Hi,
Basically i have two questions regarding Synchronisation.

1) What should be the Accuracy class of the Synchronising Potential transformer.

2) Should we provide the higher rating capacity Circuit breakers at the point of Synchronization than the normal current expected to flow thro it?

is there any standards and their contents available for the synchronising procedures and components....

 

Responding to Anonymous' Aug 26, 2:44pm queries.... Following are the 'Basic' answers:

A1) Accuracy is less important than ratio! Both protection and metering grades can be used. Insure that both are alike! Also insure that their burden capacity is more than the burden of the synchronizing relay and connecting leads!

A2) No!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Mr. Corso,

So you're saying there are two grades of potential transformers: protection and metering. Do these grades have any other designation, such as 'Class 1' or 'Class B', etc.?

Wouldn't the sizing/rating of the "generator breaker" be a function of the maximum expected output of the prime mover (which should be equal to or less than the maximum rating of the generator)?

To the originator of this thread, synchronizing components are sometimes specified by the utility which the generator is to be synchronized to; some have very specific requirements and even "recommend" the specific synchronizing check relays.

Synchronizing standards are also sometimes specified by utilities.

If you want recommendations for equipment, you can consult a vendor's representative/salesperson; they are always a good resource for equipment. There are many manufacturers who will supply generic electrical schematics which depict components used in the synchronizing circuit. Basler Electric (you can search control.com for their website address).

You should be aware that the potential transformers used in the synchronizing circuit are also quite commonly used in the protective relaying and metering circuits.

markvguy

 

Responding to markvguy's Aug 29, 12:37am comments & queries.... in the order given:

1) Instrument Transformer Types.
Although your question referred to PTs, CTs are included. Of course, both the PT and CT must accurately represent the primary "quantities" they mimic. The operative word is "accurately!" Metering transformers, especially for billing, must be extremely accurate. Furthermore, they only monitor nominal values for their lifetime. On the other hand, relaying transformers must accurately, albeit occasionally, monitor "primary 'quantities" that could reach 20 times (or more) their nominal
rating. Such an extreme is not the case for PTs, but there are stringent requirements as well: a) frequency range; voltage range; c) wave form; d) excitation variation.

2) Classification
There are three categories: a) application (eg, indoor, outdoor); b) insulation and cooling method (eg, dry, oil immersed, epoxy cast); can d) construction (eg, wound, window, bar.)

3) Accuracy
ANSI/IEEE Std 57.13 covers this in great detail: Group 1, covering PTs (eg, insulation, pri/sec ratio, allowable connections); Group 2 covers CTs that are fully insulated; and Group 3 covers CTs that have graded insulation. In addition Accuracy classes based on load burden (or impedance) are provided for both general and metering-use PTs, as well as for both 100% and 150% rated CTs.

4) Circuit Breaker Sizing
I'm not sure if you mis-interpreted my "No" reply to Anonymous question as improper, so allow me to expand. By code, nominal current rating is dictated by namplate rating. But CBs must also withstand the maximum short-circuit interrupting and momentary duties they could be exposed to!

That said, I took Anonymous' question to mean must the breakers withstand the maximum short-circuit current developed as if the machines were in parallel. Or, must the breakers be rated to withstand the maximum current resulting from an out-of-phase closure. For the 'basic' (to me it means typical) case the answer is NO! However, for very large machines a study must be done.

5) Synchronizer Recommendations.
While having some duplicate functions, there is a world of difference between a synchronizer and a sync-check relay. I'm sure it wasn't your intent to suggest otherwise.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])