A performance comparison for wind power

integration into the grid system

Felipe Valencia∗, Julian Patino† and Jairo Espinosa‡

†Departamento de Ingeniera Elctrica y Electrnica

Universidad Nacional de Colombia sede Manizales

Facultad de Ingeniera y Arquitectura∗‡Departamento de Energıa Electrica y Automatica

Universidad Nacional de Colombia sede Medellın

Facultad de Minas

Cr. 80 No. 65 - 223, Medellın, Antioquia

Email: [felipe.valencia∗, julian.patino†, jairo.espinosa‡]@ieee.org

Abstract—The integration level of wind power into the gridsystem over the world has been growing at a very fast rate inthe last few years and is still keeping this pace. In the future, windpower is expected to be at least comparable to the conventionalpower generation systems. Large-scale wind farms will need to beintegrated into traditional power grid systems, creating the needto establish technical standards to make this integration feasible.This paper covers the performance comparison of a power systemwith and without the inclusion of a large-scale wind farm. Theglobal system model is presented along to a case of study andthe simulation results.

I. INTRODUCTION

For the last twenty years, there has been a growing interest

in the employment of renewable energy resources, given the

environmental and economic problematic of the fossil-based

sources. Electricity is one of the most !exible forms of energy,

since it can be transported long distances with few losses

and can be easily transformed into other forms of energy.

Electricity demand is expected to increase, especially with the

advent of more electric-powered and plug-in hybrid vehicles.

Wind power has a great potential for providing electrical

energy that is clean and incrementally free. Its attractiveness as

an electricity supply source has fostered ambitious targets for

wind power in many countries around the world. Its bene"ts

include:

• Very low lifetime emissions of harmful gasses, particu-

larly CO2

• Signi"cant economically exploitable resource potential

• No cost uncertainties from fuel supply price !uctuations

• Increased diversity and security of supply

• Modular and rapid installation

• Opportunities for industrial, economic and rural develop-

ment.

Currently, wind power represents the 2.5% of the world

electrical capacity, and this source is undergoing one of the

fastest rates of growth of any form of electricity generation

[1]. The resource potential is large, with many countries

having wind regimes that could serve as a signi"cant energy

source. Ambitious goals for wind power development have

been set by many countries.

Wind power stations have some features that make their

operational behavior different with respect to the traditional

power plants. A wind farm is a group of wind turbines in the

same location used to produce electric power. The integration

of a relatively small amount of wind farms into the utility

grid does not normally present any large operational problem,

preserving the reliability and the quality in the energy supply.

However, integration of large scale wind power may have

severe impacts on the power system performance and system

operation. Large wind farms are typically placed where good

enough wind regimes exist, and often far away from the main

load centres and big power grid links. In addition to the

increased employment of wind farms, several questions arise

regarding the integration of large-scale wind farms into the

power grid that must be answered, both at the wind power

generation technology and the grid operation levels. Some of

the issues are [2]:

• Fault ride through system requirements

• System frequency and frequency response requirements

• Transmission system voltage and reactive power capabil-

ity requirements

• Wind power forecast

• Remote operation.

In the "rst integration experiences, the wind power plants

were disconnected from the grid under system disturbances.

But with the rising of large-scale wind farms and the high

penetration of wind power into the grid, this procedure is

not feasible because because it will diminish the system

capabilities to withstand the disturbances [3]. Therefore, the

grid codes have speci"ed the requirements for the wind farms

under the steady state and dynamic conditions, stressing the

need for voltage and frequency regulation capabilities under

978-1-4673-4655-9/12/$31.00 c© 2012 IEEE

the steady state condition. Under the speci�ed voltage dip

conditions caused by the faults within the grid, the wind farms

have to stay connected and to ful�ll the recovery requirements.

Several researchers around the world have tackled these

issues in the last years [4] [5]. The comprehensive report in

[6] provides analysis of the technical, economic and regulatory

issues concerning the large scale integration of wind energy

into European energy markets. Discussions regarding power

and energy balancing, grid connection and system stability,

grid infrastructure extension and reinforcement, power system

adequacy, market design, demand side management and stor-

age were presented. The study �nished with recommendations

for the power system operation with large-scale wind farms.

The main problems arising from the wind farms connection

to the grid were reviewed in [7] and the suggestions for

modi�cation of network codes have been made with the

purpose of integrating wind power plants without affecting

the quality and stability of the system. A review of the main

technical requirement for grid integration of wind farms is

presented in [8].

This paper focuses on the comparison of the power system

performance both before and after the inclusion of a large-

scale wind farm. First, an introduction to wind power systems

and the grid integration issues is presented. Section II explains

the power system model. Section III shows the case of study

and the simulation results. At last, some conclusions are

presented.

II. POWER SYSTEM MODEL

Consider an electric power system with n hydraulic gen-

eration units, m wind generation units, and l load centers. In

this work, the eight-order dq model (1)-(8) of the synchronous

machine is used to represent the dynamics of each hydraulic

generation unit [9]. In (1)-(8) the subindex f denotes the rotor

quantities, the subindex s denotes the stator quantities, the

subindex k denotes the damper quantities, the subindex mdenotes the leakage quantities, the superindex ′ indicates that

the quantities are re ected to the stator, and x = dxdt.

φdi(t) = Vdi(t) + ωi(t)φqi(t)−Rsiidi(t) (1)

φqi(t) = Vqi(t)− ωi(t)φdi(t)−Rsiiqi(t) (2)

φ′

fdi(t) = V ′

fdi(t)−R′

fdii′

fdi(t) (3)

φ′

kdi(t) = V ′

kdi(t)−R′

kdii′

kdi(t) (4)

φ′

kq1i(t) = V ′

kq1i(t)−R′

kq1ii′

kq1i(t) (5)

φ′

kq2i(t) = V ′

kq2i(t)−R′

kq2ii′

kq2i(t) (6)

δi(t) = ωi(t) (7)

ωi(t) =1

2Hi

[−Diωi(t) + (Pmi(t)− Pei(t))] (8)

withVi the voltage, ii as the current, Ri the resistance, ωi the

speed, δi is the angular position, Di the damping coef�cient,

Hi as the inertia coef�cient, Pmiis the mechanical power,

Pei is the electric power, and φ is the leakage ux of the ith

generation unit, with

φdi= Ldi

idi+ Lmdi

(i′fdi+ i′kdi

)

φqi= Lqi

iqi+ Lmq

i(i′kq1

i

+ i′kq2i

)

φ′

fdi= L′

fdii′fdi

+ Lmdi(idi

+ i′kdi)

φ′

kdi= L′

kdii′kdi

+ Lmdi(idi

+ i′fdi)

φ′

kq1i

= L′

kq1i

i′kq1i

+ Lmqiiqi

φ′

kq2i

= L′

kq2i

i′kq2i

+ Lmqiiqi

Li being the inductance of the generation unit i, i = 1, . . . , n.The wind generation units were modeled as induction ma-

chines by using their six-order dq model (9)-(14) [10].

φqj = Vqj − ωjφdj −Rsiqj (9)

φdj = Vdj + ωjφqj −Rsidj (10)

φqf′j = V ′

qfj − (ωj − ωr)φ′

dfj −R′

f i′

qfj (11)

φdf′j = V ′

dfj + (ωj − ωr)φ′

qfj −R′

f i′

dfj (12)

δj = ωj (13)

ωj =1

2Hj

(Tej − Fωj − Tmj) (14)

where ωr is the electrical angular velocity, F is the combined

rotor and load viscous friction coef�cient, and

φqj = Lsjiqj + Lmji′

qfj

φdj = Lsjidj + Lmji′

dfj

φ′

qfj = L′

fji′

qfj + Lmjiqj

φ′

dfj = L′

fji′

dfj + Lmjidj

Tej = 1.5pj(φdj iqj − φqjidj)

Tmj = cpj(λj , βj)ρAj

2ν3j

with pj the number of pole pairs, cpj(λj , βj) the performancecoef�cient of turbine, Aj the turbine swept area, λj the tip

speed ratio of the rotor blade tip speed to wind speed, βj

the blade pitch angle, and νj the wind speed of the jth

wind generation unit, j = 1, . . . ,m, ρ being the air density.

An expression for cpj(λj , βj) is provided in [11]. Since the

wind generation units were considered as induction machines,

capacitor banks should be added at each wind generation unit

bus in order to provide the reactive power demanded by the

network.

The model of the power system is completed by adding

the interconnection model. Each line was considered as a

π equivalent circuit, so then it is possible to compute the

power supplied by each generation unit solving the system

of equations associated with the power system balance. In

the next section, a case of study compares the performance

of the system when composed solely of hydraulic generation

units with the performance of the system when a considerable

amount of power is provided by wind generation units.

III. CASE OF STUDY

A. Case Description

In order to compare the performance of the system with and

without power produced by wind farms, the two areas and

Fig. 1. Simulation Scheme

four machines power system proposed in [12] was selected

(see Figure 1).

This system consists of two symmetric areas linked together

by two transmission lines. Each area has two identical gen-

eration units making the system fully symmetrical. However,

in the case presented in this paper generation unit G4 was

changed by a wind farm equipped by three identical wind

generation units (see Figure 1 for identifying the wind farm).

Thus, the total power produced by the wind farm is equivalent

to 14

of the total power produced by the power system. In

addition, the system of Figure 1 has two load centers (one per

area). These loads have been modeled as constant impedance

loads and their values are such that the power ows from area

1 to area 2.

With the purpose of to compare the behavior of the system

whit and without a wind farm, a case where a three phase fault

at bus 7 followed by a line trip was considered. The fault-

cleaning-time was 10 cycles of the 60Hz wave and happens

at 8 seconds (simulation time). The tripped line was one of

the lines connecting the two areas and occurs at 16 seconds

(simulation time). In order to avoid the main issues related

with power oscillations multi-band power system stabilizers

(MB-PSS) were included in the control loop of generation

units G1−G3. A block diagram of the MB-PSS is presented

in Figure 2. The parameters of the MB-PSS were computed

by the root-locus method.

The control loop of generation units G1−G3 is completedwith the automatic voltage regulators (AVR) and the hydraulic

governors (HG). The AVR used in this paper was static

exciters and the HG used was a PID governor system with a

servomotor. For the control of the speed of the wind generation

units, a proportional-integral-derivative (PID) controller was

tunned. Moreover, for the control of the voltage at bus 4, astatic synchronous compensator (STATCOM) was used. The

control loop for the wind generation units was completed with

the inclusion of the protection scheme of the wind farm. Such

a protection scheme includes:

† Instantaneous, positive-sequence, and unbalanced AC

overcurrent.

† Positve-sequence AC under and overvoltage, negative and

Fig. 2. Block diagram of the multi-band power system stabilizer systemsused in the simulations

zero-sequence AC unbalanced voltage.

† DC overvoltage, and under and over speed.

The models mentioned in Sections II and III were imple-

mented in Matlab/Simulink software using the SymPowerSys-

tems toolbox.

B. Simulation Results

Figure 3 shows the behavior of the voltage at buses 1 −4 when only hydraulic power plants are considered for the

electric power production in the system presented in Figure

1. In this Figure it is shown that the voltage of all generation

units is the same and is close to 1 pu. As time evolves and

the disturbances depicted in Section III occur, the magnitude

of the voltages at buses 1 − 4 increase. Note that despite of

the disturbances the system remains stable and the magnitude

of the voltages in steady-state belongs to the allowed values

according to the regulatory frames.

However, when the wind farm was included the magnitude

of the voltages at buses 1 − 3 was lesser than the voltage at

bus 4 corresponding to the connection node of the wind farm.The behavior of the voltages at buses 1− 4 including a wind

farm producing 14of the total power produced by the system

is presented in Figure 4.

Furthermore, the effects of the disturbances in the behavior

of the system are also affected when the wind farm is included.

In Figure 4 note that only the hydraulic generation units

respond to restore the voltage pro!le after the three phase

fault is cleaned. The voltage at bus 4 remains almost constantafter the cleaning of the three phase fault. Also, note that the

voltage pro!le diverges near to 16s (simulation time). This

divergence is due to the outage of the wind farm because

of the activation of the under speed protection of the wind

generation units at 15.89s. Recall that the wind farm produces14of the total power produced by the system. Figure 5 presents

the behavior of the speed of the wind generation units.

Figures 6 and 7 show the voltages at buses 7 and 8 with andwithout the inclusion of the wind farm respectively. In both

Figures the voltage at bus 8 is bigger than the voltage at bus

7, being lesser the difference between the voltages in Figure

7 than the difference between voltages in Figure 8. Moreover,

0 2 4 6 8 10 12 14 16 18 200.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

Time [s]

Voltage [

pu]

Evolution of the terminal voltages

VG1

VG2

VG3

VG4

Fig. 3. Evolution of the voltages at buses 1− 4 when only hydraulic powerplants are considered

0 2 4 6 8 10 12 14 16 18

0.7

0.8

0.9

1

1.1

1.2

1.3

Time [s]

Voltage [

pu]

Voltage at buses 1−4 with a wind farm

VG1

VG2

VG3

VWG1

VWG2

VWG3

Fig. 4. Evolution of the voltages at buses 1− 4 considering a wind farm

from Figure 7 it is possible to conclude that the three phase

fault has similar effects in both area 1 and area 2 when only

hydraulic generation units are considered. When the wind farm

is added area 1 is more affected than area 2, but the collapse

of the system is generated by the loss of generation in area 2.

From Figures 3-7 it is possible to conclude that the stability

margin of the system is reduced when a considerable amount

of power is generated by wind farms compared with the

stability margin of the system when only hydraulic generation

units are considered. This is due to the protections used to

shield the wind generation units. So, if the system is under

stress the wind farms may also induce more pressure to the

system instead of cooperate with the system restoring process.

Hence, the increasing use of wind farms to produce electric

power may weaken the power systems.

Figures 8 and 9 show the power ow from area 1 to area 2in both cases considered in this paper. From the behavior of

the power ow it is possible to conclude that the oscillation

modes of the system have more damping in the case in which

only hydraulic generation units are considered than in the case

0 2 4 6 8 10 12 14 16 180.99

1

1.01

1.02

1.03

1.04

1.05Speed of the wind generation units

Time [s]

Speed [

pu]

ωWG1

ωWG2

ωWG3

Fig. 5. Behavior of the speed of the wind generation units

0 2 4 6 8 10 12 14 16 18 200.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

Time [s]

Voltage [

pu]

Voltages at the interconnection buses

Vbus7

Vbus8

Fig. 6. Evolution of the voltages at buses 7 and 8 when only hydraulicpower plants are considered

0 2 4 6 8 10 12 14 16 180.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Time [s]

Voltage [

pu]

Voltage at buses 7 and 8 when a wind farm is included

Vbus7

Vbus8

Fig. 7. Evolution of the voltages at buses 7 and 8 when a wind farm isconsidered

in which the wind farm is included. Moreover, the power

exported by area 1 when the wind farm is included is higher

than the case where only hydraulic generation is considered.

0 2 4 6 8 10 12 14 16 18 20150

200

250

300

350

400

450

500

550

600

650

Time [s]

Pow

er

[MW

]

Behavior of the power flow from area 1 to area 2

Fig. 8. Behavior of the power �owing from area 1 to area 2 when onlyhydraulic power plants are considered

0 2 4 6 8 10 12 14 16 18−200

−100

0

100

200

300

400

500

600

700

800

Time [s]

Pow

er

[MW

]

Power flow from area 1 to area 2

Fig. 9. Behavior of the power �owing from area 1 to area 2 when onlyhydraulic power plants are considered

This makes the link between the areas critical because the

increasing in the power �ow across the lines connecting buses

7 and 8 brings the system closer to its stability margin.

IV. CONCLUSION

In this paper a comparison of the power system performance

with and without the inclusion of a wind farm was made. For

such a comparison detailed models of the generation units

(hydraulic and wind-based) have been considered. Moreover,

the voltage and the speed control loops also were taken

into account. The comparison was carried out analyzing the

response of the system when a three phase fault with a span

of 10 cycles of the 60Hz wave occurs at bus 7.

From the simulation results it is possible to conclude that

the inclusion of large amounts of wind-based power reduces

the stability margin of the system, mainly because of the

sensibility of the protection schemes used to protect the wind

generation units. So, when the system is stretched the large

wind farms may become in a source of stress for the system

instead of a source of energy for restoring the normal operation

of the system.

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