Page 882 - Industrial Power Engineering and Applications Handbook

26/834 Industrial Power Engineering and Applications Handbook

Then the overvoltage developed in the event of a failure of 26.9 illustrates a typical layout of a switching and
any one unit (assuming the capacitor units to have external protection scheme for an HT capacitor system, suggesting
type of fuses) as determined in the said example the recommended switching and protective devices. Based
= 10.98% on this, an application engineer can plan a more stronger
protective scheme for a particular system.
This overvoltage is higher than permissible. The fault condition
must be removed before one full unit fails. Say, we permit
only one half of a unit to fail for an alarm and maximum three 26.1.2 Protection of series capacitors
quarters for a trip. Then overvoltage when one half of the unit
has failed from Example 25.2. Series capacitors are subject to higher voltage variations
as a result of generally fluctuating line current. During a
= 5.2%
fault in the line, such as a short-circuit, the voltage variation
and overvoltage when three quarters of the unit has failed across the capacitors may be so high that it may cause an
instantaneous failure of the capacitors unless the capacitors
6~$~4-5~: are selected for a higher insulation level. IEC 60143-1
x 100 = 8%
6x8x4-6x$x4+5x$ for series capacitors suggests that the short-time maximum
overvoltage across the capacitors should not exceed 2.15
which is well within the limits for a trip
and I, for an alarm from equation (26.2) times the capacitors’ voltage rating for 10 seconds. Since
an overloading results in an overvoltage, overvoltage
3X+X8 protection becomes even more essential. IEC 601 43- 1
- X 6.82 = 0.45 A permits a temporary overloading as in Table 26.2 based
6x8x4-6~3~4+5~3
on which overvoltage protection is provided.
and I, for a trip The following are a few common methods for the
protection of series capacitors.
- 3Xax8
X 6.82 = 0.69 A
6~8~4-6~:~4+5~$ 1 Protection against overloads
One can use a 5/1 ACT and provide the desired setting in a For overloading during normal operation, which may be
1 A relay for alarm and trip.
due to load fluctuations or failure of a few capacitor
elements, normal overload protection will suffice, as
9 Frequency variation and voltage variation discussed for shunt capacitors (Section 26.1.1(3)). This
When capacitors are used to improve the p.f. of a varying protection may be provided in conjunction with the line
frequency load, such as for an inductive heating or varying fault protection scheme, discussed below.
voltage loads for improving the regulation of a power
system, they would be subjected to excessive overloading 2 Protection against line faults
due to frequency variation in the first case, since kVAr 0: There may be a number of ways by which the series
f(equation (23.4)), and voltage fluctuations in the latter.
Capacitors performing such duties must therefore be capacitors can be protected against line faults. Commonly
protected, against overloads by an IDMT overload used methods are briefly discussed below.
protection scheme.
(i) Using a dampening circuit
The theory of protection is based on a rapid voltage rise
10 No-volt protection across the series capacitors during a line fault. This voltage
On supply failure capacitors must drop out and should rise is used to achieve the required high-impedance
not switch on automatically on resumption of the supply requirement of the system during a fault, i.e. restoring
to avoid an overvoltage as a result of the trapped charge. the near-natural impedance of the line as if it were without
Even sudden voltage dips may cause the charged capacitors series capacitors. A simple R-L dampening-cum-discharge
discharge into the terminal equipment and damage them. circuit is illustrated in Figure 26.10. This consists of a
On LT, to achieve the required protection, the capacitors resistor and reactor combination, which helps to limit
may be switched through contactors which have a no- the discharge current and hence the overvoltage across
volt coil and drop out on failure of the supply. On HT an the capacitors during a fault by almost offsetting the
instantaneous undervoltage relay, with a low drop-off capacitor fault current, IC (fault), with the Inductive current,
value (say, 30-60%) may be used with the interrupting I,, through the dampening circuit, i.e. by achieving
device, which may be a breaker or a vacuum contactor. IL IC (fault)
Reclosing may be through a lockout relay with a time
delay, for at least the safe discharge time or less, if the This realizes a near-zero or very small fault current through
control is by a p.f. correction relay, whose time setting is the capacitors and protects them besides restoring the
low and the capacitors are provided with suitable discharge fault level of the system to its original level.
devices.
Figures 26.7 and 26.8 illustrate general layouts (ii) Connecting a spark gap across the capacitor banks
respectively for an 11 kV and 33 kV p.f. improvement If a breaker alone is used to bypass the capacitors during
system with switching and protective devices. Figure a line fault it may fail to discharge its required duty due