Grounds on Cap Banks

1. Grounds on Cap Banks

2. Why do we ground some cap banks and not others? • We ground 12kV Cap • We do not ground 23 and 25kV banks • We ground 34.5kV and up capacitor banks

3. Why do we ground 12kV banks • We use a multi-grounded system at 12kV and we connect loads either single phase or grounded wye. • This means that if we don’t ground a wye connected cap bank we can get phase reversal an high voltage on a phase if an upstream cutout blows. • See the phase reversal power point.

4. Why do we ground 34.5kV and up • Grounded wye banks used on a grounded wye system makes easier duty on the circuit breaker as far as transient recovery voltage, because each capacitor looks like a single phase case and there is no interaction between phases. • The 34.5kV always used a static wire and this has served as a neutral. So it is good to use ground wye as it is easier duty on the circuit breaker as each cap acts as a single phase circuit. • You need to be careful on systems where you don’t carry the neutral, such as the 25, 23, 46kV

5. Let’s look at why? Let’s look at a true delta system with a load connected ungrounded wye ( the load could be connected delta) but the key is it is ungrounded.

6. Lights motor Delta system This is a typical delta fed system such as that that runs a coal mine fan. The feed is delta the motor is ungrounded wye connected and the indication system for a fault is a set of light. Note the light bank is grounded. All the lights have voltage of phase to ground under normal conditions.

7. Lights motor Delta system Now fault a phase(since it is a delta system there is no fault current) when you do, that light goes out and the remaining two lights get bright. You know you have a fault. If the two lights that get bright are not rated for phase to phase voltage they will fail. The motor is no problem as it is not grounded so the fault does affect the motor. In fact this is the reason coal mines like to use this connection as it can continue to operate the fan, but they know one phase is contacting the ground and can fix the problem at their convenience instead of shutting down the mine. Now if you substitute a capacitor bank for the light system and have it grounded it will subject the capacitors to an over voltage. If the caps are rated for only phase to ground voltage and the fault is there for a long time they will fail.

8. For that matter any load on an ungrounded system with a connection to ground will subject the load on the unfaulted phases to 1.732 pu overvoltage. A typical example is surge arrester, but we are forced to provide a ground connection to them as lightning wants to go to ground. But it is bad ju-ju to mix and match system grounding. If the system is ungrounded hook the caps up ungrounded Wye or Delta. If the system is grounded hook up the Caps as grounded. This is why we have ungrounded caps on the 23 and 25kV systems as even though they are grounded at their source there is a good chance of neutral pull during a fault. This is recognized by the surge arresters we use What about systems such as the 46kV where we ground cap banks but don’t carry the 46 neutral/shield wire and you have substations such as Hutchins with a cap. There is Neutral pull and you do subject the caps to higher voltages on the unfaulted phases, this needs to be kept in mind when you apply these caps and relay time.

9. From 1700-10 on intermediate surge arresters

10. Delta connected Cap bank on a delta system on 281 out of Kingwood