y-y connection

Status
Not open for further replies.
B

bully

Member
We are doing an installation in the
Caribbean where we are creating our
power and distribution. We have
parallel generators that are non
separate derivate system (neutral is
floating in the generator) serving a
parallel generator switchgear. The
voltage generated is 277/480V-3PH-4W.
Each generator is protected with
electronic circuit breakers.

The generator switchgear neutral bar
and ground bar are bonded together.
From the generator switchgear, a
branch electronic circuit breaker is
serving 3000 KVA wye-wye transformer
with 3PH-4W 277/480V primary and
12,470/7200 3PH-4W secondary . The
primary 480V feeder from the branch
breaker to the 3000 KVA transformer is
5W(a,b,c,n and eq ground).
The power distribution to each group
of houses is at 12470/7200V-3PH-from
the secondary side of the 3000 KVA
transformer.
We found out that the neutral
conductor of the primary
feeder(480V)is bonded at the primary
side of the 3000 KVA transformer,and
the secondary feeder(12,470V)is bonded
at the secondary side.
My problem is that the switchgear
branch breaker serving the 3000 KVA
transformer is tripping.

I believe that the primary neutral
bonding should be deleted;this neutral
should be floating, not bonded and the
secondary neutral bonding should
remain. In this way the breaker will
not trip.
Please,fellas, let me know if I am
correct
 
bully said:
I believe that the primary neutral bonding should be deleted;this neutral should be floating, not bonded and the
secondary neutral bonding should remain.
Click to expand...

I agree that the primary neutral connection should be floating. For single source systems I see nothing but disadvantages with your present arrangement.
 
I agree with everything that has been said here....but as a still fairly inexperienced Engineer, I have to ask a question: why would a wye-wye transformer be used for a distribution application like this rather than a wye-delta? Perhaps they just used what they had on hand? Edumacate me, please. :wink:
 
y-y connection

Two question:
1. What is the voltage on the secondary feeding each group of houses
2. Are the primaries of step down transformers fed by 12470 volts single phase or 7200V single phase?
If it is duel voltage single phase secondary off wye-wye being fed form wye source the neutral does need to be connected to the primary windings. The electronic circuit breakers might be seeing a phase current unbalance.
 
Understand, I am well out of my element here. :)

1. How can the bonding connection between the generator and primary-side of the transformer be removed and still have an effective ground-fault clearing path?

2. Is the neutral between the gen. and trans. being used by the windings of the transformer?

3. Why are EGC's being used?
 
y-y connection

Wye-Wye Connection
As with the delta-delta connection, the bank ratio is the same as the individual transformer and the bank output voltage is in phase with the bank input voltage. The only extra possible problem is that the neutral point may shift causing unbalance of the line-to-neutral voltages. This can be caused by unbalanced loads (different load currents in each of the three phases).
There are two solutions. Use a 4-wire system and tie the neutral points together forcing the line-to-neutral voltage to be balanced, or add a tertiary winding connected in delta forcing the voltages to sum to zero.
 
Status
Not open for further replies.
Top