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| ARTICLE |
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| Year : 2009 | Volume
: 26
| Issue : 2 | Page : 115-136 |
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Design and Control of Voltage Regulators for a Standalone Power Generation
Gaurav Kasal, Bhim Singh
Department of Electrical Engineering, Indian Institute of Technology, Delhi Hauz Khas, New Delhi, India
Correspondence Address:
Gaurav Kasal
Department of Electrical Engineering, Indian Institute of Technology, Delhi Hauz Khas, New Delhi
India
 DOI: 10.4103/0256-4602.49099
 Abstract | | |
This paper presents various configurations of voltage source converter (VSC) based voltage regulators (VRs) for a stand alone power generation employing an isolated asynchronous generator (IAG). A set of VRs are designed and their performance is simulated using Simulink and Power System Blockset (PSB) toolboxes to demonstrate their capabilities as a voltage regulator, a harmonic eliminator, a load balancer and a neutral current compensator. It also discusses their merits and demerits, to select a suitable topology of the voltage regulator according to IAG system requirements. Keywords: Asynchronous generator, Midpoint capacitor, Two leg VSC, Voltage regulator, Voltage source converter,
Zigzag transformer.
How to cite this article:
Kasal G, Singh B. Design and Control of Voltage Regulators for a Standalone Power Generation. IETE Tech Rev 2009;26:115-36 |
| 1. Introduction | |  |
The rapid depletion and the increased cost of conventional fuels have given a thrust to the research on isolated asynchronous generators (IAGs) as alternative power sources driven by various prime movers based on nonconventional energy sources. These energy conversion systems are based on constant speed prime movers, constant power prime movers and variable power prime movers. In constant speed prime movers (biogas, biomass, biodiesel etc) based generating systems, the speed of the turbine is almost constant therefore the frequency of the generated voltage remains constant. However there may be small variations in frequency because of a variation in slip of IAG due to varying consumer loads. Such types of standalone generating systems, based on asynchronous generators, require continuous supply of reactive power for maintaining constant voltage with varying consumer loads. With the advancement in power electronics a number of reactive power controllers are available such as, static var compensator (SVC), thyristor controlled reactor (TCR) and static synchronous compensator (STATCOM) etc. In the last decade, a number of publications have reported on voltage controllers and various schemes of voltage regulation for an asynchronous generator based isolated generating system and driven by a constant speed prime mover such as biomass, biodiesel, biogas etc [1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46] . These voltage regulators (VRs) are sources of reactive power and are based on passive compensation [9],[11],[12],[16],[17],[20],[21],[26],[30] or on active compensation [28],[10],[13],[14],[15],[18],[19],[22],[23],[24],[25],[27],[28],[29],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46] to regulate the voltage at the generator terminals under varying loads and driven by constant speed prime movers. Brennen and Abbondanti [2] have reported for the first time the back-to-back thyristors in parallel with capacitor and star connected inductor in series with back-to-back thyristors and delta connected capacitors for an asynchronous generator. Elder et al. [6] have examined a variety of static VAR sources and operating schemes with single phase supply and have concluded that the switched capacitor VAR source has the highest potential for reducing the cost of such isolated generating system. Mishra et al . [9] have investigated the performance of IAG and a voltage regulator under resistive, reactive and motor loads and have reported that there is no possibility of failure of self-excitation even at the startup of motors using saturable core reactor to act as a controllable static VAR generator for self-excited induction generator. Muljadi and Lipo [14] have proposed a new series compensated induction generator/battery supply topology which provides a constant voltage and frequency at the terminal with minimum current harmonic distortion. Singh and Shilpakar [27] have given a mathematical modeling of solid state voltage regulator with IAG to predict the transient behavior of the IAG system. Kuo et al . [36],[37] have analyzed the voltage regulation and current harmonic suppression of IAG under unbalanced and/or nonlinear loading conditions using a current controlled voltage source inverter. Ahmed et al . [41],[42] have proposed a single phase static Var (SVC) compensator composed of the thyristor controlled reactor (TCR), thyristor switched capacitor (TSC) and the fixed excitation capacitor (FC) to regulate smoothly the generated output voltage of the standalone single phase induction generator with a variable inductive load. A PI (Proportional Integral) controller is employed to adjust the equivalent capacitance of the single phase SVC. Singh et al . [42],[43],[44],[45] have developed a mathematical model and a control strategy for a voltage regulator using a static synchronous compensator (STATCOM) for IAG system. Basic topology of STATCOM consists of a 3-phase current controlled voltage source converter (VSC) and an electrolytic capacitor at its DC bus. The DC bus capacitor is used to self support a DC bus of STATCOM and takes very small active power from IAG for its internal losses to provide sufficient reactive power as per requirements. Here STATCOM is a source of leading or lagging current and can be designed in such a way to maintain constant voltage across the IAG terminals with varying loads. In this paper various STATCOM based VR topologies are presented which are based on two leg VSC, three leg VSC for three phase three wire IAG system and three leg VSC with capacitor mid point, four leg VSC, three single phase VSC and integrated VSC which consists of two or three leg VSC with different transformer configuration such as star delta transformer, zigzag transformer, and T-connected transformer for three phase four wire IAG systems. Section 2 deals with the classifications of the various VR topologies. Section 3 deals with the design of the VR for IAG systems. Section 4 deals with the control schemes. Sections 5 and 6 present the performance of a set of VR for IAG system and concluding remarks.
| 2. Classification of the STATCOM Controllers | | |
[Figure 1] demonstrates the classification of the various topologies of the STATCOM based voltage regulators (VR). These VRs are classified as three phase three wire VRs and three phase four wire VRs. These VRs are based on the two leg VSC, three leg VSC, four leg VSC, three single phase VSC, three leg with midpoint capacitor based VSC and transformer based VRs. In the following section, detailed system description is presented for different STATCOM based voltage regulators.
2.1 Three Phase 3-Wire Voltage Regulators
Mainly two types of VR topologies are discussed for three phase 3-wire isolated asynchronous generator (IAG). The first one is based on three leg voltage source converter (VSC) and another one is based on a two leg VSC with mid point capacitor.
2.1.1 Two Leg Voltage Source Converter (VSC) Based Voltage Regulator
[Figure 2] shows an isolated generating system which consists of a constant speed prime-mover, and an asynchronous generator along with two leg VSC based VR. Two legs of VSC are connected to each phase of the generator through interfacing inductors while the third phase of the generating system is connected to the mid point of the capacitors. Mid point capacitors require equal voltage distribution across both the capacitors and voltage rating at the DC link of the VSC is comparatively higher than the 3-leg VSC based topology. However switch counts are reduced in this topology of VR.
2.1.2 Three Leg Voltage Source Converter (VSC) Based Voltage Regulator
[Figure 3] shows an asynchronous generator system based isolated generating system along with three leg VSC based STATCOM based voltage regulator. The VR consists of a three-leg IGBT (Insulated Gate Bipolar Junction Transistor) based current controlled voltage source converter, DC bus capacitor and AC inductors. The output of the VSC is connected through the AC filtering inductor to the IAG terminals. The DC bus capacitor is used as an energy storage device and provides self-supporting DC bus of VR. This DC side capacitor supplies the real power difference between the load and IAG during the transient period. In the steady state the real power supplied by the IAG should be equal to the real power demand of the load plus a small power to compensate for the losses of the VR. Thus DC capacitor voltage is maintained at a reference value for its satisfactory operation.
2.2 Three Phase 4-Wire VSC Based Voltage Regulators
Here 3-phase 4-wire configurations voltage regulators are classified for IAG system. These VRs are based on four-leg VSC, 3 single phase VSCs, three leg VSC with mid point capacitor, three-leg VSC with different transformer configuration (Zigzag, star delta and T connection) and two leg VSC with different transformer configuration (Zigzag, star delta and T connection).
A brief discussion on all these VRs is as follows.
2.2.1 Three-Leg VSC with Mid Point Capacitors Based Voltage Regulator
[Figure 4] shows the three-leg VSC with mid point capacitors as a voltage regulator for an asynchronous generator system feeding consumer loads and each leg of the VR is connected through an interfacing inductor at the point of the common coupling (PCC). A neutral terminal for the loads is created through the mid point of the equal voltage distributed DC bus capacitors and neutral point of the excitation capacitors.
2.2.2 Four-Leg VSC Based Voltage Regulator
[Figure 5] shows the four-leg VSC as a VR for an asynchronous generator based isolated power generating system. Here the fourth leg of the VR is used to compensate the source neutral current and the other three legs are connected to each phase of the generator through interfacing inductors similar to a three leg VSC for a three phase three wire IAG system. It maintains the constant DC link voltage at the DC bus terminal of VR.
2.2.3 Three Single Phase VSC Based Voltage Regulator
[Figure 6] shows three single phase VSCs with transformers as a voltage regulator. The VR consists of three single phase IGBT based current controlled voltage source converters, DC bus capacitor and three single phase transformers. The output of the VSC is connected through the transformers to the IAG terminals. The DC bus capacitor is used as an energy storage device and provides self-supporting DC bus of VR. However by adjusting the turn ratio of the transformer the voltage level of the DC link of VR can be optimized.
2.2.4 Integrated Three-Leg VSC with Transformer Based Voltage Regulator
[Figure 7] and [Figure 8] show the three-leg VSC with star delta transformer based [47],[48],[49] voltage regulators in an isolated and non-isolated VR configuration respectively for asynchronous generator based stand alone system. In [Figure 7] a non-isolated VR configuration of star delta transformer is shown where a transformer is connected in shunt with the three-legs VSC. In [Figure 8] an isolated VR configuration of star delta transformer is shown where tertiary winding provides the optimum voltage level of DC link of VR. [Figure 9] and [Figure 10] show the three-leg VSC with zigzag transformer [50],[51],[52],[53] based voltage regulators in non-isolated and isolated configurations respectively for an IAG system. [Figure 9] shows the non-isolated VR configuration where a zigzag transformer is connected in shunt with the three-leg VSC similar to the star delta transformer. In [Figure 10], an isolated VR configuration of the zigzag transformer is presented where all three legs of the VSCs are connected to the three phases of thegenerator through zigzagtransformer with tertiary winding. A zigzag transformer is used to create the neutral conductor for the consumer loads. The zigzag transformer acts as a path for zero sequence components of load currents while VSC serves the purpose of harmonic elimination, load balancing and reactive power compensation for voltage regulation of IAG. The zigzag transformer consists of three tertiary winding transformers with the turn ratio of 2:2:1 in an isolated type of VR configuration. The tertiary winding of the transformers facilitates the selection of the optimum voltage level of DC bus VSC. [Figure 11] and [Figure 12] show the non-isolated and isolated T-connected transformer [54],[55] based neutral current compensation schemes of VR. The T-connected transformer is a special configuration of two single phase transformers. Out of two transformers one transformer is a simple single phase transformer, while the other one has a tertiary winding. The T-connected transformer provides the path of flow of zero sequence current components.
2.2.5 Integrated Two-leg VSC with transformer based voltage regulator
[Figure 13] and [Figure 14] show two-leg VSC with star delta transformer based voltage regulators in non-isolated and isolated configurations respectively for an IAG system. Here a star delta transformer with tertiary winding in an isolated configuration serves the purpose of providing the path for load neutral current, while tertiary winding facilitates optimum voltage level at the DC link of VSC. [Figure 15] and [Figure 16] show the non-isolated and isolated two legs VSC with zigzag transformer based 3-phase 4-wire configuration of voltage regulators for an isolated asynchronous generator based generating system respectively. Two legs of VSC are connected to the two phases while the third phase is connected at the mid point of the DC bus capacitors of VSC. A zigzag transformer is used to provide the path for load neutral current. The zigzag transformer acts as a path for zero sequence components of load currents while two leg VSC serves the purpose ofharmonic elimination, load balancing and reactive power compensation. The zigzag transformer consists of three tertiary winding transformers with the turn ratio of 2:2:1. Hence it is regarded as an open circuit for the positive and negative sequence currents. The tertiary winding of the transformers facilitates the selection of the optimum voltage level of the DC bus capacitors of VSC. [Figure 17] and [Figure 18] show the two-leg VSC with nonisolated and isolated T-connected transformer based voltage regulator for an isolated asynchronous generator system. The transformer acts as a path for load neutral current while VSC serves the purpose of harmonic elimination, load balancing and reactive power compensation. The transformer winding turn ratio is designed in such a way so that it forms a neutral terminal for the load.
| 3. Design of Voltage Regulators for an IAG | | |
This section deals with the design of the voltage regulators (VRs) for an IAG system. The VRs consist of switching devices (IGBT), electrolytic capacitor at DC bus and filtering AC inductors. The VAR to be supplied by the VR to the IAG from no load to full load for varying consumer loads is the deciding factor for designing component rating of the VR. To compute the component rating, a design example is presented for a 7.5kW, 3-phase, 415V, 50 Hz Y-connected squirrel cage asynchronous machine for all VSC based topologies of VRs. When the voltage at the terminal of the generator is higher than the reference value then the VR responds as an inductor and if it lowers a reference value then it behaves as a capacitor. Here mainly two-leg VSC based VR for a three phase three wire IAG systems and four-leg VSC based VR for a three phase four wire load which can give a general guidelines to design another VSC based configuration of VRs.
3.1 Design of Two-Leg VSC Based VRs
This section presents the detailed design of two-leg VSC based VRs for an IAG driven by a constant speed prime mover. The two leg VSC and its voltage waveforms are shown in [Figure 19] and the design procedure is focused on, to determine the value of interfacing inductors, DC link capacitors and the voltage across the DC link capacitors along with the rating of the devices. The design of the inductor and capacitor depends upon the voltage and current ripples.
3.1.1 Design of the Interfacing Inductor
In PWM switching of the converter, VcontrolA, Vcontrol B and Vcontrol C can be assumed to be constant during one switching frequency time period. At the zero crossing of Vcontrol A therefore,
where 'ma' is the modulation index and the converter ACterminal voltage vector is defined from the line to neutral voltages van, vbn and vcn which can be calculated as follows:
where vaN, vbN and vcN are the converter pole voltages against the mid point of the DC capacitor and vNn is the voltage between the neutral point (n) and the midpoint of the DC capacitor (N,C).
As the phase "C" is connected to the mid point, its voltage is vcN = 0. The possible voltages vaN and vbN are dependent on the state of switches with the following pole voltages [60] .
From the waveform analysis of [Figure 19] and the equation given above one can find out the inductor of phase 'A' as follows:
peak to peak inductor current ripple is
The vAn waveform is shown in [Figure 19].
By substituting values from eqs (10) of area 'A' the value of inductor is,
where iLripple is peak to peak inductor current ripple, fs is switching frequency, 'KL' is the overloading factor, ma is modulation index, Vdc is the voltage across DC link.
In a similar way value of interfacing inductor Lbn and Lcn can be calculated.
By substituting the values of all parameters, the value of the inductor can be calculated as given in [Table 1]:
In a similar way if one analyzes the voltage waveform of VNn as shown in [Figure 19]. Then peak to peak
Therefore
3.1.2 Design of the mid point DC link capacitor and its voltage
Voltage across each capacitor must be more than the peak voltage for satisfactory PWM control as [56],[57] .
where ma is the modulation index normally with maximum value of 1. The current which flows through the phase connected to the mid point capacitor is equal to flow through capacitors. Therefore the ripple in capacitor voltage vdc1 can be estimated as:
where Iavg is the average current which flows through the DC bus capacitor (Cdc1) and Is is the required rms compensator current. 3) Voltage and current rating of the devices
From [Figure 19] and switch positions as given in equation (7), it is observed that in all four positions of the switch maximum voltage across the other switch will be equal to Vdc.
Similarly if switch Sa+ and Sb+ are ON then voltage across Sa+ and Sb+ is equal to Vdc. Therefore voltage rating (Vsw) of the device can be calculated under dynamic condition as:
where Vd is 10% overshoot in the DC link voltage under dynamic condition.
Rated current which flows through the two-leg VSC is Is. The peak value of the current is √2 Is where Is is the required current flows through the VR considering the safety factor of 1.25 the maximum device current can be calculated as:
From this voltage (Vsw) and current rating (Isw) of the IGBT switches can be estimated. Here one design example for a two-leg VSC based VR is carried out for feeding 0.8 pf lagging reactive load for an IAG system. It is reported that for feeding a 0.8 pf lagging reactive load, IAG requires reactive power of 140-160% of rated generated power. Therefore the VAR rating of VR should be for a generator of around 11.25kVAR 7.5kW. Therefore the current rating of the VR corresponds to additional reactive power required from no load to full load at 0.8 lagging PF load and it is calculated as:
where 'V' is IAG line voltage and Is is VR line current. After substituting the value of reactive power (QAR) and V, current rating of the VR is Is = 15.65 A. Now for determining the value of various parameters such as inductor and DC link capacitors peak value of the current is Is(pk) = √2*15.65 A = 22.13A and Isavg = 0.9 * 15.65 = 14.08 A. Now if one designs the converter for switching frequency of 12.8 kHz, peak to peak current ripple of inductor current is 5%, DC link voltage ripple of 2%. The value of modulation index (ma) is equal to '1' and overloading factor (KL) varies during transient condition from 120% to 180% of the steady state [44],[45] . Considering 5% current ripple peak to peak iLripple(pp) = 0.05* Is(pk) = 0.05*22.13 = 1.1 A. Here design values of the filter inductor and filter capacitor using design formulae from eqs (11), (12) and (14) can be calculated respectively as Lan = Lbn = 8.8mH, LNn = 5.2 mH, Cdc1 = Cdc2 = 1655uF. The DC link voltage is calculated from eq (15) as Vdc/2 = Vdc1 = Vdc2 = 677V. Device voltage and current ratings from the eqs (18) and (19) are as, Vsw = 1833V and Isw = 30A. From above calculation filter inductor, filter capacitor and voltage across DC link voltage is calculated along with estimating the rating of the IGBT based switches. It is observed that the value of filter inductor (LNn) for third phase which is connecting to the mid point of the capacitor differs from the other two phases (Lan, Lbn) because of no switching operation of the devices at this phase. The value of filter capacitors (Cdc1,Cdc2) is estimated by considering voltage ripple across it. However voltage and current rating of the switching devices is decided by considering the overshoot during the transient conditions. On the basis of these calculations component rating is selected for two leg VSC based VR, this selection is based on considering the safety factor and availability of the component. These are given in [Table 1].
3.2 Design of four-leg VSC based VR
This section presents the design of four-leg voltage source converter [Figure 20] for a three-phase four wire topology similar to three-leg VSC for determination of the filter inductor, DC link capacitor and rating of the switches.
3.2.1 Design of the Filter Inductor
Similar to the two-leg and three-leg VSC, all three inductors of each phase can be designed. However an inductor of fourth leg for compensating neutral current can be selected to limit the switching harmonics as given below [56],[59] . Here voltage across LNn varies from 0 to Vdc so average voltage across it is Vdc, and peak to peak inductor ripple current
Let us consider that in the worst nonlinear load condition neutral current is 1.73 times the phase current [56],[57] along with this, if an inductor design for compensation for third harmonic is as.
3.2.2 Design of DC Side Capacitor
The selection of DC side capacitor is similar to three-leg VSC [56,59]. Using principle of energy storage and value of DC bus capacitor is almost similar like three leg VSC. However the voltage rating across the DC link is estimated as [57] .
3.2.3 Voltage and Current Rating of the Devices
Similar to two-leg VSC, the voltage and current rating of devices can be estimated. Calculated and selected values are given in [Table 2] for four leg VSC as shown in [Figure 20].
By similar design procedure various components of other topologies of the VR can be achieved for IAG system.
| 4. Control Strategies | | |
The control scheme of the VR is based on the indirect current control with generation of in-phase template using line voltages shown in [Figure 21]. Here by sensing line voltages the templates of the phase voltages are extracted using line to phase transform (detailed explanation of this transform is given in the Appendix). The control scheme is based on the generation of the reference source currents (i rsa, i rsb, i rsc) consisting of two components. For this purpose, two components of the reference source currents are computed - one is in phase component (Idm) which is responsible for maintaining the constant DC link (Vdc) of VSC and the other one is in quadrature (Iqm) to phase component and it regulates the magnitude of the generated voltage (Vtm) constant. The in-phase component of source current is determined by computing the in-phase component templates (ua, ub, uc). These components (ua, ub, uc) are obtained through the line voltage templates (uab, ubc, uca) which are derived by dividing line voltages (vab, vbc, vca) by amplitude of the terminal line voltage (Vtm). A PI (Proportional Integral) controller is used to regulate the terminal voltage Vtm to its reference value Vtmref. The output of terminal voltage PI controller (Iqm) is multiplied with the phase quadrature templates (wa, wb, wc) which results in quadrature components of the reference source currents (i rqa, i rqa, i rqa). These quadrature templates (wa, wb, wc) are derived from line templates (uab, ubc, uca). To provide self-supporting DC bus of the VSC, its DC bus voltage is sensed and filtered using a low pass filter to compare with the DC reference voltage (Vdcref). The error voltage is processed in another PI controller. The output of the PI controller (Idm) is considered the amplitude of the active current component of the source currents (i rda, i rdb, i rdc). Multiplication of in-phase unit templates (ua, ub and uc) with output of the PI controller (Idm) yields the in-phase component of the reference source currents (i rda, i rdb, i rdc). The instantaneous sum of quadrature and in-phase components provides the reference source currents (i rsa, i rsb and i rsc), which are compared with the sensed source currents (isa, isb and isc). These current error signals are amplified and fed to the PWM current controller to generate the gating signals for IGBTs of the VSC (voltage source converter) used as in VR. The control strategy of the two-leg voltage controller based VR is realized similar to three-leg VSC, through derivation of reference source currents (i rsa, i rsb) while main difference between two topology to derivation of active component of current as shown in [Figure 22]. Reference source currents consist of two components one is in phase or active power component (i rda, i rdb) for the self supporting DC bus of VSC while the other one is in quadrature or reactive power component (i rqa, i rqb) for regulating the terminal voltage. The amplitude of active power component of the source current (Idm) is estimated using two PI controllers among which, one is used to control the voltage of DC bus of VSC while another one is used for equal voltage distribution across the mid point DC bus capacitors. The output of the first PI controller is estimated by comparing the reference DC bus voltage (Vdcref) with the sensed DC bus voltage (Vdc). The output of the second PI controller is estimated by comparing the voltages across both capacitors (Vdc1) and (Vdc2). This voltage error signal is processed using this second PI controller. The sum of output of both PI controllers (Idm1) and (Idm2) gives the active power current component (Idm) of the reference source current. The multiplication of Idm with inphase unit amplitude templates (uad, ubd) yields the in-phase component of instantaneous reference source currents. These (uad, ubd) templates are sinusoidal functions, which are derived by unit templates of in-phase with line voltages (uab, ubc, uca). These templates (uab, ubc, uca) are derived by dividing the AC voltages vab, vbc and vca by their amplitude Vt. To generate the quadrature component of reference source currents, another set of sinusoidal quadrature unity amplitude templates (uaq, ubq, ucq) is obtained from in-phase unit templates (uabd, ubcd, ucad). The multiplication of these components (uaq, ubq) with output of the PI (Proportional Integral) AC voltage controller (Iqm) gives the quadrature, or reactive power component of reference source currents. The sum of instantaneous quadrature and inphase component of source currents is the reference source currents (i rsa, i rsb), and each phase source current is compared with the corresponding reference source current to generate the PWM switching signal for VSC of the controller. In a similar way, control schemes of all other topologies of VRs can also be formulated for IAG system.
| 5. Matlab Based Modeling | | |
The performance of proposed VRs is simulated on MATLAB using Simulink and PSB (Power System Blockset) toolboxes. A 7.5kW, 415V, 50Hz asynchronous machine is used as the generator including the saturation characteristics, which is determined by synchronous speed test [45] . An excitation capacitor bank is used to generate the rated voltage at no load while additional demand of the reactive power of the generator during load variation is met by VR. An inbuilt block of universal bridge is used to realize the proposed VR. Modeling of the control strategy is carried out using Simulink and function blocks. Generated reference currents are compared to the sensed source currents and error signal is compared with the fixed frequency triangular wave to generate the switching signals of IGBTs of VSCs. A discrete PI controller is used to regulate the terminal voltage and the DC link voltage of VR Simulation is carried out in MATLAB version of 7.3 using ode (23tb/stiff/TRBDF2) solver mode in discrete mode at 5e6 step size.
| 6. Results and Discussion | | |
Performance of the VR of IAG system has been verified under the conditions of varying consumer loads. Different transient waveforms are shown to demonstrate the performance of the proposed VR topologies of IAG system for supplying balanced/unbalanced, linear/nonlinear loads respectively. Simulated transient waveforms of the generator voltage with (Vabc), generator current (iabc), capacitor current (icca), load currents (ilabc), controller current (icabc), neutral current of source (isn), load (iln) and compensator current (icn), terminal voltage (Vt), DC link voltage (Vdc, Vdc1 and Vdc2), speed of the generator (wg), etc are given in different dynamic conditions.
6.1 Performance of Three Phase 3-Wire Voltage Regulator
In this section performance of 3-phase 3-wire voltage regulator is presented for an asynchronous generator based isolated system. Here performance of two leg voltage source converter with mid point capacitor based VR topology has been simulated and verified for said asynchronous generator based isolated generating system.
6.1.1 Performance of Two Leg Voltage Regulator for an IAG System
[Figure 23] shows the transient waveforms of three-phase generator voltages (vabc), generator currents (iabc), capacitor current (icca), threephase nonlinear load currents (ila, ilb and ilc), threephase VR currents (ica, icb and icc), amplitude of IAG terminal voltage (Vt), DC bus voltage and its reference value (Vdc), voltage distribution across mid point capacitor (Vdc1 and Vdc2) and generator speed (wg) demonstrating the response regulating the IAG terminal voltage while supplying nonlinear load of 5 kW. At 3.3 s. three-phase nonlinear load is applied and it is found that with application of the consumer load, VR responds in a desirable manner and maintains constant voltage at the generator terminal. Along with this, the DC link voltage and voltage across both midpoint capacitors of VR also remain equal and constant.
6.2 Performance of Three-Phase 4-Wire Voltage Regulator
Performance of three-phase four-wire voltage regulators have been presented for an asynchronous generator based stand alone generating system. Performances of the VRs are demonstrated with the linear and nonlinear loads.
6.2.1 Performance of Three Leg VSC with Mid Point Capacitor Based VR
[Figure 24] shows the performance of this VR for asynchronous generator based isolated generating system and feeding four wire nonlinear loads. At 2s, a balanced nonlinear load is applied and it is observed that the VR maintains the constant voltage and along with it also maintains sinusoidal current. At 2.15s, one phase and 2.3s another phase of the load is opened but the currents of the generator remain balanced and sinusoidal. Along with this, it is also observed that equal voltage across both the capacitors is maintained along with maintaining constant DC link voltage of VR.
6.2.2 Performance of Three Single Phase VSC Based VR
[Figure 25] demonstrates the performance of VR for an isolated generator feeding 3-phase 4-wire linear loads. On the application of three single phase 2.5kW, linear loads between each phase and neutral, the terminal voltage is regulated by the VR. At 2.6s one phase and later on at 2.8s the other phase is opened, the load becomes unbalanced on the IAG system while on the generator side source currents remain balanced and sinusoidal which demonstrates the load balancing and neutral current compensation aspects of the VR. Along with this it also observed that by adjusting the turn ratio of the single phase transformer, the voltage level at the DC link is optimized to a suitable level.
6.2.3 Performance of Integrated Two Leg VSC Based VR
[Figure 26] demonstrates the performance of the two-leg VSC with zigzag transformer based VR for isolated asynchronous generator feeding 3-phase 4-wire linear loads. At 2.5s, on the application of 2.5kW loads between each phase and neutral, the voltage is regulated by the controller. At 2.65s, one phase and later on at 2.8s, the other phase is opened, the load becomes unbalanced on the system while on the generator side source currents remain balanced which demonstrates the load balancing aspects of the VR. Similar to this, various transformer based VR configurations using star delta and T-connected transformers can also be investigated with two-leg and three-leg VSC based VR for feeding three-phase four- wire loads.
| 7. Conclusions | | |
Various topologies of the voltage regulators have been classified for an asynchronous generator based stand alone generating system driven by constant speed prime movers. A set of VRs have been designed and their performance have been studied for IAG system. For three-phase three-wire IAG system two topologies of VR have been demonstrated one is based on three leg VSC while another one is based on the two leg VSC. A topology which is based on the two leg VSC, requires higher voltage rating of the switches and equal voltage distributed DC link, however less number of switching devices are required compared to three leg VSC based topology of VR. In three phase three wire IAG system there are a number of configurations of the VRs which have been presented for a three phase four wire IAG system. These VRs are classified as three-leg VSC with a capacitor mid point, four-leg VSC, three single phase VSC and integrated VSC (VSC with various transformer configuration such as star delta transformer, zigzag transformer and T-connected transformer) etc. In the first three configurations of VRs, the neutral terminal for the load is created using VR and the star point of the excitation capacitor, however in case of integrated VSC based VR the load neutral current path is independent from the source neutral and it is provided though the various transformers such as star delta, zigzag and T-connected transformers.
| 8. Appendix | | |
[Figure 27] shows a complete algorithm of proposed transformation (line to phase unit templates) is presented in detail. As shown in [Figure 2], uab, ubc, uca are unit templates in phase with line voltages (vab, vbc and vca) while wab, wbc and wca are in quadrature with these templates. Here ua, ub and uc which are unit templates in phase with phase voltages (w.r.t to a hypothetical neutral) and wa, wb, wc are in quadrature with them.
By Clark's transformation, line voltage templates are transforming to two phase templates as:
Now two phase to three phase transformation i.e. transformation uab, wab in to wab, wbc, wca shown in [Figure 2] may be made as
Therefore quadrature templates in phase with linevoltages can be expressed in simplified form by putting eq (3) in eq. (4) and (5).
While unit templates (fabd, fbcd fcad) in phase with line voltages (vab, vbc and vca) are given as:
where
From [Figure 2] it is clearly observed that templates in phase with phase voltages ua, ub, uc are in phase with ubc, uca, uab respectively.
where
van, vbn, vcn are phase to neutral voltages with respect to hypothetical neutral terminal 'n'. Now using 2-phase to 3-phase transformation templates wa, wb, wc are in quadrature to ua, ub, uc are derived using uab, uab as follows:
Therefore
In this manner, the template in phase with phase voltages (ua, ub, uc) and in quadratures to them (wa, wb, wc) are derived by sensing line to line voltages (vab, vbc and vca).
Authors
 Gaurav Kumar Kasal was born in Bhopal, India, in Nov, 1978. He received the B.E (Electrical) and M.Tech. degree from the National Institute of Technology (NIT) Allahabad and National Institute of Technology (NIT) Bhopal, India respectively in 2002 and 2004. Since Dec 2004, he has been pursuing the Ph. D. degree with the Department of Electrical Engineering, Indian Institute of Technology (IIT) Delhi, New Delhi, India and have published 35 international research papers in journals and conferences. He is working as a Sr. engineer in R&D department of Delta energy system (India). His field of interest includes power electronics and drives, switching power supply, renewable energy generation and applications.
 Bhim Singh was born in Rahamapur, India, in 1956. He received the B.E (Electrical) degree from the University of Roorkee, Roorkee, India, in 1977 and the M.Tech. and Ph.D. degree from the Indian Institute of Technology (IIT) Delhi, New Delhi, India, in 1979 and 1983, respectively. In 1983, he joined the Department of Electrical Engineering, University of Roorkee, as a Lecturer, and in 1988 became a Reader. In December 1990, he joined the Department of Electrical Engineering, IIT Delhi, as an Assistant Professor. He became an Associate Professor in 1994 and Professor in 1997. His area of interest includes power electronics, electrical machines and drives, active filters, FACTS, HVDC and power quality. Dr. Singh is a fellow of Indian National Academy of Engineering (INAE), the Institution of Engineers (India) (IE (I)), and the Institution of Electronics and Telecommunication Engineers (IETE), a life member of the Indian Society for Technical Education (ISTE), the System Society of India (SSI), and the National Institution of Quality and Reliability (NIQR) and Senior Member of Institute of Electrical and Electronics Engineers (IEEE).
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22], [Figure 23], [Figure 24], [Figure 25], [Figure 26], [Figure 27]
[Table 1], [Table 2]
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