WO2003010777A2

WO2003010777A2 - A nuclear power plant and a method of regulating the speed of a generator of the plant

Info

Publication number: WO2003010777A2
Application number: PCT/IB2002/002750
Authority: WO
Inventors: Roland Leslie John Bolton; Petrus Daniël KEMP; Michael Christiaan Nieuwoudt
Original assignee: Pebble Bed Modular Reactor Pty Ltd
Current assignee: Pebble Bed Modular Reactor Pty Ltd
Priority date: 2001-07-26
Filing date: 2002-07-12
Publication date: 2003-02-06
Anticipated expiration: 2004-01-26
Also published as: WO2003010777A3; WO2003010777B1; AU2002319558A1

Abstract

The invention provides a nuclear power plant having a closed power generation circuit which includes a reactor and a power turbine which is drivingly connected to a generator. The plant further includes a speed controller for regulating the speed of the generator. The speed controller includes a recirculation valve arrangement whereby the flow of working fluid through the reactor can be controlled. Further, the speed controller includes a variable resistor bank which is disconnectably connectable to the generator to permit the speed of the generator to be controlled by adjusting the electrical load thereon. The invention extends to a method of regulating the speed of the generator.

Description

A NUCLEAR POWER PLANT AND A METHOD OF REGULATING THE SPEED OF A GENERATOR OF THE PLANT

THIS INVENTION relates to nuclear power. More particularly it relates to a nuclear power plant and to a method of regulating the speed of a generator of the plant.

In a nuclear power plant having a power turbine drivingly connected to an electrical generator there is a need, particularly when the generator is not connected to an electrical distribution grid, to control the speed of the power turbine and hence the generator.

According to the invention in a nuclear power plant comprising a closed loop power generation circuit which includes a reactor and a power turbine drivingly connected to a generator there is provided a method of regulating the speed of the generator which includes connecting a variable resistor bank electrically to the generator and adjusting the electrical load on the generator via the variable resistor bank to regulate the speed of the generator.

The method may include monitoring the actual speed of the power turbine and generator, comparing the actual speed with a desired speed and generating an error signal in response thereto the error signal being fed to a controller, typically a proportional integral (PI) controller, which controls the value of the resistor bank. The resistor bank may have a nominal setting and the method may include, if the resistance of the resistor bank varies by more than a predetermined amount from its nominal setting, adjusting the mechanical power transmitted to the generator. Hence, the speed of the generator will typically be controlled primarily using the variable resistor bank and, if required, also by adjusting the mechanical power transmitted to the generator.

Adjusting the mechanical power transmitted to the generator may include adjusting the power output of the power turbine.

When the power generation circuit includes a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, at least one high pressure compressor recirculation line which extends from a position downstream of the high pressure compressor to a position upstream of the high pressure compressor and in which at least one high pressure compressor recirculation valve is mounted and at least one low pressure compressor recirculation line which extends from a position downstream of the low pressure compressor to a position upstream of the low pressure compressor and in which at least one low pressure compressor recirculation valve is mounted, adjusting the mechanical power produced by the power turbine may include adjusting the position of at least one of the or each high pressure compressor recirculation valve and the or each low pressure compressor recirculation valve thereby to adjust the mass flow rate of working fluid through the reactor and hence the mechanical power produced by the power turbine. Operation of the recirculation valves may be regulated by a recirculation actuator which in turn receives a signal from the controller.

The recirculation actuator may distribute the recirculation command in a ratio between the or each of the high pressure compressor and low pressure compressor recirculation valves which is determined by means of a gain determination algorithm, the two gain factors being determined by a surge margin controller which determines the factor to which the total recirculation flow is distributed across the high pressure compressor and low pressure compressor recirculation valves, the factors being controlled in such way that the difference between the surge margin of the high pressure compressor and the low pressure compressor is minimal.

The nominal setting of the resistor bank may be 50% of its full capacity. '

The invention extends to a nuclear power plant which includes a closed loop power generation circuit which includes a reactor and a power turbine which is drivingly connected to an electrical generator; and a speed controller for regulating the speed of the generator.

The speed controller will typically be used to regulate the speed of the generator when it is not connected to an electrical distribution grid.

The speed controller may include a variable resistor bank connectable to the generator, and a resistor bank value controller for controlling the resistor bank and thereby the electrical load on the generator.

The power generation circuit may include a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, at least one high pressure compressor recirculation line which extends from a position downstream of the high pressure compressor to a position upstream of the high pressure compressor and in which at least one high pressure compressor recirculation valve is mounted and at least one low pressure compressor recirculation line which extends from a position downstream of the low pressure compressor to a position upstream of the low pressure compressor and in which at least one low pressure compressor recirculation valve is mounted, the speed controller including high pressure compressor recirculation valve and low pressure compressor recirculation valve actuators whereby the positions of the high pressure and low pressure recirculation valves are adjustable to adjust the mechanical power generated by the power turbine, and a surge margin controller.

The speed controller may include a comparator for receiving signals corresponding, respectively, to the actual speed of the generator and the desired speed of the generator, and generating an error signal in response thereto, the resistor bank value controller being in communication with the comparator to receive the error signal and adjust the value of the resistor bank in response thereto. The resistor bank may have a nominal setting intermediate its minimum and maximum settings, the resistor bank controller being configured to generate a recirculate signal should the value of the resistor bank vary by more than a predetermined value from its nominal setting, the recirculation valve actuators being in communication with the resistor bank controller to receive the recirculate signal and to control the operation of the recirculation valves in response thereto.

The controller may include a surge margin controller for determining the factor to which the required recirculation flow is distributed across the high pressure recirculation valve and the low pressure recirculation valve.

The surge margin controller may include a low pressure compressor surge margin monitor, a high pressure compressor surge margin monitor and a comparator which is in communication with the monitors to receive signals corresponding, respectively, to a low pressure compressor surge margin and a high pressure compressor surge margin, and to generate a signal, the surge margin controller including a proportional integral controller which is in communication with the comparator to receive the signal and to calculate gain signals which are fed respectively to a high pressure recirculation flow controller and a low pressure recirculation flow controller which in turn are in communication with the recirculation valve actuators.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings. In the drawings, Figure 1 shows a schematic representation of part of a nuclear power plant in accordance with the invention;

Figure 2 shows a schematic representation of a resistor bank value controller forming part of a speed controller of the nuclear power plant;

Figure 3 shows a schematic representation of a recirculation valve actuator forming part of the speed controller of the nuclear power plant; and

Figure 4 shows a schematic representation of a surge margin controller forming part of the speed controller of the nuclear power plant.

In Figure 1 of the drawings, reference numeral 10 refers generally to part of a nuclear power plant in accordance with the invention.

The nuclear power plant 10 includes a closed loop power generation circuit, generally indicated by reference numeral 12. The power generation circuit 12 uses helium as the working fluid and includes a nuclear reactor 14, a high pressure turbine 16, a low pressure turbine 18, a power turbine 20, a recuperator 22, a pre-cooler 24, a low pressure compressor 26, an inter-cooler 28 and a high pressure compressor 30.

The reactor 14 is a pebble bed reactor making use of spherical fuel elements. The reactor 14 has a working fluid inlet 14.1 and a working fluid outlet 14.2.

The high pressure turbine 1 6 is drivingly connected to the high pressure compressor 30 and has an upstream side or inlet 1 6.1 and a downstream side or outlet 16.2, the inlet 16.1 being connected to the outlet 14.2 of the reactor 14.

The low pressure turbine 18 is drivingly connected to the low pressure compressor 26 and has an upstream side or inlet 18.1 and a downstream side or outlet 18.2. The inlet 18.1 is connected to the outlet 16.2 of the high pressure turbine 16.

The power turbine 20 is drivingly connected to a generator 32. The power turbine 20 includes an upstream side or inlet 20.1 and a downstream side or outlet 20.2. The inlet 20.1 of the power turbine 20 is connected to the outlet 18.2 of the low pressure turbine 18.

The plant further includes a variable resistor bank 33 which is disconnectably connectable to the generator 32.

The recuperator 22 has a hot or low pressure side 34 and a cold or high pressure side 36. The low pressure side of the recuperator 34 has an inlet 34.1 and an outlet 34.2. The inlet 34.1 of the low pressure side is connected to the outlet 20.2 of the power turbine 20.

The pre-cooler 24 is a helium to water heat exchanger and includes a helium inlet 24.1 and a helium outlet 24.2. The inlet 24.1 of the pre-cooler 24 is connected to the outlet 34.2 of the low pressure side 34 of the recuperator 22.

The low pressure compressor 26 has an upstream side or inlet 26.1 and a downstream side or outlet 26.2. The inlet 26.1 of the low pressure compressor 26 is connected to the helium outlet 24.2 of the pre-cooler 24.

The inter-cooler 28 is a helium to water heat exchanger and includes a helium inlet 28.1 and a helium outlet 28.2. The helium inlet 28.1 is connected to the outlet 26.2 of the low pressure compressor 26.

The high pressure compressor 30 includes an upstream side or inlet 30.1 and a downstream side or outlet 30.2. The inlet 30.1 of the high pressure compressor 30 is connected to the helium outlet 28.2 of the inter-cooler 28. The outlet 30.2 of the high pressure compressor 30 is connected to an inlet 36.1 of the high pressure side of the recuperator

22. An outlet 36.2 of the high pressure side of the recuperator 22 is connected to the inlet 14.1 of the reactor 14.

The nuclear power plant 10 includes a start-up blower system, generally indicated by reference numeral 38, connected between the outlet 34.2 of the low pressure side 34 of the recuperator 22 and the inlet 24.1 of the pre-cooler 24.

A low pressure compressor recirculation line 46 extends from a position between the outlet or downstream side 26.2 of the low pressure compressor 26 and the inlet 28.1 of the inter-cooler 28 to a position between the start-up blower system 38 and the inlet 24.1 of the pre-cooler 24. At least one normally closed low pressure recirculation valve 48 is mounted in the low pressure compressor recirculation line 46. A high pressure compressor recirculation line 50 extends from a position between the outlet or downstream side 30.2 of the high pressure compressor and the inlet 36.1 of the high pressure side 36 of the recuperator 22 to a position between the outlet or downstream side 26.2 of the low pressure compressor 26 and the inlet 28.1 of the inter- cooler 28. At least one normally closed high pressure recirculation valve 51 is mounted in the high pressure compressor recirculation line 50.

A recuperator bypass line 52 extends from a position upstream of the inlet 36.1 of the high pressure side 36 of the recuperator 22 to a position downstream of the outlet 36.2 of the high pressure side

36 of the recuperator 22. A normally closed recuperator bypass valve 54 is mounted in the recuperator bypass line 52.

The plant 10 includes a high pressure coolant valve 56 and a low pressure coolant valve 58. The high pressure coolant valve 56 is configured, when open, to provide a bypass of helium from the high pressure side or outlet 30.2 of the high pressure compressor 30 to the inlet or low pressure side 18.1 of the low pressure turbine 18. The low pressure coolant valve 58 is configured, when open, to provide a bypass of helium from the high pressure side or outlet 30.2 of the high pressure compressor 30 to the inlet 20.1 of the power turbine 20.

The nuclear power plant 10 also includes a speed controller, generally indicated by reference numeral 70. The speed controller 70 includes a resistor bank value controller 72 (Figure 2), a recirculation valve actuator 84 (Figure 3) and a surge margin controller 92 (Figure 4).

Referring now to Figure 2 of the drawings, the actual speed of the power turbine 20 and generator 32 are measured and fed as an input signal 74 to a comparator 76. In addition, a signal 78 corresponding to a desired set point speed of the power turbine and generator is fed through a pre-filter 80 to the comparator 76. The signals 74 and 78 are compared and an error signal 82 is fed to a PI controller 83 which controls the electrical load on the generator by adjusting the value of the resistor bank.

Hence, if the actual speed of the power turbine 20 is higher than the desired set point speed, the value of the resistor bank will be increased to increase the electrical load on the generator and thereby slow the power turbine and generator down. If, on the other hand, the measured speed of the power turbine and generator is below the desired set point speed then the value of the resistor bank will be decreased to decrease the electrical load on the generator and permit the speed of the power turbine 20 and generator 32 to increase.

To permit the value of the resistor bank to be either increased or decreased as required, the resistor bank will have a nominal setting which is typically 50% of its full capacity.

The resistor bank will typically have a power range of approximately 3.2 MW. If the value of the resistor bank varies by more than a predetermined amount, typically 0.5 MW, from its nominal setting, the high pressure recirculation valve 51 and/or low pressure recirculation valve 48 are actuated with a view to adjusting the mechanical power generated by the power turbine 20 and transmitted to the generator 32 to permit the variable resistor bank setting to move towards its nominal value.

Control of the operation of the recirculation valves 51 , 48 is regulated by the recirculation actuator 84 which is shown schematically in Figure 3 of the drawings.

As mentioned above, if the value of the resistor bank varies by more than a predetermined amount from its nominal setting, the controller sends a recirculation signal or command 86 to the recirculation actuator 84. The recirculation actuator 84 distributes the recirculation command, which represents the total mass flow through both the high pressure recirculation valve 51 and low pressure recirculation valve 48, by means of a high pressure recirculation gain 88 and a low pressure recirculation gain 90. The two gain factors are determined by the surge margin controller, which is generally indicated by reference numeral 92 in Figure 4 of the drawings.

The surge margin controller 92 determines the factor to which the total recirculation flow is distributed across the high pressure recirculation and low pressure recirculation valves 51 , 48. The factors are controlled in such a way that the difference between the surge margin of the high pressure and low pressure compressors is minimal. The sum of the two factors is one in order to ensure that the surge margin controller has only a small effect on the system as the transfer function of both components are almost equal.

In this regard, signals 94, 96 corresponding to the low pressure compressor surge margin and high pressure compressor surge margin, respectively, are fed to a comparator 98. The comparator 98 processes the signals 94, 96 and a signal 1 00 is fed therefrom to a PI controller 102. The PI controller 102 then calculates the gain signals 88, 90 which are fed, respectively, to a high pressure recirculation flow controller and a low pressure recirculation flow controller 1 04, 1 06

(Figure 3), respectively.

Hence, the speed controller 70 has three major elements, namely, the resistor bank value controller 72, the high pressure recirculation and low pressure recirculation valve recirculation actuators which are controlled by the recirculation actuator 84 and the surge margin controller.

In use, when the power conversion unit is in speed control mode, the electrical load on the generator is maintained below 6.5 MW and the working fluid inventory of the power generation circuit is approximately at or above 40% of the maximum continuous rating of the working fluid inventory level. For this condition, the high pressure recirculation and low pressure recirculation valves are used to maintain the power produced by the power turbine at low levels. The mechanical power produced in the power turbine is adjusted to the electrical load of the generator by means of the speed controller which controls the setting of the resistor bank and the mass flow rate through the high pressure recirculation valve and low pressure recirculation valve.

The primary control element of the speed controller is the resistor bank 33. As mentioned above, the resistor bank has a power range of approximately 3.2 MW and has a fast response time

(approximately 10 ms). An error signal corresponding to the difference between the power turbine and generator speed and the set point power turbine generator speed is calculated.

This error signal is input for the PI controller 83 which controls the value of the resistor bank. If the value of the resistor bank varies by more than a predetermined amount, typically 0.5 MW, from its nominal setting, the bypass command for the high pressure recirculation and low pressure recirculation valves is adjusted at a rate of about 0.05 kg/s. After the adjustment of the recirculation command is started, it continues until the resistor bank has returned to its nominal setting. The recirculation command signal from the controller is passed to the recirculation actuator which signal represents the total mass flow through both the high pressure recirculation and low pressure recirculation valve.

In order to reduce the risk of damage to the high pressure turbine and low pressure turbine, the surge margin controller determines the factor to which the total recirculation flow is distributed across the high pressure recirculation and low pressure recirculation valves. The factors are controlled in such a way that the difference between the surge margin of the high pressure and low pressure compressors is minimal. The sum of the two factors is 1 .0 in order to ensure that the surge margin controller has only a small effect on the system, as the transfer functions of both components are almost equal.

The speed controller also has the capability to fulfil the synchronization requirements, i.e. synchronizing the generator with an electrical distribution grid. The following initial steps will be performed prior to synchronization, namely, the resistor bank 33 is controlled at its nominal setting, the temperature of the hot inlet side 34.1 of the recuperator 22 is controlled at approximately 10° below its nominal set point and the power turbine generator is stabilized at grid frequency.

Once the steps are completed, an automatic synchronizer of the generator can take over the control of the speed set point. During the synchronization, the hot inlet side temperature of the recuperator and the setting of the resistor bank will be monitored. If these values are no longer within safe operating margins, the synchronization will be stopped and the control will be passed back to the speed controller.

Although as shown in Figure 1 of the drawings and as described above, use is made of a single low pressure recirculation valve 48 and high pressure recirculation valve 51 a more likely arrangement will consist of two sets of compressor recirculation valves. The sets will typically consist of low pressure and high pressure compressor recirculation control valves and low pressure and high pressure compressor circulation valves. Typically the control valves will be used for speed control in the manner described above. However, it will be appreciated that both valves in one or both sets of valves could be used.

Claims

1 . In a nuclear power plant comprising a closed loop power generation circuit which includes a reactor and a power turbine drivingly connected to a generator there is provided a method of regulating the speed of the generator which includes connecting a variable resistor bank electrically to the generator and adjusting the electrical load on the generator via the variable resistor bank to regulate the speed of the generator.

2. A method as claimed in claim 1 , which includes monitoring the actual speed of the power turbine and generator, comparing the actual speed with a desired speed and generating an error signal in response thereto the error signal being fed to a controller which controls the value of the resistor bank.

3. A method as claimed in claim 2, in which the resistor bank has a nominal setting and the method includes, if the resistance of the resistor bank varies by more than a predetermined amount from its nominal setting, adjusting the mechanical power transmitted to the generator.

4. A method as claimed in claim 3, in which adjusting the mechanical power transmitted to the generator includes adjusting the power output of the power turbine.

5. A method as claimed in claim 4, in which when the power generation circuit includes a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, at least one high pressure compressor recirculation line which extends from a position downstream of the high pressure compressor to a position upstream of the high pressure compressor and in which at least one high pressure compressor recirculation valve is mounted and at least one low pressure compressor recirculation line which extends from a position downstream of the low pressure compressor to a position upstream of the low pressure compressor and in which at least one low pressure compressor recirculation valve is mounted, adjusting the mechanical power produced by the power turbine includes adjusting the position of at least one of the or each high pressure compressor recirculation valve and the or each low pressure compressor recirculation valve thereby to adjust the mass flow rate of working fluid through the reactor and hence the mechanical power produced by the power turbine.

6. A method as claimed in claim 5, in which operation of the recirculation valves is regulated by a recirculation actuator which in turn receives a signal from the controller.

7. A method as claimed in claim 6, in which the recirculation actuator distributes the recirculation command in a ratio between the or each of the high pressure compressor and low pressure compressor recirculation valves which is determined by means of a gain determination algorithm, the two gain factors being determined by a surge margin controller which determines the factor to which the total recirculation flow is distributed across the high pressure compressor and low pressure compressor recirculation valves, the factors being controlled in such way that the difference between the surge margin of the high pressure compressor and the low pressure compressor is minimal.

8. A method as claimed in any one of claims 3 to 7, inclusive, in which the nominal setting of the resistor bank is 50% of its full capacity.

9. A nuclear power plant which includes a closed loop power generation circuit which includes a reactor and a power turbine which is drivingly connected to an electrical generator; and a speed controller for regulating the speed of the generator.

10. A plant as claimed in claim 9, in which the speed controller includes a variable resistor bank connectable to the generator, and a resistor bank value controller for controlling the resistor bank and thereby the electrical load on the generator.

1 1 . A plant as claimed in claim 10, in which the power generation circuit includes a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, at least one high pressure compressor recirculation line which extends from a position downstream of the high pressure compressor to a position upstream of the high pressure compressor and in which at least one high pressure compressor recirculation valve is mounted and at least one low pressure compressor recirculation line which extends from a position downstream of the low pressure compressor to a position upstream of the low pressure compressor and in which at least one low pressure compressor recirculation valve is mounted, the speed controller including high pressure compressor recirculation valve and low pressure compressor recirculation valve actuators whereby the positions of the high pressure and low pressure recirculation valves are adjustable to adjust the mechanical power generated by the power turbine, and a surge margin controller.

1 2. A plant as claimed in claim 1 0 or claim 1 1 , in which the speed controller includes a comparator for receiving signals corresponding, respectively, to the actual speed of the generator and the desired speed of the generator, and generating an error signal in response thereto, the resistor bank value controller being in communication with the comparator to receive the error signal and adjust the value of the resistor bank in response thereto.

1 3. A plant as claimed in claim 1 2, in which the resistor bank has a nominal setting intermediate its minimum and maximum settings, the resistor bank controller being configured to generate a recirculate signal should the value of the resistor bank vary by more than a predetermined value from its nominal setting, the recirculation valve actuators being in communication with the resistor bank controller to receive the recirculate signal and to control the operation of the recirculation valves in response thereto.

14. A plant as claimed in claim 1 3, in which the controller includes a surge margin controller for determining the factor to which the required recirculation flow is distributed across the high pressure recirculation valve and the low pressure recirculation valve.

1 5. A plant as claimed in claim 1 4, in which the surge margin controller includes a low pressure compressor surge margin monitor, a high pressure compressor surge margin monitor and a comparator which is in communication with the monitors to receive signals corresponding, respectively, to a low pressure compressor surge margin and a high pressure compressor surge margin, and to generate a signal, the surge margin controller including a proportional integral controller which is in communication with the comparator to receive the signal and to calculate gain signals which are fed respectively to a high pressure recirculation flow controller and a low pressure recirculation flow controller which in turn are in communication with the recirculation valve actuators.

1 6. A method as claimed in claim 1 , substantially as described and illustrated herein.

1 7. A nuclear power plant as claimed in claim 9 substantially as described and illustrated herein.

1 8. A new method or plant substantially as described herein.