POCO Transformer primary

[texie]

What are the technical reasons that POCO transformers supplying a typical 208Y/120 or 480Y/277 service are usually connected as a Y on the primary side?

 

[GoldDigger]

What are the technical reasons that POCO transformers supplying a typical 208Y/120 or 480Y/277 service are usually connected as a Y on the primary side?

One possible reason would be the lower voltage across the primary winding. More current but less need for voltage withstanding from the insulation. Might cost less for the same power rating?
Another would be if they are better suited to supplying an unbalanced (specifically phase-to-neutral) load on the secondary side.

In short, I really don't know.

 

[hurk27]

O course it depends upon what system the POCO uses as some areas of the country have gone to delta local feeds, but a delta wired primary would require each transformer to have a line to line voltage primary and a wye would only have to be 1.732 less, also with a delta wired primary there is no way to cause the primary OCPD to open if you have a primary to secondary fault in the transformer this is why they always bond the primay H0 to the secondary X0, I been told that even with a delta primary system they still use a wye primary transformer to act as a zig-zag to provide this primary to secondary fault protection but I can't confirm this.

One other point is most of the transformers we use are a common core so we can't bond the H0 to the X0, but since they use separate tanks without a common core they can, what I have never understood is with a pad mounted transformer how they can bond the H0 to the X0 unless it also doesn't have a common core?

Our local feeds around here are all 12kv/7200 wye's so since 7200 volt cans are common they also are used to make up our 3-phase bank's in a wye on the primary side, even stranger yet is for a three phase bank 208/120 secondary they use the same transformer they use for a single phase 120/240 volt service by tyeing all the X0's together and only using one X1 bushing from each transformer to get the 208 to me this only gives you half the KVA rating of the transformer???

 

[AdrianWint]

The usual reason to use a WYE winding for a HV winding is insulation.

If you consider a DELTA primary..... all points on the winding are 1/(root3)*lineVolts above groundf & must be insulated for such.

With a WYE winding, one end is at 1/(root3)*lineVolts but the other end (the star point) is at ground potential ... so its possible to grade the strength of the insulation on the conductor to be appropriate to the voltages that it must withstand in service. This results in a cost saving.

There are problems with a WYE/WYE transformer:

i). It will pass harmonics (particuarly triplen harmonics) from the secondary back into the primary. A delta transformer will not (the triplens get 'trapped' in the delta)

ii). Loss of the primary stra point neutral connection will result in unbalanced voltages on the secondary side even though the primary side voltages are balanced.

iii). The HV distribution system for the HV side must use 4 wires & not the 3 that would be fine with a delta primary.

iv). If the secondary of the transformer which is feeding the HV circuit is also WYE then the HV distribution circuit has an earth point at both ends. This can lead to strange distribution of zero sequence earth currents in the event of a phase/ground fault on one of the HV lines (it will return to BOTH transformer star points)


Adrian

 

[meternerd]

Clearing phase to ground faults is one reason. Wye primary has all three phases referenced to ground due to the grounded neutral. Any phase will draw fault current when a ground fault occurs. Delta primaries are usually not grounded, so a ground fault can remain undetected until a second phase grounds, then it becomes a phase to phase fault.

 

[AdrianWint]

Clearing phase to ground faults is one reason. Wye primary has all three phases referenced to ground due to the grounded neutral. Any phase will draw fault current when a ground fault occurs. Delta primaries are usually not grounded, so a ground fault can remain undetected until a second phase grounds, then it becomes a phase to phase fault.

I don't think thats quite correct. Its quite possible to have a delta primary & still have ground fault protection by phase overcurrent as long as the secondary of the feeding transformer has a ground reference.

The configuration of the primary winding has no bearing on the abilitity to do ground fault detection on the secondary of that transformer.

 

[robbietan]

the poco in this neck of the woods uses a wye primary so that they have a neutral. metering at line to neutral is much cheaper than line to line

and grounding the neutral at every other pole for safety

 

[meternerd]

I don't think thats quite correct. Its quite possible to have a delta primary & still have ground fault protection by phase overcurrent as long as the secondary of the feeding transformer has a ground reference.

The configuration of the primary winding has no bearing on the ability to do ground fault detection on the secondary of that transformer.

I was speaking from a POCO perspective. The primary I'm referring to is the distribution system. Most systems are 12,470/7200 Wye. Primary ground faults are cleared by fuses or reclosers. Secondary ground faults are cleared by customer breakers. They should trip long before a primary fuse blows. In an ungrounded delta primary configuration, a ground on any single phase of the primary will not produce fault current, since there is no reference to ground. Ground detection is usually provided to identify grounded phases. Most transmission systems are delta due to the cost savings of three wires instead of four. Transmission relay protection is a whole other subject, not pertinent to this forum. If I'm wrong, please educate me. I'm old, but I hope I'm still teachable. Been with utilities for all of my 40 years of employment. Only had to deal with code the last 16, since we are also a water utility.

 

[GISdude]

COMMON NEUTRAL ON DISTRIBUTION

COMMON NEUTRAL ON DISTRIBUTION

One thing I'd like to throw out there. In the system around here, we have a DELTA on the distribuiton side (80% 3 wire), BUT FOR REALLY LONG stretches I'll see a common neutral ran along the 3 primary wires AND AT THE SECONDARY ZONE ON THE JOINT POLES. This is what alerted me that it was a delta on the distribution side.

Really, really confusing when I was a rookie estimator. I've never asked the senior engineers/lineman why it was done this way.

 

[AdrianWint]

I was speaking from a POCO perspective. The primary I'm referring to is the distribution system. Most systems are 12,470/7200 Wye. Primary ground faults are cleared by fuses or reclosers. Secondary ground faults are cleared by customer breakers. They should trip long before a primary fuse blows. In an ungrounded delta primary configuration, a ground on any single phase of the primary will not produce fault current, since there is no reference to ground. Ground detection is usually provided to identify grounded phases. Most transmission systems are delta due to the cost savings of three wires instead of four. Transmission relay protection is a whole other subject, not pertinent to this forum. If I'm wrong, please educate me. I'm old, but I hope I'm still teachable. Been with utilities for all of my 40 years of employment. Only had to deal with code the last 16, since we are also a water utility.

Ah, Ok .... we are talking at slightly crossed purposes! When I talked about 'primary' I was referring to the HV winding of a single transformer not the entire HV circuit which feeds it. I guess this is slightly different terminology that which we use this side of the pond.

I agree that its common to run the HV circuit as a delta, thus saving the cost of one leg of cable, however it is still possible to provide ground fault protection on this circuit in two ways:

a). Make the secondary of the feeder TX a Wye (but don't send the neutral - its not needed) & connect the star point to ground.

b). If the secondary of the feeder is delta then a zigzag earthing transformer can be used to create an artificial star point and use this to make the delta system earth referenced. The problem with this method is that it results in ground fault currents upto 1.5 times the line-line fault current.

 

[kwired]

What are the technical reasons that POCO transformers supplying a typical 208Y/120 or 480Y/277 service are usually connected as a Y on the primary side?

Though you wouldn't normally see a single phase service supplied by a 120 volt or a 277 volt transformer, it could happen.

You will see delta secondaries made up with same transformers they use for 120/240 single phase services and the primary is wye connected.

I do agree that you would need higher insulation rating for the units to be connected phase to phase instead of phase to ground if you were to make up delta connected primary side, or even phase to phase instead of phase to neutral for supplying a single phase service. Plus you would need three conductors instead of two if you wanted solid ground connections to the can or other equipment.

 

[meternerd]

Ah, Ok .... we are talking at slightly crossed purposes! When I talked about 'primary' I was referring to the HV winding of a single transformer not the entire HV circuit which feeds it. I guess this is slightly different terminology that which we use this side of the pond.

I agree that its common to run the HV circuit as a delta, thus saving the cost of one leg of cable, however it is still possible to provide ground fault protection on this circuit in two ways:

a). Make the secondary of the feeder TX a Wye (but don't send the neutral - its not needed) & connect the star point to ground.

b). If the secondary of the feeder is delta then a zigzag earthing transformer can be used to create an artificial star point and use this to make the delta system earth referenced. The problem with this method is that it results in ground fault currents upto 1.5 times the line-line fault current.

Hmmm............never seen that done. I guess what I'm used to seeing is a three phase Wye system with a grounded neutral. Whenever a single phase tap is needed, we just run one phase and a neutral. Any ground faults will usually blow the tap fuse rather than tripping the entire circuit. Not so with delta, since a single phase tap would require two phases and no neutral. But..I don't want to hijack the post, so I'll shut up.

 

[mbrooke]

One thing I'd like to throw out there. In the system around here, we have a DELTA on the distribuiton side (80% 3 wire), BUT FOR REALLY LONG stretches I'll see a common neutral ran along the 3 primary wires AND AT THE SECONDARY ZONE ON THE JOINT POLES. This is what alerted me that it was a delta on the distribution side.

Really, really confusing when I was a rookie estimator. I've never asked the senior engineers/lineman why it was done this way.

Running an equipment ground has nothing to do with how the XO is grounded at the sorce substation. The ground above the phases is most likely for lightning protection, as for the secondary run it is either for the low voltage or to provide a lower impedance path back the the substation grounding grid. Keep in mind regardless of system configuration, having a redundant grounding system is of major advantage both in equal potential, ground fault detection and lighting strike protection.

In California the norm per regulations is to have loads connected either phase to phase or phase to an isolated neutral. The goal is to keep currents from flowing through the earth or ground system. As for the substation transformer XO, ungrounded, solid ground, impedance ground ect can be found in California from what I have heard.

I was speaking from a POCO perspective. The primary I'm referring to is the distribution system. Most systems are 12,470/7200 Wye. Primary ground faults are cleared by fuses or reclosers. Secondary ground faults are cleared by customer breakers. They should trip long before a primary fuse blows. In an ungrounded delta primary configuration, a ground on any single phase of the primary will not produce fault current, since there is no reference to ground. Ground detection is usually provided to identify grounded phases. Most transmission systems are delta due to the cost savings of three wires instead of four. Transmission relay protection is a whole other subject, not pertinent to this forum. If I'm wrong, please educate me. I'm old, but I hope I'm still teachable. Been with utilities for all of my 40 years of employment. Only had to deal with code the last 16, since we are also a water utility.

You are correct about fault detection on solidly grounded systems verses ungrounded systems. Most transmission systems are not delta but wye and have a lighting guard wire which also doubles as neutral/ground; sub transmission sometimes does not have the ground/neutral wire run because its easier/cheaper to just use a Delta primary step down transformer at the sub on the receiving end and have the earth for fault detection. Comes out cheaper that way. Regardless of whether or not the ground/neutral wire is run, today substation transformers are almost always solidly grounded to the substation ground grid regardless where they are at. Ungrounded systems are a thing of the past in the utility industry (ie, ungrounded transformer secondaries); even where 3 wires are the norm such as California utilities and sub-transmission lines the norm is to ground the secondary solidly at the source substation.

 

[mbrooke]

To answer the original question:

Even though wye-wye has many disadvantages it still is favorable to the utilities because:

>Single bushing transformers are cheaper.
>Ferroresonance is usually not a problem, signle phaseing and switching will not be of concern.

>Standardization of equpment make it exclusive

>Windings are cheaper to insulate
>Changing a pole top unit is easier, less clutter on the primaries.
>Zero degree phase decplacement.

 

[meternerd]

I guess since there are only three wires to our subs, we assume it's delta. The only real transmission expeience I have is with 500KV DC. Whole different animal.

About the single bushing transformers being cheaper, here in California, the transformer tank of overhead transformers is not grounded, so all of our transformers are two bushing and the neutral is not bonded to the tank. Underground is different. Bonded neutrals. We still use Wye on most of our distribution. We do have a small section that is 14,400 Wye with no neutral.

OK....NOW I'll shut up.:angel:

 

[kwired]

I guess since there are only three wires to our subs, we assume it's delta. The only real transmission expeience I have is with 500KV DC. Whole different animal.

About the single bushing transformers being cheaper, here in California, the transformer tank of overhead transformers is not grounded, so all of our transformers are two bushing and the neutral is not bonded to the tank. Underground is different. Bonded neutrals. We still use Wye on most of our distribution. We do have a small section that is 14,400 Wye with no neutral.

OK....NOW I'll shut up.:angel:

A wye has to have a neutral. It may not be directly connected to any load in some instances and it may not be grounded in some instances but there is a neutral.

 

[mbrooke]

I guess since there are only three wires to our subs, we assume it's delta. The only real transmission expeience I have is with 500KV DC. Whole different animal.

About the single bushing transformers being cheaper, here in California, the transformer tank of overhead transformers is not grounded, so all of our transformers are two bushing and the neutral is not bonded to the tank. Underground is different. Bonded neutrals. We still use Wye on most of our distribution. We do have a small section that is 14,400 Wye with no neutral.

OK....NOW I'll shut up.:angel:

The load transformer will be of course delta or ungrounded wye on the primary in the receiving substation, however the supply transformer from the originating substation may be grounded a variety of ways on the secondary. I guess terminology is whats the confusion here.

Whooa, wait you guys dont ground the transformer tank?:jawdrop:Keep in mind that even though its 2 bushing you still need to ground the transformer to a rod, plate or system EGC in case of a fault.

Im not getting the underground part. Is the customer pad mount wye grounded primary (16kv-120/208) or are you referring to the transformer in the 66/ 16kv substation?

 

[mbrooke]

Just for those wondering how its done around here:

>5-30kv from generator;

>generator step up transformer: 5-35kv to 345kv, Delta Y grounded; 345kv transmission backbone with lighting wire, VTs connected P-G everywhere.

> 345kv to 115kv: Y auto transformer grounded with delta tertiary, (a few subs use the tertiary for 13.8kv distribution by adding a zig-zag for a neutral); 115kv with lightning guard, all VTs P-G.

> lower level generation: gen voltage to 115kv, Delta Y grounded.

>115kv to either 13.2, 13.8 or 23kv distribution, Delta Y grounded; distribution system is 3 phases with a multi grounded neutral and single bushing transformers, all loads wye with a few delta connected on large Kva banks.


>115kv to older 69kv segments: Y grounded auto transformer, some have delta tertiary. 69kv one tall towers has a lightning guard, 69kv on short wooden poles is 3 wire only.

>69kv to 13.8kv distribution: Delta Y grounded
>69kv to 23kv or 27.6kv sub-transmission: Delta Y grounded, some have a 3rd 4.8kv tertiary for back in the day distribution; sub-transmission is 3 wire on wooden poles.

>115kv to 23kv or 27.6kv sub-transmission: Y grounded Y grounded, 4.8kv delta tertiary, was used to supply 3 wire ungrounded delta distribution years ago, now left unconnected. 23kv or 27.6kv is 3 wire on wood poles.

>27.6kv to 23kv distribution: Delta Y grounded
>27.6kv to 13.8kv distribution: Delta Y grounded
>27.6 or 23kv to 4.8kv distribution: Delta Delta; this system is no longer used and was 3 wire with all loads delta connected.

Of note 27.6kv sub-transmission is being phased out for being non standard, and 23kv 3 wire sub transmission are now mostly converted to 4 wire distribution with a multi grounded neutral and use single bushing 13.2kv transformers. Years ago 13.8kv was also considered sub-transmission and was 3 wire but is now exclusively 4 wire distribution.

 

[meternerd]

The load transformer will be of course delta or ungrounded wye on the primary in the receiving substation, however the supply transformer from the originating substation may be grounded a variety of ways on the secondary. I guess terminology is whats the confusion here.

Whooa, wait you guys dont ground the transformer tank?:jawdrop:Keep in mind that even though its 2 bushing you still need to ground the transformer to a rod, plate or system EGC in case of a fault.


Im not getting the underground part. Is the customer pad mount wye grounded primary (16kv-120/208) or are you referring to the transformer in the 66/ 16kv substation?

OK....this is WAY off the subject of the OP, but what the heck......

Nope....the tank is not grounded. It is treated like a phase. Any internal phase to tank fault will not blow a fuse, since the transformer case is "floating". The reasoning is (in California, anyway) that no equipment within proximity of the primary is to be grounded, because it reduces the possibility of an incidental contact from phase to ground by an unfortunate lineman. We also insulate the guy wires with "Johnny Balls" to keep any ground from entering the work area. We use EPG grounding when working on live poles. The neutral is treated as a phase as well. The primary and secondary bushings are isolated from the tank and tied to the common primary neutral, which is grounded. Primary voltage is 12,470/7200 with a grounded neutral.

Underground padmount transformers are the same as far as primary voltage, 12,470/7200. The X0 and H0 primary and secondary neutrals are one common point. The tank is bonded to the neutral and also to a driven ground rod. On newer transformers, the neutral bond to the tank is removable to facilitate megger testing. Secondary voltages on three phase are 120/208 and 277/480. We no longer offer 240 or 480 delta. Substations are all wired Delta primary and 4 wire grounded Wye secondary regardless of supply voltage. We have only 60 KV and 115 KV supplies.

As stated above, we have a small section of 14,400V 3 Wire Wye that has no neutral run from the substation, but is grounded at the substation (not ours). We treat it as delta.

 

[kwired]

OK....this is WAY off the subject of the OP, but what the heck......

Nope....the tank is not grounded. It is treated like a phase. Any internal phase to tank fault will not blow a fuse, since the transformer case is "floating". The reasoning is (in California, anyway) that no equipment within proximity of the primary is to be grounded, because it reduces the possibility of an incidental contact from phase to ground by an unfortunate lineman. We also insulate the guy wires with "Johnny Balls" to keep any ground from entering the work area. We use EPG grounding when working on live poles. The neutral is treated as a phase as well. The primary and secondary bushings are isolated from the tank and tied to the common primary neutral, which is grounded. Primary voltage is 12,470/7200 with a grounded neutral.

Underground padmount transformers are the same as far as primary voltage, 12,470/7200. The X0 and H0 primary and secondary neutrals are one common point. The tank is bonded to the neutral and also to a driven ground rod. On newer transformers, the neutral bond to the tank is removable to facilitate megger testing. Secondary voltages on three phase are 120/208 and 277/480. We no longer offer 240 or 480 delta. Substations are all wired Delta primary and 4 wire grounded Wye secondary regardless of supply voltage. We have only 60 KV and 115 KV supplies.







As stated above, we have a small section of 14,400V 3 Wire Wye that has no neutral run from the substation, but is grounded at the substation (not ours). We treat it as delta.

Is transformer tank also isolated from grounded secondary conductor?

How far apart are ungrounded conductors, if they don't want a lineman to be able to contact an ungrounded and a grounded conductor that is kind of understandable, but based on same theory they should have all ungrounded conductors at least 7 - 8 feet apart just to prevent a lineman with a long arm span from being able to contact both at same time - and that is not counting any tools or other equipment he may have in his hands.

I am not a lineman but have seen many times where they put insulating devices on exposed conductors, both grounded and ungrounded when necessary. I have also seen many times where the lines are de energized and ground jumpers are installed between all conductors, on both sides of the work if it is capable of being fed from either direction.