DTC-SVM Control Strategy for Induction Machine based on Indirect Matrix Converter

in Flywheel Energy Storage System

SHALINI , NARAYANA ENGINEERING COLLEGE

Abstract- Flywheel energy storage system (FESS) improves the quality of the power

transmitted to the grid by wind generators. Presently, FESSs, containing back to back

converter and a direct torque control (DTC) that controlled induction machine (1M), are

mainly considered for this kind of application. This paper investigates a low-speed FESS with

DTC_SVM control strategy for an 1M based on indirect matrix converter (IMC). In this

system, DTC-SVM increase efficiency of the energy storage system and facilitate the active

power output of the wind farm by providing situation to shift quickly and repeatedly between

motoring and generating operation modes in induction machine. The space vector modulation

(SVM) strategy in indirect matrix converter provides full control of both the output voltage

vector and the instantaneous input current displacement angle. The reference power of the

FESS that used as the input of the system can be obtained by calculating the active power of

wind farm and the target power. The model of the system is simulated in

SIMULlNKlMATLAB software for different FESS operations such as: charge, discharge and

standby modes. The results of simulation show effectiveness of the proposed strategy.

Keywords-flywheel energy storage system (FESS); induction machine (1M); indirect matrix

converter (IMC);direct torque control (DTC); space vector modulation (SVM).

INTRODUCTION

Urban transportation is one of the most important consumer of energy and

an emission source of greenhouse gases. Utilizing induction motors with high

performance and less maintenance have resulted in growth of using this drive in the

electric transportation industry. Deployment of induction motor in transportation

applications requires a driving method with fast response, high reliability and simple

implementation approach [1]. Different control methods including direct torque control

(DTC), field oriented control (FOC), scalar control and volt per hertz (V/Hz) control

methods have been proposed to control inverter fed induction motors. Nowadays, FOC

and DTC methods are more utilizable than the other methods. If the magnetic flux and

torque could be controlled indirectly using stator current components, there is no reason

that the magnetic flux and torque could not be controlled directly without using interface

current control loops. This issue is the difference between DTC and FOC i.e. FOC uses an

indirect method of torque control. Using direct torque control method leads to faster

response to torque vanatlon in DTC compared with FOe.

Therefore, DTC is more suitable for transportation applications i.e.

induction traction motor drives [2-5]. Creating a suitable condition for energy recovery

during speed reduction in braking is one of the most important concerns in operation of

the electric transportation systems. Electric brakes are implemented in different methods,

such as plugging, dynamic braking and regenerative braking [6]. This paper discusses

transient and steady states analysis of induction traction motor, considering energy

recovery in regenerative braking. Moreover, the performance of space vector modulation

based direct torque control (SVM-DTC) method is analyzed and simulated using MA

TLAB/Simulink, in order to control and drive of induction traction motor. A. Induction

traction motor characteristics One of the most important features of a traction motor is its

lower weight and dimension compared with conventional motors. The other difference is

its special design to tolerate long term overloads. Favorable characteristics of electric

traction motor are: High density of torque and power Vast constant power operation

region High efficiency Minimum number of sensors Maximum torque of three phase

induction motor is as follows:

T =3PV2 RrlS (1) p Ws s (RrIS) 2+(wsLr?

The motor dynamic torque equation is as follow: Tm - Te = -J(dw i dt) (2) Where J is the

moment of the inertia and w is the rotor speed. The torque-speed characteristic of

induction motor is shown in Fig. 1. The torque-speed characteristic contains generating,

motoring and braking mode of operation. Traction motor operates in generating mode

during braking or speed reduction, if the stator flux vector lags behind the rotor flux

vector. Considering this issue, traction motor should not be separated from the inverter

during energy regeneration; because this results in loss of stator flux vector and hence the

linkage flux decreases to zero rapidly. Therefore, an appropriate driver should be

provided in which by applying a suitable switching pattern, the stator flux vector locates

in a suitable position related to the rotor flux vector.

Thus, an appropriate negative electromagnetic torque is produced for

energy regeneration. With regard to (2), speed increases and decreases when the

difference between mechanical and electromagnetic torque is less and more than zero,

respectively. B. DTC Method DTC method provides suitable switching pattern for VSI

through two control loops of stator flux and electromagnetic torque. It is necessary to

estimate these quantities to form the control loops. Nevertheless, the differences between

estimated and reference values are utilized in a different way compared with FOe. DTC

does not need current controllers and uses two and three level hysteresis comparators to

control flux and torque, respectively. Fig. 2 shows the block diagram of DTe. The

hysteresis controllers determine the flux and torque variation with considering that the

flux and torque error is out of specified limits or not. Look-up table is another part of

DTC structure. Inputs of look-up table are: flux and torque hysteresis controllers output

and the specific sector in which flux vector is rotating. Consequently, an appropriate

voltage vector based on the switching strategy is produced

INDUCTION MOTOR

An induction motor (IM) is a type of asynchronous AC motor where power is

supplied to the rotating device by means of electromagnetic induction. Other commonly used

name is squirrel cage motor due to the fact that the rotor bars with short circuit rings resemble

a squirrel cage (hamster wheel).An electric motor converts electrical power to mechanical

power in its rotor.

There are several ways to supply power to the rotor. In a DC motor this power is

supplied to the armature directly from a DC source, while in an induction motor this power is

induced in the rotating device. An induction motor is sometimes called a rotating transformer

because the stator (stationary part) is essentially the primary side of the transformer and the

rotor (rotating part) is the secondary side. Induction motors are widely used, especially

polyphase induction motors, which are frequently used in industrial drives.

The Induction motor is a three phase AC motor and is the most widely used machine.

Its characteristic features are-

Simple and rugged construction

Low cost and minimum maintenance

High reliability and sufficiently high efficiency

Needs no extra starting motor and need not be synchronized

An Induction motor has basically two parts – Stator and Rotor

The Stator is made up of a number of stampings with slots to carry three phase windings.

It is wound for a definite number of poles. The windings are geometrically spaced 120

degrees apart. Two types of rotors are used in Induction motors - Squirrel-cage rotor and

Wound rotor

Torque

Space Vector PWM

The Space Vector PWM generation module accepts modulation index commands and

generates the appropriate gate drive waveforms for each PWM cycle. This section describes

the operation and configuration of the SVPWM module.

A three-phase 2-level inverter with dc link configuration can have eight possible

switching states, which generates output voltage of the inverter. Each inverter switching state

generates a voltage Space Vector (V1 to V6 active vectors, V7 and V8 zero voltage vectors)

in the Space Vector plane (Figure: space vector diagram). The magnitude of each active

vector (V1to V6) is 2/3 Vdc (dc bus voltage).

The Space Vector PWM (SVPWM) module inputs modulation index commands

(U_Alpha and U_Beta) which are orthogonal signals (Alpha and Beta) as shown in Figure.

The gain characteristic of the SVPWM module is given in Figure . The vertical axis of Figure

represents the normalized peak motor phase voltage (V/Vdc) and the horizontal axis

represents the normalized modulation index (M).

The inverter fundamental line-to-line Rms output voltage (Vline) can be approximated (linear

range) by the following equation:

………….. (1)

Where dc bus voltage (Vdc) is in volts

Space Vector Diagram

DIRECT TORQUE CONTROL (DTC):

Direct Torque Control (DTC) is a method that has emerged to become one possible

alternative to the well-known Vector Control of Induction Motors [1–3]. This method

provides a good performance with a simpler structure and control diagram. In DTC it is

possible to control directly the stator flux and the torque by selecting the appropriate VSI

state. The main advantages offered by DTC are:

Decoupled control of torque and stator flux.

Excellent torque dynamics with minimal response time.

Inherent motion-sensor less control method since the motor speed is not required to

achieve the torque control.

Absence of coordinate transformation (required in Field Oriented Control (FOC)).

Absence of voltage modulator, as well as other controllers such as PID and current

controllers (used in FOC).

Robustness for rotor parameters variation. Only the stator resistance is needed for the

torque and stator flux estimator.

These merits are counterbalanced by some drawbacks:

Possible problems during starting and low speed operation and during changes in

torque command. Requirement of torque and flux estimators, implying the consequent

parameters identification (the same as for other vector controls).

Variable switching frequency caused by the hysteresis controllers employed.

Inherent torque and stator flux ripples.

Flux and current distortion caused by sector changes of the flux position.

Higher harmonic distortion of the stator voltage and current waveforms compared to

other methods such as FOC.

Acoustical noise produced due to the variable switching frequency. This noise can be

particularly high at low speed operation.

Fig: Block diagram of FESS control shame using DTC-SVM for the 1M based on

IMC

Indirect Matrix Converter:

In "Fig. 3", is shown the indirect matrix converter (IMC) topology. The converter includes a

current source rectifier with bidirectional switches, which is directly, without a DC link

capacitor, connected to a voltage source inverter. For the input converter, soft switching

commutation is obtained by synchronizing the input and output converter pulse

widthmodulation patterns. This commutation is simpler than commutation of direct matrix

converter [11].

The following sections demonstrate the step modulation strategies of the indirect

matrix converter. First, the modulation strategies which can be used to produce a DC voltage

are presented. Second, the procedure to obtain the output converter duty cycles is shown.

Eventually, synchronizing the commutation of the input IMC to output converter is presented.

Fig: Indirect matrix converter

SIMULATION RESULTS:

The proposed flywheel energy storage system has been simulated with MATLAB.

"Figs 9-15" illustrate the simulation results of DTC-SVM strategy based on IMC. The

parameters of flywheel energy storage system (FESS) are listed in "Table I".

required power for grid can be positive, negative or zero. According to the "Table II", "at O<t

FESS is positive and flywheel start to works as a motor .At the first speed of flywheel is zero,

then by transforming energy from wind generator to the flywheel through induction motor

speed of flywheel continues to increase from zero to 420 rad/s("Fig. 7"). Accordingly, the

FESS stores up the surplus power in the form of kinetic energy and energy storage follows

the evolution of the speed and energy storage has become more ("Fig. 8"). At 1 "s", the

reference power become negative because there is a less power producing with wind rather

than required power for grid. In this situation induction machine works as a generator brake

so phase of rotor current changes .Furthermore, induction machine has received mechanical

energy from flywheel and turns it into electric energy. During discharging mode, the 1M

rotor speed decreases from 420 rad/s to 80 rad/s and flywheel has discharged until the speed

drops to minimum. The corresponding abc rotor current in charging, discharging and standby

modes is shown in "Fig. 9". The FESS at t=l.8 s switches to standby mode, due to the

reference power of FESS amounts to zero. In this mode, the relative speed between stator

field and the rotor of induction machine will equal zero. As seen in "Fig. 7,9", during standby

mode, no voltage is sent to rotor and the current of the rotor amounts to zero, which makes

flywheel stay at a stable speed.

Fig: Three phase rotor currents of induction machine.

Fig: a) Input Voltage and current FESS. b) Spectra and total harmonic distortion (THD) of

input current of IMC.

Conclusion

When a FESS is applied in a wind farm, two parameters are important; high dynamic

velocity and the high efficiency of control method. In this paper, direct torque control with

space vector modulation (DTC-SVM) based on indirect matrix converter is applied to the

induction machine of the FESS. The direct torque control has high operation speed. The

results indicate that the power FESS follow reference power with fast dynamic. The THD for

both the input current of matrix converter and stator current was obtained 6.23 and 2.09

respectively. In addition, the phase form of the input current and the input voltage of matrix

converter indicate unit shift coefficient on the input of the system as well as low loss of the

method for FESS. The recommended operation of an 357 participation is a future challenging

task In flywheel energy storage system.

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