Month: February 2024

This article Paralleling two synchronous AC generators presented phasor diagrams for two synchronous generators in parallel, and Paralleling two synchronous AC generators – simple AVR discussion discussed a simple automatic voltage regulator system that can be used to share two identical machines.

To recap, the first configuration presented follows.

Above is a phasor diagram of two machines of the type discussed in parallel. This is just after ACB closure, and they are not well adjusted in this case to allow visibility of the various phasors. Continue reading Paralleling two synchronous AC generators – phasor diagram with AVRs

Paralleling two synchronous AC generators explained synchronisation and parallel operation of two generators, discussing sharing of real and reactive power.

In that discussion, the engines were speed governed, but voltage was manually controlled by adjusting excitation current. The phasor diagram showed that there is a relatively large source impedance (dominated by the synchronous reactance), EMF is around 1.5 times Vt, and terminal voltage Vt will vary widely with changes in load as a consequence. Continue reading Paralleling two synchronous AC generators – simple AVR discussion

This article discusses adding a second synchronous AC generator in parallel with a running synchronous AC generator.

For explanation, a simple context is used, two identical diesel engine driven generators with governed speed, no droop compensation, otherwise largely manual control, simple equivalent circuit of the machines. The complexity of other factors like  harmonic currents etc are not discussed.

The simple model is useful as it underlies practical machines and is a good configuration to introduce the main principles of the operation.

To avoid typos, some of the text uses Python formatting of complex numbers.

Prerequisites for paralleling three phase synchronous generators

Prerequisites for paralleling three phase machines are:

1. same phase sequence (eg both A-B-C);
2. nearly the same voltage; and
3. nearly the same frequency, and eventually same phase.

(1) does not apply to a single phase machine. (1) is resolved at the time of wiring up the machines and circuit breakers used to connect them to the bus. Unless changes have been made to machines or wiring, sequence should not need to be checked. After any reconfiguration or wiring changes phase sequence should be checked by a competent person.

Phase sequence is checked using an instrument like one of these above. The left one is a small three phase motor, the right one is electronic and shows sequence with an LED. Continue reading Paralleling two synchronous AC generators

This article offers a phasor diagram for one phase of a synchronous generator and explains the diagram.

System characteristics:

• Load: vt=240, s=20000VAr; pf=0.85; and
• Generator (s): s=20000VAr; pf=0.9; ra=0.04pu; xs=0.7pu.
  .

Above is the phasor diagram for generator #1 in a pair of parallel generators. Green is flux, cyan is armature current, magenta is voltage. Continue reading Synchronous generator – phasor diagram and explanation

An expression found in most texts on synchronous machines relates power to the power angle δ. δ is the angle between the terminal voltage and the generated EMF (under the applicable load current).

\(P=\frac{|E|\cdot|V|}{X_s}sin \delta\\\)

V and E are RMS values, albeit sometimes with phase.

This article gives a derivation that exposes the underlying assumptions that are not usually mentioned.

 

Above is the phasor diagram of the machine in generator mode. Continue reading Synchronous machine power formula