT ratio

selection:

Current rating: 300A

Voltage rating (LL): 22kV

Frequency: 50Hz

Ratio of three-phase CTs: 300:5 in Y-connection.

In the test, secondary side currents Ia1, Ib1, and Ic1 are replicated by Omicron CMC356 Current

Output A as shown in Fig. 3. Terminals 1, 2 and 3 from CMC356 Current Output A are

connected with Z01, Z03 and Z05 of SEL-751A respectively. Terminals Z02, Z04 and Z06 at the

back of SEL-751A are shorted and connected to Z07. Z08 of SEL-751A is connected with

neutral output of Current Output A of CMC356.

Figure 3 Connections from Omicron CMC356 to SEL-751A

Question 1: Calculate the currents Ia1, Ib1, and Ic1 in Fig. 2 at the secondary sides of CTs under

normal operating conditions.

Question 2: Compare phase angles between IA1 and Ia1, IB1 and Ib1, IC1 and Ic1 in Fig. 2.

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SEL-751A overcurrent protection SEL-751A can fulfill instantaneous overcurrent protection and time-delay overcurrent protection.

In this case, instantaneous trip current is set equal to 12.5A, which is around 2.5 times the rated

current (5A). Pick-up current for time-delay overcurrent protection is set at 7.5A, which is 150%

of normal operating current.

For the time-delay overcurrent protection, SEL-751A relay provides two groups of inverse

curves, one being U.S. curves and the other being IEC curves.

In this experiment, IEC inverse curves are adopted. Two example curves are shown in Figs. 4-5,

where time-dial setting can be set continuously, though only several choices are given in the

figures. In contrast, in electro-mechanic relays these values are limited and discrete.


Figure 4 IEC inverse curve C1


Figure 5 IEC very inverse curve C2


Curves C1 and C2 can also be obtained analytically from the following formulas:

trip 0.02

0.14t TDS

PSM 1

(C1)

trip

13.5t TDS

PSM 1

(C2)

where TDS is time-dial setting, PSM is the multiple of plug-setting or pick-up current setting,

which is equal to ratio of current at secondary side of CT to pick-up current, and C1 is standard

inverse and C2 is very inverse.

Question 3: Assuming that TDS=0.1, PSM=1.5, use equation (C1) to calculate trip time by

751A. Check this trip time against value found from curve C1 in Fig. 4 and see whether they

tally with each other.

Question 4: Assume that there is a three-phase fault on the feeder. The setting of SEL-751A are

a pick-up current of 7.5A, an IEC standard inverse curve of C1 selected, and a time-dial selection

of 0.2 chosen. Assume that fault currents at primary side are 0A1I 900 45 (A) ,

0

B1I 900 165 (A) , and 0

C1I 900 75 (A) . Use the curve given in Fig. 4 to work out

expected trip time by SEL-751A for such fault. Also use formula (C1) to calculate the trip time

by SEL-751A and see whether two results are the same.

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Relay trip signal is the output from A07 and A08 or OUT103 which can be seen at the back of

SEL-751A. A07 and A08 are connected to CMC356 Input 1of Binary/Analog Input as shown in

Fig. 3. Input contact between A07 and A08 normally stays open. When the relay trips, the

contact will become closed. This will be detected by CMC356 and sent to Test Universe.

Software Omicron Test Universe Test Universe has many functions. The main user interface is shown in Fig. 6, from which you

can find some basic functions, such as Quick CMC Ramping, State Sequencer, and AuxDC. In

this experiment, State Sequencer is used to examine the relay performance. AuxDC is set to

command CMC356 to generate 110V DC to power SEL-751A.

When you click on State Sequencer as shown in Fig. 6, you will be able to see some similar user

interface as shown in Fig. 7. This is the main interface that will be used in the following

experiments.


Figure 6 User interface of Test Universe

Figure 7 Examples of producing currents from Current Output A of CMC356 by using State

Sequencer of Test Universe


Software AcSELerator QuickSet Settings for SEL-751A can be modified and updated using the following interface. In this test,

settings in Global and Group1 need be studied and modified to suit the overcurrent protection.

Figure 8 AcSELerator QuickSet interface

Test procedures

Part 1 Overcurrent protection

Step 1

Open the user interface Test Universe from Desktop. Turn on the relay power with the

demonstrator aside by accessing AuxDC from the user interface of Test Universe. You need to

choose 110V DC to command CMC356 to produce such voltage to power SEL-751A.

Open the user interface AcSELerator QuickSet from Desktop. Go to menu Communications. Then choose Connect to establish connections between AcSELerator QuickSet and SEL-751A.

Go to Menu File and choose open SEL-751A Laboratory4 Setting. Then you will be led to a user interface as shown in Fig. 8, where you can change the settings. Check the settings against

information found in the section of Introduction to the system. Your demonstrator will check with you why the settings are so. Information on SEL-751A settings can be found in the

APPENDIX.

Pick-up current of time-delay overcurrent protection is set at 7.5A. Pick-up current of

instantaneous overcurrent protection is set at 12.5A. Chosen relay curve is C1. During

experiment, you only need to change the time-dial settings.

After checking the values, you may go to menu File and choose send to send the settings of Global, Set 1 and Logical 1 to SEL-751A. Then you may manually check from the front panel of SEL-751A whether your settings have passed to it correctly. To this step, the relay is

ready to respond according to the setting for overcurrent protection.


Step 2

Go back to Test Universe. Open State Sequencer as shown in Fig. 6. Go to menu File and choose Open a file named as OverCurrent_FreqProtectionL4.seq, which is stored in subdirectory Laboratory4 on Desktop.

After the opening of the settings, you should be able to see similar interface as shown in Fig. 7.

From here, you may proceed to your experiment.

Step 3

Input each of the values as given in Table 1, run the program, and check the report whether trip

happens or not. You need to use both State Sequencer in Test Universe and AcSELerator

QuickSet to fulfill the following settings.

Save each of the test report generated by Test Universe for checking by your demonstrator. You

may use long-format to save the report. Please use file names such as OC_YourName_Case1 etc

to indicate which experiment the report is for.

You need to press Target Reset button on the front panel of SEL-751A for next test if the relay trips for last test.

Table 1 Overcurrent protection

Case Ia1(A) Ib1(A) Ic1(A) Frequency

(Hz)

TDS Recorded

Trip

Time

Expected

Trip

Time

1 08 0 08 120 08 120 50 0.05

2 09 0 09 120 09 120 50 0.05

3 09.5 0 09.5 120 09.5 120 50 0.06

4 010 0 010 120 010 120 50 0.06

5 011.25 0 011.25 120 011.25 120 50 0.08

6 013.0 0 013.0 120 013.0 120 50 0.08

Question 5: For cases in Table 1, you may use either formula (C1) or curve C1 in Fig. 4 to

obtain expected trip time. Check them against recorded trip time by Test Universe. Fill in the

values in Table 1.

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Question 6: Is the recorded relay trip time in agreement with the time found from formula (C1)

or from curve C1? If not, explain why. (Hint: Check the instantaneous protection setting for Case

6.)

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Part 2 Frequency protection

The settings in Part 1 include frequency protection. Check and see whether upper-limit frequency

is 51Hz and lower-limit frequency is 49Hz. If not, change them to these values.


Input each set of values as shown in Table 2 and conduct experiment. Save each of the test report

generated by Test Universe for checking by your demonstrator. You may use long-format to save

the report. Use file names such as Freq_YourName_Case1 etc to indicate which experiment the

report is for.

Table 2 Over/Under frequency protection

Case Ia1(A) Ib1(A) Ic1(A) Frequency

(Hz)

Recorded

Trip Time

1 05 0 05 120 05 120 51.5

2 05 0 05 120 05 120 48.5

3 05 0 05 120 05 120 52

4 05 0 05 120 05 120 48

5 05 0 05 120 05 120 50

6 05 0 05 120 05 120 50.5

Question 7: Is the recorded trip time for Cases 1-4 different? What happened in Cases 5 and 6?

Why is it so?

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Appendix

Instantaneous overcurrent protection: settings and logic

Instantaneous overcurrent element logic


Time-delay overcurrent protection: settings and logic


Frequency protection: settings and logic

Laboratory 4 Test Manual

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