978-1-4799-3807-0/14/$31.00 ©2014 IEEE

Research of Transients in Induction Generator for Small-Scale Wind Power in Off-the-Grid by Using

EMTP - ATP

Rusnok Stanislav Department of Electrical Power

Engineering VSB – Technical University of

Ostrava Ostrava, Czech Republic

rus109@vsb.cz

Sobota Pavel Department of Electrical Power

Engineering VSB – Technical University of

Ostrava Ostrava, Czech Republic

sob133@vsb.cz

Mach Veleslav Department of Electrical Power

Engineering VSB – Technical University of

Ostrava Ostrava, Czech Republic

mah30@vsb.cz

Abstract—The article deals with the analysis of transients in

induction generator, which operates on small-scale wind power in Off-the-Grid. It deals with issues of self-excited and also deals with transients that are usually occur during the operation of the induction generator. Transients were assessed using a simulation model that was created for this purpose in the EMTP - ATP. The same transients were also measured. The measurements were compared with the results of the simulations.

Keywords—induction generator, Off-the-Grid, transients, simulation, EMPT - ATP

I. INTRODUCTION Wind power has its origins in the incident solar radiation.

The solar energy heats the air near the earth's surface. Due to different warming in various fields, there are significant differences in temperature of air zones. The result is a horizontal air flow, known as wind.

Small-scale wind power is the name given to wind generation systems with the capacity to produce up to 10 kW of electrical power. Isolated networks, which may otherwise rely on diesel generators, may use wind turbines as an alternative possibility. Individuals may purchase these systems to reduce or eliminate their dependence on grid electricity for economic reasons, or to reduce their carbon footprint. Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas [1].

II. THEORETICAL ANALYSIS The induction machine can operate as a generator if it gets reactive power from the network in which is operating. Reactive power is a necessary for the excitation of the magnetic field. When the induction generator is operating Off-the-Grid, so is used excitation by remanent field using capacitors that are connected to the motor terminals. The

principle of induction generator excitation is apparent from the no-load characteristics of the induction generator (Fig. 1) [2].

Fig. 1 The process of excitation of the induction generator operating in Off-the-Grid

Assume that the induction generator is not connected to a network or to capacitor. Alternator is working to speed (1)

pf

n 11

60= (1)

Now we connect induction generator with the capacitors. Electromotive force Ue10C (which is induced in the winding by remanent magnetization) causes in capacitors current I1C’. This current also flows through the stator windings and will cause a corresponding magnetic field that induces an electromotive force Ue1’ in the stator. This electromotive force increases the

This work was supported by project SGS SP 2014/49.

size of the capacitor current to I1C’’ etc. – the whole process is repeated until the electromotive force of the alternator will not have a value Ue1, which corresponds to the intersection of both characteristics (Fig. 1). At this point there is a balance between the voltages of the alternator and the capacitors (2)

CI

IL⋅

=⋅⋅1

1C1C11 ω

ω (2)

Where C is the capacitance and L1 is inductance of the induction generator and can be calculated by (3)

1

μ1σ1 ω

XXL

+= (3)

If the induction generator is excited, so it can be loaded. It is necessary to vary the speed of the rotor of the induction generator, so that the size of the slip agrees with the size of the load. Frequency will be constant (f1 = const). The work of the induction generator can be assessed using the equivalent circuit (Fig. 2).

Fig. 2 Equivalent circuit of the self-excitation induction generator

III. SIMULATION

Induction machine is an important part of our model. The induction machine has following parameters, see below:

TABLE I. PARAMETRS OF INDUCTION MOTOR

P2N = 1100 W Rs = 11.03 Ω

U1N = 400 V Lσs = 0.0203 H

f1 = 50 Hz R’r = 4.77 Ω

IN1 = 3.25 A L’σr = 0.0203 H

nN = 1360 rpm Lμ = 0.28 H

J = 0.018 kg.m2 pp = 2

MN = 7.72 Nm MZ = 12.352 Nm

I0N = 2.687 A cosφ0N = 0.22

cosφK = 0.778 IkN = 11.375 A

Now we will calculate the individual parameters of the equivalent circuit (Fig. 3), that we will need to set the model.

Fig. 3 Equivalent circuit of induction machine

Magnetization current:

A 62.2975.0687.2sin 0N0Nμ =⋅=⋅= ϕII (4)

Current that represents core losses:

A 59.022.0687.2cos 0N0NFe =⋅=⋅= ϕII (5)

Magnetic reactance:

Ω=== 17.8862.29.230

μsfμ I

UX (6)

Resistance that represents core losses:

Ω=== 53.39159.09.230

Fesf

Fe IUR (7)

Magnetic inductance:

H 28.050217.88

2 1

μμ =⋅⋅=⋅⋅= ππ f

XL (8)

Total active resistance (short circuit state):

Ω=⋅=⋅= 8.15375.11778.09.230cos

kNksf

k IUR ϕ (9)

Resistance of rotor recalculated to stator

Ω=−=−= 77.403.118.15´ skr RRR (10)

Total stray reactance:

Ω=−=−= ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ 76.1228.15

2375.11

9.2302

2k

kNsfσ RI

UX

(11)

Total stray inductance:

H 0406.050276.12

2 1σσ =⋅⋅=⋅⋅= ππ f

XL (12)

Now total stray inductance divided to the two same parts

H 0203.02σσrσs === LLL (13)

Dialog windows where we set the model parameters are shown in Figure 4 and Figure 5.

Fig. 4 Dialog window of the motor model

Fig. 5 Detail of dialog window for individual parameters

Complete simulation model is shown in Figure 6. This model have three capacitors set to C=46 μF, which are connected to star (Y).

Fig. 6 Overall model of wind power plant with induction motor

IV. MEASUREMENTS Measurement schema is shown in Fig. 7. Voltages and

currents were measured in each phase using the measurement kit connected with the computer. We measured the voltage and current in one phase also by oscilloscope. The whole model consists of an induction generator (n=const) with excitation capacitors connected in Y connection and with load which is represented by active resistance (R=const) connected also in Y connection.

Fig. 7 Measurement schema

V. RESULTS FROM SIMULATION AND MEASUREMENTS The following figures show the transients for each

operating states which were considered.

A. Excitation of the induction generator The figures 8 and 9 show voltage waveforms during self-

excitation of the induction generator for remanent voltage 35V.

Fig. 8 Excitation for remanent voltage 35 V - Simulation

Fig. 9 Excitation for remanent voltage 35 V - Measurement The figures 10 and 11 show voltage waveforms during self-excitation of the induction generator for remanent voltage 300V.

Fig. 10 Excitation for remanent voltage 300 V - Simulation

Fig. 11 Excitation for remanent voltage 300 V - Measurement

B. Loading of the induction generator The figures 12 and 13 show voltage and current

waveforms during loading of the induction generator for rated voltage. First, the induction generator operates in no-load state and then the generator is loaded. The load is represented by three active resistances of 105 Ω. Resistances are connected in Y connection.

Fig. 12 Induction generator loading - Simulation

Fig. 13 Induction generator loading - Measurement

C. Disconnection of one phase of the induction generator The figures 14 and 15 show voltage and current

waveforms during disconnection of one phase of the induction generator. This situation represents a significant unbalance.

Fig. 14 Disconnection of one phase – Simulation

Fig. 15 Disconnection of one phase - Measurement

CONCLUSION A possible way how to solve operational state and

transients in the induction generator was described in the article. This generator is used for the operation of small-scale wind power in Off-the-Grid. The process of self-excited of induction generator is an interesting area in transients and we had devoted it significant attention. Authors created a simulation model in the EMTP – ATP for research of transients. Knowledge of the behavior of the induction generator in Off-the-Grid is very important because in this way we are able to determine the conditions for optimum operation of and reliable and efficient supply of electric power. Off-the-Grid mode of induction generator is very specific. The induction generator can completely lose remanent magnetism. This can occur in extreme cases, when insufficient knowledge of operational capability of the machine. In this case, the induction generator is unable to supply electrical network by electrical power. The plausibility of the simulation and therefore the model was verified by measuring. The results show that we have achieved relatively high precision of the model. The model will be used in the creation of software that will be able for the small-scale wind power with an induction generator drive connecting and disconnecting the load for the actual the weather conditions. Control of connecting and disconnecting the load will be carried out so that the induction generator is still working in his optimum, so with maximum efficiency. It will also ensure that the induction generator will never lose remanent magnetism.

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