WO2009134770A1

WO2009134770A1 - Apparatus and method for increasing efficiency in power generation plants

Info

Publication number: WO2009134770A1
Application number: PCT/US2009/041933
Authority: WO
Inventors: Jiuping Pan; Milovan Grbic; Dennis Kron; Lars Gertmar; Mikael Dahlgren
Original assignee: Abb Research Ltd.
Priority date: 2008-04-28
Filing date: 2009-04-28
Publication date: 2009-11-05

Abstract

It is presented an apparatus for reducing reactive power consumption of auxiliaries in an electrical power generation plant. The apparatus comprises: at least one adjustable speed drive with an associated motor; at least one reactive power consuming auxiliary device; at least one sensor for measuring a reactive power; and a controller. The controller is connected to the at least one sensor and the at least one adjustable speed drive, for controlling the at least one adjustable speed drive such that a reactive power of the at least one adjustable speed drive at least partly compensates a reactive power consumed by the at least one reactive power consuming auxiliary device. A corresponding method and computer program product are also presented.

Description

APPARATUS AND METHOD FOR INCREASING EFFICIENCY IN POWER

GENERATION PLANTS

FIELD OF INVENTION

The present invention relates generally to electrical power generation plants, and more particularly to controlling reactive power consumption in auxiliaries of electrical power generation plants to increase efficiency.

BACKGROUND Thermal power generation plants remain one of the main means for generation of electricity, in e.g. fossil fuelled plants and nuclear plants. It is estimated that up to 8 to 10 percent of its generated power is consumed for its internal electric consumption in so-called auxiliary loads. The auxiliary loads comprises e.g. fans, compressors, pumps, that are necessary for the operation of the plant, performing tasks such as feeding water, feeding air for combustion, transport of fuel and waste, etc .

In a typical large scale plant, the most energy consuming loads are fan, pump and compressor drives, typically rated 1 — 5 MW and consuming 5 to 40 GWh per year. In view of demands for energy efficiency it would be beneficial to provide more power with the same CO₂ emission. Consequently, it is desirable to reduce the power consumption of the auxiliary loads. Not only reduction of active power consumption would be beneficial; it would also be beneficial to reduce reactive power consumption. SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and method to reduce the reactive power required for auxiliaries of a power generation plant.

According to an embodiment of the invention there is provided an apparatus for reducing reactive power consumption of auxiliaries in an electrical power generation plant. The apparatus comprises: at least one adjustable speed drive with an associated auxiliary motor; at least one reactive power consuming auxiliary- device; at least one sensor for measuring reactive power,- and a controller. The controller is connected to the at least one sensor and the at least one adjustable speed drive, for controlling the at least one adjustable speed drive such that a reactive power of the at least one adjustable speed drive at least partly compensates a reactive power consumed by the at least one reactive power consuming auxiliary device.

This allows for a reduction of consumption of reactive power in the auxiliary devices. Furthermore, the reactive power capacity out of the plant can be increased. The reduction of reactive power in auxiliary device and/or increased reactive power capacity out of the power plant results in less stress of the main generator being used for reactive power. Hence, more active power can be produced by the power plant. Due to the large capacity of typical electrical power generation plants, even seemingly small reductions of power usage result in significant savings.

The at least one sensor may be configured to measure reactive power associated with the at least one adjustable speed drive and the at least one reactive power consuming auxiliary device. This allows for efficient control of the reactive power consumed by the auxiliaries as a whole.

The apparatus may further comprise a sensor for measuring a total supplied reactive power for the electrical power generation plant, wherein the controller may be configured to control the adjustable speed drive to contribute to a supplied reactive power of the electrical power generation plant to approach a desired supplied reactive power. In other words, by using appropriate sensors, the adjustable speed drives can be controlled to assist in achieving an overall desired value of reactive power.

The at least one reactive power consuming auxiliary device may be a constant speed motor .

The at least one adjustable speed drive and the at least one reactive power consuming auxiliary device may be connected to an auxiliary power bus.

The auxiliary power bus may be connected to a main power line of the power generation plant via a power transformer, and the at least one sensor may comprise sensors for measuring voltage and current of at least one side of the power transformer.

Each of the at least one adjustable speed drive may comprise: a rectifier; an energy storage device connected to the rectifier; and an inverter connected to the energy storage device. The controller is connected to the rectifier to control the reactive power produced by the adjustable speed drive. The rectifier may comprise a voltage source converter.

The controller may be connected to the at least one sensor and the at least one adjustable speed drive via a data bus .

A second aspect of an embodiment of the invention is a method for reducing reactive power consumption of auxiliaries in an electrical power generation plant. The method comprises the steps of: measuring reactive power,- and controlling at least one adjustable speed drive, with an associated auxiliary motor, such that a reactive power of the at least one adjustable speed drive at least partly compensates a reactive power consumed by at least one reactive power consuming auxiliary device of the power generation plant .

A third aspect of an embodiment of the invention is a computer program product comprising software instructions that, when executed in a controller of a power generation plant, performs the method according to the second aspect .

It is to be noted that any feature of the first aspect may be applied to the second and third aspects.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, device, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic diagram illustrating the use of an embodiment of the present invention in an electrical power generation plant, and

Figs 2a-c are phasor diagrams illustrating how, in the embodiment of Fig 1, the reactive power in the auxiliaries can be eliminated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig 1 is a schematic diagram illustrating the use of an embodiment of the present in an electrical power generation plant, typically a thermal power generation plant, e.g. a fossil fuel power plant or a nuclear power plant. A main power generator 101, as known in the art, generates electricity from mechanical energy, e.g. from steam rotating a steam turbine. A conventional main transformer 103 converts the electrical power from the main generator 101 to a voltage which is suitable for transmission .

An auxiliaries transformer 105 converts electrical power from the main generator 101 to a voltage which is suitable for auxiliary loads. This voltage is supplied on an auxiliary bus 106. The auxiliary loads comprises e.g. fans, compressors, pumps, that are necessary for the operation of the plant, performing tasks such as feeding water, feeding air for combustion, transport of fuel and waste, etc.

According to an embodiment of the present invention, there are constant speed drive motors 107a- z and adjustable speed drives 109a-z, also known as variable speed drives, with associated motors 117a- z. All these motors are used for auxiliary loads. Note that there may be any number of constant speed drive motors 107a- z as long as there is at least one. Analogously, there may be any number of adjustable speed drives 109a- z with associated motors, as long as there is at least one. In this embodiment, each adjustable speed drive 109a-z comprises a rectifier llla-z, a storage device 113a-z, such as a capacitor and an inverter 115a-z. The rectifiers llla-z can comprise a voltage source converter. With this arrangement, the phase of the power consumed can be controlled by controlling the rectifiers llla-z. Hence, the size of reactive power associated with the adjustable speed drives 109a-z for powering the motors 117a- z can be controlled. Figs 2a-c are phasor diagrams illustrating how, in the embodiment of Fig 1, the reactive power in the auxiliaries can be eliminated.

In Fig 2a, the phasor diagram shows the apparent power consumed by one or more constant speed drive motors 107a- z (Fig 1) . P_c represents the active power consumed, while Q_c represents the reactive power consumed, due to e.g. the coils in the motors. S_c represents the resulting apparent power.

In Fig 2b, the phasor diagram shows the apparent power associated with one or more rectifiers llla-z in adjustable speed drives 109a-z, controlled to compensate for the reactive power of the constant speed drive motors 107a-z. In analogy with Fig 2a, P_A represents the active power consumed, Q_A represents the reactive power and S_A represents the resulting apparent power, but in this case relating to the one or more adjustable speed drives 109a- z. Note that Q_A has the same length as Q_c, but is opposite in direction.

Fig 2c shows the combined power consumption for the constant speed drive motors 107a- z and the motors with adjustable speed drives 109a-z. As can be seen, since Q_A and Qc are identical but of opposite direction, when a vector addition is performed using S_A and S_c, the resulting vector S_c+A is equal to P_c+A. In other words, the reactive power Q_A eliminates the reactive power Q_c.

It is clear from Fig 2a-c that if another angle of the apparent power S_A of Fig 2b is chosen, S_c+A would not be purely real. In other words, by selecting the angle of S_A/ the combined reactive power Q_c+A can be affected. Returning to Fig 1, in one embodiment, there are two sensors 120a-b, each of which can measure the reactive power for all auxiliaries. The sensors 120a-b can for example comprise a voltmeter, amperemeter and phase meter for deriving the reactive power. In this embodiment, the sensors 120a-b are comprised in a device 119 for protection of the auxiliaries transformer 105. It is to be noted that only one of the sensors 120a-b is needed to be able to measure the reactive power, not necessarily two sensors 120a-b as shown in Fig 1.

The sensors 120a-b provide a signal containing at least the reactive power consumed to a controller 125. The controller 125 can comprise a commercially available CPU (Central Processing Unit) , DSP (Digital Signal Processor) or any other electronic programmable logic device. The controller 125 may be an integrated unit or it may be distributed over several modules, wherein the distributed modules would communicate with each other as necessary.

Using the signal from the sensors 120a-b containing the reactive power consumed by the auxiliaries, the controller 125 can send a signal to the adjustable speed drives 109a-z to adjust the reactive power of these adjustable speed drives 109a- z, in order to reduce or eliminate reactive power consumed by the auxiliaries, as illustrated in Figs 2a-c. This operation is repeated to reach the desired level and to adjust to any changes in operating conditions.

In the operation just described, called a first operating mode, the controller 125 is configured to reduce or eliminate the reactive power consumed by the auxiliaries. In a second operating mode, other sensors can be arranged to provide signals containing a reactive power to the controller 125. In one embodiment, there are two sensors 124a-b which can measure a reactive power produced by the entire power generating plant. The sensors 124a-b can for example comprise a voltmeter, amperemeter and phase meter for deriving the reactive power. In this embodiment, the sensors 124a-b are comprised in a device 123 for protection of the main transformer 103. It is to be noted that for the sensors 124a-b to be able to measure the reactive power, only one of the sensors 124a-b is needed, not necessarily two as shown in Fig 1. The sensors 124a-b sends a signal to the controller 125. Consequently, the controller 125 can in this operating mode adjust the reactive power of the adjustable speed drives 109a-z to achieve, or approach, a desired reactive power of the entire power generating plant. This desired reactive power could be a set value, either as commanded from a plant operator, either independently or as a result of a command from a transmission system operator. The reactive power can be a result of desired voltage control of the power generating plant. As for the first operating mode, the control cycle is repeated to reach the desired state or to allow for any variations in operating conditions.

In the second operating mode, alternatively or additionally, a second set of sensors 122a-b measures a reactive power of the main generator 101 and supplies this in a signal to the controller 125. These sensors 122a-b can be embodied in a device 121 for protection of the main generator 101. Optionally, all sensors 120a-b, 122a-b, 124a-b are connected to the internal power buses using transformers, for added security.

Optionally, all sensors 120a-b, 122a-b, 124a-b can communicate with controller 125 over a data bus 118, simplifying connection between sensors 120a-b, 122a-b, 124a-b and controller 125. Additionally, the signals from the controller to the adjustable speed drives 109a-z can be sent over the same data bus 118, or a different data bus (not shown) .

Advantageously, the adjustable speed drives 109a-z are dimensioned with enough spare capacity to potentially handle alert and emergency situations of the power generation plant and the adjacent power system. Moreover, although not an exceptional case per se, when disconnecting or significantly reducing a load from the power generation plant, the voltage will rise. This phenomenon is known as the generator load rejection and if this effect is expected, it is desired to select and dimension any adjustable speed drives 109a- z with this in mind. In particular, the rectifiers 109a-z interfacing the auxiliary bus 106 should have the correct specifications .

It is to be noted that the present invention can be applied in power generation plants with three electrical phases, as well as any phase configuration, including one and two phases .

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. An apparatus for reducing reactive power consumption of auxiliaries in an electrical power generation plant, said apparatus comprising: at least one adjustable speed drive (109a-z) with an associated auxiliary motor (117a-z) ; at least one reactive power consuming auxiliary- device (107a-z); at least one sensor (120a-b, 122a-b, 124a-b) for measuring reactive power,- and a controller (125) ; c h a r a c t e r i s e d i n t h a t said controller (125) is connected to said at least one sensor (120a-b, 122a-b, 124a-b) and said at least one adjustable speed drive (109a-z) , for controlling said at least one adjustable speed drive (109a-z) such that a reactive power of said at least one adjustable speed drive (109a- z) at least partly compensates a reactive power consumed by said at least one reactive power consuming auxiliary device (107a-z) .

2. The apparatus according to claim 1, wherein said at least one sensor (l20a-b) is configured to measure reactive power associated with said at least one adjustable speed drive (109a-z) and said at least one reactive power consuming auxiliary device (107a-z) .

3. The apparatus according to claim 1 or 2 , further comprising a sensor (124a-b) for measuring a total supplied reactive power for the electrical power generation plant, wherein said controller (125) is configured to control said adjustable speed drive (109a- z) to contribute to a supplied reactive power of the electrical power generation plant to approach a desired supplied reactive power.

4. The apparatus according to any one of the preceding claims, wherein said at least one reactive power consuming auxiliary device (107a-z) is a constant speed motor (107a-z) .

5. The apparatus according to any one of the preceding claims, wherein said at least one adjustable speed drive (109a-z) and said at least one reactive power consuming auxiliary device (107a-z) are connected to an auxiliary power bus (106) .

6. The apparatus according to claim 5, wherein said auxiliary power bus (106) is connected to a main power line of said power generation plant via a power transformer (105) , and said at least one sensor comprises sensors (120a-b) for measuring voltage and current of at least one side of said power transformer (105) .

7. The apparatus according to any one of the preceding claims, wherein each of said at least one adjustable speed drive (109a-z) comprises: a rectifier (llla-z) ; an energy storage device (113a-z) connected to said rectifier; and an inverter (H5a-z) connected to said energy storage device, wherein said controller (125) is connected to said rectifier to control the reactive power produced by said adjustable speed drive (109a-z) .

8. The apparatus according to claim 7, wherein said rectifier (llla-z) comprises a voltage source converter.

9. The apparatus according to any one of the previous claims, wherein said controller (125) is connected to said at least one sensor (120a-b, 122a-b, 124a-b) and said at least one adjustable speed drive (109a-z) via a data bus (118) .

10. A method for reducing reactive power consumption of auxiliaries in an electrical power generation plant, comprising the steps of: measuring reactive power,- c h a r a c t e r i s e d i n that said method comprises the step of: controlling at least one adjustable speed drive (109a-z), with an associated auxiliary motor, such that a reactive power of said at least one adjustable speed drive (109a-z) at least partly compensates a reactive power consumed by at least one reactive power consuming auxiliary device (107a-z) of said power generation plant.

11. A computer program product comprising software instructions that, when executed in a controller (125) of a power generation plant, performs the method according to claim 10.