Effect of LTC on utility voltage

[AdmiralSnackbar]

I'm hoping for some clarification on the following:

In classical transformer theory, an ideal transformer will behave in the following way v1=v2*(n1/n2). So if the voltage on one side of the transformer goes up due to an increase in the number of windings, the voltage on the other side will increase. However, in performing powerflow simulations, I notice that an increase in voltage on one side of the transformer will decrease the voltage on the other side, specifically when looking at a 115/69 kV transformer with an LTC on the low-side. Can someone please explain this phenomena to me? I've been told that an LTC can't change a strong high-side utility voltage so the low side voltage will decrease but i'd like a little more background on the theory behind that.

Thanks!

AdmiralSnackbar

 

[davidbeach]

In increase in the voltage (system) or an increase in the tap voltage. Very different.

If I have a 115kV-13.22kV transformer (8.7:1 ratio) and I connect that to a system running at 120kV, I have a low side voltage of 13.8kV. Then, if I raise the tap to 120kV, I've changed the ratio to 9.08. But my system voltage didn't change so my low-side goes down to about 13.2kV.

 

[bwen08]

An online LTC is a voltage regulating device. This is designed to maintain a set voltage within a bandwidth, i.e. 120V +/- 2V. An increase in the voltage on the transformer's high side winding will be observed by the LTC controller. The controller will attempt to correct this undesired voltage causing the LTC to tap and return the voltage within the desired range.

To expand on davidbeach's comment, the LTC is connected to the low voltage windings and changes the transformer's ratio. A transformer may have as many as 165 different ratios, for example, 5 taps on the highside and 33 taps on the low side.

 

[crshears]

The OP wrote:

In classical transformer theory, an ideal transformer will behave in the following way v1=v2*(n1/n2). So if the voltage on one side of the transformer goes up due to an increase in the number of windings, the voltage on the other side will increase.

As I've stated before, higher maths are not my strong suit, but something in the second sentence does not seem to comport at all well with the equation given in the first...

My prose analysis of the equation above would be given thus:

1] If the turns ratio of a given transformer remains fixed, an increase in primary voltage will be reflected by an increase in the voltage on the secondary, and

2] if the number of turns on the secondary is increased, its output voltage will rise while the input voltage remains unaltered.

Also:

I've been told that an LTC can't change a strong high-side utility voltage so the low side voltage will decrease but i'd like a little more background on the theory behind that.

What I've seen in years of power system operation is that not only will an underload tapchanger's voltage regulation scheme respond to secondary voltage drop due to an increase in load by tapping up to compensate but, as davidbeach and brianr134 alluded to, it will also see increases in secondary voltage due to pass-through increases in primary voltage and tap down.

Our IESO often directs my utility to operate specific ULTC's in 500/115 kV autotransformers to a higher tap than would normally be called for as a way to limit the occurrence of high voltage on the 500 kV system during light load / overnight periods, even if it means pushing the 115 kV system voltage to its upper limit as a consequence. That being the case, the term "strong" is a relative one.

Another example that may be helpful is the placement of low-tension capacitors [28 or 44 kV] in service at a substation; as load increases, the system operator will observe a decline in grid voltage, and direct the transformer station operator to place reactive resources [often static capacitors] in service. The operator will place the ULTC in manual control, lower the LT bus voltage to its lower limit, then place a cap in service, which typically drives the LT bus voltage to its upper limit. The TS operator then taps down the ULTC one more tap to bring the voltage to within the normal bandwidth before returning control of the ULTC to auto.

As a result, the HT system voltage on that portion of the grid will rise, the effect being inversely proportional to the impedance of the system equipment. IESO's therefore direct the deployment of reactive resources based on geographic / electrical topology frames of reference.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[AdmiralSnackbar]

Thank you. This has really helped, and yes, I misspoke regarding the equation I listed when trying to convey my question.

Thanks again.

AdmiralSnackbar

 

[cranky108]

crshears, On a classic transmission/distribution transition, the transformer and LTC will act like what mr Beach said. However in a transmission network, the LTC will act to redirect var flows by changing the effective ratio of the transformer. In this case it can be used to make some changes to the voltage on both sides of the transformer, by redirecting var flow.
The assumption here is that transmission voltage is not that critical as long as it stays in a bandwith, because the transmission/distribution transition LTC will bring the voltage into line with the customers needs.

 

[crshears]

Mostly agreed, cranky108; but for the purposes of this thread I didn't want to drag extraneous matter into the mix.

The reason I only mostly agree is that if a static cap is placed in service on the LT bus of a "classic transmission/distribution transition," the VAR flow through the winding of the supply transformers behave in exactly the same way as autotransformers in a transmission network, such that if the reactive output of the cap exceeds the total reactive demand of the station load, the balance of the reactive will flow from the station out onto the system, which causes the local system voltage rise alluded to in my earlier post.

For that matter, the situation in a transmission network is sometimes not just analogous, but identical, except for the higher voltage levels; if a 230 kV cap is placed in service at a transmission station with 500/230 kV autotransformers and the reactive support provided by the cap exceeds the reactive demand of the 230 kV system load, the VARs will flow out into the 500 kV network.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]