PPT Secondary Conductor Protection

[mls1]

We have a large hydrogenerator excitation system in which the PPT (power potential transformer) that feeds the exciter is fed from a a fused tap off the generator IPB. The PPT secondary conductors are then fed to the exciter cabinet via cables in tray. The AC breaker they land on has no overcurrent protection. This seems to violate NEC 240.21 on several levels. First, the transformer is delta/wye connected so primary protection cannot be use to protect the secondary conductors. Second, the conductors must be sized to a protective device upon which they terminate but there is no protective device at the termination, and third the conductors must be installed in a raceway but cable tray is a cable support system, not a raceway. Is there something specific to PPT installation that excludes the requirements for transformer secondary conductors in the NEC?

 

[RRaghunath]

If I remember right, NEC says the primary fuse rating shall not exceed 250% of transformer rated current if there is no overcurrent protection at the transformer secondary breaker.
Could you check please - the fuse rating may not be > 2.5 x transformer full load current.

 

[waross]

Check the scope of your code. As a generating station you may be exempt from the code.
A question, not a challenge:

First, the transformer is delta/wye connected so primary protection cannot be use to protect the secondary conductors

Is that now a rule in the NEC?
I am familiar with the CEC up to 2015. I haven't bothered to upgrade since I retired.
No such rule in the CEC up to 2015.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[mls1]

The 250% protection is for the transformer only per Article 450 but the secondary conductors are covered under a different article, 240.21. In that article it is OK to use primary protection for secondary conductors if the transformer is delta/delta or single phase, single voltage. For any other transformer configuration the secondary conductors are considered a tap and must be protected as such.

It is correct that the powerhouse would be exempt from the NEC but the owner has adopted the NEC as the design standard for the powerhouse. They, of course, can make exceptions since they are the AHJ but our directive is to follow the NEC. It looks like the old system had CTs on the secondary conductors which would have allowed the relaying to protect both the PPT and conductors but those have been moved to the high side. That appears to be where the problem starts but I'm still curious if there is something else specific to regulator/PPT transformers that addresses this such as an IEEE standard.

 

[waross]

If the transformer is considered as part of the excitation system, the NEC may apply to the complete excitation system rather than components of the system.

if the transformer is delta/delta

I don't have the NEC. Did you mean wye/wye?
In a wye/wye transformer the primary current is proportional to the secondary current.
In a delta/delta transformer the primary protection will see only 50% of a single phase overload on the secondary.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[mls1]

The current is different in delta/wye but not delta/delta. I'm not sure why wye/wye is not included in the allowance but here is the exact text from the code:

(1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device

 

[waross]

Thank you for the exact wording. mis1.
A 10 KVA single phase load on a delta secondary will put a 5 KVA load on each primary phase.
That works out to be twice the turns ratio.
Primary protection will protect the transformer secondary, but not the secondary conductors.
But, the code is the code and it seems like more and more committee members are appointed to support the commercial interests of manufacturers or contractors rather than for technical ability.
I have had technical discussions with a couple of code committee members over the years and was appalled.
Yes, there are still some very knowledgeable committee members and my respect, apologies and sympathy to them.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!