GB2196804A

GB2196804A - Improvements in or relating to electric generating systems

Info

Publication number: GB2196804A
Application number: GB08623527A
Authority: GB
Inventors: Fathy Elsayed Ahme Abdel-Kader
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1986-10-01
Filing date: 1986-10-01
Publication date: 1988-05-05
Also published as: GB8623527D0

Abstract

The load characteristic of a generator is improved by the connection of the primary P of a step-up autotransformer A to its output terminals T4,T5, a capacitor C4 being connected to the secondary S of the autotransformer. The generator may be an induction type, or preferably a reluctance type. If a three phase generator is used, a star or delta connected three phase transformer is coupled across its outputs and star or delta connected capacitor are coupled across the transformer secondaries (Fig. 3). The primary turns of the transformer are selected to produce saturation at no load. Compared to prior art use of capacitance without the autotransformer (Fig. 1), the present system gives a reduction in the required capacitance value, new-constant output voltage, reduction of no-load current and iron loss, and increase of generating output current and power. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electric generating systems The present invention relates to electric generating systems and more particularly to stand alone generators.

Stand alone generators are used in, for example, isolated locations and basically comprise a prime mover and a generator the rotor of which is normally mechanically coupled thereto. Typical prime movers are diesel engines or hydro power. The prime mover is designed to turn the rotor at a predetermined speed thereby determining (for a given pole arrangement) the frequency of the output voltage. The problems lie in the maintenance of the output voltage with increasing electrical load placed on the generator.

Known generators are of the induction or reluctance type using capacitor excitation to produce the output current and these are found to have considerable economic advantages over alternators with d.c. excitation.

The disadvantage with present designs of generator of the induction or reluctance type is that the output voltage falls with increasing load current and it is an object of the present invention to provide a stand alone generator with an improved load characteristic.

The present invention therefore provides an electric generating system for connection to a prime mover to be driven thereby the generating system comprising an electric current generator including at least one stator winding producing at least one phase of output voltage across a pair of output terminals and including an autotransformer the primary winding of which is connected to the output terminals and in which a capacitor is connected to the secondary winding of the autotransformer.

Preferably the primary winding of the autotransformer is connected directly across the pair of output terminals and the capacitor is connected directly across the secondary of the autotransformer.

Preferably the generator is of the reluctance type but may be of the induction type.

The generator may be single phase or may be three phase. If a three phase generator is used then a three phase autotransformer is used connected in a star or delta connection.

Three capacitors also in a star or delta connection are connected respectively to a secondary winding on the autotransformer.

The value of capacitor used is preferably lower than that conventionally used without the autotransformer and is selected dependent on the turns ratio of the autotransformer.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings in which: Figure 1 shows a known generator system; Figure 2 shows a single phase generator system according to the present invention; Figure 3 shows a three phase generator system according to the present invention; Figure 4 shows a graph of no-load characteristic for the generator of Figure 1 and Figure 3; Figure 5 shows a graph of autotransformer magnetising reactance variation at no-load; Figure 6 shows a graph of no-load output voltage variation with autotransformer output turns ratio; Figure 7 shows a graph of load voltage variation with load current for the generators of Figures 1 and 3; and Figure 8 shows a graph of autotransformer magnetising reactance variation with load cur rent.

With reference now to Figure 1 there is shown a three phase generating system diagrammatically represented by a star connected stator (armature) winding W1, W2, W3, connected to output terminals T1, T2, T3. The winding is of conventional form and may be an induction or reluctance type generator.

The arrangement shown in Figure 1 provides conventional shunt capacitor (C1, C2, C3) excitation and this has the disadvantage of suffering from poor voltage regulation high no-load losses and a failure to generate in overload conditions.

The generating system according to the pre sent invention is shown for simplicity in Figure 2 for a single phase generator. A reluctance generator is preferred since induction generators are slip dependent and the generated fre quency therefore varies with load whilst the reluctance generator gives the following ad vantages.

1) Constant output frequency independent of load.

2) No rotor copper loss.

3) Simple construction and low manufactur ing costs.

The armature winding W4 is connected to the output terminals T4, T5. The primary winding P of an autotransformer A is con nected across the output terminals T4, T5 and a capacitor C4 is connected across the secondary winding S. Effectively therefore the secondary winding S is connected in series with capacitor C4 and the primary winding P will carry a current at no load conditions de pendent on the voltage at the output terminals T4, T5.

The size of the autotransformer (KvA rating) is selected to correspond to the rating of the generator (since the voltage and required cur rent are given by the manufacturer) and the primary turns are selected to produce satura tion in the autotransformer at no load. The winding ratio of the autotransformer, is se lected in accordance with the value of capaci tance C4 selected, the value of capacitance C4 being calculated from the value previousiy used in the arrangement of Figure 1 but reduced in value. In general the reduction is as a ratio of the square of the turns ratio of the autotransformer.

For a 240 volt generator therefore a turns ratio 1:2 will allow 450 volt rated capacitors to be used and these may then have a capacity of one quarter of those used in Figure 1.

The operation is explained as follows. The autotransformer secondary S is effectively in series with the capacitance C4. Thus, as the magnetising current increases the series element of the autotransformer reactance falls due to saturation of the core. An effective increase of shunt capacitance occurs and the generated output voltage is boosted.

With reference now to Figure 3 a three phase generating system is diagrammatically shown. The principles of operation and selection of autotransformer size and capacitor.

Practical considerations however apply in the choice of autotransformer ratio that too high a turns ratio will result in a required increase voltage rating of the capacitance for a given output voltage.

With reference now to Figure 4 the conventional operation of the system of Figure 1 is shown on the right hand graphs (solid line for output voltage and dotted line for stator current) and for the system of the present invention using the autotransformer. The no-load results show that a) the capacitance threshold needed for generation is reduced from 18,F to 7/LF and b) the level of voltage generated stabilises independent of capacitance at 222 volts in the autotransformer system whereas in the conventional system it continues to rise.

With reference to Figure 5, the graph illustrates the fall in autotransformer reactance as the magnetising current increases.

Figure 6 shows the variation of generated output voltage with autotransformer turns ratio and that the optimum peak operating value is a function of capacitance.

Figure 7 contrasts conventional generator operation with that of the autotransformer system. The useful range of operation is increased by a factor of 4 with perfect voltage regulation characteristics with shunt capacitance of 12,LxF.

It should be noted that even with 16,F, conventional shunt excitation gives poor regulation and a limited output.

Figure 8 shows the autotransformer reactance variation with load current.

The proposed system offers the following proven advantages in operation: a) a substantial reduction in the required shunt capacitance.

b) voltage regulation control to give a nearconstant output voltage independent of load current.

c) a substantial reduction of no-load current, iron loss and increased efficiency.

d) an increase of generating output current and power.

Claims

1. An electric generating system for connection to a prime mover to be driven thereby the generating system comprising an electric current generator including at least one stator winding producing at least one phase of output voltage across a pair of output terminals and including an autotransformer the primary winding of which is connected to the output terminals and in which a capacitor is connected to the secondary winding of the autotransformer.

2. An electric generating system as claimed in Claim 1 in which the primary winding of the autotransformer is connected directly across the pair of output terminals and the capacitor is connected directly across the secondary of the autotransformers.

3. An electric generating system as claimed in Claim 1 or Claim 2 in which the generator is of the reluctance type.

4. An electric generating system as claimed in Claim 1 or Claim 2 in which the generator is of the induction type.

5. An electric generating system as claimed in Claim 1 in which the generator is single phase.

6. An electric generating system as claimed in Claim 1 in which the generator is three phase.

7. An electric generating system as claimed in Claim 6 in which a three phase autotransformer is connected in a star or delta connection and in which three capacitors also in a star or delta connection are connected respectively to a secondary winding on the autotransformer.

8. An electric generating system as claimed in any one of the preceding claims in which the value of the capacitor is selected dependent on the turns ratio of the autotransformer.

9. An electric generating system substantially as described with reference to Figures 2 to 8 of the accompanying drawings.