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.

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.

● 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:

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    IA1  = 900上 — 450 (A) , IB1  = 900上 —1650 (A) ,  and   IC1  = 900上750 (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.

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

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.

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.)

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

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