Solar power based generation is increasing as days are progressing ... hereby aims to work on similar lines to design and implement a three phase inverter with.
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 90 (2016) 673 – 680
5th International Conference on Advances in Energy Research, ICAER 2015, 15-17 December 2015, Mumbai, India
Design and Implementation of a Three Phase Inverter for Renewable Energy Source with Unified Control Strategy Swathy Pillaia, Sushil Thaleb, * b
a Student, Department of Electrical Engineering, Fr. Conceicao Rodrigues Institute of Technology Associate Professor, Department of Electrical Engineering, Fr. Conceicao Rodrigues Institute of Technology
Abstract Microgrids are becoming widespread because of their evident benefits which comprise of the improved reliabilities they can operate with and their lower emission levels over the conventional generation Thus the face of electricity generation is experiencing modifications as technological improvements are emerging to mitigate the problems of increasing cost and the pollution caused due to excessive use of fossil fuel based electricity generation to meet our ever growing demands. Microgrids utilize renewable energy sources (RES) viz. photovoltaic cells, fuel cells, wind etc. instead of using the conventional fuels. As the energy requirement of the world is growing enormously and will continue to rise as year’s progress, RES’s are surely a solution to the above problems. This paper deals with design of photovoltaic (PV) based three phase grid connected voltage source converter with unified control strategy (UCS).The UCS takes into consideration the general feedback requirements for desired response and performance from the microgrid and at the same time includes a feedforward control for DC bus control. Simulation results attained are been presented in this paper. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
© 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICAER 2015. Peer-review under responsibility of the organizing committee of ICAER 2015
Keywords: Microgrids ; Unified control
Solar power based generation is increasing as days are progressing because it is evidently one of the most abundant sources available in our country. Adding to its advantages are the facts that they can be installed in almost every location and the maintenance required is also less in comparison to other RES’s. Grid connected Photovoltaic (PV) systems are gaining more significance over standalone configuration. The evident reason being that there is no longer the unavoidable necessity of an energy storage system (ESS) to nullify the ineffectiveness of the PV source during the nighttime or low insolation period. On the other hand in interconnected systems there always lies a fear of cascaded
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICAER 2015 doi:10.1016/j.egypro.2016.11.236
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tripping and a brownout or a blackout. To solve this problem and thus aid us in at least powering our critical loads in such a crisis situation, the concept of Microgrids [1] was developed. Microgrids are basically low- or medium-voltage controlled and monitored power systems that can include numerous distributed energy resources (DERs), and local loads. The most captivating feature of a microgrid is its ability to isolate itself from the utility during blackouts or even when the power quality of the grid reduces below certain standards [1]. Thus microgrids can mainly operate in two modes viz. grid-connected and islanded modes. As micro-sources are relatively small in comparison to utility, the voltage and frequency of the microgrid are dictated by the utility in the grid-connected mode and the microgrid act as controllable current source. On the other hand in islanded mode the microgrid should produce a controlled voltage and frequency output. This objective can be realized most successfully and appropriately if the microgrid acts as controllable voltage source. [2]- [3] The project proposed hereby aims to work on similar lines to design and implement a three phase inverter with unified control strategy for balanced as well as unbalanced ac system conditions. The unified control strategy takes into consideration the general feedback requirements for desired response and performance from the microgrid and at the same time includes a feedforward control for DC bus control. This paper deals with a PV based VSC system feeding the grid in the same context. The current work includes simulation based studies on 2 kVA grid connected VSC system for balanced grid conditions. 1. System Description The feedback control for interconnection with balanced ac systems is developed taking into account the above mentioned roles the microgrid has to operate in. The dc-link voltage controller helps in balancing system power flow in grid connected mode [2]. Fig.1 represents the block diagram for the proposed grid connected PV system. Microgrid exchanges power with the utility depending on its generation and local load demand.
Fig 1. Block diagram for the proposed grid connected PV system
The exchange includes exporting excess power to the utility when power produced by microgrid is greater than that required loads or importing power when microgrid is incapable of producing the required power. Thus the active and reactive power which is to be exchanged with utility needs to be controlled. The active power being shared depends in turn on power available from the source. The microgrid operates in grid connected mode; once the static transfer switch (STS) is closed else it operates in standalone mode or voltage control mode. The micro source which is used
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here is PV and is interfaced with the entire system with maximum power point tracking (MPPT) enabled boost converter. Since PV array has comparatively low conversion efficiency, it becomes essential to extract maximum power from the installation. 1.1. Solar PV and MPPT technique For simulation study PV array of 2 KWp rating was used. Fig. 2 represents a PV array feeding a three phase load via boost converter and an inverter. Boost converter is used to step up PV array voltage to DC link voltage required for the application. The rating of PV array used and PV IV characteristics of module and array are as seen in Table 1 fig. 3(a) & fig. 3(b) respectively.
Fig. 2. PV array with constant voltage MPP method Table 1. PV array rating Parameter Value Number of cells per module
36
Number of modules in series
20
Voc of Array
21x20 = 420V
Isc of Array
1.2x5 = 6 A
Vmpp, Impp
367.5 V, 5.65 A
(a) Fig. 3.
(a) Module I-V and P-V characteristics
(b) (b) Array I-V and P-V characteristics
Maximum power point (MPP) tracking techniques are utilized in PV array powered systems to obtain maximum power from the array. The Constant voltage method is been used for simulation purpose to track MPP as shown in Fig. 2.
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This method assume a linear relation between the voltage corresponding to maximum power and the PV panel’s opencircuit voltage as shown by equation (1) [4] Vmpp = k x Voc
(1)
Where k is a proportionality constant. The value of ‘k’ is dependent on the characteristics of the PV array being used, it is calculated in advance by determining V and V for the specific PV array in different environmental conditions viz. various insolation level ,temperature level . The value of k has been quantified to be between 0.71 and 0.78 [5].In this technique as seen from Fig. 2, the PV array voltage (Vpv) is compared with a constant reference voltage (Vref) which matches with the maximum power point array voltage (Vmpp) .The subsequent error signal is used to drive a boost converter which interfaces the PV array to the load via a three phase inverter. The method was adopted due to its simplicity as compared to the other methods in implementation. In addition the method is reliable, inexpensive and fast too [4]-[5]. MPP
OC
(a)
(b) 2
Fig. 4. PV panel output with (a) constant insolation 1000 W/ m ;(b) varying insolation
The output of the PV panel is not constant one as it is hugely affected by insolation conditions and temperature. Fig. 3. shows the dependency of PV panel output on insolation condition, as seen from the figure as insolation varies from 800 W/m2 to 400 W/ m2 the PV panel output current varies proportionately and thus power available from source dips. PI controller values have been set as kp = 4.5 and ki = 0.2 to obtain the desired response and the switching frequency for the boost converter is 20 kHz. As seen from Fig. 3, the constant voltage MPPT technique tracks Vmpp and thus is capable of extracting maximum power from PV array. 1.2. Microgrid operating modes and control Microgrid can be controlled as stated to operate in either standalone mode or grid connected mode. The microgrid is connected to the utility through a voltage source converter viz. a three phase inverter. A multiloop control based microgrid operating in grid connected mode consists of a faster inner current loop for controlling current injected and a comparatively slower outer voltage loop for DC link voltage control [3], thus it basically operates as controlled current source whereas in standalone mode as the microgrid itself has to maintain voltage and frequency it acts as controlled voltage source. The control can be implemented in desired reference frame. The control is exercised in synchronous reference frame [2] also called dq frame as it transforms the control variables to dc values; thus controlling becomes easier to achieve. Proportional–integral (PI) controllers have an acceptable behaviour when dealing with dc variables hence PI compensators are used for the control. Phase locked loops are used for achieving grid synchronization.
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Compensator values for both voltage and current control loops with which the simulation has been implemented have been represented in the Table 2. Eq. (2) represents the closed loop transfer function of the system where Ʈi represents the time constant of the closed loop system which should be appropriately chosen to attain the required response. Fig. 5. represents the control loop for a voltage source converter (VSC) system. In case of an islanded system in addition to given current loop. Fig. 6 shows the bode plot for a grid connected VSC system, and as seen from the bode plot the system is inherently stable.
(a)
(b)
Fig. 5. (a) Control loop for controlled frequency VSC system; (b) Control loop for current controlled VSC system Table. 2. Simulation parameters for VSC system
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Fig. 6. Bode plot for current controlled VSC system
Fig. 7 shows the voltages and currents at different points in the system. The power sharing between the VSC and utility can be seen from Fig. 8. The system is subjected to varying load conditions to study the behaviour of the system. As seen there is a smooth transition from standalone case to grid connected system. When the local load on the VSC system is increased at t = 0.06 sec, the corresponding increase in current can be seen. At t = 0.08 VSC system gets connected to the grid and VSC starts delivering excess power to the grid The increase in current being injected when local load on the VSC is reduced can also be seen from Fig. 8.
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Fig. 7.Simulation results for grid connected VSC system for constant insolation condition
Fig. 8. Real and Reactive power sharing
1.3. DC link controller In the grid connected mode, the VSC’s DC-bus voltage is considered to be provided by an ideal, DC, voltage source, But, in several applications like PV based systems or fuel-cell systems, the VSC DC side is connected with a a (DC) power source instead of being connected to a constant DC voltage source. Therefore the DC-bus voltage is not fixed and a need arises to regulate the same [6]. Fig. 9 illustrates the variation in DC bus voltage when PV panel is subjected to varying irradiations at constant load conditions.
Fig. 9. DC link voltage variation as insolation varies
The purpose of the DC controller is therefore to regulate the DC-bus voltage (VDC). To achieve this a feedback system is used which compares VDC with a reference value and accordingly adjusts active power (Ps) which VSC shares with utility, so that the gross power exchanged with the DC-bus capacitor is kept at zero. The power balance equation for the proposed system is given by equation (3) [3].
where
(3)
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Fig. 10. DC bus controller Fig. 10 represents the DC bus controller. As seen is compared with and error is passed on to the compensator. Power available from source is added as a feedforward signal so that any variation in source can be quickly reproduced in Psref and thus the effect of any variation in source output can be diminished. Compensator includes an integral term to reduce steady state error but since the plant also has an integral term, the compensator must include a zero to have a stable system. The bandwidth of the DC link control is chosen to be 1/ 5 times of the inner current loops bandwidth. Fig. 11 verifies that an almost constant DC Link voltage can be achieved by this control technique.
Fig. 11. DC link voltage in varying insolation condition
Fig. 12. Simulation results for grid connected VSC system when subjected to varying insolation condition
Fig. 13. Real and Reactive power sharing when subjected to varying insolation condition
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Varying insolation levels impact the power availability from the source as stated earlier. Fig. 12 & Fig. 13 represent the simulation results obtained when the PV panel was subjected to varying irradiation levels. As seen from the Fig. 12 at t = 0.02 sec the system is grid connected and starts feeding available power from the source to loads and excess power to the utility. At t=0.04 sec isolation varies from 1000 W/ m2 to 600 W/m2 and thus the power produced by the PV reduces to 1400 W but still the system has to feed a load of 1500 W therefore the deficit power is being provided by the utility as seen from Fig. 13. Fig. 13 represents the active and reactive power sharing between utility and RES. When irradiation levels are back to nominal value of 1000 W/m2, the power starts flowing from PV to the grid. 1.4. Hardware Setup Fig.14 represents the hardware setup used for the experimentation part. Three phase IGBT based inverter stack (415 V, 5 A) from Semikron is being used for the setup. For testing purpose batteries are been used. Capacitor of 15µF, 450V and 6mH inductor was used for the setup. The hardware results once obtained will be presented.
Fig. 14. Hardware Setup for experimentation
1.5. Conclusion PV based grid tied inverter is an extensively popular distributed generation and lot of research is being done in this field to deploy commercial and economical microgrids for the end users. Currently simulation based study on the operation of system under various conditions like varying loads insolation have been represented. The future work consists of the attaining the hardware results of the same and further research on grid connected PV inverters when an unbalance exists at PCC. References [1] [2] [3] [4] [5] [6]
Lasseter, R.H.; Paigi, P. "Microgrid: a conceptual solution", Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annual, vol. 6, 25 June 2004. Frede Blaabjerg, Remus Teodorescu, Marco Liserre, Adrian V. Timbus, “Overview of Control and Grid Synchronization for Distributed Power Generation Systems” IEEE TRANS. Industrial Electronics, vol. 53, no. 5, October 2006 Amirnaser Yezdani, and Reza Iravani, “Voltage Source Converters in Power Systems: Modeling, Control and Applications”, IEEE John Wiley Publications. Eftichios Koutroulis, Kostas Kalaitzakis, Nicholas C. Voulgaris, “Development of a Microcontroller-Based, Photovoltaic Maximum Power Point Tracking Control System,” IEEE TRANS. Power Electronics, vol. 16, no. 1, January 2001. Trishan Esram, and Patrick L. Chapman, “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques,” IEEE TRANS. Energy Conversion, vol. 22, no. 2, June 2007. Amirnaser Yazdani, Reza Iravani, “A Unified Dynamic Model and Control for the Voltage-Sourced Converter under Unbalanced Grid Conditions”, IEEE TRANS.Power Delivery, vol. 21, NO. 3, July 2006
