Tuesday, May 17, 2016

Diesel generators

Sometimes at my job I work with diesel electric generators. As a testing engineer I perform inspection and test of it. For testing we use a mobile load bank. Our mobile load bank is just resistive and it looks like giant heater. It has bank of heater and fan. Its maximum load is 1MW. 

Diesel generators aren’t complex machines. They have two parts. One is a diesel engine and second is an electric generator. 

From electrics and electronics stuffs a diesel engine has lots of sensors. The most typical is for an oil pressure and a temperature, a coolant level and a temperature and a pick-up for speed. 

A generator gives the sin wave output voltage with specified voltage amplitude and frequency. In our case this is always 400V between phases and 50Hz. 

The generator frequency depends from a diesel engine speed. We control frequency by controlling the diesel engine speed. Because most of generators are four pole electric machines we need 1500 rpm for frequency of 50Hz. 

Most of diesel generators that we test are older than me. This old diesel engines have fixed fuel injection. Because of this its frequency varies with changing a load. As we gain load the frequency goes down. Most of old diesel generators can take no more than 80% of load and for this load frequency goes down for 1Hz. 

New generation of diesel generators has an electric control fuel injection and them frequency is perfect 50Hz all the time. Just when we apply high jump of load the frequency changes a little. This change depends from a PID controller for speed control. When we apply high load, for example form 0% to 75%, frequency goes down because of suddenly high load. Depends of a PID controller it can be slow comeback to the 50Hz without over frequency or quick come back to the 50Hz with some peak of over frequency. 

For a generator voltage control a generator has voltage regulator. A generator is synchronous electrical machine and its output voltage depends from a magnetic field in a rotor. Voltage regulator is uses to control that magnetic field. It works as controlled AC DC converter. With DC voltage output it controls magnetic field in a rotor. 

Also as frequency, old diesel generators has fixed voltage regulator. Output voltage changes with changing frequency. In some case voltage goes down when frequency goes down and in some cases are oppositely. 

New generation of diesel generators has an electric controlled voltage regulator and output voltage is always perfect 400V. 

I reconstructed one voltage regulator and this interesting electronic story will be in the next post. 

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