Voltage and Current Inrush Control of Wind Driven Embedded Wound Rotor Induction Generator

IPASJ International Journal of Electrical Engineering (IIJEE) Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm

A Publisher for Research Motivation........ Email: [email protected] Volume 4, Issue 2, February 2016 ISSN 2321-600X

Volume 4, Issue 2, February 2016 Page 15

ABSTRACT

With the depletion of fossil fuel reserves and an increase in interest in renewable energy generation systems, embedded renewable energy generation (EREG) has become increasingly common. A number of reasons limit the addition in capacity of generating units at the substation busbar (embedded generation). Voltage related a problem such as steady-state voltage rise is a major limiting factor. Techniques such as power factor control; network reinforcement and load control to regulate the voltage has been used to limit the steady-state voltage rise. In this paper, a new scheme is proposed which limits the voltage rise problem of induction generators used in wind energy conversion systems (WECS) and simultaneously reduce the magnetising current to a large extent at generator cut-in. Keywords: Ac regulators, induction generator, optimum voltage control, power distribution, wind generation.

1. INTRODUCTION The world wide concern about the environment and depleting fossil fuel reserves has led to increasing interest in alternative sources of energy such as wind, micro-hydel, solar PV, wave energy etc. The increase in the generation capacity can be done either by installing large independent generating systems connected directly to grid or connection of some embedded generation systems on local distribution systems. Embedded generation system may be based on wind, solar PV or micro-hydel energy conversion systems. The most common of these are wind energy conversion systems (WECS). Connecting an embedded generator to an existing distribution network may lead to several local problems such as steady-state voltage rise, large voltage dip at the generator cut-in, etc. This is due to the fact that active voltage regulation is generally not provided beyond the last HV sub-station. Consequently the allowable limit of addition of embedded generators may be greatly reduced and many potential generating sites may be left underutilized. This problem is more aggravated when WECS is used in embedded generation. This is due to a highly intermittent nature of the wind. Hence for determining the capacity of a embedded generator some probabilistic approach can be followed to determine the effect of embedded generator in the system. The factors to be taken into account are primary substation busbar voltage, consumer loading and wind generator output variations. If all the factors simultaneously provide acceptable conditions at all the times, the generation capacity can be selected. An extreme case may be considered in which low consumer loading, high substation bus bar voltage and high generator output occur simultaneously.

2. STEADY STATE VOLTAGE RISE AND CONTROL Figure.1 shows a typical embedded induction machine wind turbine connected to a local HV/MV substation through and MV line

Let, PG = active power of turbine QG = reactive power of turbine PL = consumer’s active power load QL = consumer’s reactive power load Z = R + jX = line impedance IR = current through the line Vs = connecting point voltage SR =Voltamperes through the line

Voltage and Current Inrush Control of Wind Driven Embedded Wound Rotor Induction

Generator

Haroon Ashfaq

Department of Electrical Engineering, Jamia Millia Islamia, New Delhi.




SR = PR – jQR = PG – jQG – (PL + jQL) PR = PG – PL , QR = QG + QL

(1)

*G

RR*G

RR V

jQPVSI

(2)

VG = VS + ZIR (3)

= *G

RRS V

jQPjXRV

=

*GV

RRQRXPj

*GV

RXQRRP

(4)

From phasor diagram of Fig. 1,

S

RRG V

RQXP δsin V (5)

Since the voltage angle δ is very small, the term S

RR

VRQXP is negligibly small,

Therefore, the approximate value of voltage rise is given by,

*G

RR

VXQRPΔV

(6)

Z=R+jX

PG

QG

IR

SR

PL + jQL

Line impedance

VS VG

Connection point

Consumer load

Embeded generator

Substation

VG

VS

IR

IRR

IRX

Fig.1. Voltage rise for an embedded system

A number of methods may be employed to limit the steady-state voltage rise. Eq.6. shows following three possibilities to reduce the voltage rise ∆V: (1) Reduction of line impedance Z = R +jX; (2) Load control (3) Reduction of generator active power output PG; (4) Increase of generator reactive power input, QG.

R*GR I VS




(1) Reduction of Line Impedance:

Line impedance may be reduced either by physically moving the connection point closer to the source substation or increasing the size of existing distribution feeder. Additional new line or cable is required to move the connection point of generation. Although full generation output at all time, is obtained without voltage rises, the cost increases. The other option of replacing conductors of lower impedance is also costlier.

(2) Load control:

From (6), load management can be done either by connecting a large load, sufficient to cover variations in the voltage due to increase in the output of the embedded generator or of properly fast switching of a number small loads. Tariff system is to be regulated during these surplus energy periods [ 1,2].

(3) Reduction of PG:

In order to maintain satisfactory voltages, the generation output may be reduced at critical times by adjusting the wind turbine set point or disconnecting some wind turbines as desired. This practice has proved quite successful economically.

(4) Increase of generator reductive power import, QG:

Increase of QG can be obtained by use of static VAr compensators, switched capacitor banks or inverters in case of variable speed wind turbines. This method of power factor control enables full generation output. Power factor control is usually favorable when additional VAr requirements are low. Large VAr flows require higher equipment thermal ratings system losses are increased and interference with power factor sensitive transformer tap changer schees occurs. Therefore capital cost increases considerably.

3. CONTROL OBJECTIVES OF THE WIND GENERATION SYSTEM The two control aspects to be taken into account are voltage rise, and current inrush.

A. Voltage Rise

The reactive power of the wind generator can be changed by using an ac regulator connected in between the substation bus and the stator terminals of the embedded generator. With the constant monitoring of the substation bus voltage, the firing angle of the ac regulator is controlled to achieve proper level of flow of reactive power between the substation bus and the generator.

B. Current Inrush Control

The wind turbine is run idle without grid connection at low wind speeds. At the generator cut-in if the stator terminals of the induction generator are directly connected to the substation bus a large voltage fluctuation due to the current inrush of the generator will occur at the substation bus. Also, this would put a lot of extra wear on the gearbox. An ac regulator is used in between the stator terminals and the substation bus to provide soft starting and control the magnetizing inrush current at generator cut-in. the connection of ac regulator also reduces the system stresses in case of islanding in which the substation bus accidentally disconnects from the main grid. A block diagram of control scheme is shown in Fig. 2.

dcchoppe

rGear Box

Wind Turbine

Wound RotorInduction Machine

Substation busbarac regulator

Fig.2. A simple block diagram of the proposed control scheme




4. RESULTS AND DISCUSSION In Fig.3. the generator is seen working on a optimum point. A condition arises such that a voltage rise occurs, which is seen as a change in reference signal (generated at maximum allowable voltage, i.e., 1.02pu or 2% above the rated substation busbar voltage), the value of reactive power intake is regulated using an ac regulator. Simultaneously the chopper duty cycle is altered to operate the generator at a desired slip.. Fig. 4a and 4b show the inrush currents of a 1.5 kilowatt induction generator connected to a small grid. In this case the wind turbine and the generators are never decoupled and only the generator-substation busbar connection is taken off at low wind speeds, hence the current inrush is only due to the magnetizing current of the generator).

Fig.3. Change in generator output voltage with change in reference signal.

(a)

(b) Fig.4. Current inrush of 1.5kW induction generator




(a) Without ac regulator (b) With ac regulator

AC regulator Tacho-generator ZCD

Personal computer

ZCD

P.T P.T

Distributionbusbar

α

WRIG

Control

δ

δ

Nf

Fig.5. Complete control scheme

5. CONCLUSION A new scheme is proposed which limits the voltage rise of an embedded wound rotor induction generator connected to a WECS. Also, the magnetizing current is reduced at the generator cut-in. As compared to other existing steady state voltage limiting methods, the proposed scheme is most simple and economical.

The thermal energy from the external rotor resistance of the wound rotor induction generator can be used in thermal storage heaters, which may further increase the efficiency of the system.

References [1] N.C. Scott, D.J. Atkinson, and J.E. Morrell, “Use of load control to regulate voltage on distribution networks with

embedded generation,” IEEE Trans. Power Syst., vol. 17. no. 2, pp. 510-514, May 2002. [2] J.W. Smith and D.L. Brooks, “Voltage impacts of distributed wind generation on rural distribution feeder,” in Proc.

Transm. Distrib. Conf. Expo., vol. 1, pp. 492-497, 2001. [3] Baggu, M.M., Chowdhury, B.H. ; Kimball, J.W., “Comparison of Advanced Control Techniques for Grid Side

Converter of Doubly-Fed Induction Generator Back-to-Back Converters to Improve Power Quality Performance During Unbalanced Voltage Dips”, IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 3 , Issue 2, pp. 516-524, Sep. 2014.

[4] Singh, B., Murthy, S.S.; Reddy, R.S.; Arora, P., “Implementation of modified current synchronous detection method for voltage control of self-excited induction generator”, IET journal of Power Electronics, Vol. 8 , Issue 7, pp. 1146 – 1155, 2015.

[5] R. Datta and V.T. Rangnathan, “Variable-speed wind power generation using doubly fed wound rotor induction machine – a comparison with alternative schemes,” IEEE Trans. Energy Convers., vol. 17, no. 3, pp. 414-421, Sep. 2002.

[6] M. Karrari, W. Rosehart and O.P. Malik, “Comprehensive control strategy for a variable speed cage machine wind generation unit” “IEEE Trans. Energy Convers., vol 20, no. 2, pp. 415-423, June 2005.

[7] Bakhsh, F.I., Islam, S. ; Ahmad, S., “Simulation for Performance Analysis of Grid-Connected Induction Generators with Input Voltage Control”, Third IEEE International Conference on Advanced Computing and Communication Technologies (ACCT), pp. 312 – 315, 2013.

[8] Miao-miao Cheng; Shiojima, D.; Isobe, T.; Shimada, R., “Voltage control of induction generator powered distributed system using a new reactive power compensator SVC-MERS”, IEEE International Conference on Power Electronics and Motion Control (EPE/PEMC, pp. DS3b.7-1 - DS3b.7-8, ), Sept. 2012.




[9] de Resende, J.T. , “Control of the generated voltage by a three-phase induction generator self-excited by capacitors using control techniques”, IEEE International Conference on Industrial Technology, pp. 530 – 535, Dec 2003.

[10] Guocheng Wang, Qingzhi Zhai ; Jianhua Yang, “Voltage control of cage induction generator in micro hydro based on variable excitation”, Proceedings of IEEE International Conference on Electrical Machines and Systems (ICEMS), pp. 1 – 3, Aug 2011.

[11] R. Datta, and V.T. Rangensthan, “A method of tracking the peak power point for a variable speed wind energy conversion system, “IEEE Trans. Energy Convers., vol 18, no. 1, pp. 163-168, Mar. 2003.

[12] S. Hari, P. Trevor, and I. Syed, “Effect of pitch control and power conditioning on power quality of variable speed wind turbine generators,” in Proc. Australasian Universities Power Engineering Conf., pp. 95-100, 2001.

[13] M.G. Simoes and B.K. Bose, “Fuzzy logic based intelligent control of a variable speed cage machine wind generation system, “IEEE Trans. Power Electron., vol. 12, no. 1, pp. 87-95, Jan. 1997.

AUTHOR

Haroon Ashfaq was born in in the Aligarh, India, on January 17, 1978. He received the B.Tech. M.Tech. and Ph.D. degrees in Electrical Engineering from AMU, Aligarh, India. He is currently working as Assistant Professor in the Department of Electrical Engineering, Jamia Millia Islamia, New Delhi. His research interests include renewable energy, hybrid systems, electric drives, switch gear and protection.

Documents

Voltage and Current Inrush Control of Wind Driven Embedded Wound Rotor Induction Generator