Transcript Grounds on Cap Banks

Grounds on Cap Banks
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

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.

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
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.
Delta system
Lights
motor
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.
Delta system
Lights
motor
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.
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.
From 1700-10 on intermediate surge arresters
Delta
connected
Cap bank on
a delta
system on
281 out of
Kingwood