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Protection Riddle No.88 - High impedance protection CT Mis-match
For high impedance restricted earth fault protection it is known the neutral CT ratio of the power transformer must much with the line CT, But during Test and commisining i have got a problem, as the neutral CT of Power transformer has CT ratio 100/1A but the line CT has 75-150/1/1/1A CT? how can i resolve this problem?
Author : Addis - From: Ethiopia
Sun, March 4th, 2012 - 16:03
There isn't any recommended solution; you must use similar line and neutral CT or low impedance (biased type) differential relay. Of course using of interposing CT for ratio matching can be an urgent/temporary solution, but other CTs mismatching such as internal resistance and magnetizing current can be caused protection instability against out of zone earth fault and generally it is not recommended.
A number of design considerations must be taken into consideration when designing a high-impedance REF scheme.
The most important considerations are described here:
- The ratio of the phase and neutral CTs must always be the same.
- In general, the CTs should have the same saturation characteristics.
- The kneepoint voltage must be higher than the stabilization voltage for external faults.
- The voltage across the relay and CTs (all in parallel) should be kept at safe levels while still being sufficiently high to allow operation of the relay when required. The magnetizing current of the CTs depends on the voltage across it, but too high a voltage results in higher magnetizing current that leads to a less sensitive scheme.
- In most cases, a metal oxide varistor (MOV) or surge arrestor is connected across the parallel connection of the CTs and relay to clamp the voltage to a safe limit, without affecting relay operation. The MOV protects the relay against high voltages developed during inzone faults. Sufficient current still flows through the relay to ensure operation.
Author : Hamid - From: Iran - Firoozabad Fars
Tue, March 13th, 2012 - 13:01
Currently i don't have chance to make the CTs ratio the same, else the solution is to use "low impedance (biased type) differential relay" and my question is how can i change the high impedance restricted earth fault relay to low impedance (biased type) differential relay what is the idea behind? and is it permanent solution I mean how much the power transformer is protected by doing so? can you give me brief things on the difference and similarity of the two protections?

thanks inadvance 
Author : Addis - From: Ethiopia
Tue, March 13th, 2012 - 17:52
The high-impedance REF relay is normally a currentoperated relay with a resistor in series that provides stabilization. Generally, it may be one of two different types. The first type has internal resistors and has a voltage setting. In this type, the resistors are effectively switched in and out to change the setting and therefore the value of the stabilizing voltage. The second type has an external variable resistor where the setting is calculated in ohms and applied by changing the resistance of the variable resistor. Any mismatch in CT ratio will result in spill current, part of which will flow through the relay. Spill current cannot be related to current that flows on the primary side and flows on the secondary side. It may, therefore, cause the flow of “fictitious” current that results from CT inaccuracy. Not all spill current will necessarily flow through the relay; some of the current also flows through CTs (phase and/or neutral) not carrying primary fault current. Effectively, the spill current flowing through the relay raises the voltage across the relay and CTs, causing more magnetizing current to flow. In the case of a through fault, equilibrium is reached between the voltage, relay current, and magnetizing current. Generally, relay current is far less than the operating current, as explained in the calculation of the stabilizing voltage.
Another important factor in the design of an REF scheme is the minimum allowable knee-point voltage of the phase and neutral CTs. This value is necessary during the design phase of the high-impedance REF scheme to ensure adequately specified CTs.

Low-impedance REF protection is provided with new numerical or microprocessor-based protection relays. Generally, relay manufacturers employ different methods to provide REF protection. In most cases, operation of the low-impedance REF protection is based on the fundamental current, after filtering removes all harmonic currents. The most important difference between classical highimpedance REF protection and new low-impedance REF protection is the input impedance. As with all numerical relays, the input impedance of the low-impedance REF is very low compared to high-impedance relays. For example, a lowimpedance relay typically has an input impedance of 0.1 VA.
At 1 A nominal rating, this computes to 0.1 W. On the other hand, for a high-impedance REF relay with a voltage setting of 100 V and a 20 mA operating current, the input impedance is 5 kW. This is a significant difference. Low-impedance REF protection does not have the same inherent stability against CT saturation for external faults as does high-impedance REF protection. A second significant difference is that the operating current of the low-impedance REF protection is not realized by CT connection. With low-impedance REF, the relay measures all four CTs necessary to realize the element.

Author : Hamid - From: Iran
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