EEL 3086 SWITCHGEAR AND PROTECTION
EXPERIMENT 2
DIFFERENTIAL PROTECTION OF A THREE-PHASETRANSFORMER
Objective
•To analyse the differential protection scheme as applied to a three-phase power transformer
•To describe the operation and setting of differential protection.
•To explain the transformer supply voltage and current waveforms during the differential relay operation.
•To evaluate the differential protection performance.
Introduction
The differential protection scheme can be used to protect both the primary and secondary windings of a three-phase transformer against earth faults and phase-to-phase faults. This is possible because the efficiency of the power transformers is high and the magnetizing current is negligibly small. In a differential protection scheme a circuit compare the current entering the protective equipment to the current leaving the equipment, in each phase. Any difference of current of sufficient magnitude operates a relay, which in turn indicates fault clearance. Figure 1 shows a simplified diagram of a single-phase differential protection scheme.
The currents entering and leaving the protected equipment (Ipin and lpout) are sensed through two identical current transformers. When there is no fault in the protected equipment, currents Ipin and Ipout are equal and the currents at the transformer secondaries are also equal (Isin = Isout) because the current transformers are identical. When the current transformers are connected with the polarities indicated in Figure 1,the secondary currents flow round the circuit and no current flows in the coil of the protective relay (IR = 0), which can be an overcurrent relay. However, when a fault occurs in the protected equipment, currents lpin and lpout are no longer equal. Consequently, currents Isin and Isout are also no longer equal. The current resulting from the difference between these two currents (Isin – Isout) flows in the protective relay coil. This trips the protective relay, there by, initiating fault clearance. Similar differential protection scheme can be employed for the protection of transformers. In this case, when the turns ratio of the protected transformer is not unity, the primary and secondary currents are different, and thereby, current transformers with different turns ratio are required for the CT secondary currents to be equal and the residual current IRto be zero under no fault condition. When protecting three-phase power transformers, some additional considerations must be taken into account:
• There is a 30° phase shift between the primary and secondary currents of a three-phase power transformer that is connected delta-wye or wye-delta and supplies a balanced load.
• When a three-phase power transformer is connected delta-wye or wye-delta, the zero sequence current on the wye side of the power transformer has no replica on the delta side.
The 30° phase shift must be compensated and the zero sequence current on the wye side of the power transformer must be eliminated, for the CT secondary currents to be equal under no fault condition. This is achieved by proper connections of the current transformer secondary windings. A general rule for connecting the current transformers states that the CT secondary windings should be connected in delta when the power transformer windings are connected in wye, and vice versa. Figure 2 shows typical connections of the current transformers for three-phase power transformers connected delta-wye and delta-delta. Note that it is assumed that the ratios of the current transformers have been selected so that the secondary currents supplied by the two groups of current transformers are equal, thereby ensuring balance of the currents in the differential protection system.
In practice, it is very difficult to maintain perfect balance of the currents in a differential protection system protecting a three-phase power transformer. This is mainly due to the following factors:
• Change in the power transformer turns ratio (on transformers with a tap-changing facility).
• Current transformer mismatch (difficulty in having current transformers with ratios that perfectly balances the differential protection system).
• Transformer magnetizing current.
All these factors unbalance the differential protection system and produce a residual current IR in the differential relay coil. This residual current increases as the line currents flowing through the three-phase power transformer increase. Therefore, the current setting of the differential relay must be increased to prevent undesired relay tripping, thereby reducing the system sensitivity.Differential relays with bias coils are often used in transformer differential protection systems to reduce the negative effect of current unbalance on the system sensitivity.Figure 3 shows the bias characteristic of a differential relay. This characteristic shows that the current required to trip the differential relay (differential operating current) increases as the current flowing through the transformer increases. Note that, in general, the sensitivity of transformer differential protection systems is less than that achieved in differential protection systems protecting the stator windings of a synchronous generator.
Figure 3: Typical bias characteristics of a differential relay
Transformer magnetizing inrush, discussed in the first exercise of this unit, is another source of unbalance in transformer differential protection systems. This is because the magnetizing inrush current in the energized winding of a transformer is not replicated in the other windings of the transformer. This appears as a current unbalance to the differential protection system, which superficially, cannot be distinguished from a current unbalance caused by a fault in the transformer. When magnetizing inrush is severe, the current unbalance may easily exceed the current required to trip the differential relay and cause undesired disconnection of the power transformer. Fortunately, transformer magnetizing inrush is a transient phenomenon occurring on transformer energization. Undesired transformer disconnection can therefore be avoided by adding a short time delay to prevent the differential protection system from tripping on transformer magnetizing inrush.
Note that after the magnetizing inrush, the magnetizing current stabilizes to a very low value. This current, however, causes a slight current unbalance that is stable under normal operating conditions. To preserve the system stability, this slight current unbalance must be taken into account when setting the differential relay operating current.
To obtain additional information on transformer differential protection, refer to section 16.7, entitled "Differential protection", in the third edition of the Protective Relays Application Guide published by GEC Alsthom Measurements Limited.
Procedure Summary
In the first part of the exercise, set up the equipment in the EMS Workstation and the Protective Relaying Control Station.
In the second part of the exercise, connect the equipment as shown in Figures 4 and 5. In this circuit, power transformers connected delta-wye are protected by a differential protection system which mainly consists of a current sensitive relay and line current transformers. When a fault occurs in the power transformers, the overcurrent) relay trips. This energizes time delay relay TD1. Once the time delay is elapsed, contact TD1-A closes to initiate a trip current in the coil of control relay CR1. Contact CR1-C closes to memorize the fault and light up the corresponding reset button. Contact CR1-B opens to open contactor CR2, thereby disconnecting the power transformers from the three-phase power source.
Open contactor CR3 to prevent operation of the differential protection system. Check whether or not the differential protection system is perfectly balanced when no load is applied to the power transformers. Adjust the current setpoint of the overcurrent relay.
With the power transformers supplying power to a balanced three-phase load, initiate earth faults at the primary and secondary windings of the power transformers and observe what happens in the differential protection system.
You will close contactor CR3 to allow operation of the differential protection system. You will verify whether or not the differential protection system is stable on transformer magnetizing inrush. You will initiate earth faults and a phase-to-phase fault in the power transformers, and observe the operation of the differential protection system.
Equipment Required Protective Relaying Control Station: (record the equipment rating)
- AC/DC current sensitive relay: ______
- EMS: ______
Workstation (record the equipment rating)
- Power supply: ______
- Interconnection module: ______
- Universal fault module: ______
- Faultable transformers: ______
- Transmission grid – A: ______
- Current transformers: ______
- Resistive loads: ______
- AC ammeter: ______
- AC voltmeter: ______
PROCEDURE
CAUTION!
High voltages are present In this laboratory exercise! Do not make or modify any banana jack connections with the power on unless otherwise specified!
Setting Up the Equipment
1.Ensure that the Protective Relaying Control Station is connected to a three-phase power source.
Make sure the DC Power Supply of the Protective Relaying Control Station is turned off.
Install AC/DC Current sensitive relay in the Protective Relaying Control Station.
2.Make the following settings on the Universal Fault Module:
TD1 time delay ...... -1 s
SST1 time interval ...... ~5 s
SS12 time interval ...... ~10s
Note: The control knobs for adjusting the time delay and time intervals are located on time delay relay TD1 and solid-state timers SST1 and SST2 in the Universal Fault Module.
3.Install the Interconnection Module, Power, Supply, Universal Fault Module, Faultable Transformers, Transmission Grid "A", Current Transformers, Resistive Load, AC Ammeter, and AC Voltmeter in the EMS Workstation.
Make sure the Power Supply is turned off and its voltage control knob is set to the O position. Connect the Power Supply to one of the three-phase power outlets on the back panel of the Protective Relaying Control Station.
On the Current Transformers module, make sure that all switches are set to the I (close) position to short-circuit the secondary’s of the current transformers.
Differential Protection of a Three-Phase Power Transformer
4.Connect the Interconnection Module installed in the EMS Workstation to the Interconnection Panel of the Protective Relaying Control Station using the supplied cables.
Connect the equipment as shown in Figures 4and 5
Note: Since a single AC/DC Current Sensitive Belay is available, terminals A2 and A3 are connected to terminal A4 to avoid disturbing the operation of the differential protection system.
5.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 ...... I (close)
Switch S2 ...... O (open)
Figure 4 Connection diagram of the equipment in the EMS workstation
Figure 5 Connection diagram of the equipment in the protective relaying control station
On the AC/DC Current Sensitive Relay
INPUT switch ...... ……… AC
MODE switch ...... OVER CURRENT
Current setpoint ...... minimum (fully CCW)
Hysteresis ...... ……... ~7.5%
On the Universal Fault Module
INITIATE FAULT button ...... released position
FAULT DURATION switch ...... ….0.05 - 5 s
Make sure that the current transformers are connected as shown in Figures 4and 5then set the switches of current transformers CT1 to CT6 on the Current Transformers module to the O (open) position.
6.On Control Relays 2 of the Protective Relaying Control Station, set the time delay of control relay TD1 to approximately 2 s.
Note: -Access to the time delay adjustment knob of control relay TD1 is through a panel on top of the Protective Relaying Control Station.
Turn on the DC Power Supply of the Protective Relaying Control Station.
On Transmission Grid "A", set switch S3 to the O (open) position to open contactor CR3. This will prevent operation of the differential protection system and allow the operation of the AC/DC Current Sensitive Relay to be observed.
7.On the Resistive Load module, set all toggle switches to the O (open) position to temporarily disconnect the load (resistors R1, R2, and R3) from the secondary windings of the power transformers.
Turn on the Power Supply and set the voltage control knob so that the line-to-neutral voltage at the secondary windings of the power transformers
Record the circuit voltages and currents in the following blank spaces.
E1 = ___ V I1 = ___ A
E2 =___V I2 = ___ A
E3 = ___ V I3 = ___ A
Is the differential protection system perfectly balanced? Briefly explain?
8. Adjust the current setpoint of the AC/DC Current Sensitive Relay to approximately 110% of the residual current (I3) measured in the previous step. To do so, slowly turn the current setpoint adjustment knob clockwise until the tripping indicator (red LED) of the AC/DC Current Sensitive Relay turns OFF.
On the Resistive Load module, set the resistance of resistors R1, R2, andR3 to the value indicated in Figure 2.
9.On the Faultable Transformers, set fault switch FS1 of transformer T1 to the 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.
Record the circuit voltages and currents in the following blank spaces.
E1 = ______VI1 = ______A
E2 = ______VI2 = ______A
E3 = ______VI3 = ______A
Describe what happens when an earth fault occurs near the middle of one of the power transformer primary windings.
On the Faultable Transformers, set fault switch FS1 of transformer T1 to the O position to remove the fault.
10.On the Faultable Transformers, set fault switch FS3 of transformer T1 to the I position to insert an earth fault near the neutral end of the secondary winding of transformer T1. While doing this, observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.
Record the circuit voltages and currents in the following blank spaces
E1 = ______VI1 = ______A
E2 = ______VI2 = ______A
E3 = ______VI3 = ______A
1
Describe what happens when an earth fault occurs near the neutral end of one of the power transformer secondary windings.
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the O position to remove the fault.
11.On Transmission Grid "A", set switch S3 to the I (close) position to close contactor CR3. This will allow operation of the differential protection system.
12.On Transmission Grid "A", set switch S1 to the O (open) position to open contactor CR1 and remove power from the power transformers.
Energize the power transformers by setting switch S1 on Transmission Grid "A" to the I (close) position. While doing this, observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.
13. Repeat the previous step at least ten times.
Does the residual current (I3) sometimes exceed the current setpoint of the AC/DC Current Sensitive Relay on transformer energization?
1
EEL3086 SWITCHGEAR AND PROTECTION
Is the differential protection system stable on transformer energization?
Yes No
Note: The AC/DC Current Sensitive Relay is fairly insensitive to the residual current resulting from the transformer magnetizing inrush. A time delay relay is, however, included in the differential protection system to prevent transformer disconnection in case a high magnetizing inrush would trip the AC/DC Current Sensitive Relay.
14.On Transmission Grid "A", set switch S1 to the I position to close contactor CR1 and energize the power transformers.
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the I position to insert an earth fault near the neutral end of the secondary 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 fault been cleared by the differential protection system?
Yes No
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the O position to remove the fault.
15.On the Faultable Transformers, set fault switch FS1 of transformer T1 to the 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 fault been cleared by the differential protection system?
Yes No
On the Faultable Transformers, set fault switch FS1 of transformer T1 to the O position to remove the fault.
16.On Control Relays 1 of the Protective Relaying Control Station, press the RESET button of control relay CR1 to reset the differential protection system.
On the Universal Fault Module, depress the INITIATE FAULT button to produce a phase-to-phase fault at the secondary windings of the power transformers. While doing this, observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.
Describe what has happened.
Has the fault been cleared by the differential protection system?
Yes No
Does the differential protection system protect the power transformers against earth faults as well as phase-to-phase faults? Briefly explain.
Turn off the Power Supply.
17. Turn off the DC Power Supply of the Protective Relaying Control Station. Remove all leads and cables.
CONCLUSION
In this exercise, you learned that differential protection can be used to protect the primary and secondary windings of a three-phase power transformer against earth faults and phase-to-phase faults. You also learned that the sensitivity of transformer differential protection is limited by several factors explained in the discussion of this exercise. You saw that transformer magnetizing inrush unbalances the circulating current circuit of a differential protection system, and may cause undesired transformer disconnection. You saw that transformer disconnection on a magnetizing inrush can be prevented by adding a time delay relay in the differential protection system.
REVIEW QUESTIONS
1. A differential protection system protects
- Power transformers against magnetizing inrush.
- The primary and secondary windings of a power transformer against earth faults
- Power transformers against phase-to-phase faults
- Both b and c
2.In general, when using differential protection to protect a three-phase power transformer, the secondary windings of the line current transformers should be connected in
.aWye when the power transformer windings are connected in delta, and vice-versa.
- Wye on-both sides of the power transformer
- Delta on both sides of the power transformer
- None of the above.
3. In transformer differential protection systems, a time delay relay can be used to