Grounded wye capacitor banks, composed of series- and
parallel-connected capacitor units per phase, may require overvoltage/unbalance
protection. Grounded capacitor banks provide a low impedance path to ground for
lightning surge currents and give some protection from surge voltages.
Some banks may be operated without surge arresters, taking
advantage of the capability of the capacitors to absorb the surge.
Grounded capacitor banks also provide a low-impedance path
to ground for triplen harmonic currents. These harmonic currents may cause
communication facility interference, if such circuits parallel power lines.
Additionally, an open phase produces zero sequence currents
that may cause ground relay operations. Harmonic resonance may also be a
problem.
When the neutral is grounded, recovery voltages encountered
during switching are reduced; however, careful consideration should be given to
the effect of inrush current on protective relay performance and the method of
grounding.
One Unit Phase to Ground
There is no overvoltage across the remaining capacitor units
if a failed, externally fused capacitor unit is isolated with a fuse;
therefore, unbalance relay protection is not required. The individual external
capacitor fuses shall be capable of interrupting the system available fault
current.
Internally fused and unfused capacitor banks should have
unbalance protection to avoid excessive element voltages and to trip the bank
for severe unbalance conditions.
Multiple Units in Series Phase to Ground—Single Wye
Grounded wye externally fused capacitor banks above 34.5 kV
are commonly composed of two or more series groups of parallel connected
capacitor units per phase. The presence of multiple series groups of units
limits the maximum fault current so that individual current-limiting fuses are
usually not required unless the parallel kilovar exceeds 4650 kvar or the level
specified by the manufacturer.
Internally fused capacitor units should be arranged in the
bank design to ensure that energy in the parallel group does not exceed the
interrupting capability of the internal fuses. Relatively small banks can be
designed with internally fused capacitor units and still use large capacitor
units for a simple design.
The fuseless design is not usually applied for system
voltages less than about 34.5 kV. The reason is that there shall be more than
10 elements in series so that the bank does not have to be removed from service
for the failure of one element because the voltage across the remaining
elements would increase by a factor of about E/(E− 1) where E is the number of
elements in the string.
Multiple Units in Series Phase to Ground—Double Wye
When a capacitor bank becomes too large, making the parallel
energy of a series group too great for the capacitor units or fuses, the bank
may be split into two wye sections. The characteristics of the grounded double
wye are similar to a grounded single wye bank.
The two neutrals should be directly connected with a single
connection to ground. The double wye design allows a secure and faster
protection with a simple uncompensated relay because any system zero sequence unbalance
affects both wyes equally, but a failed capacitor unit will be detected in the
neutral unbalance.
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