Operation of an Induction Generator Controlled by a VSI Circuit

Ion Catalin Petrea, Student Member, IEEE

Serban Ioan Faculty of Electrical Engineering and Computer Science

Transilvania University Brasov, Romania

E-mail: ion@leda.unitbv.ro, serban@leda.unitbv.ro

Marinescu Daniela Faculty of Mathematics and Computer Science

Transilvania University Brasov, Romania

mdaniela@unitbv.ro

Abstract—The paper analyses the operation of an induction generator (IG) controlled by a voltage source inverter (VSI) circuit. This circuit performs both voltage and frequency regulation. The IG frequency is controlled by keeping constant the VSI synchronous frequency. For the IG voltage regulation two cascaded regulators are used, which have as reference the line voltage and the VSI DC voltage, respectively. The VSI also ensures a smooth voltage buildup during the start-up process. Simulations are made in order to investigate the reliability of such a configuration. In addition, experiments were carried out using a laboratory-scale prototype.

I. INTRODUCTION

These days there is a growing concern regarding the impact of using conventional energy sources for electricity production, on both natural environment and economical markets. Furthermore, the Kyoto protocol stipulates by 2010 a gradual diminution of polluting gases that each covenanter state releases into the atmosphere. Relying on these two aspects, the need for green energy arises, as electric energy production is still a major polluter worldwide. Green energy can be obtained using renewable energy sources such as wind, solar, biomass, and micro-hydro.

For stand-alone low power systems based on micro-hydro and wind, the IG is the most suitable, due to the following advantages over the synchronous one: price, robustness, simpler starting and control. Nevertheless, two main problems should be solved, respectively voltage and frequency regulation.

Two aspects arise concerning frequency regulation. First, the mechanical power delivered by the turbines can vary, especially in wind farms. Second, and most important, the loads supplied by such systems are variable by nature, so an active power balance should be achieved rapidly.

At a first look, a turbine governor seems an appropriate solution - from the energetically efficiency point of view -, because by maintaining the produced power in range with the demanded one eliminates the need for an additional circuit in the system [1, 14]. However, such a configuration is expensive for low-power applications (few tens of kW). As the mechanical constants are high, the regulating process is slow and the overall cost significant. Also, the system’s response

under suddenly load switching is poor, resulting in voltage sags and frequency deviations.

The other solution is to use an electronic load controller [3, 4], which feeds a dumping resistance, enabling the total power supplied by the generator to match the sum between the consumer’s loads and dump load. As the active power balance is achieved, the frequency is satisfactory regulated.

For voltage control, the regulating devices have evolved from switched capacitors and saturable core reactor to linearly varying current elements (emulating capacitor) like in [12].

Properly controlled inverters are used to perform both voltage and frequency regulation. There are two main controlling techniques, according to the desired behavior of the inverter. If only reactive power control, or harmonic compensation is required the inverter and its control system form a static compensator (STATCOM) [2]. The configuration, used also in this paper, ensures the control of both, active and reactive powers of the IG and loads, and has a different control technique than a STATCOM [1, 5].

Nevertheless, the process of frequency regulation must be – for such stand-alone systems – in accordance with voltage regulation. Thus, common control circuits have been developed, in order to ensure the most efficient regulation procedure possible.

II. SYSTEM CONFIGURATION

The circuit diagram of the proposed IG control scheme is presented in Fig. 1. It consists in a three-phase IG, excitation capacitors, a VSI with adaptation transformer and inductive filter, and consumers (variable loads). It also contains the IG voltage-frequency control system.

The circuit diagram from Fig.1 follows an experimental scheme. The need for the adaptation transformer arises from the voltage limitations imposed by the VSI semiconductor devices and DC capacitor.

The hydraulic turbine (replaced in the experiments by a DC motor, properly controlled), which drives the IG, has no regulation. In these circumstances, any variation in load may accelerate/decelerate the machine, thus bringing the voltage and frequency levels outside the allowable range [3]. Thereby, the exceeding power delivered by the IG has to be consumed

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by a dumping resistance, in order to achieve constant voltage and frequency [10].

The excitation capacitors supply almost the entire reactive power necessary for the IG self-excitation process; they also sustain the rated voltage in steady-state regime. The IG control is accomplished by a VSI, which acts as an impedance controller [4]. The interconnection between the VSI and the IG is made through an adaptation transformer and an inductive filter, Lf. On the DC side of the VSI there is a capacitor -Cdc -

and a dissipative circuit for the exceeding power, also known as dump load. The dissipative circuit consists in a DC chopper and a dumping resistance. The amount of power delivered to the dump load is controlled by modifying the PWM duty cycle that drives the TD transistor.

An additional low-voltage source (Vst-up) in series with a series diode on the inverter DC side is used for the IG start-up. This source supplies the initial current required by the IG to start the self-excitation process.

A

B

C

IG

LOADSV Vdc

VvAB

RMSvAB PI

Driver

VSI control system

6

IG System Control

Vref230V

CDCRD

TD

Excitation capacitors

TransformerVSI

Dump Load

CACLf

Vst-up

Σ Σ

Fig.1 Circuit diagram of the proposed control system

III. THE IG START-UP

As the hydraulic turbine gate is gradually opened, the prime mover accelerates, thus leading to the IG start-up. The inertia moment of both turbine and IG rotor, give the start-up time. The turbine mechanical characteristic also influences this process.

The main concern during this process is to buildup the voltage at the generator leads. Several start-up techniques were developed for low-power stand-alone plants with IG [7]. The most common one is to accelerate the induction machine above its synchronous speed – with the prime mover – and then connect a suitable capacitor bank at its leads. Initially, the IG electromagnetic force (EMF) is zero, due to its lack of excitation. After the capacitors are connected, the machine will absorb reactive power to develop its magnetic field, and the EMF will gradually increase, until the steady-state regime is achieved.

The initial EMF is obtained by using several methods, as follows: using the residual magnetic flux of the IG, injecting a small current into one phase from an external source, or charging a capacitor form the excitation capacitors bank with a DC voltage. Here, a low-voltage low-power DC source is used (Vst-up) in order to help the IG self-excitation process begin. The voltage build-up takes a few seconds or less [3]. During this process, over-voltages can appear [6].

In this paper, the IG works in parallel with a VSI that is the voltage-frequency regulator. Thus, both IG and VSI have to be

synchronized at their connection moment. This paper proposes a controlled IG start-up by connecting the VSI at the beginning of the starting process, with the excitation capacitors also connected. Thus, the IG voltage will be smoothly built-up, and the start-up is continuously controlled by the VSI. For this, the VSI frequency has to rise from a low value (f0=few hertz) to the rated value (fn = 50 Hz). Fig. 2 shows the VSI frequency

rise. Practically, the IG start-up process follows a .ctfU ≅

characteristic. The rise slope depends on the start-up time, which will be chosen in concordance with the system mechanical characteristics. For low-power plants of few kW, the start-up time (tst-up) lasts for several seconds.

f0

fnfVSI

ttst-up Fig.2 VSI frequency rise during the IG start-up process

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IV. VSI OPERATION

The VSI is a three-phase PWM inverter with six transistors. Its control requires the generation of six PWM pulses, which drive the transistor bridge. The VSI operates at constant synchronous frequency (fn = 50 Hz), maintaining the IG frequency constant, excepting the start-up, previous presented. Thus, the system’s power balance is reduced to the DC capacitor voltage control. The dump load connected to the VSI DC side will be controlled so that the voltage across the CDC capacitor remains at a constant level, maintaining the system voltage in a standard variation range.

Thus, the difference between the power delivered by the IG and the loads demand will circulate through the VSI towards the CDC capacitor, which acts as a short-time energy storage element. This leads to a voltage increase on the DC capacitor.

The DC voltage variation ratio depends on the capacitance value and on the amount of power transferred from the IG towards the capacitor. The capacitor value plays a very important role during transitory regimes, when it has to handle large amounts of energy (in or out). Large capacitors ensure low voltage drops across the IG lines when dynamic loads (as induction motors) are connected to the system [1]. Likewise, for unbalanced loads asymmetrical currents will flow through the inverter lines, producing voltage variations on the CDC capacitor.

Thereby, the voltage variation on the CDC capacitor has two sources: the first-one is the exceeding active power from the IG, which is dissipated in the dump load, and the second-one is the reactive power flow through the inverter for asymmetrical and harmonics currents flow. For symmetrical three-phase reactive currents, the voltage variation on the CDC capacitor is close to zero.

Two controllers are used to regulate the system voltage: a PI controller and an on/off controller, as shown in Fig. 3. The PI controller is the leading voltage regulator. It compensates the voltage drops across the inverter arms and filter, IG leakage impedances, and other circuit elements, which usually led to a decrease of the IG voltage. The IG root-mean-square (RMS) voltage (VAB) is the feedback signal, it is compared with the 230 V reference signal (VREF), and the error feeds the PI controller, giving the reference signal (VDCref) for the second controller. The on-off controller is used to maintain constant the CDC voltage. The allowed voltage variation (ripple) across CDC capacitor (∆VDC) will give the frequency and the width of the pulses that drive the Td transistor from the dump load.

VDCref

vDC

to TDgate

On-off controller

+- VDC∆ΣRMS

vABPI

Vref230V

Σ

Fig.3 The DC capacitor voltage regulation technique

V. SIMULATION RESULTS

The proposed system was modeled and simulated using the Matlab/Simulink environment. The block diagram is shown in Fig. 4. The configuration includes the 4 kW IG, a block that models the prime mover (hydraulic turbine), the VSI with an adapter transformer, the two voltage controllers, an adequate capacitor bank, loads and measurement blocks.

The IG parameters are listed below: P = 4 kW, 400 V/50 Hz/1500 RPM. Rotor type: squirrel-cage Stator: Resistance and the leakage inductance (p.u):

045.0035.0 == lss LR Magnetizing (mutual) inductance (p.u.): 352.1=mL Rotor: Resistance and leakage inductance both referred to

the stator (p.u.): mHLR lrr 045.0034.0 '' == The excitation capacitors are star connected with C = 100µF. The adaptation transformer ratio is 1.46, bringing the 230 V

from the IG leads to 157 V on the inverter branches and 220 V on the DC capacitor – this is due to the experimental setup limitations –.

Fig.4. Simulink block diagram of IG-VSI system

A. Start-up process The start-up process follows the steps detailed in chapter III.

As the turbine accelerates, the IG voltage increases. The IG voltage build-up is shown in Fig. 5. Its rising is slow and linear due to the VSI control, and reaches the steady-state regime in approximately 8 seconds. During the voltage build-up the IG current (Fig. 6) is kept constant, in order to maintain a constant the voltage-frequency ratio. The VSI current (Fig. 7) is equal to the IG current minus the capacitor bank current, which relieves the VSI of a part of the IG magnetization current. At t = 8 s the voltage reaches the rated value (230 V). At this point the dump load begins to operate, absorbing power from the IG, through the VSI. The frequency and VSI DC voltage are shown in Fig. 8. As mentioned before the frequency rises along with the voltage until the rated value of 50 Hz.

B. Transient behavior As loads supplied by an autonomous IG are variable by

nature, their effect on voltage and frequency characteristics are

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of real interest. At t = 4 s, a pure resistive load with P = 2 kW is connected to the IG leads. Its influence on several parameters is shown in the figures below.

After the R load connection, at t = 4 s, the IG voltage has a little sag (Fig. 9); in consequence, the VSI DC capacitor voltage decreases as well, the regulators command an increase of the DC voltage in order to compensate this phenomena..

In Fig. 10, the IG and VSI RMS currents are depicted. In steady-state regime, the current circulation through the VSI is around 10 A, as it regulates the frequency by dumping the exceeding active power on a dumping resistance placed on its DC side. As the 2 kW loads is connected at t = 4s, the frequency tends to decrease; in order to stabilize it, the dissipated active power must decrease accordingly to the new regime. Thus, the current through VSI decreases to around 6 A, stabilizing in approximately 0.1 s, and keeping the IG current constant.

The system frequency variation is insignificant due to the constant frequency VSI operation, which imposes the system frequency.

0 1 2 3 4 5 6 7 8 9 10-400

-200

0

200

400

Time [s]

IG V

olta

ge [V

]

9 9.05 9.1 9.15 9.2 9.25 9.3-400

-200

0

200

400

Time [s]

IG V

olta

ge [V

]

Fig.5. IG Voltage during start-up (upper) and in steady-state (lower)

0 1 2 3 4 5 6 7 8 9 10-20

-10

0

10

20

Time [s]

IG C

urre

nt [A

]

9 9.05 9.1 9.15 9.2 9.25 9.3-10

-5

0

5

10

Time [s]

IG C

urre

nt [A

]

Fig.6. IG current during start-up (upper) and in steady-state (lower)

0 1 2 3 4 5 6 7 8 9 10-20

-10

0

10

20

Time [s]

VSI

Cur

rent

[A]

9 9.05 9.1 9.15 9.2 9.25 9.3-10

-5

0

5

10

Time [s]

VSI C

urre

nt [A

]

Fig.7. VSI current during start-up (upper) and in steady-state (lower)

0

50

100

150

200

250

300

VSI D

C V

olta

ge [V

]

0 1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

Time [s]

Freq

uenc

y [H

z]

Fig. 8. VSI DC current (upper) and frequency (lower) during start-up

VI. EXPERIMENTAL RESULTS

Using the conclusions deduced from previous simulations, experiments were carried out to validate the proposed system. The experimental test bench was accomplished on a laboratory-scale prototype, which contains a 1 kW/1500 RPM IG and a VSI controlled by a DS1102 dSPACE™ card (TI DSP TMS320C31), used for data acquisition too. As prime mover, a 1.5 kW/1500 RPM DC motor was employed, supplied by a controlled rectifier bridge.

At start-up, the IG works as a motor for a few seconds, being driven by the VSI-PWM. Then the DC motor is coupled and speeded up, and the capacitor bank connected. When the synchronous frequency is attained, the induction machine passes into generating regime. The start-up is shown in Fig. 11-14. The voltage built-up (Fig. 11) lasts for approximately 11 seconds, when a steady-state regime is attained. Fig. 12 and 13 show the IG and VSI currents. Fig. 14 shows the system frequency and the VSI DC voltage.

In order to see the transitory regime, a 200 W three-phase resistive load is connected at the IG leads. Fig. 15 shows the IG

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voltage variation, while Fig. 16 presents the IG and VSI RMS currents. The VSI operation ensures an insignificant frequency variation during transients.

It can be clearly seen that the experimental results are in concordance with the simulated ones.

Conclusively, the studied configuration ensures a high degree of stability for a small-power stand-alone power system, both during IG start-up and in normal operation.

3.5 4 4.5 5200

220

240

260

Time [s]

IG R

MS

Vol

tage

[V]

Fig. 9. The IG RMS voltage variation (2kW load connection)

3.5 4 4.5 510

11

12

13

14

IG R

MS

Cur

rent

[A]

3.5 4 4.5 5

6

8

10

12

14

Time [s]

VSI R

MS

Cur

rent

[A]

Fig. 10. IG and VSI RMS currents (2kW load connection)

0 2 4 6 8 10 12 14 16 18 20-400

-200

0

200

400

Time [s]

IG V

olta

ge [V

]

16 16.05 16.1 16.15 16.2 16.25 16.3-400

-200

0

200

400

Time [s]

IG V

olta

ge [V

]

Fig. 11. IG Voltage during start-up (upper) and in steady-state (lower)

0 2 4 6 8 10 12 14 16 18 20-4

-2

0

2

4

Time [s]

IG C

urre

nt [A

]

16 16.05 16.1 16.15 16.2 16.25 16.3-4

-2

0

2

4

Time [s]

IG C

urre

nt [A

]

Fig. 12. IG current during start-up (upper) and in steady-state (lower)

0 2 4 6 8 10 12 14 16 18 20-4

-2

0

2

4

Time [s]

VSI C

urre

nt [A

]

16 16.05 16.1 16.15 16.2 16.25 16.3-4

-2

0

2

4

Time [s]

VS

I Cur

rent

[A]

Fig. 13. VSI current during start-up (upper) and in steady-state (lower)

50

100

150

200

250

300

VS

I DC

Vol

tage

[V]

0 2 4 6 8 10 12 14 16 18 2010

20

30

40

50

Time [s]

Freq

uenc

y [H

z]

Fig. 14. VSI DC current (upper) and frequency (lower) during start-up

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2200

220

240

260

Time [s]

IG R

MS

Vol

tage

[V]

Fig. 15. The IG RMS voltage variation

0

0.5

1

1.5

2

2.5

3

IG R

MS

Cur

rent

[A]

0.2 0.4 0.6 0.8 1 1.2 1.40

0.5

1

1.5

2

2.5

3

Time [s]

VS

I RM

S C

urre

nt [A

]

Fig. 16. IG and VSI RMS currents

VII. CONCLUSIONS

This paper investigates the operation of an IG controlled by a VSI circuit. The control circuit performs both frequency and voltage regulation and supports also the start-up process.

The proposed system configuration was modeled and simulated using Matlab/Simulink environment. In addition, experiments were carried out using a laboratory-scale prototype.

Both simulation and experimental results were satisfactory and they have shown the effectiveness of the proposed control method.

Based on these results, several conclusions were drawn: - the control system can efficiently control the start-up

process; - for a smooth and stable start-up, the need for an adequate

control of U/f=ct characteristic arises; - during transient regimes, it behaves satisfactory. - the reaction time of the system is according to the Power

Quality Standards (EN 50160).

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[3] B. Singh, S. S. Murthy and Sushma Gupta, “Transient Analysis of Self-Excited Induction Generator with Electronic Load Controller (ELC) supplying Static and dynamic Loads” IEEE Trans. On Industry Applications, vol. 41, no. 5, September/October 2005

[4] B. Singh, S. S. Murthy and Sushma Gupta “An Improved Electronic Load Controller for Self-excited Induction Generator in Micro-Hydel Applications ” Industrial Electronics Society, 2003. IECON’03. The 29th Annual Conference of the IEEE, Nov.2003

[5] Jayanta K. Chatterjee, B. Bnkatesa Perumal and Naveen Reddy Gopu “Analysis of Operation of a Self-Excited Induction Generator With Generalized Impedance Controller” IEEE Transactions on Energy Conversion 2006

[6] Andrea Del Pizzo, Diego Iannuzzi, Luigi Piegari, Stefano Barato and Umberto Macri, “A simple and efficacious control system for stand alone generation plants equiped with induction machines”, The 32nd Annual Conference of the IEEE Industrial Electronics Society IECON’06 7-10 November, Paris, France

[7] James D. Bailey, “Factors Influencing the Protection of Small-to-Medium Size Induction Generators”, IEEE Transactions on Industry Applications, Vol. 24, No. 5, September 1988

[8] Ion C., Marinescu C, Optimum Voltage Regulation for Induction Generators Working in Autonomous Micro Hydro Power Plants, 10th International Conference on Optimization of Electrical and Electronic Equipments OPTIM’06 May 18-19 Brasov Romania

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[10] Serban Ioan, Ion Catalin, Marinescu Corneliu, Cirstea Marcian “Electronic Load Controller for Stand-Alone Generating Units with Renewable Energy Sources” The 32nd Annual Conference of the IEEE Industrial Electronics Society IECON’06 7-10 November, Paris, France

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[12] Kuperman A., Rabinovici R., „A capacitor emulating solid-state voltage regulator for autonomous induction generators”, ACEMP'04, Istanbul, Turkey, 2004

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