AC Electrics -Alternators — Page 206, Lesson 216

I want to walk you through how real load sharing works in a paralleled AC alternator system, and then we’ll start looking at reactive load sharing. Let’s start with the hardware. In the load sharing loop, we have current transformers — one for each generator. When current flows through these transformers, voltage is induced in them, and that causes a current to flow in what’s called the Load Sharing Loop. Each current transformer is connected in series with the others in that loop. And here’s the key: each current transformer also has an Error Detector wired in parallel with it. Now, let’s assume we’ve reached a condition of balanced load — that’s the starting point shown in Figure 12.12. In that balanced condition, the current output of each current transformer is 5 amperes. Because they’re all equal, no current flows through any of the error detectors. Everything is in equilibrium. Now let’s introduce an imbalance. Imagine that the drive unit of Generator No. 1 increases its torque output. That means Gen 1 will take a bigger share of the total load, and the other two generators will decrease their share by a proportional amount. So the output of the No. 1 current transformer rises to 7 amperes. To keep the average current in the circuit at 5 amperes, the outputs of the No. 2 and No. 3 transformers drop to 4 amperes each. Here’s where Kirchhoff’s first law comes in. Kirchhoff’s first law says that the sum of currents entering a junction equals the sum leaving it. In this circuit, the difference between each current transformer’s output and the average current — that 5-ampere average — gets pushed through the error detectors in opposite directions. So for Gen 1, the error detector sees a positive difference; for Gens 2 and 3, it sees a negative difference. That error signal gets amplified and sent to the speed governors. The signal tells the Constant Speed Drive Unit — the CSDU — for Gen 1 to reduce torque, which means reduce speed. And it tells the CSDUs for Gens 2 and 3 to increase torque, to increase speed. This adjustment continues until the current in each transformer is once again equal — back to that 5-ampere balance — and the real load is once again balanced. So the key takeaway: real load sharing is controlled by matching CSDU speed, which is the same as matching torque. Now let’s move to reactive load sharing. The reactive load sharing circuit looks very similar to the real load sharing circuit, and it works in a similar fashion. But it is a completely separate circuit. The sensing of out-of-balance loads by the current transformers works the same way. However, this time the error detector needs to know the difference between the reactive loads carried by each generator — not the real power loads, but the reactive component. That’s where we’ll pick up next.