automated load tap changers


Thread Starter

souvik ghosh

I have two general questions:

1. What are the effect on automated load tap changers in case it is connected to inverters which are being fed by PV Cells when

a. There is no power on the utility side and an excess power generation on the PV side?

b. There is less generation on the PV Side and a very high demand on the utility side?

2.Which side of the transformer in this tap changer connected to? Is it the PV/Primary/Low Voltage Side or it is the Utility/Secondary/High Voltage Side?
What is it that you think an automated load tap changer does, and how do you think using an automated load tap changer will enhance or improve the output of PV cells?
Souvik Ghosh... following are answers to your questions:

(1a) Typically Grid-connected PV-systems are shutdown upon detection of Grid-loss!

(1b) If the voltage on the Grid-side is too high, the PV-side is shutdown!

2) If the Xfmr is 2-winding, the Tap-Changer can be connected to the Grid-side winding or the PV-side winding! If there are two PV-side banks, then the Tap-Changer is usually installed in the Grid-side winding!

Phil Corso
souvik ghosh,

I was very sincere in asking what you thought an automated load tap changer does. My understanding of a load tap changer is that it is a device that is capable of changing transformer turns ratios while current is flowing through the transformer winding(s) and through contacts ("taps"). Current is directly proportional to real power, KW. Many transformers have adjustable taps, but they can't be moved while current is flowing through the transformer. Such transformers have to be isolated in order to change their taps.

By increasing or decreasing the transformer turns ratio one can adjust the voltage change through the transformer (usually slightly), but has little effect on the load (real power, KW) passing through the transformer. It's a kind of a coarse voltage adjustment; each tap changes the voltage by a specific amount, so the changes are kind of like "bumps" in the voltages when the taps are moved.

If used as a main transformer (usually a step-up transformer) on the output of a synchronous generator, changing the taps of transformer can affect the reactive current seen in the synchronous generator's stator windings.

"Load tap changer" is kind of a misnomer, in that it doesn't mean that the load (real power, KW) can be changed by any appreciable amount. It just means that the transformer taps can be changed while current is flowing through the transformer.

An "automated" load tap changer is usually a load tap changer that is adjusted by some controller that is set to maintain some setpoint (voltage; VArs; power factor) automatically (without human interaction).

Your question prompted a bit of research, because a PV "cell" produces DC (direct current) and to connect them to an AC grid requires a power inverter to change the DC to AC (alternating current). From the limited research I did it seems most PV power inverters "generating" AC and connected to an AC grid operate at primarily unity power factor (zero VArs), and it seems that unless there is some very special electronics the PV inverter "generator" can't change/vary the power factor.

I even found one article that claimed that PV power inverters are causing utilities to have to produce more VArs (operate their synchronous generators at lower power factors--lower efficiencies), which means they are making less money. (Utilities don't directly bill most of their Customers for VArs, though it is factored in to their KWh rates.)

So, I'm still curious what it is you think an automated load tap changer does and how it can be used to improve or enhance the output/performance of PV cells connected to an AC grid through power inverters. The purpose of my question was--and still is--to try to understand how you think they work and how you think they might work with PV cells connected to an AC grid so that I, or someone else here on, might be better able to respond to your query and provide useful information. For my part, at least, I don't understand how a load tap changer on a transformer connected to the output of a power inverter converting DC from PVs to AC can enhance or improve or appreciably affect the PV cell or power inverter. If you can help us understand your question(s) better, we can probably provide better answer(s).

I have little direct experience with PVs and power inverters so there may be something really important or germane to this "generator" and it's operation that I'm not familiar with and so could learn from this thread. The little research I've done has piqued my interest, since PV power connected to AC grids is becoming more and more commonplace, and seemingly can begin to have something of a negative impact on grid operation, and even stability.

Looking forward to your clarification(s)!