[ieee 2011 ieee international conference on advanced power system automation and protection (apap) -...

2011The International Conference on Advanced Power System Automation and Protection

APAP2011 www.apap2011.org

*Corresponding author (email: [email protected])

Optimization and control for thermal power plant based on plantwide control

Hao Fei, Gu Quan

Nanjing Nari-relays Electric Co.,LTD, Nanjing 211102, China;

Abstract: In order to achieve the economic distribution of the power unit and to ensure the area voltage quality and meet the separation of generation and the economic dispatch, according to the design and equipment configuration of the direct transfer mode of the dispatching system. On the supervisory level, a new power plant control architecture based on plantwide control is proposed. The plant-level optimal load distribution system and the voltage and reactive power control system on the plant side will be integrated into the electrical power plant network control system and constitutes the collaborative optimization controlsystem with the voltage and reactive power on the plant side. The design not only realizes their function, but also interacts with each other information. It adopts the integrated idea, which makes the acquiring information and executing command more fluently and achieves the active power’s economic dispatch and the reactive power’s optimization distribution. At the same time, on the optimization and control level, in order to get more information about the power plant side from the dispatching level, excitation system model based common information model (CIM) is built in the network control system, which is de-signed based on the analysis of current excitation systems. and embedded into existing CIM by inheritance and association. it will be a bridge between the voltage and reactive power control system and energy management system, Which will be a better guarantee of the power plant side of the AVC control for the accuracy and security.

Keywords: plantwide control, common information model, voltage and reactive power control, network control sys-tem, excitation system

1 Introduction

Plantwide control deals primarily with the structural issue of the control system, and determines what to measure and control, which inputs to use, and how to pair these sets of variables by quantifying methods, in order to look for using the simplest method to satisfy the system control requirement [1]. In order to improve and achieve the plantwide control and optimization for the thermal power plant, it will change the current power grid direct dispatch mode, which the power grid side sends the generating order to the power unit of the power plant. This dispatch mode is no good for the power plant to achieve the plantwide control and optimization, so the power plant side needs more decision-making power to dispatch the total active power order and voltage target, which receive from the center dispatching of power grid, according to the run-ning state and generating efficiency of each power unit. The plant-level optimal load distribution acquired the online operating data, fitted out real-time characteristic curve of each unit’s coal consumption, and achieved the dynamic problem of optimal load distribution, finally sent the distri-bution result to the coordination control system[2]. So as to enhance the region's power supply voltage level and im-

prove the regional power grid power quality[3]. Voltage and reactive power control system (AVC) by the power plant side receives the bus voltage target orders transferred through the central scheduling in real-time. It combines with the local bus voltage measuring and the whole power plant of the operation of generation units, in accordance with the established allocation strategy, dispatched the ra-tional allocation of reactive power to the appropriate units, fast and accurately tracking of the voltage target, to improve the power plant of high-voltage bus voltage level. In the past, the plant-level optimal load distribution system and the voltage and reactive power control system were independ-ent, one was a apartment of supervisory information system (SIS), another installed in the electric network control sys-tem (NCS). Such configuration will bring difficulty, if the power plant dispatch mode will changed from the direct scheduling to the plantwide mode. In this paper, the plant-level optimal load distribution system will integrate with the voltage and reactive power control system, by way of a part of the network control system. The design not only realizes the function of their own, but also interacts with each other information; it adopts the integrated idea, which makes the acquiring information and executing command more fluently, and achieves the active power’s economic dispatch and the optimization distribution of the reactive

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power.

2 Structure of optimization and control system

The configuration for the thermal power plant is shown in figure 1.In according to the plantwide control method, it could be separated into four control levels by the way of top-down analysis [4-6]. The top of the system was dis-patching control level, which was belong to the power grid cooperation, and installed the automatic generation control software and the automatic voltage control software to achieve the adjust of the active power and reactive power automatically for the whole power grid. The second level was NCS supervisory control level, which was responsibil-ity for receiving the active power and the bus voltage orders

Figure 1 Integrated optimization of power plant control system structure

from the first control level, and acquiring the thermal power plant running data for optimization and control meanwhile. The third level is the plant-level optimal load distribution system and the voltage and reactive power control system, which could make apartment of the NCS. the fourth level is the regular control level, which consisted of the unit coor-dinate control system and the excitation system.

The excitation system was the executing end of the volt-age and reactive power control system, so it could be nec-essary to build the model of the excitation system, which would be improve the control accuracy and security of the AVC, and send more information to the upper control level. The research of plant-level optimal load distribution sys-tem was based on the curve of each unit’s coal consumption,

the accuracy of the curve effected the optimal result of the distribution, so it was necessary to update the coal con-sumption, in according to each unit’s operation. The formu-la used to calculate the standard coal consumption curve as follows:

0

ii

i

qb

Q �� (1)

0

i ii i i

i

p qB p b

Q �� (2)

In order to acquire the online simulated data of the charac-teristic curve between the coal consumption and load and correct the data real-time, it would be necessary to get the data as follows: pi, qi, , Q0. Where qi is the heat consump-tion rate of the power unit, Q0 is the low temperature fever heat of the standard coal. The is the boiler efficiency, pi

is the power output of the unit. Which could be acquired from the generator measure and control set, qi, Q0, andcould be acquired from the DCS. Otherwise, it is necessary to send the real burning coal quantity of each power unit to NCS. Meanwhile, the environment protection data can be received from the corresponded measuring set. The plant-level load optimal distribution system can formulate better and more accurate unit’s power distribution policy based on these data above. The superiority of the structure in this paper is that the whole information supplied by the plant-level optimal load distribution and the voltage and reactive power control sys-tem for the NCS, can be used for separate system. Both of them are in the NCS platform, and can share all the data on the network, and can share and use the analysis result di-rectly. Otherwise, there are the environment protection data in the NCS platform, both of them can use these data, such as the emission smoke quantity, sulfur dioxide information, nitrogen and oxygen substance information. Using the method of online identification could acquire the simple model of the environment protection data, and forecast the variety trend, in virtue of the data of real-time data, such as active power, reactive power, and other information of the unit. The excitation system was the executing end of the voltage and reactive power control system. In order to im-prove the accuracy and security, and supply more infor-mation for the upper dispatching end convenient, it could be necessary to build the excitation system model based on the CIM standard. Because the power grid equipment models in the dispatching system formed, in according to the CIM standard.

3 Excitation system model based on CIM

IEC61970-301 standard defines the application program interface (API), common information model is a important part of the standard, and regulates the meaning of the API.


Figure 2 shows the simplified relationship between the synchronous machine and the other models. In the figure the excitation system and the synchronous machine has the in-divisibility relationship, but there is no excitation system model in the CIM[7-8]. In order to get more information for the power plant voltage and reactive power control system, and supply more model and data information for the dis-patching end, it needs to build the excitation system model on the power plant side. Firstly, it needs to integrate the excitation system into the CIM. In the current CIM, the classes including the regulat-ing and control information drives from the “Regulat-ingCondEq”, each class of “RegulatingCondEq” builds the relationship of the control class, and the control class con-tains all of the description about it. Otherwise, a special class of “RegulatingSchedule” is a necessary part of the class of “RegulatingCondEq”. So the relationship of the excitation system model and the orient CIM shows as fol-lows: the class of “ExcitationSystem” drives from the class of “RegulatingCondEq”, which could drives the control information and description, meanwhile adds the descrip-tion of “RegulatingSchedule”. Figure 3 shows their rela-tionship.

Fig 2 Synchronous Machine Model in the CIM

Fig 3 Description of the Excitation System Model in the CIM

According to the modeling principle and method of the CIM and the excitation system characteristic and makeup, a

detail model for the excitation system can be build, and the relation diagram shows as the figure 4. The class of the “ExcitationSystem” divides into the class of “SeperateExci-tationSystem” and “SelfExcitationSystem”. It is necessary to build a series class as follows: “Exciter”, “PowerTrans-former”, “Converter”, “Rectfier”, “AutoVoltageRegulator”, “PowerSystemStabilizer” .The class of “PowerTransformer” borrows from the CIM. Because the self-excitation system needs the power transformer to get the excitation current from the generator terminal and the plant bus, and acquire the excitation current from the generator neutral side and the outlet side by the converter, it is necessary to build the class of “PowerTransformer” and “Converter” for the self-excitation system . The separate excitation system needs the AC vice exciter to supply excitation current, so it is necessary to build a class of “Exciter”. Both the seperate excitation system and the self-excitation system need the rectifier, so it is necessary to build the “Rectfier” class. For the regulation of the excitation system, the control mode of “AVR+PSS” is used widely, the auto voltage regulator achieves the excitation voltage automatic regulation, and the power system stabilizer controls the exciter output by the excitation regulator. So it is necessary to build the class of the “AutoVoltageRegulator” and “PowerSystemStabilizer” to accomplish the regulation of the excitation system. Thus the whole modeling of the excitation system is over and can use it in the power plant side voltage and reactive control system. In order to acquire better control effect for the voltage and reactive power control system, it is necessary to build the dynamic excitation system model to achieve the fore-casting control and avoid the over- regulation. The dynamic model of the excitation is simple, and it consists of a series of classical control factor, such as proportion regulation, limited range[9]. the controller adopts the PI. So the transfer function model of excitation system can be used as the dy-namic model. After there are both the static and dynamic


Fig 4 Detail Description of the Excitation System Model

model, Offline learning and online identification could be done for the excitation, which could get the key parameter for the voltage and reactive power control system, such as the thermal power plant system feedback coefficient, gener-ator feedback coefficient, the system impedance and per regulation quantity of the reactive power. These parameters can improve the system accuracy largely.

4 Application of optimization and control sys-tem

The structure shown on figure 1, contains four control levels. The top is the electric dispatching centre, which be-longs to the power grid side. At the thermal power plant side, it should comply with the order from the upper level. The order includes the active power and reactive power needs for the thermal power plant. The plant-level load optimal distribution system is responsible for the distribution of the active power and the voltage and reactive power control is responsible for the distribution of the reactive power. The last one receives the high bus voltage order from the AVC installed on the electric dispatching centre mostly. Because the excitation system, which is the main system. In order to achieve the distribution of reactive power to each excitation control system, it needs to change the high bus voltage or-der into the whole plant reactive power demands. According to the theory of the voltage and reactive control system, where realU is the high bus voltage. The bus supply the re-

active power to the power grid system is realQ� where

preU is the high bus voltage before adjustment, and

preQ� is the total reactive power of the bus. So their rela-tionship is follows:

( ) ( )prerealreal pre

real pre

UQQ

U XU U

� � � �� (3)

According to the formula 3, when the order from the dis-patching centre is tgtU the total reactive power of the bus

is tgtQ� the formula 4 could be built as follows:

( )tgt

tgt real tgt real tgt

real

QQ

U U U UX U

� ��

�� (4)

After acquiring the total reactive power of the power plant, the next requirement is the distribution, the total reactive power should be divided into each power unit. The distribu-tion has four control strategies. The first is “equal power factor control”, the second is “similar apparent power”, the third is “equal reactive power reserve”, and the last one is “similar reactive power margin”. The high bus voltage of the thermal power plant is correspond to the reactive power output of each generator. Maybe several power generator connect one high bus voltage. Actual operation experience has shown that the terminal voltage of each generator is proportional to its reactive power output, so it is necessary to control each unit’s reactive power reserve.

2

2( ) ( )i i i igen i tgt

i tgt i tgt

Q X P XU i k U

k U k U�

� � � � ��

(5)

According to the distribution of the reactive power for each unit, the formula 5 could give the calculation of the terminal voltage of each unit. Where ( )genU i is the terminal voltage of unit, iX is the power transformer’s impendence that connect the unit, ik is the power transformer proportion, iPis the unit active power, iQ is the unit reactive power. Through the formula 5, the active power and the reactive power exist the relationship of constraint each other.

In most of the AGC applications, it is assumed that there is no interaction between the power/frequency and reactive power/voltage control loops. It may be permissible only when the speed of the excitation systems is much faster than that of the LFC system, but in practical systems, during dy-namic perturbations, there does exits some interaction be-tween these two control channels. So it is necessary to in-vestigate the damping effects of voltage control, assuming that i) reactive power/voltage control loop has a much faster response than power/frequency control loop and, thus, tak-ing the area voltage perturbation to be directly available as a control variable. And ii) area voltage perturbation does not have any effect on the area load. Considering these assump-tions unrealistic, a realistic load frequency control (LFC)


model was developed by including the excitation control in one area and voltage-perturbation as the input in the other [10-11]. In the thermal power plant, the change in load de-mand from the upper AGC is respond to the power unit’s coordinated control system, and its executive control means are the speed governor control loop and the fuel quantity. For voltage-dependent load characteristics on stabilizing intersystem oscillations, the exciter and speed governor control loops could be introduced into the plant-level load optimal distribution system.

Through the formula 5, it can build a constraint relation-ship between the active power and the reactive power, and build a connection bridge between the plant-level load op-timal distribution system and the voltage and reactive power control system meanwhile. They can send and receive in-formation for each other easily, because they are in the same application platform (NCS). Otherwise, the dynamic and static excitation system models are introduced into the voltage and reactive power control system, and coordinated control system are introduced into the plant-level load op-timal distribution system. At the supervisory and mak-ing-decision control levels, the simulation and forecast con-trol can be built based on these control system and loops. When the upper dispatching level (AGC, AVC) sends orders to the thermal power plant side, the supervisory and mak-ing-decision control levels can make the control decision according to the real measuring data and the models, which can solve the coordination of the frequency and the voltage, and prevent the unit from over-regulation.

5 Conclusion

In this paper a new thermal power plant structure puts for-ward, which concerns the active power output and the reac-tive power output. The plant-level load optimal distribution system and the voltage and reactive power control integrates into the NCS platform as its advanced application. The su-periority of this structure has four points as follows:

Firstly, when the thermal power plan would change its dispatching mode from direct dispatch to plant-level distri-bution, the structure in this paper could save the costs and expense, reduce the difficulty of the project construction, and utilize the old equipment and measurement set at most.

Secondly, because the plant-level load optimal distribu-tion system and the voltage and reactive power control sys-

tem on plant side integrate into one platform, they not only achieve function themselves, but also sends information and analysis result to each other, which can improve their con-trol and optimization level. Adopting the integrative design idea makes the acquiring information and executing orders fluently. Thirdly, the static and the dynamic models drive from the configuration of the excitation system, and use the model in the voltage and reactive power control system on plant side. The identification of the key parameters can supply war-ranty for accuracy control. Fourthly, the unit coordinated control system and the excitation system model are introduced into the supervisory and making-decision control level, the simulation and fore-cast control can be built based on these control system and loops. Before the control orders send to the executive con-trollers separate, the system can determine that whether the order is right or not, and judge the order that the control result in the active power and the reactive power over regu-lation.

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