REF PROTECTION

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The following table shows current in LCTs and NCT. During internal and External fault.

TABLE FOR INTERNAL FAULT
Fault
Ir(C.T)
Iy(C.T)
Ib(C.T)
In(C.T)
Irelay
R- E
0
0
0
0.14
0.14
Y- E
0
0
0
0.14
0.14
B- E
0
0
0
0.14
0.14
R- Y
0
0
0
0
0
Y- B
0
0
0
0
0
B- R
0
0
0
0
0

                                              TABLE FOR EXTERNAL FAULT
Fault
Ir(C.T)
Iy(C.T)
Ib(C.T)
In(C.T)
Irelay
R- E
0.14
0
0
0.14
0
Y- E
0
0.14
0
0.14
0
B- E
0
0
0.14
0.14
0
R- Y
0.08
0.08
0
0
0
Y- B
0
0.08
0.08
0
0
B- R
0.08
0
0.08
0
0

Difference B/W Low and High Impedance REF

LOW IMP. REF HIGH IMP. REF
The input impedance of the low-impedance REF is very low. The input impedance of the high-impedance REF is very high.
Low-impedance REF protection has low inherent stability against CT saturation for external faults. High-impedance REF protection has high inherent stability against CT saturation for external faults.
The operating current of the low-impedance REF protection is not realized by CT connection. The relay measures all four CTs necessary to realize the element. The operating current of the High-impedance REF protection is realized by CT connection

Choice B/W High And Low Impedance Ref:

If the existing equipment is of such a nature that the same ratios are not available for both phase and neutral CTs, you should use low-impedance REF protection, because this type of protection can handle different CT ratios for phase and neutral CTs. However, if the same ratios are available for both phase and neutral CTs, further investigation should reveal whether high-impedance or low-impedance REF is the most suitable for the application.

REF scheme sensitivity is a problem only on star windings with resistance earthing, because the fault current is a function of fault position, phase-to-neutral voltage, and earthing resistance value. For faults close to neutral, the fault current is very small. The relay operating current and CT magnetizing current are important in determining the winding coverage.

In cases where there is always sufficient fault current to operate the REF relay, the choice between high-impedance and low-impedance REF is not important. Issues such as available CT ratios for the phase and neutral CTs may dictate the choice.

For poor-quality CTs that require larger magnetizing current than a better-quality CT at the same voltage, the lowimpedance REF element is more sensitive. Where you use good-quality CTs, however, the high-impedance REF relay is more sensitive.

Methods Used To Avoid False Tripping

A) This relay makes use of the direction change of the operating current for in-zone and external faults. It derives a zero sequence operating current from the phase CTs (Ir = Ia + Ib +Ic) and a polarizing current from the neutral CT (In). It then compares the direction of operating (Ir) and polarizing (In) currents.

B) The basic principle of operation for this relay is to compare the residual (restraint) current Ir = Ia + Ib + Ic with the diff. current Id = Ia + Ib + Ic– In,

Where,

Ia, Ib, and Ic = the respective phase currents

In = the neutral current flowing in the transformer as a result of the fault

C) Similarly to method A above, this relay uses the residual current calculated from the three phase CTs where Ir = Ia + Ib+ Ic and the neutral current In for the REF protection. During an in-zone fault, neutral current will always flow irrespective of the transformer winding connection and earthing arrangement. The residual current depends on the transformer winding connection and earthing arrangement. In this case, if residual current exists, it will be in phase with the neutral current. During an external fault, the neutral and residual currents will be equal in magnitude and 180° out of phase.

D) This relay calculates the differential current as Id = Ia + Ib +Ic + In and the residual current as Ir = Ia + Ib + Ic. Restraining current is the maximum of the positive-sequence, negative sequence, or zero-sequence current in the residual current.

Relays scale the CT ratios automatically between the phase and neutral CTs to compare the different values on an equal basis.

Low Impedance REF (87N)

Low-impedance REF protection is provided with new numerical or microprocessor-based protection relays. In the biased low-impedance REF, the three phase currents and the neutral current become the bias inputs to a differential element.

A very important advantage of low-impedance REF protection is the fact that the CT ratios for the phase CTs and neutral CTs do not have to be the same.

Most low-impedance REF relays use an operating and a restraint current. The difference between different relays lies in the way these relays determine the restraint quantities and in the CT saturation detection algorithm of each relay. Note that, in the case of low-impedance REF protection, there is no inherent immunity to CT saturation, as is the case with high-impedance REF protection. The following different methods are used to determine the restrain and operating current:

1. Use of the residual current Ir = Ia + Ib + Ic as the restraint current and the differential current Id = Ia + Ib + Ic – In as the operating current.

2. Use of the residual current Ir = Ia + Ib + Ic as the operating current and the neutral current In as the restraint current.

Design Considerations

Because of the inherently unstable nature of the low impedance REF element, it may misoperate during external faults, especially in the case of faults not involving earth as phase-to-phase and three-phase faults, when one of the phase CTs saturates. The following methods can be used to avoid false tripping 

REF PROTECTION


REF takes advantage of the large current in the neutral conductor to provide sensitive and fast protection for transformer faults close to the earth point. REF protection applied to transformers may be referred to as “unit earth-fault protection,” and the “restricted” part of the earth-fault protection refers to an area defined between two CTs. Generally, REF protection can be applied in one form or another to all transformer windings, even delta-connected windings. It can be of the High Impedance type, or of the Low Impedance type.
High Impedance REF(64)

    The residual current of three L.C.Ts. is balanced against the output of a C.T. in the Neutral conductor. This scheme can be used with any O/C relay as the fault current is measured by C.T. connection.
    If the star point of winding is grounded with resistance to limit the ground current (In Higher rating m/c). The ratio of NCT & LCT is different. In this case a IPCT (Inter Posing C.T.) is used.

Design Considerations:

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 knee point 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 in zone faults. Sufficient current still flows through the relay to ensure operation.