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Abstract--This paper proposes an optimized dispatch strategy

for active power cutback control in a wind farm. The strategy is considered as the supplement of the control strategy of the active

power control system for wind farm clusters. Based on the

priority list method, the control strategy is designed to realize the

dynamic control sequence of wind turbines considering

regulation capacity, regulation speed and response time. In this control approach, wind farm is supposed to respond to dispatch

command from centre control station in an optimal and

economical way. The strategy achieves the control target of wind

farm output constraint by pitch angle control and rotor speed

control instead of cutting off turbines. At last, the feasibility and validity of the presented strategy are tested on a wind power

system.

Index Terms--acti ve power control, priority list method, wind

farm

I. INTRODUCTION

Wind power generation has been significantly developed

during the last decades. Due to the intermittency and volatility

of wind power, it brings great challenge to the safe and stable

operation of power system. According to the “Technical

Requirements for Wind Farms Connected to the Grid” issued

by the State Grid, “Wind farm should be qualified with active

power adjustment ability and its active power output can be

controlled according to the dispatch command.” Hence, how

to control the active power output of wind farm actively is

concerned by the grid and the wind farm operators [1].

Recently, several research groups explored dispatch

problems of power system connected with wind farms[2][3].

On the other hand, active power dispatch problem in a wind

farm is also being studied by experts and scholars at home and

abroad[4]. The power control systems in wind farm have been

proposed in [ 5] and [ 6]. Among the studies at wind far m

control level, the power dispatch problem in a wind farm is

received great attentions. Reference [ 7 ] presents a new

integrated control system for a wind farm and its active power

controller is designed to share power reference from the grid

operator to each available machine equally. Automatic

generation control strategy in [8] is designed to dispatch the

control target to selected wind turbines according to utilizat ion

rates of their inverters.

This work is sponsored by Major Program of the National Natural Science

Foundation of China (51190103) and National High Technology Research and Development Program of China (863 Program)(2011AA05A104).

Li Lin, Yongjun Xie are with State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power

University, Beijing,102206, China (email: linli@ncepu.edu.cn; ) Ningbo Wang is with Gansu Electric Power Company, Lanzhou, 730050，

China (email: wangnb19630204@sohu.com).

Recently, reference [ 9 ] introduces a new approach that

applied the traditional unit commitment and dispatch method

to the active power control of wind farms . Reference [ 10]

proposes a classical mathematical model by designing an

objective function for min imizing wind power deviat ions

regarding the dispatch request.

This paper simply describes the active power control system

for wind farm clusters in Jiuquan wind power base and

designs the fundamental framework of monitor & control

system at wind farm level. And then considering real-t ime

operational information of wind turbines, this paper studies an

optimizing control strategy which is suitable for active power

dispatch of wind farm. Based on priority list method, three

parameters, regulation capacity, regulation speed and response

time, are comprehensively considered in the strategy. The

strategy is supposed to avoid cutting off wind turbines blindly

and improve the control ability of wind farm. This paper

focuses on the situation when wind farm output needs to be

constrained.

II. MONITOR &CONTROL SYSTEM FOR WIND FARM

A. The Active Power Control System for Wind Farm Clusters

At present, an active power control system for wind farm

clusters is constructed in Jiuquan wind power base[1]. The

control system framework has four layers, as shown in Fig.1.

Jayu Pass Center

Control Station

Yumen Master

Control Station

(Main or standby)

330kV

Yumen

Slave

Control

Station

Tianyun

Liuyuan

Wind

Farm

Terminal

Control

Station

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Gansu Province

Dispatch Center

Guazhou Master

Control Station

(Main or standby)

330kV

Jiayu Pass

Slave

Control

Station

330kV

Shandan

Slave

Control

Station

330kV

Guazhou

Slave

Control

Station

330kV

Liangzhou

Slave

Control

Station

Jieyuan

Wind

Farm

Terminal

Control

Station

Shisanli-

jingzi

Wind

Farm

Terminal

Control

Station

Changma

Wind

Farm

Terminal

Control

Station

Yumen

Wind

Farm

Terminal

Control

Station

Beida-

qiaodong

Wind

Farm

Terminal

Control

Station

ZhongDian

Juquan

Wind Farm

Terminal

Control

Station

Zhangye

Thermal

Plant

Terminal

Control

Station

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Wind Farm

Supevisory

Terminal

Unit for

Operator in

Thermal

Plant

Fig. 1 Framework of Active Power Control System for Wind Farm Clusters

1): Center Control Station

The station is designed to realize the real-t ime monitor and

control of the whole wind power base. Its main functions

Priority List Method for Active Power Cutback

Control in a Wind Farm

Li LIN, Member,IEEE, Yongjun XIE and Ningbo WANG

IEEE PES ISGT ASIA 2012 1569528117

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include coordinating control, real-time calcu lating and

distributing planned values, automatic rep lying request of

output change of wind farm, switching operation and control

mode and so on.

2): Master Control Station

The station is designed to collect and exchange informat ion

among wind farm, slave control station and center control

station.

3): Slave Control Station

The function of this station is to monitor each interface flow

at 330kV level and upload operating conditions, fault

conditions and overload conditions to the center and master

control station.

4): Terminal Control Station

The station is designed to monitor active power output of

wind farm and dispatches wind farm output plan according to

the selected operating mode of the system automat ically.

The system has been successfully implemented in the wind

power base and the controllability and effectiv ity have been

demonstrated. However, optimal control strategy in wind farm

level has not been studied deeply.

B. Monitor &Control System for Wind Farm

Fig.2 The framework of monitor &control system for wind farm

The function of monitor &control system for wind farm is

supposed to realize the optimized power control of wind farm

response to the dispatch command. The framework is designed

as shown in Fig. 2.

The main functions of power control system shown in Fig. 2

are to receive power prediction signals and operation

informat ion of wind turbines dynamically, controlling the

wind turb ines and compensation device optimally, and

sending feedback signals to superior control station. To realize

those functions, the active power control strategy of the

decentralized controller is one of the important parts.

III. THE ACTIVE POWER CONTROL STRATEGY FOR WIND FARM

A. Passive control mode of wind farm

In the ordinary situation, each wind turbine in wind farm

operates over the maximum power point tracing curve, while

does not response to the active power variation of the grid.

B. Active control mode of wind farm

As the wind power penetration is increasing greatly, wind

farms with the ability of power control would be more suitable

for the requirement of safe and stable operation of power

system. It is infeasible to increase the power output of wind

farm temporarily because of the special characteristics of wind

energy. So this paper focuses on the power dispatch problem

when output power of wind farm needs to be constrained.

Methods for controlling power output of wind farm include

cutting off a row of wind turbines, cutting off the whole wind

farm and regulat ing active output of wind turbine by pitch

angle control and rotor speed control. Regulating act ive power

output of wind turbines is more economical than cutting off

turbines or wind farm directly, which would ensure more

turbines online. So this control method is used in this paper for

output power constraint of wind turbine. The active control

strategy of wind farm is shown as follows.

After receiv ing dispatch command for output constraint

from dispatch centre, the system at wind farm control level

analyzes and calculates the actual constrained value dP with

wind power prediction considered. Besides, the signals which

request for connecting to the grid or increasing output should

be postponed.

Then, considering regulation capacity S , regulation speed

V and response time T of turbines, the control center

dispatches control signals to selected wind turbines following

priority list which is detailed studied below. At last, the

selected wind turbines would operate with the power control

method mentioned above. The proposed control strategy aims

to obtain a minimum deviat ion between the total active power

output of the wind farm and the dispatch command, while

another control objective is to realize the optimal act ive power

dispatch in a wind farm. Considering dispatch accuracy, this

paper takes 15 minutes as one control period. The process of

the calculations of priority list and wind turbine output power

references is studied as follows and turbine i is chosen as a

representative.

1): Calculation of Relevant Parameters of turbines

a. Regulation Capacity Si

The regulation capacity is the adjustable capacity of a wind

turbine with controlling pitch angle actively. Considering

wind velocity variation, turbine i needs to reserve a certain

amount of control margin of p itch angle to prevent wind

turbines online abnormally disconnected from power grid.

Suppose maximum pitch angle of turbine i max max

i i , where max

i is the actual maximum p itch

angle, and is reserve marg in of pitch angle . max

i is

determined by the possible increasing value of active power

output of wind turbine during a control period:

*)(2

1

*)(2

1

1

max

max

3

1

2

max

max

3

2

optiP

optiP

CS

CSP

，

，

(1)

where 1v and

2v are wind velocit ies at the first and last

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moment of a control period. v is the approximate value of

wind fluctuation during the adjacent control period and

2 1v v v . The wind power coefficient

0.255( ) 0.5( 0.022 2)

RC f

f

p

RCC e

(2)

where fC is the blade design constant,

opt is the tip speed

ratio and is wind power generator efficiency.

Assuming that wind velocity 1v is constant during the

control period, regulat ion capacity of turbine i is

3 max

1

1[ ( ) ( )]*

2i w p i p iS S v C C . (3)

And then max

i can be calculated based on (4)

max( )i i i iP P S P (4)

where

max 3 max

1

1( ) ( )*

2i i w p iP S v C (5)

and iP is the active power output of turbine i corresponding to

wind velocity 1v .Through the formulas above, regulation

capacity iS can be calculated at last.

Because of the difficu lties to determine regulation capacity

of wind turbines operating over the maximum power point

tracking curve, this paper focuses on wind turbines operating

in constant rotor speed region.

b. Regulation Speed iV

The regulation speed is defined as the maximum rate of

change of the pitch angle when the wind turbine operates at its

rated power. Regulation speed can be determined by

/i iV P t (6)

where P is a certain power variation and it is the time cost

by pitch angle variation. The parameter can be obtained in the

manual book or in the test.

c. Response Time iT

Response time iT is the t ime ranging from the moment

turbine i receiving control signal to pitch angle control startup,

which can be obtained in the manual book or in the test.

2): Calculation of Regulation Performance Index

The regulation performance index iK is relevant to the

three parameters above, which can determine the priority

levels of wind turbines. The index iK can be calcu lated by (7)

as follows:

i i ii S V TK aK bK cK (7)

whereiSK ,

iVK and iTK are the indexes representing

regulation capacity, regulation speed and response time of

turbine i. a , b and c are the weighting coefficients and

1a b c . This paper takes regulation capacity as prior

factor and sets 0.8 0.1 0.1i i ii S V TK K K K .

As there is no normative standard for optimal control of

wind farm at present, this paper references the requirements of

AGC for the calculat ion of iSK ,

iVK and iTK [ 11 ]. The

assessment process of the indexes is shown as follows.

a. Index of Regulation Capacity

Set regulation capacity index iSK as 1.0 when regulation

capacity takes 10% of the rated capacity of wind turbine. And

when regulation capacity rises or drops 1% of the rated

capacity, iSK increases or decreases 0.015:

1.0 ( / 0.1)*1.5iS i NiK S S (8)

where NiS is the rated capacity of turbine i.

b. Index of Regulation Speed

Suppose that the required regulation capacity per minute

must be more than 10% of the rated capacity. Set 1.0iVK

when regulation capacity per minute miniVS equals to 10% of

the rated capacity. And increase iVK by 0.01 while regulat ion

capacity per minute rises 1% of the rated capacity.

min1.0 ( / 0.1)*1.0i iV V NiK S S (9)

c. Index of Response Time

If the system requests response time must be less than 30s,

set 1.0iTK when 30iT s . 0.5 is used to balance orders of

magnitude of three indexes.

1.0 (1.0 / 30)*0.5iT iK T (10)

3): The Determination of Priority List

According to the regulation performance index of each

turbine, the priority list can be determined finally. Using the

priority list obtained, the units are committed based on their

priority with the highest priority being on first to control

followed by other units in the list accordingly. As shown in

section Ⅳ , the studied wind farm includes three turbines. The

indexes and the priority level of turb ines can be shown in

Table I as fo llows. TABLE I

REGULATION PERFORMANCE INDEX AND REGULATION

PRIORITY OF TURBINES

Unit Serial Num. iK Regulation priority

1 1.23 2

2 1.31 1

3 1.15 3

4): The Calculation of the Output Power References

The decentralized controller determines the wind turbines

participating in output constraint and dispatches the control

targets to selected wind turbines according to the priority list.

Suppose the number of turbines participating in output

constraint is j considering the total adjustable capacity equal to

the dispatch command valuedP approximately. We can get

1

1

j

d i j

i

P S P

(11)

where jP is regulation capacity of the selected turbine which

is in the lowest priority level, namely j jP P where P is

the decreased power value required. Other selected turbines

should take regulation capacity as the adjustable capacity

value, namely i iP S .

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The adjustable capacity P will be sent to each selected

turbine when dispatch command is received. Then ref iP P

will be taken as new output reference as refP is the output

reference when wind generator operates in ordinary control

mode. The control target for output constraint can be achieved

by rotor speed control and pitch angle control.

C. Flow Diagram of the Control Strategy

Flow diagram of the strategy is shown as follows.

The Operation Information

of Wind Turbines

Si Vi Ti

Priority List

Dispatch

Command

Confirm the dispatch command

value and postpone the

connecting signal of wind

turbines

Operate over the

Maximum Power

Point Tracing Curve

JUDGE

According to the priority list,

the adjustable wind turbines can

be selected and ordered

Send the control signals

containing power references to

the selected wind turbines

limit output based on rotor

speed control and pitch

angle control

iSKiVK

iTK

No Demand

iK

When the strategy cannot satisfy the dispatch demand of

output constraint in a certain period, wind turbines should be

cut off following a principle then.

IV. SIMULATION RESULTS

A test system is presented to test the proposed methodology,

using a small wind farm with three equivalent turbines

(6*1.5MW nominal capacity for each). In the test system each

wind generator would be connected to its own transformer

without the need of any internal wind park grid

reconfiguration. Line capacity restrictions are also not

considered. The wind farm is connected to an infinite bus bar

which represents the grid, as shown in Fig.3.

Fig.3 The test system

The system simulated and analyzed based on Matlab/

Simulink tool.

Wind speed and the priority level of each turbine are shown

in Table Ⅱ. The controller receives dispatch command at 10s

and 7.36MW need be reduced for safe and stable operation of

the power system in this simulat ion. The result would be

compared with conventional control strategy which constrains

wind farm output by cutting off turbines. TABLE Ⅱ

WIND SPEED AND REGULATION PRIORITY OF TURBINES

Unit Serial Num. Wind Speed(m/s) Regulation priority

1 24.5 2

2 16 1

3 12 3

TABLE Ⅲ UNIT COMMITMENT WITH TWO STRATEGIES

Unit Serial Num. Proposed Strategy Conventional Strategy

1 1 1

2 1 1

3 1 0

(a) Output performance with priority list strategy

(b) Output performance with cutting turbine strategy

Fig.4The variation of total output of wind farm with active control (MW)

Table Ⅲ shows unit commitment of wind turbines with the

two control strategies alternatively applied(1 represents

online). And Fig.4 presents the simulation results of the two

strategies respectively. It is obvious that the proposed method

in this paper is more effect ive as the performance in Fig.4 (a)

is much more accurate than in Fig.4 (b) and the number of

turbines which needs to be cut off is reduced.

V. CONLUSION

Considering the fundamental thought of active power

control system of wind power base and classical unit

commitment and dispatch strategies , a priority list method is

proposed for active power output constraint at wind farm

control level. The proposed strategy determines the dynamic

priority list of adjustable turbines considering regulation

capacity, regulation speed and response time. Through

sequential control of turbines, this strategy can realize fewer

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turbines disconnected from the grid and improve the ability of

optimal operation of wind farm. The simulation results show

the validity and feasibility of the method proposed.

VI. REFERENCES

[1] Li Xueming, Xing Zhou, Chen Zhenhuan, Chen Yonghua, Wang Fujun

and Luo Jianbo, “Design of Large Clusters of Wind Power Active

Intelligent Control System,”Automation of Electric Power Systems, vol. 34, pp. 59-63, Sep. 2010

[2] Liu Jun, Zhou Xichao, “Wind farm automatic generation control based on ultra-short-term wind-power forecasts,” Electric Power, vol. 44, pp. 74-77,

Feb. 2011 [3 ] Jung-Hyun Choi, Seon-Ju Ahn, Jin-Woo Park,Seung-Il Moon, “Active

power limitation of wind farm to reduce system operating cost ,” in Proc. 2009 Transmission & Distribution Conference & Exposition: Asia and

Pacific Conf., pp. 1-4 [4] Li Lin, Shiqian Wang, “Investigation of the strategy of wind farm power

regulation considering system frequency regulation demand,” in Proc.

2011 4th International Conference on DR PT, pp. 1138- 1143 [5] Hui Jing, Gu Xin, “Research on centralized power control strategies for

large wind farms,” East China Electric Power, vol. 36, pp. 57-61, Jun. 2008

[6 ] Mark Cardinal, “Monitoring, Control, and Automation of Large Wind Plants,” in Proc. 2008 IEEE/PES Transmission and Distribution Conference and Exposition, pp. 1-6

[7 ] Jose Luis Rodriguez-Amenedo, Santiago Arnalte, Juan Carlos Burgos,

“Automatic generation control of a wind farm with variable speed wind turbines,” IEEE Trans. On Energy Conversion, vol. 17, pp. 279-284, Feb. 2002

[ 8 ] Qiao Ying, Lu Zongxiang, “Wind Farms Active Power Control

Considering Constraints of Power Grids,” Automation of Electric Power Systems, vol. 33, pp. 88-93, Nov. 2009

[ 9 ] Moyano C.F., Lopes J.A. Pecas, “Unit Commitment and Dispatch

Strategies for a Wind Park ,” Power Engineering, Energy and Electrical Drives, 2007. 2007 , Page(s):12-17

[10] Liu Wei, “Study on Optimum Dispatching of Wind Farm Clusters,” M.D. dissertation ,Beijing Jiaotong University, 2011

[11] Lin Wanjing, Liu Rao and etc., “AGC Requirement Determination and Unit Selection in the Generation Market,” Automation of Electric Power Systems, vol.19, pp. 17-21, Oct. 2004

I. BIOGRAPHIES

Li Lin successively received her B.Sc. degree, M.Sc. degree and Ph.D.

degree in electrical engineering from North China Electric Power University, Beijing, China in 1991, 1997 and 2009. Now she is an associate professor in the School of Electrical and Electronic Engineering, North China Electric Power University. Her research mainly focuses on power system analysis,

operation and control. Yongjun Xie received a B.E. degree in electrical engineering from

Shandong University of Science and Technology, China, in 2010. He is

currently pursuing a M.Sc. degree at North China Electric Power University. His research interests are in power system analysis, operation and control and power system including wind power.

Ningbo Wang is a senior engineer in Gansu Electric Power Company. He

mainly focuses his research on wind power technology and power system planning.

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