Synchronization of an Alternator to a Power Grid

The setup in this virtual laboratory can be used for the following experiments:

DC Machine starting and speed control

Control of AC-DC converters for electrical machine control

Effect of load and excitation control on synchronous alternator output voltage.

Principle of synchronization of a synchronous machine to the power grid.

The target groups for these experiments are:

Senior Under-Graduates (Electrical Engineering)

Post-Graduates (Power Electronics, Power Systems, Electrical Machines and Drives)

Practising Power Engineers

Before we begin our experiments, let us list out the requirements on the user side

Web browser : Mozilla Firefox(3.0+), Internet Explorer(6+), Konquerer
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A lot of enthusiasm !

General Description of the Setup

The setup consists of a controlled prime mover (DC motor - separately excited) coupled to a synchronous generator which is to be connected to a power grid. The synchronous generator is excited by a controlled AC-DC power electronic converter. A separately excited DC motor is used as a prime mover. The armature of the DC motor is fed from a (controlled) three phase thyristor rectifier. The DC voltage to motor field is also provided by a similar controlled rectifier. All the interconnections of the components are made inside the panel. A measurement card is enclosed inside the panel, which is used to measure and scale down the voltages and currents of the motor and generator.

Equipment Description and Ratings

Synchrnous generator : 1 kW, 1.25 kVA, 3 phase, 415 V, 1500 rpm, 0.8-0.9 PF, 50 Hz, cylindrical Rotor, Excitation max. 110 V without damper bars shaft extension on both sides

DC motor : 3 HP, 220V, 1500rpm, shunt-wound, foot mounted, self fan cooled.

AC-DC Converters for (a) DC machine armature (b) DC machine field and (c) Synchronous Machine Exciter

Load Bank : Three switchable load banks are provided each having a rating of *1440 W, 166VAr, 3ph*.

Operating System : Presently, we are using QNX neutrino 6.3 version. QNX is used to run the programs in hard real time. In our set up QNX is used to generate control signals to hardware devices like 3 Phase Thyristor full bridge rectifier,etc.
QNX is a commercial Unix-like real-time operating system, aimed primarily at the embedded systems. As a microkernel-based OS, QNX is based on the idea of running most of the OS in the form of a number of small tasks, known as servers. This differs from the more traditional monolithic kernel, in which the operating system is a single very large program composed of a huge number of "parts" with special abilities. In the case of QNX, the use of a microkernel allows users (developers) to turn off any functionality they do not require without having to change the OS itself; instead, those servers are simply not run. This gives the QNX a capability to run programs in real time based on priority.
Linux is an UNIX based operating system. It is not a real time operating system like QNX. In our set up Linux machine serves as a interface between the QNX system, which controls the hardware, and the user. The server is set up on this Linux machine which sends and receives data from the user to QNX system and vice-versa

Voltage and current measurement card : This card takes the voltages and currents as inputs and converts them into digital format. The card can measure following :

This voltage in digital format then goes to PCI card.

PCI card : Advantech 1710 HG

Before you proceed for the actual experiment it is useful to revise at the theory behind the experiment. This section will take you throgh the topics which you should be aware of before going further. You are directed to references for more exhaustive coverage.

We list the various steps to perform the experiments. You will be led from one step to another when you are actually doing the experiment (there will be adequate flexibility for you to try out some safe variations on your own). However you can get a flavour and some idea what to expect by reading about the same first. It is strongly recommended that go go through these steps before actually entering the "Perform Experiments" page, the link for which is given below. First identify the various components in the setup:

Synchronous and DC machines

AC-DC convertors for (a) DC machine armature, (b) DC machine field and (c) Synchronous Machine Exciter

Slip-rings and commutator of the machines.

Field failure relay

Synchronization Lamps

Load (lamp load)

Three Phase Inductor (representing system reactance) between machine and grid.

Voltage and current measurement Card

Hall effect Current sensors

Digital Control Module, PCI card

Switch ON the main power supply by pressing the appropriate button on your menu.

Switch on the ac supply to the power electronic controller supplying power to the field winding of the motor. Using the knob in the menu, apply the rated voltage to the field winding. The field failure protection relay will allow you to energize the armature winding only when the field is energized. You will hear the engaging of the relay as soon as sufficient voltage is applied to the field.

Switch on the ac supply to the power electronic controller supplying power to the armature of the motor. Using the knob on the controller, slowly increase the voltage to the armature till the rated value. Also, apply the rated voltage to the field winding of the generator.

Note down the meter readings, speed and direction of rotation. Reduce the voltage applied to the armature of the motor to zero. Put switch in position B to interchange the connection to the DC armature winding. Run the machine again and note reversal of direction.

Bring speed back to zero. Now energize switch to supply the field winding of the synchronous generator.

See how the excitation voltage is controlled (you need not start rotating the prime-mover). Do not exceed the machine field current rating.

Manually adjust the control voltage of the static exciter to see the variation in field voltage. Observe the waveform of the field voltage on the scope.

Start the DC machine (prime mover) again by increasing armature voltage. Increase the speed of the prime mover gradually. Start the exciter of the synchronous machine. As speed increases observe the variation of the stator voltage if the exciter voltage is constant. Verify proportionality.

Using the power electronic controllers supplying power to the field and armature of the DC motor, vary the speed. Get the speed near 1500 rpm so that when the synchronous generator field is energized, voltage of 50 Hz will be induced at the armature terminals of the synchronous machine.

Load the machines with a lamp load. Note down the variation in terminal voltage and speed.

Note down the voltage magnitude versus the load in Watts.

Unload the machine by switching OFF the loads. Observe the change in speed while doing the same.

Go into to "auto" mode wherein the voltage of the synchronous generator is maintained constant by using feedback control.

Keep the set point of voltage at 100 V and activate.

Give a step change of 100 V. Observe the armature voltage and speed of the synchronous machine.

Repeat the loading of the synchronous machine and see whether voltage is regulated or not.

Monitor the field current and voltage dueing the step change and loading process.

Check if the phase sequence is correct. For this you need to see the pattern of glowing of lamps connected between the terminals. If all of them glow and fade together then phase sequence is correct. If they are glowing one after the other then phase sequence is incorrect. If it is incorrect correct then go to second step. Otherwise go to third step.

Alternative 1 : Phase sequence reversal (direction of DC machine). This will require a shutdown of the synchronous and dc machine.
Alternative 2 : Interchange the wires connecting any two phases (this is done by pressing the DPDT switch).

Remember you have to see the system voltage and frequency to which you are going to synchronise and then adjust the frequency and voltage of the alternator. Control frequency by controlling speed of the the prime mover. Also adjust the armature voltage by controlling excitation control.

Specify the maximum synchronization angle (between +/- 10-20 degrees) and press the "enable synchronization signal". The machine should synchronize during the dark period of the lamps.

Ramp up dc motor speed (i.e. prime mover power) and field excitation. Observe the "system frequency" as you do this.

Also vary the load to see the changes in the frequency. Compare this with the change you have observed when only single machine was handling the load.

Shutdown Sequence for Synchronous Machine

De-synchronize the alternator (in case it is synchronized to the grid) by reducing the alternator stator current to zero. This can be done by -
adjusting the prime mover input to reduce real power
and/or
reducing the field current (by changing voltage reference to the automatic voltage regulator if you are in "Auto" mode) to change reactive power.

Switch OFF the grid connection once the current is zero.

In case the machine is not synchronized to the grid but loaded by lamp load, switch off the lamp load in steps.

Reduce Field current to zero (this can be done in "auto" mode or in the "maual mode").

Switch OFF the synchronous machine switch.

Shutdown Sequence for DC Motor

Reduce armature voltage of DC machine to zero (which will reduce speed to zero). Switch OFF the armature voltage switch.

Reduce field voltage of DC machine to zero (you will hear the field failure relay disengage). Switch OFF the field voltage switch.

Don't reduce field supply to zero when the armature voltage supply is ON. A large amount of current will flow into the armature which will damage the winding.

Switch OFF the main power supply by pressing the approriate button on your menu.

Perform the Experiment

Having performed the experiments it is natural to ask are there alternatve ways of doing this? Or are there associated concepts which may be learnt, but which were not apparent from the procedure above. This tab describes some of the alternatives possible.

DC motor speed-torque characteristics and efficiency calculations

Asymmetry in no-load speed of DC machine in different directions (compensation of armature reaction)

Transient responses : estimation of machine inertia. This can be determined by plotting speed decay vs time as the unloaded machine is suddenly switched off.

Obtaining resistance of armature and field of DC motor

Obtaining V curve for a synchronous machine.

Obtaining field resistance of synchronous machine

Estimation of field inductance of synchronous machine. (This can be done by plotting the respose to the step response.)

Calculation of maximum power that can be supplied by the alternator if the excitation is fixed.

P controller replaced by PI controller in voltage regulator (what happens to steady state error) of synchronous machine

Basic Electrical Machinery

I Nagarath & D P Kothari, Electric machines, Tata Mc Graw hill. Year

A. E. Fitzgerald, Charles Kingsley,Jr., Stephen Umans, Electric Machinery, Sixth edition, Tata Mc-Graw Hill, 2003

Power Systems operation and Control

O.I Elgerd, Electric energy systems theory-An Introduction, Second edition, Tata McGraw Hill, 1982

A.R.Bergen and V. Vittal, Power Systems Analysis, Pearson Education Asia, New Delhi, 2002

P.Kundur, Power System Stability and Control, MGraw Hill, 1993

Power System Operation and Control, NPTEL Web Course

AC-DC Converters

N. Mohan, T.M. Undeland W.P.Robbins, Power Electronics: converter, Applications & design, John Wiley & Sons,1989

M.H.Rashid, Power electronics, Prentice Hall of India, 2004 B.K. Bose Power Electronics & A. C. Drives, prentice Hall, 1986

Digital Control and Implementation

G. Franklin, J.D. Powell and M. Waterman. Digital Control of Dynamic Systems, Third edition, Pearson Asia, 2003