PT Secondary Fuse

[charz]

PT: wye grounded - wye grounded.
Ungrounded System.

The secondary of the PT (L1, L2, L3, N) four wires is connected to the Numerical relay. The Numerical relay calculates the zero sequence voltage to detect the ground faults. (No broken delta connection to detect the ground fault). There is no damping resistor at the secondary side.
In our case, the phase- phase voltages are normal, but the phase - neutral voltages are not consistent with each other. Due to inconsistent phase to neutral voltages, the numerical relay operates on ground fault.

1. what happens if the neutral at the Primary of the PT is floated i.e disconnected from ground?
As I understand, In broken delta secondary connection, the relay can not detect ground fault. Will the phase to neutral voltages will be stable? I think no.

2. what happens if the neutral at the secondary of the PT is floated i.e disconnected from ground?
My understanding is that secondary neutral is grounded only for safety point. If secondary neutral is not grounded, can it cause distortion of secondary voltages?

Sometimes the PT secondary fuse gets blown? If there is a short circuit at the secondary loading side, the secondary fuse gets blown. What are other cases the PT secondary fuse gets blown?

 

[charz]

I may add to my previous post that this phase to neutral voltages gets normal once there is load on the power transformer. During no load, this phenomena returns. I read from previous threads suggesting to put damping resistors, I'm unable to understand how the resistors cause loading effect.

 

[waross]

Put a scope on your phase to phase and phase to wye point voltages. With an unloaded wye:wye connection you may be seeing badly distorted voltages with no load.
At no load the current is entirely non-linear exciting current. The non-linear current is reflected as a distorted voltage wave form.
I have seen obvious errors in the ratio between phase to phase and phase to neutral voltages (noticeably NOT the normal root 3 ratio) when the voltages were measured with a de'Arsonval based Volt meter.
The ratio between the RMS current and the average current is 1.1 for a sine wave.
The ratio between the RMS current and the average current is NOT 1.1 for a distorted waveform.
As a result, meters that respond to average current and scale it to indicate RMS do not give true readings on most distorted wave forms.
The damping resistors cause enough real current to swamp out the effect of the distorted exciting current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ShooterBooter]

1. You need to ground the neutral on the PT primary side. Without a grounded neutral you will not get a earth fault relay to work trough the PT.
I assume the network is isolated or hiZ earthed.

-SB

 

[podobing]

Charz,
I have seen the PT system that you describe used for ground fault detection in medium voltage (4160 volt) systems that are ungrounded. The neutral of the PT primary bank must be grounded or else you cannot detect a ground fault. As you said, the main reason for grounding the secondary neutral is for safety, so I would not disconnect it for that reason. I assume that the primary voltage rating of each PT is rated for the line-line voltage of the system. That is a must.

I am not familiar with a numerical relay, but the systems that I have seen use three light bulbs, each connected across the secondary of each of the three PT's. Normally, all three lights are dimly lit. When a ground fault occurs, one of the light goes out and the other two get brighter. This alerts the operator that they have a ground fault, but does not cause the circuit breaker to trip. That is the (only) reason for running an ungrounded supply system in the first place - so that it does not trip on the first ground fault.

If the secondaries of the PT's were connected in a broken delta, with a (250 ohm or so) resistor used to close the delta, the voltage measured across the resistor (zero sequence voltage) would be an indication of a ground fault. In this system, one side of the resistor is grounded for safety reasons. Using this voltage as an indicator, you could either alarm or trip on the occurrence of a ground fault.

If you want to leave the PT secondaries in a grounded wye configuration, I would install a resistor (or a 120 volt panel light) across each of the secondaries to provide a "token" load on each of these PT's. That may resolve your unwanted tripping issues.

However, if you trip on the occurrence of a ground fault, maybe the supply system should be high resistance grounded. That may be a good compromise and you would get the benefits of having a "grounded" system.

Happy Holidays,
Podobing