MULTIMEDIA UNIVERSITYFACULTY OF ENGINEERING

LABSHEET

EET4116 POWER STATIONS

PS1: Overcurrent Protection of a Substation Power TransformerPS2: Coordination of Hydro-Thermal Power System

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Experiment # 1Overcurrent 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 lecturer.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 scheme including earth fault protection for 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.

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

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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

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 sSST1 time interval 3 sSST2 time interval 10 s(Note: These settings have already been done by the Lab technician)

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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) - - - - - OTransformer T3 Fault switches (FS1 to FS3) - - - - - O

On Transmission Grid “A”Switch S1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - O

Fig. 2 Connection diagram of the equipment in EMS workstation

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Fig. 3 Connection diagram of the equipment in the Protective Relay Control Station

On the AC/DC Current Sensitive RelayINPUT switch - - - - - - - - - - - - - - - - - - - - - - - - - ACMODE switch - - - - - - - - - - - - - - - OVER CURRENT

On the Universal Fault ModuleINITIATE FAULT button - - - - - - - - released positionFAULT 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.

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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.

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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 is happening.

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 is happening.------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------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.

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Conclusions

In this experiment you should have learnt the following: How to design a simple overcurrent protection scheme for a three-phase power

transformer. How to evaluate the overcurrent protection system using the workbench available

in the Lab. How the overcurrent protection acts as a backup protection. How 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.

Exercises

1. Evaluate the performance of the overcurrent protection which you have designed in this experiment.

2. Distinguish between overcurrent protection and differential protection of a transformer.

3. Explain how the tripping of overcurrent relay is blocked at the time of inrush current of a transformer.

4. Enumerate the various type of protections used to protect a large power transformer.

Marking Scheme

Lab (5 %)

Assessment Components

Details

On the Spot Evaluation (2.5%)

You will be evaluated on the spot based on the theory concerned with the lab experiments and the observations.

Lab Report (2.5%)

You will each have to submit a lab report along with the recorded experimental data.

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Experiment #2

Hydro-Thermal Coordination in a Power System

(It is an assignment type exercise.)

Objective: To develop a MATLAB program to solve a hydro-thermal coordination

problem for a system consisting of one hydro plant and one thermal plant. To evaluate the performance of the program by conducting experiments

using the computers available in the lab. Students can conduct the experiments at any time as long as the lab is available.

Reports are due at the specified date.

Preparation of the report:The report must include the following: A brief introduction to the hydrothermal coordination problem The MATLAB program code Limitations of your program Optimal scheduling, optimal fuel cost and the drawdown for each period Conclusions

Marking Scheme:

Lab/ Assignment (10 %)

Assessment Components

Details

Program code - 3% MATLAB Code

Optimal scheduling result - 3%

Accuracy of results

Report - 4% Quality of report (i.e., content and format).

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