AC Electrics - Alternators 12

aircraft. The VSCF also incorporates a built-in test facility which can provide fault isolation information to the ground engineer.

Self-excited Generators

A self-excited generator is one which has some permanent magnetism in its exciter generator. On initial rotation, the flux from these Stationary Permanent Magnets causes an induced AC voltage and therefore current to flow in its rotor. The rotor output is then fed directly to a rotating rectifier which in turn supplies the rotating field coils of the main generator with a DC supply.

The output of the main generator stator is tapped to provide a regulated supply to the exciter field so enabling the voltage to be controlled.

Load Sharing or Paralleling of Constant Frequency Alternators

When running two or more constant frequency alternators in parallel they must be controlled in order that each one takes a fair and equal share of the load.

This “load sharing” or “paralleling” requires that two parameters are regulated:

•Real Load.

•Reactive Load.

Real Load

Real Load is the actual working load output available for supplying the various electrical services and it is measured in kilowatts (real power or true power).

Real Load is directly related to the mechanical power or torque which is being supplied to the alternator drive by its prime mover, i.e. the engine or CSDU.

Real Load Sharing is achieved by controlling the Constant Speed Drive Unit (CSDU) and adjusting the torque at its output shaft so that if the torque of the two or more CSDUs is equal then the real load taken by each generator is the same.

Reactive Load

Reactive Load is the so-called Wattless Load which is the vector sum of inductive and capacitive currents and voltages expressed in kVAR (Kilovolt-Amperes Reactive). Reactive Load Sharing is achieved by controlling the Voltage Output (Exciter Field Current) of each generator that is connected in parallel. If their voltages are identical then the reactive load on each generator will be the same.

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Parallel Connection

Alternators - Electrics AC 12

To control the real and reactive load when two or more generators are paralleled there are two separate load sharing circuits, one to detect and control real load and one to detect and control reactive load.

N.B. It must be stressed that until a generator is connected in parallel with one or more generators it will not be connected into the load sharing circuits. While constant frequency alternators are operating as individual units, such as at engine start when only one alternator may be on line, their real load and reactive load sharing circuits are not connected.

Before Connecting in Parallel

AC generators, or alternators, are synchronous machines which will lock frequencies when they are operated in parallel. The system frequency thus becomes that of the alternator with the highest load.

However, if the two alternators are at different frequencies before they are connected in parallel then damage can occur as one generator tries to slow down and the other tries to speed up, so they must be at the same frequency before paralleling.

As well as being at the same frequency they must also be of the same phase sequence, i.e. at any point in time, phase A, B and C on the first generator must be identical to phase A, B and C on the second generator. The voltage of each generator being paralleled must also be the same.

Figure 12.10 Conditions required before paralleling

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Layout of a Paralleled System

Figure 12.11 shows a diagram of the layout of a three generator paralleled system. Notice that for each of the three phases of the output there is a separate bus bar, for example the No. 1 generator bus bar (Gen Bus 1) is made up of three separate bus bars A, B and C for phases A, B and C. The generator is connected to its own bus bar through a 3 phase circuit breaker called the Generator Circuit Breaker (GCB), operated automatically or controlled from the flight deck. All the electrical loads of the aircraft are shared between the three generator bus bars.

To operate the generators in parallel they are connected through their respective generator bus bars to a synchronizing bus bar via a Bus Tie Breaker (BTB). A Bus Tie Breaker is a 3 phase circuit breaker controlled automatically or manually from the flight deck.

The synchronizing bus bar takes no electrical loads at all, it is only there to allow the engine driven generators to be operated in parallel. Ground power or power from the APU generator can be connected into the synchronizing bus and from there can be fed to the load bus bars through the BTBs when the engine generators are not operating and the GCBs are open.

			12
GENERATOR	GCB	GCB	Alternators
CIRCUIT			-
BREAKER NO.1	NO.2	NO.3	ElectricsAC
BUS TIE	BTB	BTB
BREAKER NO.1
	NO.2	NO.3
GROUND
POWER OR
APU
	SYNCHRONIZING BUS

Figure 12.11 Three generator paralleled system

Real Load Sharing

The Load Controller controls the basic frequency of the AC generator (400 Hz).

After paralleling, the load controllers work together to evenly share the real load by increasing the torque input to the lower speeding alternators drive and decreasing the torque input to the higher speeding alternators to ensure each alternator takes an equal share of the load.

Current transformers sense the Real Load distribution at the output of each of the paralleled alternators.

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When current flows through these transformers, voltage is induced in them and a current will flow in the Load Sharing Loop. Each of the current transformers, which are connected in series with each other in the loop, has an Error Detector wired in parallel with it.

If it is assumed initially in Figure 12.12 that conditions of balanced load have been attained, then the current output of each current transformer can also be assumed to be 5 amperes and no current will flow through the error detectors.

		5 A
GEN1		GEN2		GEN3
7 A	5 A	4 A	5 A	4 A
2 A			1 A	1 A
TO SPEED GOVERNOR NO. 1		TO SPEED GOVERNOR NO. 2		TO SPEED GOVERNOR NO. 3

Figure 12.12 Real load sharing circuit

Now imagine that the drive unit of the No 1 alternator increases its torque output, it will take a bigger share of the load than the other two alternators which will decrease by a proportional amount.

The output of the No. 1 alternator current transformer has increased to 7 amperes so this will mean that the output of the No. 2 and 3 transformers will decrease to 4 amperes so that the average current flowing in the circuit is still 5 amperes.

According to Kirchoff’s first law the difference between each current transformer and the average current will be pushed through the error detectors in opposite directions. This signal, when amplified, will be sent to the speed governors to tell the CSDU for the No. 1 Gen to reduce torque (speed) and the CSDUs for the No. 2 and 3 Gen to increase torque (speed) until the current in each transformer is once again equal and the real load is once again balanced.

Real load sharing is controlled by matching CSDU speed (torque)

Reactive Load Sharing

The reactive load sharing circuit shown below looks very similar to the real load sharing circuit. It works in a similar fashion but it is a completely separate circuit.

The sensing of out of balance loads by the current transformers is the same but this time the error detector needs to know the difference between the reactive loads carried by each generator.

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