GB2037463A

GB2037463A - Field excitation control of synchronous machine

Info

Publication number: GB2037463A
Application number: GB7943846A
Authority: GB
Original assignee: Skoda op; Skoda np
Current assignee: Skoda op; Skoda np
Priority date: 1978-12-21
Filing date: 1979-12-20
Publication date: 1980-07-09
Also published as: GB2037463B; DE2951140A1; CS206719B1; SE7910417L

Abstract

With a view to preventing both overexcitation and underexcitation of the field of a synchronous rotating electrical machine, and thereby extending the operating range of the machine, the system comprises a regulating unit (5) for connection to the field regulator and having two inputs from a unit (4) for providing controlled limitation of the excitation of the field. The unit (4) has two inputs derived from a circuit (1) for the evaluation of any deviation of the stator current of the machine from a predetermined limit, and a third input derived from an amplifier (3) connected so as to amplify the output of a sensing element (2) of wattless current or of the output of the synchronous machine. In operation of the machine, the system causes the excitation of the field to be reduced if it is overexcited and increased if it is underexcited, in dependence on the stator current. <IMAGE>

Description

SPECIFICATION A system for regulating the field of a synchronous machine This invention relates to a system for regulating the field of a synchronous rotating electrical machine for the purpose of maintaining its stator current within a predetermined limit, the system comprising a circuit for the evaluation of any deviation of the stator current from said limit, a sensing element of wattless current or of an output of the synchronous machine, an amplifier, a unit for providing controlled limitation of the excitation of the said field, and a regulator unit.

Hitherto such a system comprised a circuit for the evaluation of any deviation of stator current from a predetermined limit and a regulator unit. This system, however, had an output signal of only one polarity and it was intended for use in the overexcitation zone of a synchronous machine where it is necessary to de-excite the field in order to decrease the stator current. In the underexcitation zone of a synchronous machine, where it is necessary to increase the excitation in order to decrease the stator current, the desired stator current limitation was provided by an underexcitation monitor. The disadvantage of such a system resides in the fact that when operating in the underexcitation zone of the synchronous machine, no decrease but only an increase in stator current can take place up to the limiting value determined by the underexcitation monitor.

The disadvantages mentioned above are overcome by the invention, according to which a first and a second input to the regulating unit, the output of which is the output of the system, are defined by a first and a second output of the controllable limitation unit, the latter having a first and a second input defined by a first and a second output of the circuit for. the evaluation of stator current deviation from a predetermined limit, and a third input which is defined by the output of the amplifier, the latter having an input defined by the output of the said sensing element of wattless current or of an output of the synchronous machine.

One practical form of system embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which Figure 1 is a block schematic diagram of the system; and Figure 2 is a graphical representation of its operation.

A negative D.C. voltage U1, the value of which is proportional to the stator current is of the synchronous machine defines the input of a circuit 1 for theevaluation of stator current deviation from a predetermined limit, represented by a D.C. voltage U2.

If the stator current exceeds the said limit lU,I > U2 then a first output voltage U3 of the circuit 1 is positive and a second output voltage U4 of this circuit is negative, their absolute values being equal. The operation will be explained on the assumption that these voltages are constant.

The steady state voltages of the circuits depending on the operating point of the synchronous machine are illustrated in Fig. 2, in which the underexcitation zone is marked KAP, and the overexcitation zone is marked IND. The output voltages U3, U4 of the circuit 1 define input signals of the unit 4. Both these signals are modulated by the output voltage U6 of an amplifier 3, to the input of which the output voltage Us of a sensing element 2 of the wattless current of the machine is connected. The characteristic of the output voltage U5 of the sensing element 2 and of the output voltage U6 of the amplifier 3 are illustrated in dependence on the operating point of the synchronous machine, in Fig. 2 in which the output voltages U7, U5 of the unit 4 are also shown. These output voltages U7, U8, define the inputs of a regulating unit 5.At the steady state in dependence on the operating point of the sycnhronous machine the output voltage U9 of the unit 5 is the amplified sum of voltages U7 and U8. The voltage U9 which is the output voltage of the overall system is then connected to the input of the field regulator of the synchronous machine.

The overall operation of the system is such that if the limit of stator current is exceeded, the synchronous machine is de-excited if it is operating in the overexcitation zone, and the excitation of the synchronous machine is increased if it is operating in the underexcitation zone. If the stator current is below the predetermined limit 1U11 < U2 then the voltages U7, U8 at the input of the unit 5 U7 > O and U, < O so that there is zero voltage at the output of the unit 5.

The advantage of the system described is that it can be used for the protection of a synchronous machine in its overexcited as well as in its underexcited state and that it therefore extends the operating region of the machine. The invention finds application especially in field regulators of large synchronous machines.

Claims

1. A system for regulating the field of a synchronous rotating electrical machine for the purpose of maintaining its armature current within a predetermined limit, the system comprising a circuit for the evaluation of any deviation of the armature current from said limit, a sensing element of wattless current or of an output of the synchronous machine, an amplifier, a unit for providing controlled limitation of the excitation of the said field, and a regulator unit, wherein a first and a second input to the regulator unit, the output of which is the output of the system, are defined by a first and a second output of the unit for providing controlled limitation of the excitation of the said field, the latter having a first and a second input defined by a first and a second output of the said circuit, and a third input which is defined by the output of the amplifier, the latter having an input defined by the output of the said sensing element.

2. A system according to Claim 1, wherein said armature current is the stator current.

3. A system according to Claim 1 or Claim 2, wherein said inputs and outputs are in the form of voltages.

4. A system for regulating the field of a synchronous rotating electrical machine the system being adapted to operate substantially as hereinbefore described with reference to Fig. 2, and wherein at least said circuit and/ or a said unit is constructed and arranged substantially as hereinbefore described with reference to Fig. 1, and adapted to operate substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompany ing diagrammatic drawings.

5. A system according to any one of the preceding claims, connected to a field regulator of a synchronous rotating electrical ma chine, the output of the system defining the input of the field regulator.

6. A synchronous rotating electrical ma chine whose field is connected to the combi nation of a system and field regulator accord ing to Claim 5.