Technology

Understanding Generator Load Sharing and the Power Factor

11 June 2021By John Pierce
Bugsy Gedlek/Boat International

Written by

John Pierce

Chief Engineer John Pierce last worked aboard a 58-meter sailing yacht and has worked as both a second and chief engineer on motor and sailing yachts during his yachting career. He has completed several refits on 50-meter-plus vessels. He holds a Chief Engineer Unlimited CoC.  If you have any questions or suggestions, email him at johnllewellynpierce@hotmail.com.

Chief Engineer John Pierce tackles the power factor concept to explain the intricacies of real, reactive, and apparent power and their applications in an engine room.

Many of my fellow yacht engineers have told me they do not understand the concept of power factor. They may be perfectly happy overhauling a compressor or internal combustion engine, but an attempt to discuss real, reactive, and apparent power with regards to generator load sharing brings out the comment, “I don’t understand it because I can’t see it.”

I’d like to try to make the subject a bit clearer without going down the mathematical trail as this is not an electrical engineering textbook, but I’d like to at the same time improve the reader’s understanding.

Grow Your Understanding

So why is it important for a yacht engineer to understand power factor with regards to load sharing? As a watchkeeper, the engineer must ensure that if two generators are running in parallel and sharing load, attention is paid not only to the real power (kW) but also the reactive power (kVAr). It is important that both are shared equally. Otherwise there is the risk of a breaker tripping and disconnecting a generator from the switchboard, which could lead to a dangerous situation, particularly during maneuvering.

Now remember that the current produced by a generator is directly proportional to the apparent power (kVA) it generates, not the real power. The apparent power is the sum (the vector sum to be exact) of real (or active) power and reactive power.

Here is an example from my own experience that illustrates the importance of this understanding. Some years ago, I took over as chief engineer on a 56-meter motor yacht with two Leroy-Somer generators with Scania prime movers. It was an old boat built in the early 1980s and synchronizing and load sharing was done manually. The amount of kW load (real power) taken by each generator was controlled by manually adjusting the governor settings by turning dials. However, there was no provision on the switchboard to adjust the kVAr (reactive power) load sharing. You may know that the amount of reactive load each generator takes depends on the level of excitation provided to the magnetic field of its rotor coil. The level of excitation provided to the rotor coil is controlled by the generator’s Automatic Voltage Regulator (AVR). If more excitation is required, the AVR causes more current to be passed through the rotor coils, increasing the strength of the rotor magnetic field (excitation). In turn, this increases the voltages generated by the stator coils, which are the generator three-phase output voltages.

During the handover, the previous chief engineer informed me that when the generators were running in parallel and the bow thruster was operated (the bow thruster was powered by a large electric motor), the breaker for the starboard generator would often trip. He was unsure of the cause, as the load was shared equally between the generators and was not excessive. He informed me that he was in the process of ordering a new generator from Leroy-Somer. I felt that the cost of a new generator was excessive when the cause of the problem was not even known.

I proceeded to investigate by running up the generators in parallel and operating the bow thruster. I noticed that even with the kW meters for each generator showing equal load, the starboard generator ammeter was showing a large current while the port generator ammeter was showing hardly any. This was the reason for the starboard generator breaker tripping issue — it was tripping on overcurrent.

Now remember that the current produced by a generator is directly proportional to the apparent power (kVA) it generates, not the real power. The apparent power is the sum (the vector sum to be exact) of real (or active) power and reactive power. See the vector triangle below.

The obvious cause of the problem in this case, since the kW loading was being shared equally, was that the reactive power (kVAr) was NOT being shared equally. The starboard generator was carrying excessive kVAr load and therefore a much greater apparent power and therefore producing a much higher current.

I located the AVR for the starboard generator and there was no need to test it as it had started to disintegrate, probably due to age, heat, and vibration. I contacted Leroy-Somer and a technician came out with a new AVR and installed it. It was not a matter of swapping it in place of the old one as the droop of the AVR had to be set up, which took up a few hours of work.

Note that in this case the starboard generator AVR turned out to be the cause of the problem. It was causing excessive magnetic excitation in the rotor of the starboard generator when running in parallel with the port generator, and therefore the starboard generator was taking more than its share of the reactive load. However, it could have been that the port generator AVR had been the cause of the problem, causing this generator to not “pull its weight” with regards to reactive power load and forcing the starboard generator to take the load.

Inductance

A reactive power load is the result of the property of “inductance” in the yacht electrical loads. Inductance is an electrical property of wire coils. A large part of a yacht’s electrical load is electric motors, and all electric motors utilize wire coils, which have inductance. It’s an electromagnetic effect. Transformers are also made of wire coils and are inductive loads.

Inductance is a phenomenon only seen in AC circuits and not DC circuits as the voltage is constantly changing in an AC circuit. Remember that a current cannot flow through a conductor without a voltage across it to “push” the current through the conductor. An AC voltage is constantly changing, growing, and then collapsing back to zero, then growing and collapsing in the opposite direction. Applying such a voltage across a wire coil causes a changing current to flow through the coil. This current generates a changing magnetic field around the wire coil. As English chemist and physicist Michael Faraday found out from his experiments, this changing magnetic field in turn generates a voltage in the coil that OPPOSES the applied voltage.

This opposing voltage has the effect of delaying the flow of current through the coil and therefore the current is said to lag behind the voltage. In a pure inductor (a pure inductor has no resistance, only inductance), the current lags the voltage by 90 degrees. By the way, this is the reason why there is a 90-degree angle between the reactive power and the real power vectors in the power triangle. This 90-degree lag can be seen in the AC curves shown below. Note how the blue curve representing current is 90 degrees behind the green curve representing voltage (diagram below).

Power Factor Correction

On some more modern boats, reactive power load has been largely eliminated by power factor correction. I was recently working as chief engineer on a brand-new 58-meter sailing yacht. This was a completely different story to the old 56-meter motor yacht I described before. On this new boat, synchronizing of the generators was purely automatic. There was no provision for manual synchronization. Most of the electric motors were driven by electronic frequency drives with power factor correction. This compensated for the inductive effect of the wire coils in the electric motors, so that the power factor was practically 1. Remember that power factor is real power divided by apparent power and for a three-phase induction motor is about 0.8. This yacht had power factor meters for each generator and in spite of all the automation and power factor correction, it was still vital to monitor the power factor on each generator and record it on the daily log. This information would indicate any problems in the system if either of the generator power factors started straying away from 1, whether they were running alone or in parallel.

An interesting fact is that in an AC circuit, a capacitor has the exact opposite effect of an inductor. A capacitor causes the current to LEAD the voltage by 90 degrees. Capacitors can be installed in inductive circuits for power factor correction.

This article will not be complete without a brief description of Cos φ. You may have heard of power factor being referred to as Cos φ. When considering the right-angled power triangle, φ (a Greek letter pronounced “phi”) is the angle between the real power and the apparent power vectors. Remembering your school trigonometry, the cosine (or Cos) of φ is the real power divided by the apparent power. Which, as we have seen before, is the definition of power factor. If there is no reactive power load, then the real power and apparent power are the same and the power factor is 1, which is the ideal scenario.

This feature is taken from the November 2020 issue of Dockwalk.

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