Why We Use Neutral Grounding Reactor (NGR) ?

Neutral Grounding Reactors For Shunt Compensated EHV Transmission Lines

The application of shunt reactors for controlling the overvoltage in power systems is a well-known practice in EHV systems. A large amount of reactive power generation by EHV transmission lines (400 kV and above) causes power frequency overvoltage during load rejection and line charging.

Thus, line connected shunt reactors are used on long EHV lines for the purpose of line energization and load rejection. They are typically rated to compensate 40 to 90 % of the line shunt capacitance. 

Use of Neutral Grounding Reactors (NGR), connected at the neutral of these shunt reactors facilitates single-pole reclosure adopted in EHV systems.

In many of the power systems, tripping and reclosing of all three phases for a single-line-to-ground fault can cause the system to lose synchronism.

Majority of the power system faults are predominantly single-phase-to-ground faults and, as such, can be successfully isolated by opening only the faulted phase (Single Pole Switching – SPS) while the remaining two healthy phases remain in service. By adopting this, the transmission line is still capable of transferring two-thirds of the power.

Additionally, SPS applications demand a smaller amount of reactive power from the system during the single-pole operation period (compared to three poles opening), thereby improving voltage stability margins.

SPS and Auto reclosing are thus adopted for maintaining power system reliability, wherein the relays trip the faulted phase breaker pole only and reclosing is done after a certain dead time. If the fault persists, the relaying scheme trips all three phases and recloses again or blocks reclosing.

When one phase of the transmission line is opened at both ends to clear the fault, this faulted phase is coupled capacitively and inductively with the other two healthy phases of the same circuit (Fig 1) and also with the second the circuit (Fig 2) in case of a double circuit line.

Thereby, the opening of the circuit breakers on the faulted phase at either end of the line does not necessarily guarantee arc extinction, even if the fault is a temporary arcing fault. This coupling feeds current to the fault arc before its extinction and thus tries to maintain the arc and it also causes a recovery voltage across the fault path after its extinction.

The arc on the faulted conductor after it has been switched off is called the secondary arc. It is much smaller than the primary fault current which exists before opening the circuit breakers. The Recovery voltage is the voltage existent across the fault path after the extinction of the secondary fault arc and before reclosure of the circuit breakers.

The dead time adopted for single-pole reclosure should be sufficient enough to allow the second arc to extinguish. Thus, the magnitude of secondary arc current and the recovery voltage are the prime factors for the success of single-pole reclosure.

As the primary cause for secondary arc current and the associated recovery voltage is the capacitive coupling between phases of the same circuit and inter circuit capacitive coupling in case of double circuit line, neutralizing this capacitive current by an inductive current will enhance the success of SPS, especially on long EHV lines.

In practice, with 400 Kv lines and above, special compensation schemes are adopted to limit the secondary arc current and recovery voltage to ensure successfully high-speed single-phase reclosing.

Various compensation schemes based on the use of additional reactors called as Neutral Grounding Reactors (NGR), connected between line reactor neutrals and ground, are being in use. 

SINGLE POLE RECLOSURE: 

Some of the approaches used by utilities for successful single pole reclosure is, use of

(a) Neutral Grounding Reactor

(b) High-speed Grounding Switches or

(c) Increase in dead time.

Neutral Grounding Reactor

One of the methods adopted for secondary arc extinction is to use a single-phase reactor in the neutral of the shunt reactors when the transmission line is compensated with shunt reactors. 

High-Speed Grounding Switches

When shunt reactors are not provided on transmission lines, the use of high speed grounding switches are an effective method for the extinction of the secondary arc current associated with single-pole switching.

Increase in Dead Time

The method of increasing the dead time for facilitating single pole reclosure can be adopted for short line lengths (less capacitive coupling) and when stability is not a major concern.