Single phase Voltage Regulators - three phase circuit

[podobing]

To all,
We have a customer that has a 12,470 volt, three phase, resistance grounded power system. The secondary of the source transformer is wye connected. The neutral grounding resistor resistor limits the line to ground fault current to 15 amperes. They want to install two single phase voltage regulators to maintain the system voltage during hard motor starts. The regulators would be connected in an "open delta" type of arrangement. The two regulators are connected:
Regulator 1: Terminal S to X1 of source transformer; SL to X2 of source transformer; L to load phase A
Regulator 2; Terminal S to X3 of source transformer; SL to X2 of source transformer; L to load phase C
X2 of the source transformer also becomes load phase B.

There is also a VCB between the source transformer and the proposed site of the regulators, but tI thought that may just confuse my explanation of the regulator connections.

I explained that I did not think this plan would work properly as the regulators would resemble a delta - delta transformer and would not be able to handle zero sequence currents in the event of a ground fault. Other than putting a short on the load circuit, I do not know how to prove or disprove what I just wrote. Can anyone help me with this problem?

Regards,
Dave

 

[stevenal]

Regulators don't respond quickly enough to affect motor starts. This does not resemble delta-delta, because regulators are autos with a single winding. I see no issue with I0, but do see a great source of voltage angular unbalance.

 

[dpc]

Agree with stevenal - the regulators will do nothing to help motor starting - they are much too slow.

 

[podobing]

I agree with the comment about operating speed also. That aside, I am more concerned about ground faults and the ability of what I referred to as (correctly or incorrectly) as open delta connected auto-transformers. The open delta arrangement is accepted for three phase, three wire loads. There are no line to neutral loads on the system, but with it being a resistance grounded system, I could loosely call that a three phase four wire system. For that type of system, three regulators is accepted. Maybe that is due to the "imbalance" that stevenal mentioned. Also,if the regulators operate in a small window, they can almost be considered bypassed.
Dave

 

[waross]

The open delta auto-transformer circuit is often used to adjust voltages. In Canada it often is seen converting 480 Volts to 600 Volts and 600 Volts to 480 Volts.
600 Volts is not a standard generator voltage. 480 Volt generators are standard and widely available. When a plant with a 600 Volt system needs temp power, the open delta auto-transformer circuit is often used to boost the 480 Volt generator output to 600 Volts.
The voltage balance is quite good, but the line to neutral voltages are off a little. For this reason the open delta auto-transformer circuit is not suitable when line to neutral loads are present.
That said, it won't work for you. The motor will have started before the voltage regulator is able to respond to the motor starting voltage drop.
Bear in mind that this is the Electric power & transmission & distribution Forum.
The voltage regulators that we work with are too slow, but you may be considering a type of voltage regulator that does have a fast enough response. If you have a specific regulator in mind, post some information and we will be glad to look it over.


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

 

[stevenal]

Open delta transformer connections are notorious for imbalance, and are generally limited to mainly single phase loads with small three phase motors. The regulators in the proposed configuration will work to regulate two of the line to line voltages, disregarding the third voltage and any angular imbalance.

 

[waross]

I agree with you stevenal however the source of the voltage and phase angle errors with an open delta service is due to voltage drops on the supply neutral and often due to different sized transformers (with differing impedances).
However OPEN DELTA AUTO-TRANSFORMER voltage correction does not use a neutral connection and the transformers are matched in size and impedance. In Canada the standard voltage is 600 Volts and our largest trading partner, the USA uses 480 Volts. The OPEN DELTA AUTO-TRANSFORMER is seen more frequently in Canada than elsewhere. It is not to be confused with an open delta service supplying a single phase load and a small amount of three phase load.
The voltage regulator scheme, if properly connected will act as an OPEN DELTA AUTO-TRANSFORMER. The issue is the speed of operation.

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

 

[podobing]

I agree that the speed of the regulator(s)is the issue regarding motor starting - no questions. After reading the responses and giving it some thought, I think what will happen is that the open delta regulators (auto-transformers) will establish a neutral due to the balanced charging current of the long cable system. This neutral will be different from the neutral (Xo) of the supply transformer if the regulators are tapped up or down. The neutral grounding resistor is connected between the Xo bushing of the supply transformer and the "neutral" created by the system capacitance. If this is true, low level ground fault current will flow through the NGR anytime that the regulator taps are "off of center". Does this make any sense?

 

[waross]

The neutral doesn't change but two phases change in relation to the neutral. The the available ground fault current on one phase does not change. The available ground fault current of the two boosted phases is limited by the impedance of the auto-transformer. It is also at a slightly higher voltage base.
However your ground current is limited by the NGR and so will not be an issue.
Do not connect the auto-transformers to the neutral. Such a connection will introduce phase shifts and phase shifts are part of your problem.
The correct connection is from A phase to B phase and from A phase to C phase. I have used and seen this circuit to convert from 600 Volts to 480 Volts and to convert from 480 Volts to 600 Volts. I have seen it used with NGRs and with solidly grounded neutrals.
Whatever the grounding method, the open delta auto-transformer circuit is not suitable for line to neutral connected loads.
One plant that I used to service had the main service and two MCCs supplied as a unit. The incoming voltage was 600/347. The plant had a lot of 480 Volt legacy motors. New equipment was 600 Volts. There was an open delta auto-transformer built into the unit and one MCC was supplied with 600 Volts and the other was supplied with 480 Volts.
As time went on equipment was changed out for different equipment. It was common to take out a 480 Volt unit and replace it with a 600 Volt unit and vice versa. We would use a pair of small dry type transformers mounted on the machine to develop the correct voltage.
Some large heavy oil plants use 347/600 volts.
When temp power is needed for a shut-down, a number of diesel generators will be rented. The rental outlets in the area have available three transformer, wye connected auto-transformer boost units to supply 347/600 volts with line to neutral loads allowed.
Then comes the other rental equipment. Smoke buster fans and welding machines are commonly available at 480 Volts. A line to neutral connection is not needed for these loads and the rental outlets have available open delta auto-transformer units to drop the voltage back down to 480 Volts.
This is just two of the many instances where I have encountered open delta, auto-transformer circuits. I have never heard of problems.
Standard, boost rated dry type, 480 V/120 V transformers are used.
It's a Canadian thing, eh! grin

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

 

[stevenal]

Waross,
So these are not fixed auto-transformers, but are two single phase step voltage regulators which would normally operate independently. Do you see a problem with them operating out of step? Or would you use a single control and keep them in step? And does this arrangement offer an advantage over a wye connected three phase regulator for this application?

 

[podobing]

Stevenal,
These are two single phase units that operate independently. Just like anything else, things happen and they can get out of step. For the reasons that I mentioned in my previous posts, I would like for the customer to use three regulators instead of two. I agree with Waross and I am sure that that open delta transformers (or auto-transformers) are used successfully in many industrial applications. The concern that I have is that the voltages provided by the open delta single phase regulators will not be symmetrical with respect to the Xo bushing of the supply transformer. For three phase balanced loads, this does not present a problem, but I am worried about current flow through the NGR because of the difference between transformer Xo neutral and the "center" of the load voltages.
Dave

 

[stevenal]

podobing,

With out a connection to ground, your regulators cannot supply ground current no matter how far off neutral they are. Likewise your three wire loads cannot supply ground current no matter how unbalanced the voltages are. I see no problem with this aspect.

 

[PHovnanian]

podobing, if I understand your customer's situation, it sounds like the plan is to feed all of their loads through the regulators and attempt to mitigate voltage dips caused by motor starting.

Are these motors the customer's motors? Or are they elsewhere on the 12 kV circuit? If it is the former case, I'd suggest spending the money to 'soften' the motor starting. Equipment (autotransformers, contactors, VFDs, etc.) would only be sized for the motor loads. And the response time becomes a non issue, as the equipment would be switched in as a part of the start sequence. If the problem is with other motors on the source (utility system), the answer involves stiffening the source. That could be a larger substation transformer, reconductoring, separating the problem motor loads onto dedicated circuits, etc. Or increasing the distribution circuit voltage.

I chased a number of these problems around a few decades ago with a utility serving a rural area. Lots of farm motor loads on long, small conductor circuits. Line regulators actually made the voltage dip problem somewhat worse. If they were operating off their neutral position, they added a small amount of series impedance to the problem. And there was the resulting voltage unbalance, which the motors didn't like either. The final solution was to convert that territory from 12.5 to 34.5 kV. The added benefit to this was eliminating a bunch of small, underloaded substations that had been put in in the past to allow 12.5 kV circuits to 'reach' the end of long lines.

 

[waross]

Ground currents with the regulators will be similar to ground currents without the regulators. The NGR will limit the ground current to 15 Amps in any event. Note, Ohms law applies and if the voltage on the boosted phases is a a few percent high the ground current will be a few percent over 15 Amps.

And does this arrangement offer an advantage over a wye connected three phase regulator for this application?

The advantage of two regulators as opposed to three regulators is often cost. It depends on the capacity needed and the regulators available. If the load is close to the capacity of three regulators, you may have to go to larger regulators with the open delta arrangement. If the load is less than about 2/3 of the capacity of three regulators in wye, then you may as well use two regulators. Another issue is availablity. For three regulators in a wye arrangement the primary winding must be rated for line to neutral voltage. For the open delta scheme the primary winding must be rated for line to line voltage. I have found that for dry type transformers for voltage adjustment applications there is a larger selection of transformers available at line to line voltages than with line to neutral voltages. At times the choice is dictated by what is available in the real world on time and at a reasonable price. At times it may be nescessary to derate a higher voltage transformer to the line to neutral voltage. This works well but wastes capacity and money. (Use a 480 volt transformer for 277 Volts or in Canada for 347 Volts.)

Do you see a problem with them operating out of step?

The regulators should be arranged to step together.

These are two single phase units that operate independently. Just like anything else, things happen and they can get out of step. For the reasons that I mentioned in my previous posts, I would like for the customer to use three regulators instead of two.

Two regulators possibly out of step or three regulators possibly out of step?

A final comment and reasurance on unbalances and out of step regulators:
I have used the open delta auto-transformer connection and seen it used mainly for motor loads. As you know motors do not tolerate unbalanced voltages well. I have never seen a motor voltage unbalance issue introduced by the open delta auto-transformer connection.

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

 

[stevenal]

If you want two or three single phase regulators to step together, you would need the controllers to be configurable in a master slave fashion. A three phase regulator has a single control with the individual phase tap changers mechanically linked.