GB2039094A - Control system for turbine generators - Google Patents

Abstract

Steam flow to a turbine (1) is controlled (10, 11, 12) by the low- ester level signal (17, 18) from three control subsystems: a turbine speed control, (5,6,8,9) including a turbine load sensor (3) and a droop setting means (4), a load limit control (15, 16) and a backward power control (19,20,21,22). A constant pre-turbine steam pressure control (13A, 14A) may also be provided. The speed setting means (8) and the droop setting means (4) are each provided with instantaneous reset means (A, B, C) for automatically setting desired signal levels under certain system operating conditions: When the generator is operated in synchronism with another electric power system, the set speed of the speed control is raised and control is effected by the load limit control or by the backward power control. In asynchronous operation, the load limit signal is raised, the droop set to zero and the speed set to correspond with a reference frequency for isochronous operation. On disconnection of the load, the set speed and the droop are instantaneously decreased to preset values. <IMAGE>

Description

SPECIFICATION Control system for turbine generators The present invention relates to control systems for turbine generators and more particularly to a control system so designed that when a turbine generator is run in parallel with another electric power system whose frequency varies over a wide range, synchronous closing of the turbine generator can be easily effected without being affected by variation in the frequency of the electric power system, the amount of electric power received from or supplied to the electric power system is controlled at a constant value, and any abnormal acceleration of the turbine rotational speed upon disconnection of the turbine generator from the electric power system or upon disconnection of the load.

Generally, in closing a turbine generator in synchronism with another electric power system, the usual practice has been such that a signal is sent to a governor unit from a synchronous closing device to adjust the frequency, voltage and phase of the generator and then the synchronous closing is effected.

The governor is usually provided with a droop which will be described later, so that when the turbine generator is closed in synchronism with an electric power system whose frequency varies over a small range, a small load is imposed on the turbine generator upon the synchronous closing and in this way the occurrence of any motoring is prevented.

On the other hand, when the turbine generator comes into a rated operation so that a point is reached where the receipt of electric power from or the supply of electric power to the electric power system is started, the amount of electric power received or the amount of electric power supplied to the electric power system is detected so that a regulating valve actuation signal is applied to the governor unit and a governor motor is operated in response to the signal, thus satisfactorily controlling the amount of electric power received or the amount of electric power sent back.

Where the load is varied over a relatively large range, by providing a dead zone corresponding to the load variation so as to control the amount of power received or the amount of power sent back to come within a relatively small range of variations, it is possible to ensure stable operation of the turbine generator.

On the other hand, since the turbine generator is always controlled by the governor unit in these operations, when the turbine generator is disconnected with the electric power system so that the load is changed abruptly and the rotational speed of the turbine generator is changed, the opening of a steam regulating valve is controlled by the operation of the governor unit so that the amount of steam flow to the turbine is controlled and the rotational speed of the turbine generator is held within a predetermined range.

However, in the case of a small-capacity electric power system, if the loads connected to the power system include one involving a wide load variation, when a turbine generator is operated in parallel with such power system, the same operating conditions as obtained with the previously mentioned system with reduced frequency variations cannot be obtained and various detrimental effects will be produced.

More specifically, when there occurs a variation in the frequency of the electric power system in parallel operation, due to the fact that the share of load is determined in relation with the system frequency, the turbine output varies with the same period as the variation period of the system frequency. For example, if a governor unit with the ordinarily set drrop of 3 to 5% is used for the parallel operation of a turbine generator with another electric power system in any area where the frequency variation and the variation period are respectively as high as 2% and 10 sec, the variation of the turbine output or the output of the turbine generator will amount to as high as 50% of the rating.In this connection, to control the rotational speed of a turbine to provide such a characteristic that the turbine rotational speed linearly decreases with increases in the output of the turbine (generator) is called as a control by speed droop, and a droop of 3%, for example, means a characteristic which decreases the rotational speed by 3% between the turbine outputs of 0% and 100%.

In such a case, if any motoring of the generator is caused after the synchronous closing, it will be made difficult to operate the turbine generator stably, and moreover if the frequency of the electric power system is decreased extremely, a condition of generator overloading will be caused thus resulting in a condition which is detrimental to stable operation of the turbine generator.

While an ordinary method of stably operating a turbine without being influenced by variations in the frequency of another system is known in the art in which the opening of a regulating valve for the turbine is maintained constant so as to limit the load and thereby to operate the turbine stably without being affected by the frequency, this method is also disadvantageous in that if the frequency varies over a wide range, the rotational speed of the governor must be set high so as to eliminate the effect of the frequency variations and there is thus the danger of causing an overspeed tripping of the turbine upon disconnection of the turbine generator.If the setting of the load limiter is decreased to overcome this danger, there is the disadvantage of decreas ing the output of the turbine generator corsid- erably as compared with the rating, though the effect of the frequency will be eliminated.

With a view to overcoming the foregoing deficiencies in the prior art, it is an object of the present invention to provide a control system for turbine generators which is designed so that when a turbine generator is operated in parallel with another electric power system involving a wide frequency variation, the turbine generator can be operated stably substantially without being influenced by variations in the frequency of the power system, that is, without bringing a governor unit into operation, the amount pf power received and transmitted backward are respectively controlled at a substantially constant value, and upon occurrence of an abnormal condition such as the disconnection of the turbine generator the governor unit is brought into action to restore the rotational speed control function for the generator.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram showing a prior art control system for turbine generators; Figure 2 is a diagram showing the relationship between the output and rotational speed of a turbine; Figure 3 is a diagram showing the relationship between the amount of power transmitted backward and the generating station load with a constant turbine output; Figure 4 is a diagram showing the relationship between the frequency variation and the turbine output; and Figure 5 is a block diagram showing a control system according to an embodiment of the invention.

In order that the invention may be better understood, a prior art control system will first be described in detail.

Referring now to Fig. 1 showing a block diagram of a prior art control system for turbine generators, its control units operate as follows.

A generator 2 is driven by a turbine 1 so that the output of the generator 2 is detected by a load sensor 3 and the output of a droop potentiometer 4 is adjusted according to the detected value. In this case, the output of the droop potentiometer 4 increases in proportion to the output of the load sensor 3 or the output of the generator 2. The rotational speed of the turbine 1 is detected by means of an electromagnetic pickup 5 and a speed detector 6 and the rotational speed and the output of the droop potentiometer 4 are compared with the output of speed setting means 8 or the preset value indicated and determined by load setting means 7.The resulting deviation or difference is applied to a speed controller 9 which performs a PID control action so that its output control signal is applied to an actuator 10 which in turn drives a hydraulic servomotor 11 and the servomotor 11 controls the opening of a steam regulating valve 1 2 for adjusting the amount of main steam flowing to the turbine 1. The described governor unit allows the turbine 1 to operate at a desired rotational speed.

In this case, if an engine controlled by the speed droop provided by such a droop potentiometer is coupled to a synchronizing device (an infinite bus) which is so large that the frequency is not influenced, the speed of the engine will not be restricted by the speed setting but will be determined by the frequency of the bus so far as the maximum synchronizing torque is not exceeded. Thus, as shown in Fig. 2, with the droop being selected 4% and the bus frequency 50 Hz, if, for example, the speed setting means 8 is set to 50 Hz (A in Fig. 2), the output of the turbine shares no load. If it is set to 51 Hz (B in Fig. 2), the turbine output shares 50% of the load. If it is set to 52Hz (C in Fig. 2), the turbine output shares 100% of the load.In other words, in Fig. 2 the share of load is determined by the position of the intersecting point of the bus frequency and the oblique line of the speed droop and the share of load increases with an increase in the preset frequency. On the contrary, if the speed setting is fixed, the output of the turbine generator will be held constant. This means that as shown in Fig. 3, if the generating station load varies, the amount of power transmitted backward or the amount of power received will be varied.

However, if the system frequency is subject to variation, since the share of load on the turbine output is determined by the relation between the turbine output and the system frequency as mentioned above, the turbine output will be varied with the same period as the variation period of the system frequency.

For example, if a turbine generator is operated in parallel with an electric power system by means of a governor unit with the conventional set droop of 3 to 5% in the area where the frequency variation and the variation period are respectively as high as 2% and 10 sec, the variation of the turbine output or the generator output amounts to as much as 50% of the rating. Such a generator operation brings about a number of detrimental effects as mentioned previously.

The prior art method has been described in detail and an embodiment of this invention will now be described in detail with reference to Fig. 5.

Fig. 5 is a block diagram shown the overall construction of a turbine generator control system in accordance with the invention and the control system comprises the following five subsystems.

(1) Actuator 10 This is an actuator whereby a controlled variable which is proportional to the voltage signal from an electric governor which will be described later, is converted to a mechanical displacement. The actuator 10 actuates a steam regulating valve 1 2 through a mechanical link mechanism and a hydraulic servomotor 11 and the amount of steam flowing to a steam turbine 1 is adjusted. The signal applied to the actuator 10 from the electric governor is automatically selected by two low signal selectors 1 7 and 1 8 which select the lowest voltage level one of the signals from a speed control subsystem, load limit control subsystem and backward electric power (received electric power) control subsystem which will be described later.

(2) Speed control subsystem This control subsystem is designed so that the difference between a preset rotational speed and the actual rotational speed detected by an eddy current type electromagnetic pickup 5 is reduced to zero so as to hold the speed at a constant value. The speed control subsystem comprises speed setting means 8 constituting a speed setting means for applying a reference voltage to a PID controller 9 and a feedback mechanism including the electromagnetic pickup 5, a speed sensor 6 for converting the output pulse signal of the pickup 5 to a voltage signal, a load sensor 3 for sensing the output signal of the turbine generator, digital setting means or droop setting means 4 and the speed controller 9 for operating on the reference voltage and the feedback signal voltage to perform a PID control.There are further provided auxiliary means comprising instaneous reset means for automatically and instantaneously changing the present value to a preliminary set value in response to an externally applied signal and these reset means include IR(A) and IR(B) incorporated in the speed setting means 8 or the speed setting means and an IR(C) incorporated in the digital setting means 4 or the droop setting means. In particular, by setting the droop to zero in the droop setting, it is possible to perform an isochronous operation.

It is to be noted that the electromagnetic pickup 5 and the speed sensor 6 may be replaced with any other speed detecting mechanism such as a tachometer generator.

(3) Load limit control subsystem This is a control subsystem for controlling the opening of the steam regulating valve 1 2 at a constant value and it serves the function of a kind of load limiter which maintains constant the steam condition at the turbine inlet and outlet, respectively, so as to maintain the output of the turbine generator below a preset value. The control subsystem comprises load setting means 1 5 and an amplifier 16.

(4) Backward power (or received power) control subsystem This control subsystem is designed so that the output of the turbine generator is controlled so as to maintain the power sent to the power system or the amount of power received therefrom is maintained constant against variations in the system frequency and the station load. The control subsystem comprises a mechanism for setting the amount of power transmitted backward (or received power) which comprises backward (or received) power setting means 1 9 for applying a reference voltage to a PID controller 22, and a feedback mechanism including a load sensor 20 for detecting the amount of power sent backward or received, a signal converter 21 and the backward/received power controller 22 for operating on the reference voltage and the feedback signal to perform a PID control action.This backward/received power control subsystem is designed so that when power is received from the power system, the phase of the output voltage from the load sensor 20 is invested by the signal converter 21 and the output signal of the PID controller 22 increases to the maximum. Thus the signal is cut off by the low signal selector 1 7 and the subsystem no longer takes part in the control of the turbine generator.

(5) Constant pre-turbine steam pressure control subsystem When the amount of steam flowing to the turbine decreases with respect to the turbine load, the pre-turbine steam pressure decreases thus giving rise to the danger of causing a drain attack due to the carry over. The purpose of this control subsystem is to prevent such drain attack and it comprises a mechanism so that when the pre-turbine steam pressure drops to a predetermined value, the preset load limit value is decreased in response to the signal supplied through a pressure transducer 1 4A and a voltage switch 1 3A and the output of the turbine generator is decreased until the pre-turbine steam pressure is restored.

There will now follow a description of the operations of this embodiment of control system comprising the above-mentioned various control subsystems, which take place upon making a parallel operation, system power interruption, restoration of parallel operation and disconnection of load.

(1) Operation upon making parallel operation.

Synchronous making or closing for parallel operation with another electric power system (i.e., closing of a turbine generator circuit breaker 13) is accomplished by means of the speed control subsystem. In this case, the load limit is set to about 10% of the rated output of the turbine generator, the amount of backward power transmission is set to about 10% of the maximum amount of backward power transmission and the droop is set to the desired value by the digital setting means 4.

In the pre-closing condition there is no load so that the output signal voltage from the speed controller 9 of the speed control subsystem is lower than the signal voltages from the load limit amplifier 1 6 and the backward power transmission controller 22 and consequently the signal from the speed control subsystem is applied preferentially to the actuator 10 by the low signal selector 1 7 and 1 8.

After the rotational speed has been adjusted by adjusting the speed setting of the speed setting means 8 in response to an adjusting signal from the synchronous closing means (not shown), the synchronous closing is effected and the speed setting of the speed setting means 8 is set to the upper limit automatically and instantaneously by the instantaneous reset means IR(A) in response to a signal indicative of the closing of the generator circuit breaker 1 3. In this case, while the speed control subsystem generates a maximum signal so as to establish a rotational speed corresponding to the set upper limit value, the output signal of the load limit control subsystem is lower than the output signal of the speed control subsystem and consequently the control of the turbine generator is automatically transferred to the load limit control subsystem.At this point, the generator is at the constant output operation where is is not affected by any variation in the frequency of the power system and any change in the generating station load is met by a change in the amount of power received.

To increase or decrease the output of the turbine generator, it is only necessary to vary the load limit setting.

The change to the backward transmission operation, the load limit setting is changed to correspond to the maximum output of the turbine generator and the backward power transmission setting is changed to a value which is lower than the load limit set value, thus causing the backward power transmission control subsystem to start controlling the turbine generator. The power receiving operation can be initiated in the like manner.

(2) Operation upon interruption of system power.

When the system power is interrupted, the generating station load is made dependent only on the output of the turbine generator. In this case, since the load limit has been set to the upper limit and the amount of backward power transmission is reduced to zero upon power interruption, the output signals from these two control subsystems increase to the maximum and consequently the speed control subsystem is automatically brought into operation to start the control.In this case, since at the droop operation, the station frequency varies depending on the magnitude of the station load, in response to a signal indicative of the opening of a system power circuit breaker the droop operation is switched to the isochronous operation by the instantaneous reset means IR(C) of the digital setting means 4 and the speed setting is changed to 50 Hz by the instantaneous reset means IR(B) of the speed setting means 8 instantaneously. In this way, the constant-frequency operation can be accomplished against variations in the generating station load.

(3) Operation upon restoration of parallel operation.

While, in the above-mentioned condition, the turbine is in the isochronous operation, in order that the parallel operation with the electric power system may be restored, it is necessary to perform the droop operation. For this purpose, the signal from the synchronous closing means is applied to the speed setting means 8 and simultaneously the desired droop is automatically provided from the isochronous speed up to a predetermined droop by the digital setting means 4. In this restored condition the set values of the control subsystem are the same as in the case of the above (1) except that the load limit is set about 100 kw higher than the thin current station load and the turbine generator is controlled in the like manner.

(4) Operation upon disconnection of load.

When the load is separated, the speed control subsystem generates a minimum signal only when the rotational speed rises in excess of the speed set value and in this way the rotational speed is prevented from rising further.

On the other hand, when the turbine generator has been controlled by the load limit control subsystem or the backward power transmission control subsystem, the speed setting has been changed to correspond to the upper limit value and consequently there is the danger of causing an overspeed tripping.

In order to prevent such a tripping, the speed setting is automatically and instantaneously switched to 50 Hz by the instantaneous reset means IR(B) of the speed setting means 8 in response to a signal indicative of the opening of the turbine generator circuit breaker 1 3. As a result, practically at the same time with the breaking the speed control subsystem comes into operation to prevent the rotational speed from rising and in this way the instantaneous speed variation rate can be held below the preset value of an emerge ozy governor.

It will thus be seen from the foregoing description that while the prior art speed control cannot control the amount of power transmitted backward (or the amount of power received) at a constant value if the system frequency is varied over a wide range and the station load is also varied, in accordance with the invention the output of the turbine generator can be varied in response to a change in the station load without being influenced by the frequency of the power system and in this way the amount of power transmitted backward (or the amount of power received) can be controlled constant with a high degree of accuracy. Thus the invention has a very great practical utility.

Claims (7)

1. A control system for a turbine generator comprising: speed control means for controlling a rotational speed of a turbine, said speed control means including droop setting means; load limit control means for controlling an output of a generator driven by said turbine below a predetermined value; backward/forward power transmission control means for controlling an amount of power transmitted backward or received at a predetermined value; lowest level signal selecting means for selecting one of control signals from said control means having the lowest signal level so as to allow said control means generating said selected control signal to perform control in preference to the remainder of said control means;; whereby when said turbine generator is operated in synchronism with another electric power system, a set speed of said speed control means is changed to near an upper limit value and said droop setting means is set to a predetermined droop to thereby allow the control operation of either said load limit control means or said backward-forward power transmission control means; whereby when said turbine generator is operated in asynchronism with said electric power system, a setting of said load limit control means is changed to near an upper limit value, the droop of said droop setting means of said speed control means is reduced to zero, and the speed setting of said speed control means is set to a rotational speed corresponding to a reference frequency to thereby effect an isochronous control operation; and instantaneous reset means whereby when said turbine generator is disconnected from a load, each of the speed setting and the droop of said speed control means is instantaneously decreased to a predetermined value.

2. A control system according to claim 1 further comprising means responsive to an output signal of said lowest level signal selecting means to adjust a steam regulating valve of said turbine.

3. A control system according to claim 1 or claim 2 further comprising contant preturbine steam pressure control means whereby when a pre-turbine steam pressure drops below a predetermined value, the set value of said load limit control means is decreased so as to decrease the output of said generator and thereby to maintain said preturbine pressure at said predetermined value.

4. A control system according to any one of claims 1 to 3 wherein said speed control means includes speed setting means, speed detecting means, a load sensor and a speed controller.

5. A control system according to any one of claims 1 to 4 wherein said load limit control means include load setting means and an amplifier.

6. A control system according to any one of claims 1 to 4 wherein said backward/forward power transmission control means includes backward/received power setting means, load sensing means, a signal converter, and an electric power controller.

7. A control system for a turbine generator substantially as described herein with reference to Fig. 5 of the accompanying drawings.