[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.9.8 230-kV bus-tie reactors.
A method to evaluate the merits of using either phase reactors or bus-tie reactors is presented in
Section 2.9.3.2, Phase Reactors vs. Bus Tie Reactors.
2.9.2.2.3 Neutral-Grounding Reactors
Neutral-grounding reactors (NGR) are used to control single line-to-ground faults only. They do not
limit line-to-line fault-current levels. They are particularly useful at transmission-voltage levels, when
autotransformers with a delta tertiary are employed. Figure 2.9.9 shows a typical neutral-grounding
reactor.
These transmission-station transformers can be a strong source of zero-sequence currents, and as a
result, the ground-fault current may substantially exceed the 3-fault current. These devices, normally
installed between the transformer or generator neutral and ground, are effective in controlling single
line-to-ground faults, since the system short-circuit impedance generally is largely reactive. NGRs reduce
short-circuit stresses on station transformers for the most prevalent type of fault in an electrical system.
If the objective is to reduce the single line-to-ground (1) fault, then Equations 2.9.14 and 2.9.14a
must be used and, after algebraic manipulations, the required neutral reactor impedance, in Ohms, is
calculated as:
X NGR = 3 V LL [(1/I SCA ) – (1/I SCB )]/3 (2.9.3)
where,
X NGR = reactance of the neutral grounding reactor,
V LL = system line-to-line voltage kV
I SCA = required single line-to-ground short circuit current after the installation of the neutral
reactor, kA.
I SCB = available single line-to-ground short curcuit current before installation of neutral
reactor, kA
FIGURE 2.9.9 Typical neutral-grounding-reactor connection.
where the parameters are defined as before, with the exception that the short-circuit currents in
question are the single line-to-ground fault currents expressed in units of kA.
A factor to be taken into consideration when applying NGRs is that the resulting X 0 /X 1 may exceed
a critical value (X 0 10X 1 ) and, as a result, give rise to transient overvoltages on the unfaulted phases.
(For more information, see IEEE Std. 142-1991, Green Book.)[14]
Because only one NGR is required per three-phase transformer and because their continuous current
is the system-unbalance current, the cost of installing NGRs is lower than that for phase CLRs. Operating
losses are also lower than for phase CLRs, and steady-state voltage regulation need not be considered
with their application.
The impedance rating of a neutral-grounding reactor can be calculated using Equation 2.9.1, provided
that the short-circuit currents before and after the NGR installation are single line-to-ground faults.
Although NGRs do not have any direct effect on line-to-line faults, they are of significant benefit, since
most faults start from line-to-ground, some progressing quickly to a line-to-line fault if fault-side energy
is high and the fault current is not interrupted in time. Therefore, the NGR can contribute indirectly to
a reduction in the number of occurrences of line-to-line faults by reducing the energy available at the
location of the line-to-ground fault.
2.9.2.2.3.1 Generator Neutral-Grounding Reactors — The positive-, negative-, and zero-sequence reactances
of a generator are not equal, and as a result, when its neutral is solidly grounded, its line-to-ground
short-circuit current is usually higher than the three-phase short-circuit current. However generators are
usually required to withstand only the three-phase short-circuit current, [1] and a grounding reactor or
a resistor should be employed to lower the single line-to-ground fault current to an acceptable limit.
Other reasons for the installation of a neutral-grounding device are listed below:
• A loaded generator can develop a third-harmonic voltage, and when the neutral is solidly
grounded, the third-harmonic current can approach the generator rated current. Providing impedance
in the grounding path can limit the third-harmonic current.
• When the neutral of a generator is solidly grounded, an internal ground fault can produce large
fault currents that can damage the laminated core, leading to a lengthy and costly repair procedure.
The ratings of generator neutral-grounding reactors can be calculated as follows:
1. The reactance value required for limiting a single line-to-ground fault to the same value as a threephase
fault current can be calculated by the following formula:
X
NGR
X" X
d
3
m0
(2.9.4)
where
X NGR = reactance of the neutral-grounding reactor,
X d = direct axis subtransient reactance of the machine,
X m0 = zero-sequence reactance of the machine,
Equation 2.9.4 assumes that the negative-sequence reactance is equal to X d . In the absence of complete
information, the value of reactance calculated using Equation 2.9.4 is satisfactory. Equations presented
in chapter 19 of the Westinghouse Electrical Transmission and Distribution Reference Book [2] can also
be used when the negative-sequence reactance of the generator is not equal to X d .
2. The short-circuit-current rating of the grounding reactor is equal to the three-phase generator
short-circuit current when the machine is an isolated generator or operating in a unit system
(Figure 2.9.10). When more than one generator is connected to a shared bus and not all of them
are grounded by a reactor with a reactance calculated from Equation 2.9.4, the short-circuitcurrent
rating of the reactor in question should be calculated by using proper system constants
for a single line-to-ground fault at the terminal of the machine being grounded. Equations in
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC