Generator’s synchronizing and what causes generators under voltage?

The idea of this post came from one of my followers, which contact me and ask for my advice and opinion with regard to one of his generator’s ACB “non close” alarm.

An alternative current power generating system comprises of a number of diesel generators, which transform diesel engine’s mechanical energy into electrical energy. Generally, electrical power can be produced by employing a single generator or by distributing the power among few generators according with the vessel’s power requirement.

The alternators are synchronized and connected in parallel and to do so, there are some electrical condition that must be fulfilled before being able to do that. There are permissible limits for voltage, frequency and phase sequence and angle deviations. Usually they are factory set and as follow:

  • Voltage: ± 5 %. The voltage value of the incoming generator must be equal or within mentioned limits with the one from the bus bar. If all other conditions are met but the voltages are not the same, there will be a voltage differential that can cause a large reactive power output (MVAR) flow. For example, if the generator voltage is higher than the bus bar voltage, when is connected to the bus bar the generator will be overexcited and will put out MVAR. If the generator voltage is less than the bus bar voltage, when is connected to the bus bar the generator will be under excited and will absorb MVAR.
  • Frequency: 0.1 to 0.5 Hz and before bus bar. The frequency of voltage produced by the incoming generator must be equal with the bus bar one. If the synchroscope would be rotating rapidly counter clockwise and the generator breaker were accidentally closed, the generator will be out of phase with the bus bar and will behave like a motor and the bus bar will try to bring it up to speed. Because of that, the rotor and stator would be slipping poles and destroy the generator by breaking the engine crankshaft or deteriorating the coupling between the engine and alternator. The same problem can occur if the generator is faster than the bus bar.
  • Phase sequence and angle: 50 max; ΔU = 40 V. If the incoming generator and bus bar are matching speed, the synchroscope will turn clockwise and in this case the high points and zero crossing of the voltage occur at the same rate of speed. This would appear as a non-rotating synchroscope (both generator and bus bar at the same frequency) with pointer stuck at about 12 o’clock. In this case the phase angle between voltage of the incoming generator and the one from bus bar is 00 .

In the case of under voltage, a protection relay will prevent the circuit breaker to close until the fault is eliminated. Possible causes of generator under voltage can be multiple, but the most common are: voltage sensing fuses failure, wire breakage fault, generator’s capacitors failure, current transformers failure, AVR failure, etc.

So, in order to find the root cause of the generator’s under voltage you should start with the easiest part by checking all the voltage sensing fuses to ensure that none of them have blown. If the fuses are ok, next should be checked the wire continuity to ensure that the signal is present from the generator to your main switchboard safety panel. Furthermore, if wires are ok and you have circuit continuity check the capacitors if your generator has any capacitors. The function of these is to induce voltage into the rotor. If there are no capacitors, check the automatic voltage regulator (AVR) for proper functioning and replace it as found necessary. The AVR is an electronic device that monitors the power output and controls voltage and often is the main reason of under voltage issues in the generator.

Besides AVR, if this is ok alternator diodes and a megger test of alternator winding need to be performed in order to check if there is any issues with it. Check for wire loose connections, breakages and water ingress traces (especially for water cooled alternators).

Moreover, current transformers must be checked to ensure that none of them are burned out.

In conclusion, in order to avoid generator’s breakdowns and damages proper maintenance and periodical checks must be carried out as per your company and manufacturer’s maintenance plan and instructions. Moreover, every engineer must be familiar with proper monitoring and how to manually synchronize the generator’s in case of automation failure or emergency situations.

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