Basic theoretical information. On aircraft with multiple engines to enhance the survivability of the electric system the parallel operation of ГПС is carried out

On aircraft with multiple engines to enhance the survivability of the electric system the parallel operation of ГПС is carried out. In parallel operation of ГПС, the dependability of consumers' supply with electricity increases, using of generators improves and the active reserve of capacity of the supply system increases. However, all that benefits for the system with parallel operation of generators apper the most fully when the load between the generators of one capacity is distributed irregurarly. Let us consider the diagram (Fig. 5.1), according to Kirchhoff's law it can be written:

; ;

where U _М – voltage of the network; U ₁, U ₂, I ₁, I ₂, R ₁, R ₂ – voltages, currents of load of the base of positive wires of generators respectively.

Fig. 5.1

These expressions represents that equality of generators' currents is possible only if there is equality between resistances of positive wires of generators.

As long as the bases of wires that come from the generator to the netwok are extremely small (about 10^-3 Ohm) then any deterioration of contacts state may lead to significant difference of loads of DCG.

To reduce the difference in load currents of generators in parallel operation circuits we use circuits of balancing that affect the МРС ВРН. Circle of balancing is pluged into special ballast resistances R _б1 and R _б2 in the " minus" circuits of generators (Fig. 5.1). The main elements of the circuit are balancing windings W _y, located together with operating winding W_p on the core EM BP. If for improvement of the quality of voltages regulation the multi-coils stabilizing transformers are used, then the balancing circuit includes windings of balancing W _y0 of these transformers.

Setting up the operation of balancing circuit by using the controled resistance R _р. Fig. 5.1 shows primary W _y0 and secondary W ₂ windings of stabilizing transformers. The ballast holders R _б1 and R _б2 are made of heat-resistant alloys of Nichrome with low temperature coefficient. If currents of generators I1 and I2 are equal, the voltage drops across the ballast holders and potentials of points a and b donot differ from each other.

The current in the balancing circuit depends on the unbalance of generators' currents and is determined with the expression:

,

where R _у is the general resistance in the balancing circuit

If I ₁ > I ₂, then the balancing current is directed to the point and, as a lower capacity and, therefore, MMF of the balancing first winding BPH, which consists of MRS working winding, will reduce the voltage of the first generator. Action of winding balancing of the second ВРН is reversed. Because of this change in voltage the irregular load of DCG decreases as long as BPH can respond to diminishing of current of balance in the circle. Full equalization of currents due to the winding balancing can not be done, if not to eliminate the basic causes that lead to the irregular distribution of load.

To ensure uniform load distribution between parallel working generators if BP and balancing circle are present, three conditions should be set:

- equality of generator voltages U ₀₁ = U ₀₂ in idle mode an at opened circuit;

- equality of holders of positive circuits of both generators R ₁ =R ₂;

- equality of balasst holders R _б1 = R _б2.

The impact of each of these conditions in the load sharing between parallel working generators can be determined on the basis of expressions for each current generator and currents unbalance:

; ;

, (5.1)

where I ₁ і I ₂are currents of generators, D I is unbalance of currents of DCG;
I _М = I ₁ +I ₂ is the current of network’s load; b is the coefficient of correction.

оFrom the expression (5.1) we see that the current imbalance DI is zero if equalities are made U₀₁ =U ₀₂; R ₁ =R₂ and
R _б1 = R _б2. In case of failure of only one of the conditions of the formula (5.1) we obtain expressions for the unbalance currents.

1. If U ₀₁¹ U ₀₂; R ₁ = R ₂ – R; R _б1 = R _б2 = R _б, then:

(5.2)

From the expression (5.2) we see that the unbalance of currents of generator does depend on the load current and has a constant value.

In open-loop circuit the balancing ofcurrent unbalance equals :

The dependence of the currents of generators on the current load network in presence of circuit of balancingand without it if conditions U ₀₁¹ U ₀₂; R _б1 = R _б2; R ₁ = R ₂are done is represented in Fig. 5.2. Fig. 5.2

2. При U ₀₁ = U ₀₂; R ₁¹ R ₂; R _б1 = R _б2 = R _б, то:

(5.3)

In this case, the current of unbalance depends on the increasing of the load current.

At opened circuit of balancing the current of unbalance D I increases and equals:

Fig. 5.3 represents the graphical dependence of the generator currents on the load current of network with conditions R ₁ R ₂; R _б1 = R _б2 and U ₀₁ =U ₀₂ Fig.5.3

3. If U ₀₁ = U ₀₂; R ₁ = R ₂ = R; R _б1¹ R _б2, then:

(5.4)

At opened circuit of balance .

Physically, this is explained because of the ballast holders enabled in the zone of the BP operation and their differences can not affect the currents of generators. Fig. 5.4 represents graphical dependences, expressing the dependence of currents of generators on current network with a condition

Fig. 5.4 R _б1 R _б2, R ₁ =R ₂та U ₀₁ =U ₀₂.

Whereas, at the circle of balancing the ballast holders inequality negatively affects the method of distribution of the load current, they must be carefully monitored during the operation. Changing the value of the ballast resistances much greatly impact on inequality current generators than the same change in resistance of plus wire of generators.