Short circuit tolerance of the main transformer of the power plant As you know, the GCB of the power plant must have the ability to cut off the short circuit in critical conditions of Out of Phase Synchronizing and control the return over voltages (TRV) caused by it. But why does the IEC 60076-5 standard consider the ability of the power plant's main transformer to pass this current to be important? As we know, the size of Out of phase current is not greater than Three-phase short circuit current, so why does the standard insist on this issue?
Saman Abbasian
3mo
Assume a synchronous generator operates at very low speeds and the equivalent impedance “seen” by the power system is the generator’s X2 reactance in series with the negative-sequence resistance R2 but, it could be validated X2 may equal to subtransient X”d and this low impedance would results in up to 500% of nominal to rush into the transformer unit and when the main breaker accidently closed or disconnected especially in out of phase 150 or 180 deg. But why? Assume when power of an armature of a DC motor is disconnected then, it turns into to a generator and in this case, the rotor tends to rotate like an induction motor due to the large magnetic flux in the airgap of and large induction current in the rotor of synchronous generator. Therefore X/R ratio of transformer would be very critical due to the total equivalent impedance including that generator X2/R2 ratio and a type manufacturing test shall be passed where it consists a unit generator power transformer for possible withstand maximum time according to IEC 60076-5 up to 2s and lets say the German standard DIN 57 532 VDE provides a better limits depending on transformer ratings up to 630kVA 2s and above up to 1250kVA 3s and above up to 3150kVA 4s and 5s for 200MVA.
Electrical Engineering Beauties
3moAccording to the relationships below, the size of the OUT OF PHASE short circuit passing current is lower than the three phase short circuit due to the involvement of the impedance of both sides of the GCB in most cases, but what makes this type of fault important is the OUT OF PHASE voltage applied to The winding of the transformer is significantly higher than the rated voltage. This issue can cause the saturation of the magnetic core, increase the leakage fluxes, and as a result, increase the destructive force vector of the short circuit.