Potential Transformer Arrangement

[living2learn]

Situation: 12kV switchgear fed from utility. This service entrance switchgear has a 12kV generator connect onto as well as one utility onto the bus and all new SEL-351 relays have been provided for the new incoming generator and utility breakers. All of the existing potential transformers are open delta type and one new wye wye PT was installed on the line side of the generator breaker.

The new generator switchgear is integrating into this one. It has a RTAC to make decisions. This is a closed transition system. The generator has SEL-751A relays and wye wye potential transformers.

The situation...

Looking at the RTAC on my new switchgear we notice a 500V imbalance from the main switchgear. This will still satisfy the sync check relay and etc... but is still concerning. This is looking at the older open delta PT and comparing it to the new wye wye pt that are installed in the same switchgear. The wye wye is on the incoming of the generator and the existing open delta is on the secondary of the utility breaker so it will sync across that breaker. The SEL relays are feeding all the voltage information into the RTAC via IEC 61850 fiber connections. The generator building is 3000' away.

What could cause this 500V imbalance? we have tested the entier system. All older PT's are within .3%, we have added real/reactive load so we know it is capacitance building up...any other thoughts. I read somewhere at sometime that "Thou shall not utilize sync'n with dissimilar PT's" not sure where I read it or if it is applicable with new digital relays now adays.

Client is forcing us to buy new wye wye PT's for his existing switchgear since we cannot confirm where the 500V's is coming from.

IT couldn't be the 30 degree phase shift from the open delta compared to the wye wye because you would think that it would be a much larger.

 

[Marmite]

How is the system earthed? Is the Yy VT primary star point earthed?
Can you post a SLD?

Regards
Marmite

 

[living2learn]

Pictures is worth a thousand words.

PT1 is open delta grounded on the secondary, PT2 is grounded wye. During normal operation with the utility supplying the facility and the generator breakers NO PT1 and PT2 read the exact same. The voltages all show up as normal and within tolerance.

When the utility goes away and we TRY and do a closed transition back to utility we sync our (2) generators up with the utility PT1, BUT the voltage on PT2 is higher by 500V's while PT3 perfectly matches PT1. So the voltage is rising from the generator plant to the switchgear.

So the problem only shows up one way - with the generators running. We have retested this by using the loads tapped off of the incoming generator circuit which consisted of inductive and resistive loads to cancel out any large capacitance in the underground circuit that could cause the voltage to rise.

We have tested the grounding between the generator building and the service entrance switchgear as well.

We are probably going to have to change PT1 to match PT2 or at least install and auxillary one. But I want to know the math/logic behind it before I spend someone elses $$$$

What could cause this?

 

[marks1080]

How exactly did you test the old PTs? Did you test all the taps? There could be a minor short between windings that is worse at primary voltages than you see at testing voltages. Have they been dobled?

One of the lovely things with IEDs is that you don't need to worry about secondary transformer configurations... you just tell the IED what it is and it does the math - which is why most new installations use WYE secondaries.

 

[Marmite]

The diagram shows PT2 as Yy, but your first post says " All of the existing potential transformers are open delta type and one new wye wye PT was installed on the line side of the generator breaker". Which is correct?

Where is the 500V being measured? It is important to know as your open delta PT's can not measure phase to neutral voltage on the primary, and they are only measuring two of the three phase to phase voltages.

Regards
Marmite

 

[waross]

Are you using digital RMS Voltmeters or old panel meters with d'Arsonval movements. The d'Arsonval movements had an issue with distorted wave forms. I have seen false voltage readings a couple of times when the difference mattered. Both times it was a wave form issue. The d'Arsonval movement responds to average values rather than RMS values. If the wave form was not a true sine wave, the ratio between the average value and the RMS value (the form factor) changed and the reading was in error.

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

 

[living2learn]

Thanks for the comments!

The testing of the PT invovled grounding and turns ratio test. The system works when on utility...which has the team convinced the PT's are working correctly...

The 500V is being seen at the HMI. The voltage is rising from the generator building to the main service entrance switchgear.

The SLD is correct. The existing PT - PT1 is open delta while the new PT's #2 and #3 are Y-Y.

All measurements are being taken directly from the PT's and connect to a SEL-351S relay which then connects to a RTAC then HMI.

 

[marks1080]

Measure the secondary voltages directly, before they get to the HMI.