Restricted Earth Fault Trip due to inrush current

In summary, the conversation discusses a problem with a transformer tripping on restricted earth fault due to magnetizing inrush current. The rated current, line CT ratio, and relay settings are provided. The inrush current and relay operating time are also mentioned, and a potential solution of changing the TMS is suggested. The customer is hesitant to make this change due to concerns about affecting normal earth fault protection and the fact that other transformers do not have this issue. The transformer's construction and materials are also mentioned.
  • #1
Sreelathag
1
0
Hello I need some help. In our 50MVA, 132/33 kV star/star transformer, at the time of charging from HV side, the transformer trips on REF (restricted Earth fault) due to magnetizing inrush current.

Rated current: 218.7 Amp
Line CT ratio: 800/400/200/1 Amp
tap selected 400/1 Amp
setting on MJT 36 Numerical relay.
Earth fault = 0.2 In
TMF = 0.15 In
Inrush current in three phases is:
Ia = 0.42
Ib = 0.05
Ic = 0.51

EF current in neutral = 0.58 In
Relay operating time = 0.94 s

Since, inrush dyeing time > 2.5 s, the relay trips.

At TMS 0.3, the relay operating time = 1.89s
At TMS 0.4, the relay operating time = 2.52s

Customer advised to change TMS to 0.4 (2.52s), he does not agree since,

i) It will affect his normal Earth fault protection time of 0.94s which is not desirable.
ii) Other similar transformer in the same substation does not give the problem.

The transformer is made with step lap core construction using Hi-B grade material using 1.6 tesla flux density.

Can anyone help me analyze and suggest what to do?

Thanks & Regards
G.Sreelatha
Asst. Manager
Vijai Electricals Limited
Hyderabad.
 
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  • #2
Last edited by a moderator:

1. What is a restricted earth fault trip due to inrush current?

A restricted earth fault trip due to inrush current is a protection mechanism used in electrical systems to detect and isolate faults that occur when there is a sudden increase in current, known as inrush current, typically caused by the energization of transformers or motors. This trip function is designed to prevent damage to the equipment and ensure the safety of the system.

2. How does a restricted earth fault trip due to inrush current work?

This trip function works by monitoring the current flow in the earth fault path. When inrush current occurs, the current flow in the earth fault path is significantly higher than normal, triggering the trip function to open the circuit and isolate the fault. This helps to prevent damage to the equipment and maintain the stability of the system.

3. What are the benefits of using a restricted earth fault trip due to inrush current?

The main benefit of using this trip function is the protection it provides to the electrical equipment and system. By detecting and isolating faults caused by inrush current, it helps to prevent damage and reduce downtime. It also improves the safety of the system by reducing the risk of electrical fires and other hazards.

4. Are there any limitations to using a restricted earth fault trip due to inrush current?

One limitation of this trip function is that it may sometimes trip unnecessarily, for example, if there is a sudden surge in current due to a lightning strike or other external factors. This can result in interruptions to the system and inconvenience for users. Additionally, this trip function may not be effective in detecting faults in certain types of equipment, such as electronic devices with low inrush current.

5. How can false trips of the restricted earth fault trip due to inrush current be prevented?

To prevent false trips, it is important to properly set and adjust the settings of the trip function according to the specific system and equipment. Regular maintenance and testing of the trip function can also help to identify and address any issues that may lead to false trips. Additionally, using advanced trip algorithms and technologies can improve the accuracy and reliability of the restricted earth fault trip due to inrush current.

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