MULTIMEDIA UNIVERSITY
FACULTY OF ENGINEERING
LABSHEET
EET4116 POWER STATIONS
PS1: Overcurrent Protection of a Substation Power Transformer
PS2: Coordination of Hydro-Thermal Power System
Experiment # 1
Overcurrent Protection of a Substation Power Transformer
Instructions:
1. Read this lab sheet carefully and understand the procedure of performing the experiment before coming to the laboratory.
2. Do not switch on the power supply unless permitted by the teacher.
3. Do not make or break any connection with the power supply on.
4. Handle the equipment with care.
5. Complete the experiment and submit the report before leaving the laboratory.
Objective
· To design an overcurrent protection including earth fault protection of a three-phase substation power transformer with overcurrent relays and current transformers.
· To evaluate the protection scheme designed using the Workstation and Protective Relay Control Station available in the Lab.
Introduction
For economic considerations, small-three-phase power transformers (less than 100 kVA) are usually protected with fuses. The main disadvantage of using fuses to protect power transformers is their long operating time under low overcurrent conditions. This delays transformer disconnection, and in certain cases, provides little protection to the transformer and rather protects the system disconnecting the faulty transformer after the fault has reached an advanced stage.
The protection schemes for large power transformers usually include differential protection and/or restricted earth fault protection. In this case, transformer disconnection is achieved through circuit breakers, and overcurrent relays can be added to the protection schemes to provide overcurrent protection. This additional protection serves as a general backup protection in case of a failure of the main protection system.
Figure 1 shows overcurrent and earth fault protection system of a three-phase power transformer implemented with protective relays. In this system, the power transformer primary currents are measured through three CTs (current transformers) and the secondary currents of the CTs are connected to the overcurrent relays. The earth fault relay is connected in the neutral circuit of the CT secondaries. This arrangement allows the measurement of each of the line currents by the overcurrent relays and the vector sum of the line current by the earth fault relay. Note that the earth fault relay must be of low impedance type to allow measurement of the line currents by the overcurrent relays.
Design of Protection Scheme
A simplified diagram of an overcurrent and earth fault protection system is shown in Fig. 1.
Assume that the current set point of the Three-Phase Overcurrent Relay is approximately 200% of the nominal full-load current of the power transformers. Based on the above information, compute the transformation ratio of the current transformers.
Design the values of the load resistances to be connected in star for full load current of the transformer. Complete the design and fill up all the details in the simplified diagram given.
Fig.1 Simplified diagram of an overcurrent and earth fault protection system
Equipment to be used for testing the protection system
Mobile Workstation, Protective Relay Control Station, Transmission Grid “A”, Interconnection Module, Universal Fault Module, Power Supply, Faultable Transformer, Resistive Loads, Current Transformers, AC Ammeter, AC Voltmeter, Three-phase Overcurrent Relay, AC/DC Current Sensitive Relay, and Connection Leads.
Testing procedure and observations
1. Ensure that the Protective Relay Control Station is connected to a three-phase power source. Make sure that the DC Power Supply to the Protective Relay Control Station is turned off. Also, make sure that all fault switches on the Three-phase overcurrent relay and AC/DC current sensitive relay are set to O (off) position.
2. Make the following settings on the Universal Fault Module:
TD1 time delay 1 s
SST1 time interval 3 s
SST2 time interval 10 s
(Note: These settings have already been done by the Lab technician)
3. Make sure that the Transmission Grid “A”, Interconnection Module, Universal Fault Module, Power Supply, Faultable Transformer, Resistive Loads (2), Current Transformers, AC Ammeter, and AC Voltmeter are installed in the EMS Workstation. See that the Power Supply is turned off and its voltage control knob is set to the O position.
4. On the Current Transformer module, make sure that all the switches are set to the I (close) position to short-circuit the secondaries of the current transformers.
5. Connect the equipment as shown in Figs. 2 and 3.
6. Make the following settings:
On the Faultable Transformers
Transformer T1 Fault switches (FS1 to FS3) - - - - - O
Transformer T3 Fault switches (FS1 to FS3) - - - - - O
On Transmission Grid “A”
Switch S1 ------O
Fig. 2 Connection diagram of the equipment in EMS Workstation
Fig. 3 Connection diagram of the equipment in the Protective Relay Control Station
On the AC/DC Current Sensitive Relay
INPUT switch ------AC
MODE switch ------OVER CURRENT
On the Universal Fault Module
INITIATE FAULT button ------released position
FAULT DURATION switch------0.3 – 30 s
On the Current Transformer Module
Current transformers CT1, CT2 and CT3 - - - O (open)
7. Set the current set point of the Three-Phase Overcurrent Relay to approximately 200% of the nominal full-load current of the power transformers, taking into account the transformation ratio of the current transformers.
8. Set the time delay of the Three-Phase Overcurrent Relay to approximately 5 s. Adjust the set point and hysteresis of the AC/DC Current Sensitive Relay to 100mA and 5%, respectively.
9. Turn on the DC Power Supply of the Protective Relay Control Station.
10. On Transmission Grid “A”, set switch S2 to O (open) position to open the contactor CR2. This will prevent operation of the protection system and allow the operation of the protective relays to be observed.
11. Turn on the Power Supply and set the voltage control knob to the 100% position. Set the primary line current (indicated by ammeter I1) should be approximately equal to the full-load current of the power transformers by adjusting the resistive loads.
12. On the Universal Fault Module, depress the INITIATE FAULT button to short-circuit one phase of the three-phase load. While doing this, observe the circuit currents and voltages as well as tripping indicator (red LED) on the Three-Phase Overcurrent Relay and AC/DC Current Sensitive Relay. Record the current and voltages in the blank spaces.
(Note: You may have to initiate this fault a few times to record the circuit currents and voltages)
I1 = ……. A I2 = …… A I3 = …… A
E1 = …… V E2 = ……V
Describe what happens when this fault is initiated.
Is the earth fault protection stable when the current through one phase of the three-phase power transformer is high? □ Yes □ No
On the Universal Fault Module, place the INITIATE FAULT button in the release position.
13. On the Faultable Transformers, set the fault switch FS1 on the transformer T1 to the position I to insert an earth fault near the middle of the primary winding of transformer T1. While doing this, observe the circuit currents and voltages as well as the tripping indicator on the Three-Phase Over Current Relay and AC/DC Current Sensitive Relay.
Record the currents and voltages in the following blank spaces.
I1 = ……. A I2 = …… A I3 = …… A
E1 = …… V E2 = ……V
Describe what happens when this fault is initiated.
On the Faultable Transformers, set the fault switch FS1 on the transformer T1 to the position O to remove the earth fault
14. On the Transmission Grid “A”, set switch S2 to the I (close) position to close contactor CR2. This will allow operation of the protection system.
15. On the Universal Fault Module, depress the INITIATE FAULT button to short-circuit one phase of the three-phase load. While doing this observe the circuit currents and the tripping indicator on the Three-Phase Overcurrent Relay.
Describe what has happened.
Has the large fault current been cleared by the overcurrent protection system?
□ Yes □ No
On the Universal Fault Module, place the INITIATE FAULT button in the released position.
16. On Control Relays 1 of the Protective Relaying Control Station, press the RESET button of control relay CR1 to reset the protection system.
On the Faultable Transformer, set fault switch FS1 of transformer T1 to I position to insert an earth fault near the middle of the primary winding of transformer T1. While doing this, observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.
Describe what has happened.
------
Has the earth fault at the primary winding of the power transformer been cleared by the earth fault protection in the overcurrent protection system?
□ Yes □ No
On the Fautable Transformer, set fault switch FS1 of transformer T1 to the O position to remove the fault.
17. Turn off the DC Power Supply of the Protective Relaying Control Station. Remove all leads.
Conclusion
In this experiment you have learned the following:
Ø Designing a simple overcurrent protection scheme for a three-phase power transformer.
Ø Evaluate the overcurrent protection system using the workbench available in the Lab.
Ø The overcurrent protection will act as a backup protection.
Ø When the current through the power transformer exceeds the overcurrent relay set point during a certain time, the transformer is disconnected from the power source.
Ø An earth fault relay can be included in the overcurrent protection to provide fast, sensitive protection of the primary side of the power transformer against earth faults.
Experiment #2
Development of a Computer Programme for Coordination of Hydro-Thermal Power System
(MATLAB program to do this experiment has to be prepared by the students. The details will be discussed in the class. After the program has been approved by the teacher the student will carry out the experiment using PC. The PC available in the Energy System Lab may be used for this purpose.)
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