Lecture 32 Electro Mechanical System 1

Assignment 9

Page 373 Problems 16-13, 16-16, and 16-19

Due Date: Tuesday 19th April, 2011

Quiz No.5 Next Week

Quiz 5

Lecture 32 Electro Mechanical System 2

Regulation curves When a single synchronous

generator feeds a variable load. We are interested in knowing how

the terminal voltage E changes as a function of the load current I.

The relationship between E and I is called the regulation curve.

Regulation curves for the 36 MVA, 21 kV, 3-phase generator are plotted.

They are given for loads having unity power factor, 0.9 power factorlagging, and 0.9 power factor leading.

The change in voltage between no-load and full- load is expressed as a percent of the rated terminal voltage. The percent regulation is given by the equation:

% regulation = [(ENL – EB)/ EB] x 100

Where: ENL = no-load voltage [V] EB = rated voltage [V]

Lecture 32 Electro Mechanical System 3

Synchronization of a Generator Often two or more generators are connected in parallel to supply

a common load in large utility systems build up during the day, generators are successively connected to supply extra power.

Connecting a generator to other generators is called paralleling. Many paralleled generators behaves like an infinite bus. Voltage and frequency are constant and can not be easily

altered.Before connecting a generator to an electrical grid, it must be

synchronized. A generator is said to be synchronized when it meets all the following requirements:

1) The generator frequency is equal to the system frequency.2) The generator voltage is equal to the system voltage.3) The generator voltage is in phase with the system voltage.4) The phase sequence of the generator is the same as that of

the system.

Lecture 32 Electro Mechanical System 4

To synchronize a generator, we proceeds as follows: Adjust the speed regulator of the prime

mover so that frequencies are close to the system frequency.

Adjust the excitation so that generator voltage and system voltage are equal.

Observe the phase angle by means of a synchroscope, which indicates the phase angle between two voltages, the pointer rotates proportional to the frequency difference a zero mark indicates a zero degree phase angle the speed regulator is adjusted so that the pointer barely creeps across the dial.

On the zero mark, the line circuit breaker is closed.

Synchronization of a Generator

Lecture 32 Electro Mechanical System 5

We seldom have to connect only two generators in parallel. It is common to connect a generator to an infinite bus that

already has many alternators connected to it. An infinite bus is so powerful that it imposes its own voltage and

frequency upon any apparatus connected to its terminals. Once connected to an infinite bus, the generator becomes part of

a network comprising hundreds of other generators that deliver power to thousands of loads.

It is impossible, therefore, to specify the nature of the load (large or small, resistive or capacitive) connected to the terminals of this particular generator.

What, then, determines the power the machine delivers? We must remember that both the value and the frequency of the

terminal voltage across the generator are fixed. We can vary only two machine parameters:1) The exciting current Ix

2) The mechanical torque exerted by the turbine

Synchronous generator on an infinite bus

Lecture 32 Electro Mechanical System 6

After we synchronize a generator to an infinite bus, the induced voltage EO is equal to, and in phase with, the terminal voltage E.

There is no difference of potential across the synchronous reactance and, the load current I is zero.

The generator delivers no power; it is said to float on the line.

Effect of varying the exciting current

Lecture 32 Electro Mechanical System 7

If we now increase the exciting current, the voltage EO will increase and the synchronous reactance will experience a difference of potential Ex given by

EX = EO – E A current / will therefore circulate in the circuit given by

I = (EO – E)/XS

The synchronous reactance is inductive, the current lags 90° The generator sees the system as if it were an inductive reactance. When we over-excite a synchronous generator, it supplies reactive

power to the infinite bus. it is impossible to make a generator deliver active power by raising

its excitation.

Effect of varying the exciting current

Lecture 32 Electro Mechanical System 8

Decrease the exciting current so that EO becomes smaller than E. Now, phasor Ex = EO – E becomes negative and points to the left. Current I = Ex/Xs lags 90° behind Ex. This puts current I 90° ahead of E, which means that the alternator

sees the system as if it were a capacitor. When we under- excite an alternator, it draws reactive power from

the system. This reactive power produces part of the magnetic field required

by the machine: the remainder is supplied by exciting current Ix

Effect of varying the exciting current

Lecture 32 Electro Mechanical System 9

Let, generator is floating on the line, EO & E are equal & in phase. If we open the steam valve of the turbine driving the generator, the

immediate result is an increase in mechanical torque. The rotor will accelerate and, consequently, EO will attain its

maximum value a little sooner than before. Phasor EO will slip ahead of phasor E, leading it by a phase angle Voltages having the same value, the phase angle produces a

difference of potential Ex = EO – E across the reactance. A current I will flow (lagging 90°), but it is almost in phase with E. The generator feeds active power into the system. Difference of potential is created when two equal voltages are out

of phase. A potential difference of 4 kV exists between EO and E.

Effect of varying the mechanical torque