I work for a utility company in southeastern New York as a Substation Electrician. I have a question about a transformer bank that my company installed on a 34.5 KV subtransmission system in our service territory.
To give a little back ground, this 34.5 KV subtransmission system originates from a 69-KV transmission station. (The source substation). It consists of two delta to wye transformer banks. (69 KV delta to 34.5 KV wye.) The wye connected windings on each transformer are grounded to the station grid only. There is no system neutral leaving the station and traveling along with the lines. Each transformer feeds one 34.5 KV transmission line. Lines A and B. These two lines run parallel on the same structures and travel to various stations. There are five distribution substations connected to this subtransmission system. The two lines are eventually tied together through a tie circuit breaker at the furthest 34.5 KV substation from the source, (34.5 KV to 13.8 KV). This creates a loop scheme connecting two of the five 34.5 KV substations in a loop. The other three 34.5 KV substations are fed from radial feeds off of line B.
About half way between the source substation (69 KV to 34.5 KV) and the furthest substation (34.5 KV to 13.8 KV), line B is T-tapped to feed line C. Line C is a radial that feeds two distribution substations. One substation has a 34.5 KV to 4.8 KV delta to delta bank, and the other has a 34.5 KV to 4.16 KV grounded wye to grounded wye bank. The bank in question is the latter one. I will call this substation D.
I was involved in a job at station D a while back to replace the three single phase transformers that made up the 34.5 KV to 4.16 KV bank. The original bank of transformers there was three 250 KVA 33 KV to 2.4 KV transformers connected delta to grounded wye. One of the transformers in the bank failed during a fault on the 4.16 KV distribution circuit. There was no spare 33 KV to 2.4 KV transformers available. With an outage on customers going on, the company decided to install three 500 KVA 19.9 KV to 2.4 KV distribution step transformers connected grounded wye to grounded wye. This new bank of transformers is fused on the high voltage side with three single phase open type cutouts so it is possible for this bank to operate single phase in the case of fuse failure or transformer failure. I am being told that this bank does not need a system neutral returning back to the 34.5 KV source. That we are relying on the station ground grid in station D to return current to the 34.5 KV source substation transformer through the earth. This substation D has only a twenty feet by twenty foot footprint and is in a very rocky mountainous area. There were ground resistance readings taken recently at station D and they were found to be less than satisfactory. The grid consists of a few pieces of 4/0 copperweld laying horizontal in a couple feet of fill. The 4.16 KV distribution system is a four-wire multi-grounded system, with its neutral running the entire length of the circuit but with no ties to any other circuit.
This transformer bank is the only piece of equipment that is connected grounded wye to this 34.5 KV subtransmission system. Everything else is connected either delta or floating wye. I do not think this system was meant to have equipment connected to it grounded wye in this manor.
My question is, is this an accepted practice in the industry to connect these transformers (single phase distribution step transformers) to a system with no system neutral that is continuous throughout the entire circuit? Does this transformer bank pose any hazzards to anyone working in close proximity to the station? Does this subtransmission system fall under the NESC rules concerning neutrals of a four-wire wye system being continuous throughout the entire circuit? Also, are there any criteria for substituting a system neutral of a four-wire system with a substation ground grid used for grounded wye transformer connections to what appears to be a three wire unigrounded wye system?
Any help anyone can give me in understanding this transformer connection would be very much appreciated.
To give a little back ground, this 34.5 KV subtransmission system originates from a 69-KV transmission station. (The source substation). It consists of two delta to wye transformer banks. (69 KV delta to 34.5 KV wye.) The wye connected windings on each transformer are grounded to the station grid only. There is no system neutral leaving the station and traveling along with the lines. Each transformer feeds one 34.5 KV transmission line. Lines A and B. These two lines run parallel on the same structures and travel to various stations. There are five distribution substations connected to this subtransmission system. The two lines are eventually tied together through a tie circuit breaker at the furthest 34.5 KV substation from the source, (34.5 KV to 13.8 KV). This creates a loop scheme connecting two of the five 34.5 KV substations in a loop. The other three 34.5 KV substations are fed from radial feeds off of line B.
About half way between the source substation (69 KV to 34.5 KV) and the furthest substation (34.5 KV to 13.8 KV), line B is T-tapped to feed line C. Line C is a radial that feeds two distribution substations. One substation has a 34.5 KV to 4.8 KV delta to delta bank, and the other has a 34.5 KV to 4.16 KV grounded wye to grounded wye bank. The bank in question is the latter one. I will call this substation D.
I was involved in a job at station D a while back to replace the three single phase transformers that made up the 34.5 KV to 4.16 KV bank. The original bank of transformers there was three 250 KVA 33 KV to 2.4 KV transformers connected delta to grounded wye. One of the transformers in the bank failed during a fault on the 4.16 KV distribution circuit. There was no spare 33 KV to 2.4 KV transformers available. With an outage on customers going on, the company decided to install three 500 KVA 19.9 KV to 2.4 KV distribution step transformers connected grounded wye to grounded wye. This new bank of transformers is fused on the high voltage side with three single phase open type cutouts so it is possible for this bank to operate single phase in the case of fuse failure or transformer failure. I am being told that this bank does not need a system neutral returning back to the 34.5 KV source. That we are relying on the station ground grid in station D to return current to the 34.5 KV source substation transformer through the earth. This substation D has only a twenty feet by twenty foot footprint and is in a very rocky mountainous area. There were ground resistance readings taken recently at station D and they were found to be less than satisfactory. The grid consists of a few pieces of 4/0 copperweld laying horizontal in a couple feet of fill. The 4.16 KV distribution system is a four-wire multi-grounded system, with its neutral running the entire length of the circuit but with no ties to any other circuit.
This transformer bank is the only piece of equipment that is connected grounded wye to this 34.5 KV subtransmission system. Everything else is connected either delta or floating wye. I do not think this system was meant to have equipment connected to it grounded wye in this manor.
My question is, is this an accepted practice in the industry to connect these transformers (single phase distribution step transformers) to a system with no system neutral that is continuous throughout the entire circuit? Does this transformer bank pose any hazzards to anyone working in close proximity to the station? Does this subtransmission system fall under the NESC rules concerning neutrals of a four-wire wye system being continuous throughout the entire circuit? Also, are there any criteria for substituting a system neutral of a four-wire system with a substation ground grid used for grounded wye transformer connections to what appears to be a three wire unigrounded wye system?
Any help anyone can give me in understanding this transformer connection would be very much appreciated.
