Scholarly article on topic 'Coordination of Generation and Transmission Planning for Power System with Large Wind Farms'

Coordination of Generation and Transmission Planning for Power System with Large Wind Farms Academic research paper on "Materials engineering"

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Energy Procedia
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{"wind farm" / "generation expansion" / "transmission expansion" / "coordinated planning"}

Abstract of research paper on Materials engineering, author of scientific article — Xiufan Ma, Ying Zhou

Abstract This paper proposes a model of coordination planning between the power plants and the power network including large wind farms. The objective function consists of expansion costs of both transmission and generation. In the part of transmission planning cost, it uses risk factor to express risky constraints, and risk assessment fee to evaluate overload risk. The risk factor has realized quantization of risk, so it is able to deal with the uncertainty of wind power output and loads. The risk assessment cost reflects transmission programming risk as an economic index, which is comparable with the network investment cost. In the part of generation planning cost, the environmental costs of all generators are used to achieve optimal comprehensive social benefits. The impacts of wind farms on system peak regulation and frequency regulation are considered in the form of constraints.

Academic research paper on topic "Coordination of Generation and Transmission Planning for Power System with Large Wind Farms"

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Energy

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Energy Procedía 16 (2012) 1979 - 1985

2012 International Conference on Future Engery, Environment, and Materials

Coordination of Generation and Transmission Planning for Power System with Large Wind Farms

Xiufan Ma, Ying Zhou

Electricity Market Research Institution North China Electric Power University Beijing, China

Abstract

This paper proposes a model of coordination planning between the power plants and the power network including large wind farms. The objective function consists of expansion costs of both transmission and generation. In the part of transmission planning cost, it uses risk factor to express risky constraints, and risk assessment fee to evaluate overload risk. The risk factor has realized quantization of risk, so it is able to deal with the uncertainty of wind power output and loads. The risk assessment cost reflects transmission programming risk as an economic index, which is comparable with the network investment cost. In the part of generation planning cost, the environmental costs of all generators are used to achieve optimal comprehensive social benefits. The impacts of wind farms on system peak regulation and frequency regulation are considered in the form of constraints.

©2011 Published by Elsevier B.V Selection and/or peer-review under responsibility ofIntesnational Materials Science; Society.

Keywords-wind farm; generation expansion; transmission expansion; coordinated planning

1. Introduction

In order to deal with the world-wide energy crisis, wind farms have been developed rapidly in recent years to generate electric power from renewable wind power. However, unlike conventional generation sources, wind power is low-controllable, stochastic, intermittent and anti-peak-shaving, and it therefore introduces uncertainties for operation and planning of power system with large proportion of wind farms. Large wind farms are generally far from the load center, and the grid in the access point grid is weak, so there is need to strengthen the construction of power transmission network to accept more wind power, which requires coordination of generation and transmission planning. With the separation of generation from transmission, the original single investor was differentiated into multiple investors. It is necessary to

1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. doi:10.1016/j.egypro.2012.01.302

research coordination of generation and transmission planning for power system with large wind farms under the interaction among multiple investors' investment strategies.

Nowadays, research on coordination of generation and transmission planning for power system with large wind farms is still in the exploration period. In the literature about planning methods of transmission system with large wind farm, [1] has analyzed how to deal with large amounts of intermittent generation resources connected to the grid. The impact of wind power integration in bulk electric system reliability analysis is conducted on the selected transmission reinforcement alternatives to determine the optimum planning option in [2]. [3-4] proposes a chance constrained formulation to tackle the uncertainties of load and wind generator in transmission network expansion planning. Based on the multi-scenario probability method, the uncertain factors, such as wind farm power output, load change and economy, are described by scenario occurrence in [5]. Wind energy exploitation indexes and energy saving indexes are proposed to evaluate the planning models in [6]. In the literature about generation expansion planning with wind power plants, [7] utilizes WASP, a state-of-the art computer model, to obtain optimal generation expansion plans for an electric utility. A forward dynamic programming algorithm is utilized to determine the optimal expansion planning of a generation system with renewable energy sources operating in parallel with a large-scale network in [8]. [9] studies the long-term competitiveness of introducing renewable energy sources alongside the conventional generating units in a generation expansion plan by linking a short-term study and a long-term planning model. [10] proposes a model of the optimal expansion planning of wind-diesel energy system based on improved genetic algorithm, using penalty function to penalize the scheme not meeting constraints.

This paper proposes a model of coordination planning between the power plants and the power network including large wind farms. The objective function consists of expansion costs of both transmission and generation. In the part of transmission planning cost: The variable characteristics of wind power are similar to those of the loads, in that both of them are uncontrollable. When the load and wind generator are considered in transmission network planning, the investment of the newly-built power lines may be very high to guarantee all the transmission lines will not be overloaded at any time. A rational choice is to relax some constraints appropriately, so that the optimal planning scheme will be obtained within the risk constraint that some lines may be overloaded. This thesis, using quantification risk assessment cost as a part of transmission planning cost to show the cost caused by these security risks, establishes power transmission system model which considers risk constraint. As a result, a comparison between the risk expense and investment expense becomes obvious. In the part of generation planning cost, the environmental costs of all generators are used to achieve optimal comprehensive social benefits. The impacts of wind farms on system peak regulation and frequency regulation are considered in the form of constraints.

2. Wind Farms

There are two modes to integrate wind farms into power system[11]: large-scale wind farm is directly connected with the power network through transmission lines; small-capacity wind farm or wind turbine is integrated into distribution network in the form of distributed generation. This article focuses on the former.

collecting power lines

box substation wind gene rat o r 0 jS9 /35k V

8"0-^CTC^rrt

step-up transformer JiflÜOkV

transmission grid

Figure 1. Typical connection mode of large wind farm with transmission network

Figure 2. Connection mode of small wind turbine with distribution network as distributed generation

A grid connected wind farm is usually composed of more than one wind turbine generators with large capacity, so it has large-scale units (50kW~5 MW) and can be installed and controlled centralized. Wind turbine generator, composed of wind turbine and generator, is the main generation equipment, and is integrated into transmission network after its output voltage boosts several times from the initial value 690V. The typical connection mode is shown in Fig. 1. The output voltage of wind turbine generator rises to 35kV through 0.69/35 kV box substation, and then access 220kV booster substation to do a second boosting. In addition to maintenance of power systems, large wind farm is always lined with the power grid

A wind farm or wind turbine generator usually connects with distribution grid as distributed generation if its capacity is small. Distributed wind power, energy storage devices, energy conversion devices, load, monitor equipments and protection installation can constitute a micro-grid system operating independently or grid-connectedly. The connection mode of small wind turbine with distribution network as distributed generation is shown in Fig. 2.

3. Concordant model of generation and transmission expansion

From the perspective of physical operating rules in power system, generation and transmission are integrated in essence. The network is a tache between electricity production and electricity transaction, but on the other hand it may be a barrier to transact reasonably and compete impartially in electricity market, which needs to construct a strong network structure. However, excessively emphasizing network construction will lead some problems such as utilization efficiency reduction, resources wasting, and overlapping investment. Therefore, it is necessary to coordinate the expansion planning between generation and transmission.

The mathematical model is

min Z = CT +CG (1)

where Z is the total construction cost of power system planning. CT and CG are transmission expansion cost and generation expansion cost respectively.

3.1 The Transmission Expansion Cost

When uncertainty of loads and wind generator is considered in transmission network planning, the investment of the newly built power lines may be very high to guarantee all the transmission lines not overloaded at any time. A rational choice is to relax some constraints appropriately, so that the optimal planning scheme will be obtained within the risk constraint that some lines may be overloaded. This chapter, using chance constraint's risk denotation and basing itself on risk factor, and using quantification risk assessment cost to show the cost caused by these security risks. As a result, a comparison between the risk expense and investment expense becomes obvious, and the planning of transmission system with large wind farm will achieve both safety and economy in this way.

The transmission expansion cost can be expressed as

CT =Cj + juCS (2)

Subject to:

B6 = p (3)

r=Prh|>PLmax M (4)

f(x)< 0 (5)

n(X)< 0 (6)

In formulation (2), CT is the total cost of transmission expansion including cost of lines added to the network and risk cost. C1 is the cost of lines added to the network. CS is risk assessment cost. ^ is risk coefficient, whose appropriate value is selected according to the needs of network reliability.

In the equality constraint (3), B is the node admittance matrix, 0 is node voltage phase angle vector. P is the nodal injected active power array.

In the inequality constraint (4), r is risk factor, and X is a given threshold value of risk factor, which is set by transmission planners according to risk preference or the development stage of the grid. Pr {•} is probability of events. PL is active power flow array. PLmax is power flow limit array.

Inequality constraint (5) demonstrates network structure constraints such as maximum number of lines that can be added to every two buses. Inequality constraint (6) describes constraints of transmission

network optimal operation, including generator output constraints, load constraints and available transfer capability limitations of lines.

3.2 The Generation Expansion Cost

When research the power system with large-scale wind farms, it is necessary to analyze comprehensive social benefits of wind power. Compared with conventional generation sources, wind power will economize the cost on fuel consumption, construction investment and contaminated environment remediation. Meanwhile, it should be considered that the peak-valley difference is tending to increase because of the connection between wind farm and grid. To reply the impact of variable wind farm output on power system, it should ensure that the system has sufficient peak modulation capacity and frequency regulation besides meeting the electric power and energy balance constraint.

According to the unit features, construction requirements and running roles in system, power source can be divided into several kinds: a) Nuclear power plants, operate as base-load supply; b) Thermal power plants, including Coal-fired units, gas units, fuel units and thermal units, operate as base-load supply or peak-load supply, and can take part in peak load and frequency regulation; c) Hydropower plants, whose unit commitment and adjustment are rapid, are suitable to be peak-load supply; d) Wind farms, whose output are low-controllable, are base-load supply not involving in proactive peak load and frequency regulation.

The generation expansion cost can be denoted as

C0=CH+Cf+Ch+Cw+C0 (7)

where Cn , C f, Ch and Cw are expansion costs of the alternative nuclear power plants, thermal power plants, hydropower plants and wind farms respectively. C0 is costs of the existing power plants.

Nn , X

Q = EfaH-c.+rf,-) (8)

Nf cr= ) j=1 (9)

Nh ^ = +bk ~ck) k=1 (10)

Nw Cw = +bi-Cl) l=1 (11)

N0 Q) = H(bm ~Cm +dm ) m=1 (12)

In the formulations (8)~(12), ai, bi, ci and dt are fixed cost, operation cost, additional income and environmental cost of the i -th nuclear power plant respectively. aj, bj, c j and dj are fixed cost, operation cost, additional income and environmental cost of the j -th thermal power plant respectively. ak , bk and ck are fixed cost, operation cost and additional income of the k -th hydropower plant respectively. al, bt and cl are fixed cost, operation cost and additional income of the l -th wind farm

respectively. bm , cm and dm are operation cost, additional income and environmental cost of the m -th existing power plant respectively.

The constraint conditions includes generator construction, electric power and energy balance, reliability requirement, system peak regulation and frequency regulation. The peak modulation capacity constraint is

Nn Nf Nh No

Z ^ + Z "A + Z akpk +Taipi

1=1 >=1 ^ '=1 (13)

> y AP max + APrmax

/ i m L

Where ai, a j, ak, al are the peak modulation depth of nuclear power plant i, thermal power plant

j , hydropower plant k and the existing power plant I. Nn, Nf, Nh, NW and N0 are the amount of alternative nuclear power plants, thermal power plants, hydropower plants, wind farms and the existing power plants respectively. APm"ax is the maximum output change of the m -th wind farm. APLmax is the maximum peak-valley load difference in system. The frequency regulation constrain is

Nn Nf Nh N0

i=i j=i k=i l=i (14)

>YS Pmax +5jL

— / , m m L m

Where Si, 5j, Sk and Sl are the maximum regulation speed of nuclear power plant i, thermal power plant j , hydropower plant k and the existing power plant l. Nn, Nf, Nh, NW and N0 are the amount of alternative nuclear power plants, thermal power plants, hydropower plants, wind farms and the existing power plants respectively. 5m is the maximum output change ratio of m -th wind farm in the

opposite direction with load variation. 5L is the maximum load variation rate in system. Lmax is the maximum load in system.

4. Model Calculation

It is difficult to calculate this model using traditional optimization methods. Heuristic algorithm can be used to solve this massive mixed integer nonlinear programming problem. An initial solution is calculated without considering the constraints of transmission and generation firstly. and then the optimum solution under the constrains will be obtained by using heuristic algorithm.

5. Conclusion

This paper proposes a model of coordination planning between the power plants and the power network including large wind farms. The objective function consists of expansion costs of both transmission and generation. An optimal scheme with minimum social total cost will be obtained .

In the part of transmission planning cost, it uses risk factor to express risky constraints, and risk assessment fee to evaluate overload risk. The risk factor has realized quantization of risk, so it is able to

deal with the uncertainty of wind power output and loads. The risk assessment cost reflects transmission programming risk as an economic index, which is comparable with the network investment cost. In the part of generation planning cost, the environmental costs of all generators are used to achieve optimal comprehensive social benefits. The impacts of wind farms on system peak regulation and frequency regulation are considered in the form of constraints.

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