Page 153 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/133
where C is the capacitance used across the device. filter circuits may be provided on the incoming side
During a turn OFF operation the stored energy, Q, of for more prominent harmonics. (For details of filter
the circuit will discharge into this capacitor and charge circuits see Section 23.9.) The main purpose of
the same to its optimum level (charging time constant inductance here is protection. rather than suppression
r= R C) and slow down the rate of rise of TRV (r.r.r.v.). of harmonics.
i.c. duldt across the static circuit and limit the voltage Due to a high time constant of the dampening circuit
spikes. similar to motor protection discussed in Section t = L,/R (R being the resistance of the circuit) it will
17. IO. 1. The higher the value of C, the lower will be also delay occurrence of the fault by which time the
the voltage (commutation) overshoots. During a switch circuit’s protective scheme may initiate operation.
ON the capacitor discharges its total energy into the R It would also add to the line impedance to contain
and prepares for the next switching operation. The the severity of the fault conditions.
power dissipation into R is proportional to the switching From the above we notice that the current surges
frequency. R also limits the peak value of the discharge can be caused either by the tripping of a current limiting
current through the static device and damps the device. when the distribution is through a large
oscillations. Here the use of C is to hold the charge transformer on which is connected the static circuits.
and then release the same into R and not to smooth the or by switching of the SCRs within the converter circuit
ripples. itself. The protective scheme for both remains the same
Ciirrerlr rrnrisirrits A similar situation will arise when and is located at the incoming of the semiconductor
a switching ON operation of the rectifier unit occurs circuits. There can be two situations. When the static
when it is a thyristor rectifier. Under load conditions. circuits are being fed through a dedicated transformer
the stored magnetic energy in the incoming supply in all probability no additional inductor will be
system. which can be the feeding transformer and the ary. Not even when there is a large transformer
line reactances similar to a fault condition discussed g a large distribution network on which is
earlier. may cause a current transient which can be connected the semiconductor circuits. It is. however.
expressed by better to carry out the trapped energy calculations to
compare these with the inductance already available
in the switching circuits of the semiconductor devices.
When there is no dedicated transformer and these
circuits are connected on the system bus directly a
where large inductor will be essential at the incoming of the
V =applied voltage static circuits, sufficient to absorb the trapped charge
L = inductance of the total circuit up to the d.c. within the transformer and the interconnecting cables
link and up to the converter unit. The size of the inductor can
di/d/ = rate of change of current, as the switching be calculated depending on the size (kVA) of the
ON is a transient condition and causes overload distribution transformer, its fault level and the
and short-circuits. This is rnaxirnurn at the characteristics of its current limiting protective device.
commencement of switching ON and becomes An inductor sufficient to absorb i,: . L of the trans-
zero on its completion. It is analogous to former and the cables may be provided at the incoming
contact making in an interrupter (Section of the static circuits.
19.1. I). The same situation will arise even
during a fault condition. Excessive rate of Voltage surges in the inverter circuit
change of current may cause an overload and
even a short-circuit. Generally, voltage surges on an LT system are of little
relevance as analysed in Section 17.7.6. Instances can.
The rate of current change must therefore be controlled however, be cited of motor insulation failures, even on
to a safe limit by providing a dampening circuit on the an LT system, when the machine was being controlled
supply side. This can be a series inductance as shown through a static drive, which might be an IGBT switched
in Figures 6.2h(a), 6.34 and 6.36. This inductance may or a thyristor (GTO) switched inverter. the reason being
not be necessary when the unit is being supplied through a steep rising switching wave generated through the
a dedicated transformer. The inductor will absorb the inverter circuit. The output of the inverter unit being in
magnetic charge and damp the rate of rise of current. the shape of a non-sinusoidal voltage waveform also adds
to the switching transients. To visualize the effects of
di fast switching in a static circuit, it is relevant to corroborate
NOW V = L -
dt these with the switching of a conventional HT interrupting
device, discussed in Section 17.7. The static devices also
L, is the additional series rcactmce. The higher the cause switching surges and their severity is also defined
value of L,, the lower will be the rate of rise of current. by their amplitude and the rise time (Figure 17.2). These
The inductor on the input side also suppresses the devices are seen to produce voltage surges with an
harmonics in the incoming supply, as high Li will amplitude up to two to three times the voltage of the d.c.
providc a high impedance path to higher harmonics. link and a rise time as low as 0.05-0.4 ps (typically) in
For suppression of harmonics, where the supply system IGBTs and 2 to 4 ps (typically) in GTOs (see Lawrence
is already substantially distorted, additional L-C et al. (1996) and the Further reading at the end of the