Cooperation of Induction SCIG With Grid Connected ACDCAC Converter

7/28/2019 Cooperation of Induction SCIG With Grid Connected ACDCAC Converter

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BULLETIN OF THE POLISH ACADEMY OF SCIENCESTECHNICAL SCIENCESVol. 57, No. 4, 2009

Cooperation of induction squirrel-cage generator

with grid connected AC/DC/AC converter

A. SIKORSKI and A. KUMA

Division for Power Electronics and Electrical Drivers, Bialystok Technical University, 45D Wiejska St., 15-351 Biaystok, Poland

Abstract. The paper presents a squirrel-cage induction machine operating as a generator principally designed for use in small wind andhydroelectric power stations. The main advantages of such a generator are its high reliability, low price and costs of operation and maintenance.The asynchronous generator coupled to the grid through the AC/DC/AC converter is capable of generating energy even at low turbine speed.The results of investigations on the AC/DC and DC/AC converters controlled by linear and nonlinear current regulators are also discussed.

Key words: asynchronous generator, AC/DC/AC converter, PWM method of control, nonlinear method of control.

1. IntroductionBoth wind and water energy can be converted into electri-cal energy in various electromechanical systems using induc-tion and synchronous generators with separate excitation andpermanent magnets [15]. In electric power stations main-ly double-fed induction generators are used. They consti-tute almost 50% of the installed power of the wind pow-er plants in operation [1]. Part of them are induction ma-chines working at fixed turbine speed [67]. The systemsare characterized by a small range of speed changes at thetime when energy is sent to the grid. In a historical per-spective, to make a better use of various wind power: me-

chanical, electromechanical and electrical changes have beenintroduced. The improvements involved using constant andvariable speed transmission mechanisms, variable pitch angleof turbine blades, changeable number of pole pairs as wellas wound-rotor motors with controlled resistance and powerelectronic converters used practically in all modern systems.A list of conditions that should be fulfilled by a system usedto produce electrical energy from wind power is given be-low.

The generator should be adapted to operate at variable an-gular turbine velocity at the largest possible range to makemaximum use of weak winds.

The generator should provide efficient conversion of windpower into electrical energy (high efficiency of the wholesystem).

The energy sent to the grid should be characterized by alow levels of higher harmonics.

The generator design should take extreme working condi-tions and emergency situations into consideration. It shouldhave suitable braking systems protecting it against stronggusts of wind and other dangers that may occur in this typeof systems.

The actual construction of the generator should ensurelong-lasting, failure-free and low-noise operation. Addi-

tionally, its design should enable easy, quick and infrequentmaintenance or repairs.

The generators should be relatively light and small in sizebut, first of all, inexpensive to buy and also cheap to oper-ate.

The above requirements are fulfilled by three types of windpower plants additionally equipped with power electronicconverters [812]:

wind power plant with squirrel-cage asynchronous genera-tor (SCAG),

wind power plant with double-fed asynchronous generator

(DFAG), wind power plant with synchronous generator (SG) andseparate excitation or permanent magnets.

With respect to their technical specifications all the abovetypes possess similar characteristics their main advantagelying in the possibility of producing electrical energy evenat very low shaft angular velocity. Efficiency of energy con-version depends on both mechanical and electrical factors.Assuming that the mechanical efficiency of a device (i.e. itsturbine and the transmission system) have the same efficiency,then the devices efficiency will be dependent on its electri-cal properties (i.e. motor and converter efficiency). Machines

are characterized by similar efficiency, however, a double-fedinduction machine shows the lowest efficiency between themdue to the way in which the energy is transferred throughits stator and rotor. The key element in the efficiency chainis the power electronic converter. In theory, all the convert-ers possess similar efficiency but in the case of the convert-er co-operating with a double-fed induction generator, onlypart of the energy is converted inside it, which should makethe conversion method work with higher efficiency. In theremaining two methods the converter transforms the wholeenergy produced by the generator. The final effect of an in-vestment depends on satisfying all the necessary technical

e-mail: [email protected]

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A. Sikorski and A. Kuma

requirements ensuring appropriate working and safety stan-dards. In turn, the economic results are determined by thecosts of the investment, operation and efficiency of energyconversion. Comparing the investment costs outlays, or actu-al cost differences, one can state that the synchronous gen-erator is the most costly, whereas the asynchronous one is

the least expensive. Also the operation and maintenance costsare the lowest for the latter. The greatest fallibility charac-terizes the machines equipped with slip rings i.e. double-fedinduction machine and synchronous machine with externalexcitation. DFAG system requires a lower power converterthan in the other two systems and hence, it is the cheap-est one [13]. Efficiency of the converter plays the key rolein the overall output of the generator, especially in SCAGand SG systems in which the whole energy is converted.The converter efficiency is directly determined by the av-erage switching frequency of power electronic devices, i.e.the control method. As regards to the generator, some cur-rent distortion is acceptable, unlike in the grid converterin which current and voltage distortions cannot be accept-ed. Considering their effectiveness, non-linear control meth-ods could be more advantageous because of lower switchingfrequency with similar properties of output such as torqueand current ripples. In the case of the grid converter, linearcurrent controllers seem to be more suitable since, at con-stant frequency modulation, it is easier to design the out-put current filter [14]. In the presented paper we analysea turbine coupled with an asynchronous squirrel cage gen-erator.

2. Concept of converter control

The way in which the generator controls system is built de-pends on its function in the electric power system. It may beworking as one of many others on a wind turbines farm, asa single generator connected to the grid and also as a gen-erator working on island [15] or as uninterruptible powersupply (UPS). The power of the turbine at winds constantspeed vw, assumes a shape characterized by a clearly visi-ble power maximum that can be utilized (Fig. 1) [2, 1618].The maximum value, depending on the wind speed, occursat different values of the turbine shaft speed. To make anoptimal use of the turbines work, it is necessary to utilizeits power in the way shown by the dotted line. The turbine

characteristics may be additionally determined by the turbineconstruction, i.e. the turbine motion and changes of its bladepitch angle.

In this paper, it is assumed that the turbine works as a sin-gle generator connected to the grid. At low turbines power,it can be assumed that the turbine, at any given shaft speed,works with its maximum power to make optimal use of it.Thus, no limits are imposed on the maximum power thatcan be sent to the grid. The turbine is coupled to the gen-erators shaft using a constant ratio gear resulting from therelation between synchronous generator speed and the maxi-mum allowable turbine speed. The idea of converter controlwhich makes it possible to perform the described above task

is shown in Fig. 2 [19]. The DC/AC converter connects thegenerator windings to the DC link represented by capacitor C.

P =f( )w mw

wm

vw6

vw5

vw4

vw1

vw3

vw2

Pw

Fig. 1. Characteristics of wind turbine power as dependent on windspeed vw

Gear

Squirrelcagedinduction

motor

Converter

AC/DC/AC

L

Grid

Turbine

wm

-

- -

-

P =f( )w mw

iG uG udc uLiL

T*=P /w mw

T*

Y*r

u *dc

Estimator

T, Yr,

Estimator

Q ,PL LQL

QL*

PLYr

T

T

Control. Control.

GI

Fig. 2. Diagram of the AC/DC/AC converter control

In order to make a maximum use of wind and turbinepower by using the relation Pw = f(m) at a given angularspeed of the shaft m, the power obtained from the turbineis determined. Next the set value of torque T at witch thegenerator should be working is determined in order to attainmaximum power at a given angular speed m. The AC/DCgrid converter transfers the energy from the DC link to thethree-phase grid. The converter is controlled by the main volt-age control loop of the capacitor (DC link). The above loopis created for two reasons:

to provide an appropriate voltage value to make it possibleto shape the sinusoidal current transferred to the grid underfixed conditions (grid voltage value, current and inductanceof grid choke) [2021],

to protect the converter against overvoltages that may occurdue to the differences between the delivered and receivedenergy from the DC link.

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Cooperation of induction squirrel-cage generator with grid connected AC/DC/AC converter

The internal control loop of the asynchronous generator isidentical to the one used in the squirrel cage motor [22]. Fig-ure 3 shows a system with direct field oriented control method(DFOC). Both flux and torque are controlled in two separatedpaths. The internal control circuits consist of controllers ofcurrent components id and iq working in the rotating refer-

ence frame dq. The controllers of torque, rotor flux and cur-rent components are made as PI type linear controllers andits parameters are designed according to the rules that resultfrom the module or symmetry criterion. Electromechanicalstate variable (flux, torque and speed) can be estimated usingprocedure described in references [2325]. There are no spe-cific requirements concerning the dynamics of the controlled

quantities. The generator works at constant flux values anda transition to the second zone flux control (decrease of flux)is rarely used. Torque set value T is variable, however thechange rate of its value is limited by the change rates of theturbine angular speed m. Thus, the dynamics of the changesof the given power and torque is several orders of magni-

tude smaller than the time constants of torque control (sev-eral to several dozen of milliseconds) possible to achieve byusing linear controllers. The reactive power (to magnetize themachine) circulates between generator and DC/AC inverter,whereas the active power is transmitted to the grid. Figure 4presents a detailed diagram of the control of the converterconnecting the generator with the grid.

Fig. 3. DC/AC converter control of the squirrel cage generator

Fig. 4. AC/DC converter (grid converter) control of the squirrel cage generator system

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The subordinate control systems were built using two-path current control loops working in rotating coordinate xy(with grid angular frequency). The capacitor voltage error(udcudc) determines the amplitude and direction ofi

x com-ponent directly proportional to the set value of the active pow-er. The component iy is directly proportional to the reactive

power generated into the grid. When a set current value i

yequals to zero then the currents flowing into the grid remainin phase with the corresponding voltages in the supply grid only active power is generated. The reference components ix,iy are compared to the transferred to the rotating coordinates

xy the real currents iL1, iL2, iL3. The output signals com-ing from PI current controllers, after passing the decouplingprocess (DN block), are transformed into the stationary coordinates and then into the three-phase system in the xy/123block. The angle O, used for the operation of current vectorrotation (transferring from into xy coordinates and viceversa), is reproduced in phase locked loop (PLL). As for theDC/AC converter, high dynamics of current component con-trollers are not required here. Rapid changes of the currentreference values can not takes place due to the type of thepreceding PI controllers. A static accuracy is ensured by thecontrollers operating at quasi-constant values.

To compare the properties of the systems, investigationswere carried out using non-linear comparator controllers ofcurrent [24]. A schematic diagram of such a regulator whichwas used instead of current controllers in Figs. 3 and 4 (shownagainst the gray background) is presented in Fig. 5. The type controller has a very good dynamics [11].

Fig. 5. Block diagram of type current controller

3. Results of tests

The operation of AC/DC/AC current controllers employingthe above algorithms was tested in the system co-operatingwith the induction machine (generator) driven by a DC motor(turbine simulator). The main data and parameters are givenin Table 1.

Table 1System parameters

Generator Grid

PN = 3.7 kW UN(RMS) = 200 V

UN = 350 V L = 30 mH

IN = 10 A

fN = 51.6 Hz

N = 324 rad/s

The converter was controlled using dSPACE DS1103board programmed in the Matlab Simulink 7.01 environment.

Figures 6 and 7 illustrate changes of voltage and phase cur-rent of both the generator and grid as well as the real torque T,rotor flux r and also active PL and reactive QL powers re-turned to the grid when non-linear (Figs. 6a, 7a) and linear

(Figs. 6b, 7b) current controllers are used. The sampling timeTp of type current controller and the frequency of PWMmodulator (in linear controllers) were chosen in such a way asto be equal, in both cases, to the average switching frequencyof the converter transistors (9.2 kHz in AC/DC and 8.2 kHz inDC/AC). Thus, it is possible to compare the converter prop-erties such as accuracy of current, torque and power shaping(ripples), when the used converters have comparable efficien-cy [24].

a)

b)

Fig. 6. Oscillograms of voltage uL, grid current iL and active andreactive powers PL and QL returned to the grid using non-linear (a)

and linear (b) current controllers

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Cooperation of induction squirrel-cage generator with grid connected AC/DC/AC converter

a)

b)

c)

Fig. 7. Oscillograms of voltage uG, generator current iG, rotor fluxr and generator torque T of the control system equipped with non-linear (a) and linear (b) current controllers and system response to

both rapid and linear change of a reference torque T

(c)

The linear controller (in comparison to the non-linear type) shows smaller current ripples, and as a result, small-er torque and power ripples in both converters (Figs. 6, 7,channel 1).

In the asynchronous generator, similarly to the DC gen-erator, there is a possibility of controlling the output volt-

age(power) by controlling the rotor flux r.The AC/DC converter is controlled in such a way ( iy = 0)that the converter current (Fig. 6) remains in the oppositephase to the voltage so only active power (QL = 0) is re-turned to the grid.

4. Conclusions

A squirrel-cage generator may prove to be a very attractivesolution for small wind and water power plants. Its main ad-vantages include high reliability due to its simple constructionrequiring neither brushes nor commutators, low price compa-rable to other machines of the same power and low operating

costs. The disadvantages of such a solution i.e. the necessityof generator excitation and problems connected with ener-gy conversion at low drive speed, have been eliminated byuse of the AC/DC/AC converter. The controlled, bidirectionalAC/DC/AC converter makes it possible, for the reactive pow-er, to flow from the grid to the generator for its excitationas well as send the active power from the generator to thegrid (Fig. 7). Another very important advantage of the so-lution is a possibility of energy generation at a theoreticallylow angular speed of the turbine (when its power compen-sates both the generator and converter losses). In the testedsystem the speed was equal to 15 rad/s. The linear controlleris characterized by lower current ripples in comparison with

other controllers, especially if high dynamics is not an es-sential property for converters used in the considered system.Considering the minimization of switching frequency, predic-tive controllers seem to be the most appropriate solution [23].However, their use requires a fast processor and, as a result,a relatively expensive equipment.

Acknowledgements. The scientific paper was financed by2009 science funds in the framework of individual researchproject W/WE/5/09.

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Cooperation of Induction SCIG With Grid Connected ACDCAC Converter