Application Notes
The Effect of di/dt on SCR’s used in Reduced Voltage Starting
Application Challenge
Customers are applying solid state reduced voltage starters on applications with increasing motor lead
lengths. This increased lead length has increased concerns of di/dt issues, especially at 4160 volts and
above.
Problem
Power cable has a value of capacitance per metre. As the length is increased, the capacitance is increased.
The voltage across a capacitor cannot be changed instantaneously, so when a step voltage is applied, a large
current tries to flow instantly into the capacitor to charge it to the value of the step voltage. This rapid
change in current is defined as di/dt. The step voltage comes from turning on the SCR part way through the
electrical cycle. If the circuit di/dt is greater than the capability of the SCR, the SCR will not be able to
spread the current across the whole silicon wafer. This causes all of the circuit current to try to go through a
small portion of the silicon wafer, resulting in localized heating and melting of the gate/cathode region, and
failure of the SCR.
Application Solution
Higher values of di/dt are caused by capacitance in the circuit. The di/dt may be avoided by removing the
capacitor, or limited by installing inductors, or other components to alter the surge impedance. Inductors
and capacitors in the same circuit can create a resonance, which can be stimulated by the harmonics
generated during the start or stop cycles of the SMC. This can result in increased peak voltages across the
SCR’s and voltage failures rather than di/dt failures.
An inductor or line reactor may be used to limit the di/dt in a system, which may protect the SCR's from
damage. The inductor can be wired inside the MV Dialog Plus bypass loop, such that it is only in the circuit
during MV Dialog Plus operation. This keeps the inductor size smaller than inductors rated for continuous
duty and eliminates the heat loss during running.
For power systems without power factor correction capacitors, or where the capacitors can be disconnected
from the bus while the MV Dialog Plus is not in bypass, there are no di/dt restrictions if the feeder
transformer is 25 MVA or smaller. For larger transformers, restrictions may apply, depending on the
impedance of the transformer. The di/dt limit is not exceeded for 30 MVA transformers >6% impedance, or
for 40 MVA transformer >8% impedance.
For power systems where power factor correction capacitors are on the same bus as the MV Dialog Plus, a
line inductor of appropriate rating may limit the di/dt to acceptable levels.
Consult the factory for cable lengths above 500m, transformers above 25 MVA, or power systems with
power factor correction.
Note: Reference Engineering report summary ER-042-98, and application note 1560D-2.3.19.
Publication 1560D-2.3.3 - January, 1999
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