Mode Of Generator

The only difference is which variable is the control variable, the other two adjust to keep the third at its setpoint; within, of course, the capabilities of the machine to produce and the grid to accept, those changes.

The AC current and voltage are sine curves that may or may not be exactly in phase. If not in phase, the current waveform can be broken down into two components, real current in-phase with the voltage and reactive current 90 degrees out-of-phase with the voltage.

The voltage times the real current is the real power (Watts). The voltage times the reactive current is the reactive power (var). The power factor (pf) is the real power divided by (voltage x current). It is unity when the current and voltage are in phase.

When you set the generator excitation different from the bus voltage, there is circulating current between the generator and bus caused by this voltage difference. This circulating current is reactive current.

If the initial power factor was initially 1.0 (voltage and current in phase), the power factor will go down due to the reactive circulating current.

First clear a misunderstanding! Even if the generator were bolted direct to the grid, its e.m.f. - behind its inductive reactance & resistance - can be different. Your statement only really applies if the generator is connected to the Grid by a transformer or reactor.

To simplify matters (an isolated system could have all the requirements of a major Grid)...

Voltage control would be applied if the generator were supplying a load alone or in parallel with similar generators, where the bus supplies loads directly - voltage control is needed to keep to acceptable voltage limits for the load. In theory, if all generators have same no-load voltage and same regulation, they will share the load.

Var control is applied when the Grid operator wants Vars to regulate the system voltage - on a small system with a limited load using Var control with setting exceeding the load Vars available from the load would lead to overvoltage.

Power factor control would be applied if the ratio of Vars to Watts was required to be a known ratio [ which enables the Grid operator to predict Vars for a given Watts]. On small generators [which may be less than e.g. 50 MW on a Grid] operation at unity power factor is usually a requirement - because the system operator does not want small units outside his control affecting the system voltage.

If you look at the power flow equation through an inductor relative to its terminal voltages and phase difference you will find that the flow of both active and reactive power depends critically on this simple feature of an AC power system - consequently power system operators always want to be in control of the voltages & Vars in their systems - despite customers mostly turning their loads on/off whenever they like.

I have known generators with Vars vs Volts control with adjustable slope, as well as the usual options you mention.

GA from me, and I'd just like to add that, depending upon the size of the grid, different units may be operating in different modes to satisfy the conditions local to the generation.

Controlling the Voltage, PF and VARs of a generator is based on, 1. the requirements of the load, 2. maximizing profits from doing the work of sending electrons down the grid and 3. the ease of reading Volt and Amps to get VARs. The latter voltage and amperage can be easily read with any meter, but the multiplying the two together don't always give power in Watts, hence the term VAR. This will be explained later.

The first two take a little more understanding.

1. As the load (current) increases it take more pressure to push the electrons down the line, this is initially seen as a drop in voltage which is increased to offset the drop. Hence, increasing actually just brings the voltage back up to the normal level which is usually done automatically to prevents brown outs.

2. Has to do with proper billing for service supplied. Although we think of Kilo-Watts (KW), what we charge the customer for is the work (Work=Force*distance) it takes to supply current, or push electrons down the wire. Work is measured in Joules (J). A Watt is a unit of power, the rate of doing work and is expressed as W=J/s or Power =work/time. Look at your electric bill, you are charged for KWH or Kilo-Watts*hrs. That is power (Work/time) x time. Time cancels and we are left with Work. OK?

Now here is the subtlety, most loads tend to be inductive in nature, motors, fans, etc. and cause the voltage to lead the current (basic inductor theory) this is a phase shift between the Voltage and Current and the more inductive loads coming on line increased this phase shift, decreasing the PF more and more. Since the power of the system in Watts=PF*VAR, the KW (power) reading is reduced which decreases the KWH value in turn reducing the $/KWH, the money to be paid to the supplier of all that work.

This may seem like manipulation but remember it is the work for pushing the electrons / current to the customer that the customer pays for, not power. So to insure accuracy and get full value for the work being done and is charge correctly, the generation unit will affect a PF change in the capacitive direction to pull the Voltage and current back in phase returning the PF to 1. Hence in a nut shell, this is why we bother with things like P F, Voltage and VAR. Hope I helped.

"Good judgment comes from experience. Experience comes from bad judgment " – Mulla Nasrudin