Voltage Control in Wind Power Generation Using Doubly Fed Induction Generators

JOURNAL TITLE ‐ MONTH YEAR 1

Voltage Control in Wind Power Generation

Using Doubly Fed Induction Generators E. Toledo*1, L. Aromataris2, G. Tarnowski3, M. Oliveira4, O. Perrone5, H. Reversat6

Energy Study Center to Development (CEED), National University of Misiones (UNaM)

Oberá, Misiones, Argentina

*[email protected]; [email protected]; [email protected]; [email protected]; [email protected]

Electric Power System Analysis Group (GASEP), National University of Río Cuarto (UNRC)

Rio Cuarto, Córdoba, Argentina

[email protected];

Abstract

In recent years around the world, conventional generation

plants are being replaced by wind power plants. The rapid

development of wind power generation brings new

requirements for the integration of wind turbines to the

Electric Power Systems. These requirements establish that

the new technologies must provide ancillary services similar

to those of conventional plants, such as voltage control in

steady state and voltage control during faults occurring close

to the wind power plant. This paper explores and compares

the performance of two different alternatives of voltage

control using doubly fed induction generator (DFIG)

topology, which is the most used wind generation

technology nowadays. This performance is investigated in a

transmission network to a disturbance which endangers the

stability of long term voltage thereof. In the first case, the

voltage control of the terminal bus of the wind plant will be

performed only through the rotor‐side converter, while in

the second case; the voltage control will also have the

additional contribution of the reactive power delivered by

the grid side converter. The results show the importance of

this additional contribution of the reactive power to voltage

system stability.

Keywords

Wind Power; Voltage Stability; Voltage Control; Doubly Fed

Induction Generator (DFIG); LVRT; Coordinated Control;

Reactive Power Control; Control Strategies; DFIG Model.

Introduction

The rapid development of wind power generation

brings new requirements for the integration of the

wind turbines to the grid. These requirements are

related to ancillary services that wind turbines can

offer, such as voltage control and the ability to remain

connected during voltage dips (LVRT). [1]

Not all wind generation technologies are capable of

providing auxiliary services. Within existing

technologies, the most widespread is the doubly fed

induction generator (DFIG), which has the ability to

contribute with reactive power to the network voltage

control [2]. In this turbine, the stator circuit is

connected directly to the network while the rotor

circuit is connected via an electronic converter. The

power passes through the converter between 20 and

30% of the nominal power range depending only on

the variation of slip and reactive power requirements.

The losses in the power electronic converter are

reduced, compared with a system where the total

power must be converted as in the wind turbine

generator which is synchronous. This reduces the cost

of the converter which is smaller in size. As it is a

variable speed wind machine, it reduces voltage

fluctuations in the point of connection to the network

and allows having an independent control of the

active and reactive power which is delivered to the

network [3]. Another benefit is that one can adjust the

rotor speed according to the speed of the wind, so that

aerodynamic efficiency is optimal. The voltage control

through DFIG has been discussed in various papers.

Dynamic simulations illustrating the voltage control

action in variable speed turbine is presented in [4].

This article compares the different steady‐state current

of the rotor‐side converter (RSC), for different

scenarios of absorption and generation of reactive

power (Q). However, it does not consider the

contribution of the reactive power that can be

delivered by the grid side converter (GSC). Capacity

2 JOURNAL TITLE ‐ MONTH YEAR

limits to deliver reactive power from the DFIG stator is

studied in [5], but this study does not consider the

contribution of the GSC. Different voltage control

algorithms are suggested in [6], in which only the GSC

is used, without considering the significant

contributions provide by the RSC. This paper

compares the behaviour of a highly loaded

transmission network whose reactive power

requirements are close to the limit available. Under

these circumstances, a disturbance will be applied

which endangers the stability of long‐term voltage.

Not only will the conventional voltage control

produced by synchronous machines be considered,

but also the voltage control provided by a wind park

made up of DFIG turbines. In the first case, it is

considered that the voltage control in the terminal bus

of the wind turbine will be performed by the rotor‐

side converter (RSC), a system commonly used for this

type of machines, providing active and reactive power

independently. The grid side converter, GSC,

maintains a constant dc‐link voltage and adjusts

reactive power absorbed from the grid by the GSC. In

the second case, both RSC and GSC converters may

control the voltage and provide active and reactive

power. The results show that in the second case, the

additional power supply can be determinant to

maintain the voltage stability of the system.

Reactive power capability of DFIG

Fig. 1 shows the diagram of a generator DFIG where

we can see a mechanical gear system that couples the

blades with the asynchronous generator. It can also be

observed the direct connection from the stator to the

transmission network. The rotor is connected to an

electronic converter back‐to‐back which comprises two

independent electronic devices separated by a bus

which maintains a constant DC voltage level. On the

rotor side converter, it can be appreciated the RSC. On

the grid side is shown the GSC. Figure 1 shows the

schematic diagram of the DFIG system.

FIG. 1 ‐ SCHEMATIC DIAGRAM OF THE DFIG

The rotor side converter, RSC, controls the active and

reactive power of the machine independently. This

control is performed through the d q axes, which are

referenced in the stator system and are orthogonal to

each other. Thus, the d component of the rotor current

is used to control the reactive power and the q

component of the rotor current is used to control the

torque of the wind turbine. This control consists of two

cascaded PI control. The first is the PQ control, which

receives the active and reactive power measured in the

network and compares them with the reference,

passing later to the PI control that outputs the

reference currents. These signals enter the second

control, that is to say the current control, which

compares these references with the measured currents

and after a new PI control, the rotor voltage is changed

[7]. The GSC maintains the DC voltage at a set value

which is independent of the magnitude and direction

of the rotor power. In this case, the converter only

exchanges active power with the network. Therefore,

the exchange of reactive power from the DFIG is done

through the stator. Nonetheless, if an additional PI

control that takes into account the error signal voltage

is added to the GSC, is possible that the machine can

also issue additional reactive power through this

converter, increasing the supply of reactive power in

situations in which it is required.

a) b)

FIG. 2 ‐ POWERFACTORY LIBRARY MODELS. A) MODEL

INTEGRATED DFIG B) DFIG DETAILED MODEL

To perform the power system simulations on the

electric system being studied and model both types of

DFIG controls, the PowerFactory DigSilent software

was used. This software has a library model of the

DFIG which includes both types of controls. One of

the models is integrated, that is to say, the generator

and GSC and RSC controls form a block in which the

user does not have access to intermediate variables.

This model controls the terminal voltage only through

the RSC, while the GSC emits or consumes only active

power (Fig. 2.a). The library also offers another

alternative where the plant components are modeled


separately (Fig. 2.b) [8]. In this way, intermediate

controls can be accessed so that the GSC can provide a

portion of the reactive power in order to cooperate

with the control voltage.

Case Study

As a case study, a system consisting of 14 buses

composed of different elements was considered. In fig.

3, it can be observed its topology.

FIG. 3 ‐ SYSTEM OF 14 BUSES

The generating park consists of two plants with

synchronous machines and a park with wind

generation. The demand is concentrated in three buses

where 50% of the load was modeled as static and the

other 50% as induction motors. The system has six

transformers with on load tap charguer(OLTC). Below

are the main features of the foregoing:

TABLE 1 ‐ SYNCHRONOUS GENERATORS

Generator 1 Generator 2

S rate

(MVA)

110 300

V rate

(KV)

16,5 18

Controls IEEEX1 (voltage

regulator)

IEEEX1 (voltage

regulator)

IEEEG3 (speed

regulator)

IEEEG3 (speed

regulator)

MAXEX2 (exitation

regulator)

lMAXEX2 (exitation

regulator)

TABLE 2 ‐ WIND FARM

Machine

type

S rate

(MVA)

DFIG

2,22

V rate

(KV)

0,69

TABLE 3 ‐ TABLE 4 ‐ MOTORS

Motor

1

Motor

2

Motor

3

Power rate

(MVA)

28 28 28

Voltage rate

(KV)

6 6 6

TABLE 5 ‐ LOADS IN GENERAL

Load 1 Load 2 Load 3

Type of load static static static

Active Power

(MW)

100 75 50

Reactive Power

(MVAr)

50 35 25

Voltage rate (KV) 6 6 6

Simulations

The simulations consider two cases:

a) The wind farm is modeled by 57 DFIG machines

which controls the terminal voltage of the converter

through CSR

b) The wind farm 57 is modeled by DFIG machines

where the terminal voltage is controlled through RSC

converters and GSC.

The fault that is applied in both simulations

corresponds to a short circuit and disconnection phase

with clearance in one of the lines of the system.

Output curves show the behavior of the network

variables for each case:

Case 1 ‐ Wind farm voltage control through CSR

The base case shows that before the disturbance

voltage levels are within acceptable limits. The

synchronous machines are generating reactive power

to the limit of its capacity. The wind farm is operating

at the nominal point of its capacity of active and

reactive power.

At the time of 1 second, the fault is applied on the line

wich connecting the bus 8 and 9. At 120 milliseconds

occurs the clearing of the fault and the outage of the

line. At 55 seconds, the limiter of overexcitation

(MAXEX2) of the synchronous generator 2 acts and

the terminal voltage drops progressively to the value

corresponding to the admissible field voltage. At 59

seconds, the MAXEX2 of the synchronous generator 1

acts and produces the same phenomenon occurred

with the generator 2 (Figure 4). During all this time the

transformers with load tap changers acted to recover

the voltage on the demand producing, in this way, the


recovery of the active and reactive power of the static

Load. This process which finally reached the limit of

tap‐changer, needed the contribution of reactive

power from generators and caused, in part, the action

of the limiters MAXEX2. The wind farm kept their

terminal voltages, active and reactive power (Figure 6

and 8) at approximately constant values until the

action of MAXEX2 limiters occurred. After this, there

is a significant increase of the reactive power to sustain

DFIG voltage values at the expense of the decrease of

the active power generated thereby. The charging

voltages (Figure 10) drop to unacceptable values

which are typical in a scenario of voltage instability.

This is shown in the following table:

TABLE 6 ‐ VOLTAGE IN THE LOAD BUSES

Voltage

before the

fault (p.u.)

Voltage

after the

fault (p.u)

Load

1

0,95 0,84

Load

2

0,99 0,83

Load

3

0,98 0,71

Case 2 ‐ Wind farm with voltage control by RSC and

GSC

The base case is the same as case 1.

At the time of 1 second, the fault is applied on the line

connecting bus 8 and 9. At 120 milliseconds occurs the

clearing of the fault and the outage of the line. At 79

seconds, the limiter of overexcitation (MAXEX2) of the

synchronous generator 2 acts and the terminal voltage

drops progressively to the value corresponding to the

admissible field voltage. In this case, the action of the

limiter occurs later than in the former case due to the

additional contribution of reactive power produced by

the wind farm (Figure 5). In the case of generator 1, the

terminal voltage is maintained due to fact that the

provided additional reactive power does not

necessitate the protective action of the synchronous

generator. Under these conditions, the terminal

voltage of the wind farm (Figure 7) can be sustained in

the setpoints improving significantly the voltage

profile of the entire process. Throughout this process,

the on load tap charguer of distribution transformers

act but they do not reach the limit. The output powers

deliver by the wind farm, show a suitable output to

the type of control used where voltage control

privileges against the emission of active power. The

portion of the active power emitted by the GSC,

suffers a decrease to result in the necessary reactive

power which maintains the terminal voltage at the set

values (Figure 9). The voltage decreases in the load but

the voltage maintained within an acceptable profile for

these conditions (Figure 11). In Table 6 the results are

shown.

TABLE 7 ‐ VOLTAGE IN THE LOAD BUSES

Voltage

before the

fault (p.u.)

Voltage after

the fault (p.u)

Load

1

0,96 0,95

Load

2

0,99 0,96

Load

3

0,98 0,95

Output Curves

Below are output curves where the figures in a) belong

to the first case; while figures in b) belong to the

second.

FIG. 4 ‐ TERMINAL VOLTAGE IN THE SYNCHRONOUS

GENERATORS 1 AND 2 (CASE 1)

FIG. 5 ‐ TERMINAL VOLTAGE IN THE SYNCHRONOUS

GENERATORS 1 AND 2 (CASE 2)


FIG. 6 ‐ TERMINAL VOLTAGE IN THE WIND PLANT (CASE 1)

FIG. 7 ‐ TERMINAL VOLTAGE IN THE WIND PLANT (CASE 2)

FIG. 8 ‐ REACTIVE AND ACTIVE POWER GENERATED BY THE

WIND PLANT (CASE 1)

FIG. 9 ‐ REACTIVE AND ACTIVE POWER GENERATED BY THE

WIND PLANT (CASE 2)

FIG. 10 ‐ VOLTAGE IN THE LOAD BUSES (CASE 1)

FIG. 11 ‐ VOLTAGE IN THE LOAD BUSES (CASE 2)

Conclusion

One of the most used technologies in wind turbines is

the doubly fed induction generator (DFIG) which is

capable of contributing to the reactive power control

in the network close to the wind plant point of

interconnection. In this type of wind turbine generator,

the stator circuit is connected directly to the network

while the rotor circuit is connected to the network

trough a back‐to‐back power electronic converter. As

it is a variable speed wind turbine, it allows reducing

voltage fluctuations in the point of connection to the

network, as well as having an independent control of

the generated active and reactive powers. The voltage

control by DFIG could be performed through the

rotor‐side converter RSC; a system which is commonly

used for such machines. This converter provides active

and reactive power independently. The grid side

converter, GSC, normally maintains a constant dc‐link

voltage and adjusts reactive power absorbed from the

grid by the GSC. However, it has been proposed in the

literature the use of the GSC to contribute to voltage

control too and to provide reactive power to the

network when it becomes necessary.

For situations of long‐term voltage stability, it has

been shown in this paper that the simultaneous use of

both RSC and GSC controls can provide additional


reactive power. Therefore, avoid voltage instability of

a system loaded to its reactive power limit, after a

strong disturbance is avoided.

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Assessment of Wind Farms, DIgSILENT GmbH,

Germany.

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AUTHOR’S INFORMATION

Eduardo J. Toledo was born in

Resistencia, Chaco, Argentina in August

1986. He received the degree in

Electromechanical Engineering from the

National University of Misiones (UNaM),

Argentina in 2011. He is now a M.Sc

student at National University of Rio

Cuarto, Argentina (UNRC). Currently, he

is researcher at the Energy Study Center to Development

(CEED). His research interests include stability analysis of

power systems, power system modeling and analysis of

voltage stability in wind farms.

Luis Aromataris was born in Mendoza,

Argentina. He received the degree in


Universidad Nacional de Rio Cuarto,

Argentina and Doctor of Engineering

from the Universidad Nacional de La

Plata, Argentina. Currently, he is

researcher at the Grupo de Analisis de

Sistemas Eléctricos de Potencia (GASEP). His research

interests include power systems stability and voltage

stability analysis with high penetrations of wind farms.

Germán C. Tarnowski received the

Electro‐Mechanical Engineer degree from

National University of Misiones,

Argentina in 2003; the MSc. degree in

Automation and Control from Federal

University of Rio Grande do Sul, Brazil in

2006, and the Industrial‐PhD degree from

Technical University of Denmark in 2012.

Since 2007 he is with Vestas Wind Systems, where he did his

Industrial PhD studies on Coordinated Frequency Control of

Wind Turbines. He holds today the position of Research

Engineer. His interests involve operation and control of

wind power plants, control systems and applications of

electrical machines and power electronic converters for

renewable energy.

Mario O. Oliveira was born in Capioví,

Misiones, Argentina in May 1979. He

received the degree in Electromechanical

Engineering from the National University

of Misiones (UNaM), Argentina in 2005

and Masters in Electrical Power Systems

from Universidade Federal do Rio

Grande do Sul (UFRGS), Brazil in 2009.

Currently, he is a researcher at the Energy Study Center to

Development (CEED) and associate professor at UNaM. His

areas of interest include protection of electrical machines,

power system modeling and troubleshooting electrical

systems.

Oscar E. Perrone was born in Venado

Tuerto, Santa Fe, Argentina in December

1954. He received the degree in


Universidad Nacional de Córdoba (UNC),

Argentina in 1982. Currently, he is a

researcher at the Centro de Estudios de

Energia para el Desarrollo (CEED),

director of Electromechanical Engineering and professor at

the Universidad Nacional de Misiones. His areas of interest

include measurements and electrical installations

https://www.researchgate.net/publication/3270652_Milanovic_JV_Reactive_power_control_strategies_for_DFIG-based_plants_IEEE_Trans_on_Energy_Conversion_222_389-396?el=1_x_8&enrichId=rgreq-7ca49909191d6cbad0b7999de57d395e-XXX&enrichSource=Y292ZXJQYWdlOzI3NTAyMzc3OTtBUzoyMTg2MTgwMzkxNDg1NDRAMTQyOTEzNDAxMDM1NQ==





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https://www.researchgate.net/publication/245525718_Contribution_of_Variable-Speed_Wind_Turbines_to_Voltage_Control?el=1_x_8&enrichId=rgreq-7ca49909191d6cbad0b7999de57d395e-XXX&enrichSource=Y292ZXJQYWdlOzI3NTAyMzc3OTtBUzoyMTg2MTgwMzkxNDg1NDRAMTQyOTEzNDAxMDM1NQ==



https://www.researchgate.net/publication/3270218_Modeling_and_Control_of_a_Wind_Turbine_Driven_Doubly_Fed_Induction_Generator?el=1_x_8&enrichId=rgreq-7ca49909191d6cbad0b7999de57d395e-XXX&enrichSource=Y292ZXJQYWdlOzI3NTAyMzc3OTtBUzoyMTg2MTgwMzkxNDg1NDRAMTQyOTEzNDAxMDM1NQ==





Jose H. Reversat was born in Jardin

America, Misiones, Argentina in

November 1963. He received the degree

in Electrical Engineering from the

Universidad Nacional de Misiones

(UNaM) Argentina in 1996 and

specialization in Plant Engineering and

Production from UNaM in 2000.

Currently, he is a researcher at the Centro de Estudios de

Energia para el Desarrolo (CEED), and adjunct professor at

UNaM. His areas of interest include power systems and

electrical installations.

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Voltage Control in Wind Power Generation Using Doubly Fed Induction Generators