Transformer protection

How many types of transformer protection are there? What is the primary protection of a transformer? What protections are required for transformer?

Transformer is an important element in power system. Transformer protection is crucial in ensuring reliable operation in power transmission and distribution systems. Transformer protection systems are designed to detect and respond to various faults and abnormal operating conditions to prevent damage to the transformer and ensure continuity of power supply.

How many types of transformer protection are there?
various protection available for transformers
1. IDMT Overcurrent Protection: (51) & IDMT Earth Fault Protection (51N)
2. Instantaneous Overcurrent Protection: (50) & Instantaneous Earth Fault Protection (50N)
3. Differential protection
4. Restricted earth fault
5. Standby earth fault
6. Over fluxing
7. Voltage protection
8. Frequency protection
9. Other Protection

IDMT Overcurrent Protection: (51) & IDMT Earth Fault Protection (51N)

Inverse overcurrent protection

IDMT Overcurrent (51) and earth fault (51N) protection needs to be provided in both the LV and HV winding of the transformer. This is essentially a back up to the faults which are happening outside the transformer. Three major parameters to be selected for this protection

  • Curve: Normal Inverse, Extremely Inverse, Very Inverse, Long Time Inverse, etc
  • Pick up setting
  • Time Dial Setting (TDS) / Time Multiplier Setting (TMS)

Since it is backup protection, TDS should be selected along with a curve in such a way that clear discrimination is maintained between downstream protection devices.

It is also important to select the curve and TDS in such a way that this relay does not malfunction during transformer energization due to inrush current.


Instantaneous Overcurrent Protection: (50) & Instantaneous Earth Fault Protection (50N)

Instantaneous Overcurrent (50) and earth fault (50N) protection need to be provided in HV winding of the transformer. This is essentially a main protection for faults within the transformer. Hence, there should not be any intentional time delay to be added.

Overcurrent and earthfault protection

It may be noted that instantaneous overcurrent (50) will not cover the complete transformer. Since the pick up setting is given atleast 125% of through fault current and higher than inrush current, some portion of the LV winding faults may not be deducted by instantaneous overcurrent protection.

It is also interesting to note, Instantaneous Earth Fault Protection (50N) is insensitive to earth fault in the LV side of the transformer except Star Grounded – Star Grounded (Ynyn) winding configuration of transformer.

In order to prevent the nuisance tripping of 50N during transformer energization, adding an intentional time delay of 100 ms is a common practice in some parts of the world. If that is the case, this is no longer called Instantaneous Earth Fault Protection (50N). It should be called Definite Time Earth Fault Protection.

Since most IEDs (intelligent electronic devices) are coming up with multiple stages, it is recommended to use three stages to prevent maloperation as well as increase sensitivity.


Transformer Differential Protection: 87

Differential protection

Transformer differential Protection, is the main protection for large transformer winding failures. Though the differential protection operation principle is simple by measuring the current enters and leave the terminals, transformer differential protection is complex due to

  1. Different Voltage at both winding and hence the current rating
  2. Unequal Rated Current & CT Rating
  3. Phase shift in the current due to vector group
  4. Transformer inrush appears only on one side of the transformer winding
  5. Zero Sequence in one side which is not reflected on other side (delta-star-grounded transformers)
  6. CT errors
  7. Current variation due to changes in taps
  8. Reactive power loss in transformer
  9. Normal Magnetizing current

Development in the Intelligent Electronic Devices (IEDs) have many features to overcome the above challenges. Let us discuss such features in upcoming post.

For low impedance differential Protection the following parameter to be set,

1. Minimum Differential Pick up
2. End Section 1
3. Slope of Section 2
4. End Section 2
5. Slope of Section 3
6. Unrestrained pick up

Transformer Restricted Earth Fault Protection: 87N (64R)

REF

Restricted earth fault is most sensitive and fastest protection to detect the earth faults in winding. Unlike differential protection, Restricted earth fault protection is insensitive to

a. Transformer inrush current
b. Current variation due to change in taps
c. Normal Magnetizing current
d. Tap changer operation

For transformer differential protection as well as Restricted Earth fault protection both low impedance and high impedance protection is possible. 

[The merits and demerits of Low impedance vs High impedance for both Differential and REF protection and remaining protections coming soon...]


Categories: : Protection

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