Contents
Chapter 1
• Differential Protection of a Transformer
Chapter 2
• REF Protection in Trasnformer
Chapter 3
• Primary Current Calculation of a Transformer
Chapter 4
• Transformer Stability Test Format
Differential Protection of a Transformer:
Generally Differential protection is provided in the electrical power transformer rated
more than 5MVA.
The Differential Protection of Transformer has many advantages over other schemes
of protection.
1. The faults occur in the transformer inside the insulating oil can be detected
by Buchholz relay. But if any fault occurs in the transformer but not in oil then it cannot be detected by Buchholz relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any
internal fault in the transformer but Differential Protection scheme detects the
same in more faster way.
2. The differential relays normally response to those faults which occur inside the differential protection zone of transformer. Differential Protection Scheme in a Power
Differential Protection Scheme in a Power Transformer:
Principle of Differential Protection
Principle of Differential Protection scheme is one simple conceptual technique.
The differential relay actually compares between primary current and secondary
current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.
Suppose you have one transformer which has primary rated current Ip and secondary
current Is. If you install CT of ratio Ip/1A at the primary side and similarly, CT of ratio
Is/1A at the secondary side of the transformer. The secondary of these both CTs are
connected together in such a manner that secondary currents of both CTs will oppose
each other.
The relays used in power system protection are of different types. Among them differential relay is very commonly used relay for protecting transformers and generators from localized faults. Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone. Most of the relays operate when any quantity
exceeds beyond a predetermined value for example over current relay operates
when current through it exceeds predetermined value. But the principle of differential relay is somewhat different. It operates depending upon the difference between two or more similar electrical quantities.
Definition of Differential Relay:
The differential relay is one that operates when there is a difference between two or
more similar electrical quantities exceeds a predetermined value. In differential relay
scheme circuit, there are two currents come from two parts of an electrical power circuit.
These two currents meet at a junction point where a relay coil is connected. According
to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing
but summation of two currents, coming from two different parts of the electrical
power circuit. If the polarity and amplitude of both currents are so adjusted that the
phasor sum of these two currents, is zero at normal operating condition. Thereby
there will be no current flowing through the relay coil at normal operating conditions.
But due to any abnormality in the power circuit, if this balance is broken, that means
the phasor sum of these two currents no longer remains zero and there will be nonzero current flowing through the relay coil thereby relay being operated. In current
differential scheme, there are two sets of current transformer each connected to
either side of the equipment protected by differential relay. The ratio of the current
transformers are so chosen, the secondary currents of both current transformers
matches each other in magnitude. The polarity of current transformers is such that the
secondary currents of these CTs opposes each other. From the circuit is clear that only
if any nonzero difference is created between this to secondary currents, then only this
differential current will flow through the operating coil of the relay. If this difference is
more than the pickup value of the relay, it will operate to open the circuit breakers to
isolate the protected equipment from the system. The relaying element used in
differential relay is attracted armature type instantaneously relay since differential
scheme is only adapted for clearing the fault inside the protected equipment in other
words differential relay should clear only internal fault of the equipment hence the
protected equipment should be isolated as soon as any fault occurred inside the
equipment itself. They need not be any time delay for coordination with other relays in
the system
Restricted Earth Fault Protection of Transformer:
Restricted Earth Fault (REF) protection is basically a Differential Protection. The only
difference in between the Differential Protection and REF Protection is that, latter
protection is more sensitive as compared to the former protection scheme.
For the sake of understanding REF Protection, we take a Transformer of configuration
DYn i.e. HV side of Transformer is Delta connected while the LV side is Start connected
and neutral is grounded solidly.
As shown in figure above, there are a total of four Current Transformers (CTs), three
CTs connected in each phase i.e. R, Y and B and one CT connected in neutral. The
secondary of these four CTs are connected in parallel. The parallel connected CT
secondary are then connected to REF Relay Coil. Basically REF protection Relay
element is an over current element.
Under balanced condition i.e. under normal operation the sum of currents through
the secondary of CTs will be zero and current in neutral CT will also be zero. But as
soon as a fault takes place in the secondary winding of Transformer, the current in R, Y
and B phase will no longer be balanced. Also under earth fault a current will flow
through the neutral CT. Because of this unbalance, the summation of current will not
be zero but it will have some finite value and hence the relay will pick up. It shall be
noted that for a fault outside the Transformer i.e. for through fault Restricted Earth
Fault Protection will not operate as in this case of through fault, the vector sum of
currents in CT secondary will be zero. This is the reason; such kind of protection
scheme is for restricted zone and hence called Restricted Earth Fault Protection.
Now, it is normal to ask that Differential Protection is also a zone protection and it shall
operate for any internal fault in Transformer, then why do we need extra Restricted
Earth Fault Protection?.
This is really a very smart question. See, what happens is, the setting of differential
protection is normally kept at 20%. So, differential relay shall pick if the differential
current exceeds 0.2 A. Now let us consider a case where earth fault occurs just near
the neutral point as shown in figure below.
current will be very less and hence the reflection of such a low current in primary side
of transformer will also will be low. Thus in such case, Transformer differential
protection may not operate as its setting is quite high at 20%. Therefore for protection
of Transformer from such a fault we need more sensitive protection scheme which is
implemented by using Restricted Earth Fault Protection. The sensitivity of REF
protection is superior as compared to Differential Protection. Normally the setting of
REF protection is kept as low as 5%. Basically the sensitivity of REF protection increases
as we are using CT in neutral of transformer and whenever an earth fault takes place it
is damn sure that current will complete its path through the neutral and hence
increasing the sensitivity of REF protection.
Stability Test of Power Transformer (Differential and REF)
The stability test of a power transformer ensures its reliable operation under various fault conditions while preventing maloperation during normal or external fault scenarios. Protection schemes such as differential protection and Restricted Earth Fault (REF) protection are critical for safeguarding transformers from internal faults. Both tests validate the proper functioning of these protective systems and help avoid equipment damage or system outages.
1. Differential Protection
Principle:
Differential protection operates on the principle of comparing the current entering and leaving the transformer. Under normal conditions or external faults, the current at both ends remains balanced. An imbalance indicates an internal fault, such as winding short circuits or inter-turn faults.
Testing Differential Protection Stability:
The stability test evaluates the system’s ability to differentiate between internal and external faults, ensuring it operates only for internal faults.
Steps for Testing:
Simulate Normal Operating Conditions:
- Apply current to the transformer through primary and secondary windings while monitoring the relay.
- Verify that the relay does not trip under normal load conditions.
Simulate External Faults:
- Introduce an external fault on the busbar or the system connected to the transformer.
- Ensure that the relay remains stable and does not trip.
Check Internal Fault Detection:
- Introduce an internal fault (e.g., within the transformer windings).
- Confirm that the differential relay detects the fault and trips appropriately.
Key Points:
- Current transformers (CTs) must be correctly matched and connected to prevent false differential current.
- Relay settings like percentage bias and sensitivity must be fine-tuned to avoid maloperation during inrush or external faults.
2. Restricted Earth Fault (REF) Protection
Principle:
REF protection specifically detects earth faults within a predefined zone, typically within the transformer windings and earth connection. It is more sensitive than differential protection for ground faults.
Types of REF Protection:
- High-Impedance REF: Uses a high-resistance relay to block false currents.
- Low-Impedance REF: Relies on sensitive current measurement without introducing high impedance.
Testing REF Stability:
The stability test ensures that the REF relay does not operate for external faults or healthy conditions.
Steps for Testing:
Simulate Normal Conditions:
- Energize the transformer and monitor the REF relay.
- Confirm that the relay does not trip during normal operation.
Simulate External Earth Faults:
- Create an earth fault outside the protected zone (e.g., in the network connected to the transformer).
- Verify that the REF relay remains stable.
Simulate Internal Earth Faults:
- Introduce an earth fault within the transformer’s protected zone.
- Confirm that the REF relay detects the fault and trips.
Key Points:
- CTs in REF protection must be accurately matched to ensure correct differential calculation.
- REF relays are highly sensitive and require precise calibration to distinguish between internal and external faults.
Challenges in Stability Testing
- Inrush Current: Magnetizing inrush during transformer energization can mimic fault conditions. Differential protection must incorporate inrush blocking or harmonic restraint mechanisms.
- CT Saturation: During high fault currents, CTs may saturate, leading to incorrect differential calculations. Proper CT selection and testing are critical.
- Coordination: Ensuring coordination between differential and REF protection is necessary to avoid overlapping or conflicting operations.
Conclusion
Stability testing of power transformer protection schemes, such as differential and REF protection, is vital for system reliability and safety. These tests ensure that protective relays operate correctly under fault conditions while remaining stable during normal or external fault scenarios. Proper configuration, calibration, and periodic testing are essential to maintain transformer protection and prevent costly equipment failures.
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