Abstract-This paper discusses modeling of a micro-grid with
PV-Wind-Diesel generator hybrid system and its operations. The PV system is modeled with a DC/AC inverter with a pre-defined Maximum Power Point Tracking (MPPT) control. Fixed speed induction generator based wind turbines are used to model the wind farm. The diesel generator controls are modeled with droop governor and automatic voltage regulator (A VR). The PV inverter controls, including MPPT and voltage droop, are modeled in the DC/AC inverter itself. The proposed MPPT control is more stable with additional input of PV cell temperature. The complete system is modeled in PSCADIEMTDC software. Performance of this system is studied on (i) fault tripping of the main grid and (ii) islanded operation of micro-grid with intermittent power from the wind and PV plants. The simulation results confirm the smooth operations of the proposed concept for disturbances from main grid as well as due to the intermittent wind and PV renewable power plants.
Keywords- PV-wind-diesel hybrid system, PV plant controls, droop control, maximum power point tracking control, islanded and grid connected operations.
I. INTRODUCTION
The renewable power plants are mostly implemented in rural areas as they cover large ground surface. The rural areas are far away from the main grid network and connection is possible through a weak transmission line. The concept of Micro-grid (MG) is proposed as effective solution for such weak systems [1]. The operation can be smoothened by the hybrid generation technologies while mmlmlzmg the disturbances due to intermittent nature of energy from PV and Wind generation. Also power exchange is possible with the main grid when excess/shortage occurrs in the MG.
A MG concept with hybrid operation of generators is studied in this paper. A model is developed to study the operations of integrated PV-Wind-Diesel generators in a MG. Simplified active and reactive power controllers are developed to the PV inverter and diesel generator. Finally the system's operations are studied with grid connected and islanding modes with intermittent power from renewable plants.
This paper includes detailed discussions on (i) literature review, (ii) MG circuit configuration, (iii) active and reactive power of the PV systems and diesel generator and (iv) simulation results. The simulation results confirm the smooth operations with proposed controller of the MG.
The authors would like to thank the Australian Research Council (ARC) discovery grant scheme, the University of Queensland, Australia and the University of Peradeniya, Sri Lanka for their supports to conduct this research work.
Micro-grid is defined as a cluster (typically two to three) of DG units and loads which operates in a coordinated and independent manner as a single entity that can be integrated into a distribution system or stand-alone system. In a more advanced MG, energy storages such as super capacitors, super conducting magnetic energy storage (SMES), Flywheel and batteries are also integrated along with generating units and loads [1]. MG appears to the main grid as a single load or source depending on whether load is higher than the generating sources in the area. MG architecture ensures that it follows grid and/or distribution codes and does no harm to existing consumers and the rest of the grid. MG concept will allow a higher penetration of renewable energy in the form of DG units without requiring redesign and reengineering of distribution system. However, control aspect of MG is critical for allowing maximum penetration of desirable energy without hampering the grid integrity.
Power control strategies of a grid connected hybrid system or MG for versatile power transfer have been proposed in [2]. The hybrid system includes PV array, Wind turbine and battery storage along with common DC bus and the versatile power transfer is defined as multimode of operation, including normal operation without battery storage, power dispatching and "power averaging". A supervisory control that regulates power generation of generating elements has been used for the hybrid system to operate in the various operation modes and a hysteresis control strategy was applied in the converter of battery.
An interesting study of a stand-alone PV-Wind hybrid system has been reported in [3]. In this work, as the hybrid system cannot always provide stable output with weather conditions, an auxiliary generation apparatus that uses elastic of energy is used to form energy storage. Rotary energy in the spring was used to power a small scale generator. However, the auxiliary generator output was limited to a maximum of 240 W. Relatively larger Wind-PV hybrid generation system design, experimental as well as simulation results have been presented in [4]. A controller which includes PWM chopping technology in charging control of batteries used in the windPV hybrid system. This control operates the system at maximum power point. Further constant voltage and limited current loops control of battery charge and SPWM conversion with front end high frequency DC to DC model together with
special microchip control, are also used in 1 kW to 5 kW of hybrid system.
Another stand-alone PV -Diesel hybrid generation system for a small island has been presented in [5]. The hybrid generation that consists of 750 kW of photovoltaic power and 300 kW of diesel power as auxiliary source was designed to supply power to private houses in the island and far away from utility distribution grid. Though this is a practical work, the control aspect and maximizing the PV yield and storage facilities have not been discussed.
Reference [6] reported a 500 W hybrid system that consists of PV and fuel cell. An electrolysed couple to the PV array is employed for hydrogen production in this hybrid configuration. A controller is designed to ensure continuous and constant power generation throughout day and night via the combination of PV and fuel stack. The details of the controller were not presented in the work.
A stand-alone PV-Wind hybrid system for a small scale application with a controller MPPT of both PV and Wind system is discussed in [7]. The wind energy generation system is based on variable speed wind turbine. The MPPT controller is based on terminal voltage or current according to the open circuit voltage or short circuit current and the control relationship of the turbine speed and dc-link voltage that was obtained using simple calculations. It is important to note that the controller was implemented without measuring the environmental conditions such as irradiance and wind speed. Power fluctuation in the stand-alone MG architecture has been suppressed using battery storage. The issues relating to sizing and control of battery storage could be identified as the drawback of the paper.
A variable structure controller to regulate output power of a stand-alone hybrid generating systems consists of PV -Wind and battery bank storage is presented in [8]. The control presented in the paper admits two modes of operations. The first mode takes place when the isolation regime is sufficient to satisry power demand. The second mode of operation takes place under insufficient insolation regimes which leads the system operations at the maximum power operating point.
Reference [9] presented power control of a grid-connected hybrid generating system which consists of PV, Wind and Battery storage. The proposed system has reported several modes of operations similar to what is presented in [2]. Though the MG configuration has been verified with test results, the hybrid system is relatively small and the islanded mode of operation is not covered in the work.
Based on the above, in this paper relatively large MG, in few MW range, with PV-Wind-Diesel hybrid system has been proposed. The PV system is modelled with predefined MPPT. While the wind energy system is modelled as fixed speed induction generator, the Diesel generator controls are modelled with droop governor and A YR. The MG architecture proposed has been analysed for both grid connected as well as islanded operations.
III. PROPOSED MICRO-GRID FOR HYBRID GENERA TrONS
Equivalent :�/�1�� to the Main Grid (MG)
generator (6 MW) Diesel generator
(I5.MW) Figure I. Micro-grid network used to study the hybrid operation of the PV
Wind-Diesel generation.
Table I Network parameters Load and PFC at the related busbars ,
Figure 1 shows simple MG to study the hybrid operation of the PV-Wind-Diesel generations. The diesel generator is engaged with Automatic Voltage Regulator (A VR) and governor droop-control. The A VR maintains the voltage at bus 2. The wind generator is a fixed speed induction generator and its input torque is applied with fluctuation to reflect the intermittent nature of wind speed. The PV system is designed with DClAC full inverter model. The inverter reactive power injection is controlled through a droop to keep Bus 3 voltage within the acceptable limit. The inverter active power injection is based on DC-link voltage regulation. The PV side is modeled as a DC current source, which varies the current injection based on the sun energy variations. Loads are connected at all three busses. Power Factor correction Capacitor (PFC) banks are connected at Bus 1 and 3 to minimize reactive power requirement from the network thus supports the voltages. Main grid is connected at Bus 1 through a long transmission line and modeled by a voltage source with equivalent grid impedance. The 33 kV network line parameters per km and other loads, PFC ratings are given in Table 1.
IV. ACTIVE AND REACTIVE POWER CONTROLS
This section discusses active and reactive power control of PV-wind-diesel hybrid system. In this paper, controls of PV inverter and Diesel generator are presented. For wind, a Fixed Speed Induction Generator is considered hence no special control is described.
A. Control circuits of the DCIAC inverter of the PV system The PV system DC/AC inverter active and reactive power
controls are shown in Figures 2 and 3. Figure 2 shows the reactive power control, which maintains the AC terminal voltage magnitude within the acceptable limits. Here the droop control is developed with a Proportional Integral (PI) regulator. The PI regulator is tuned to be slow to avoid any malfunction due droop fast action of the droop. It also maintains the steady state operation along the pre-defined voltage droop curve.
VPVmeas
VPVref VPVmag
Figure 2. Reactive power control of the PV inverter - voltage droop control.
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together with MPPT control.
QJs
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Figure 4. Three phase voltage vector PLL for the PV inverter control.
Figure 3 shows the active power control. The active power control regulates the DC-link voltage to the desired level. The PI controller is tuned to be fast enough to regulate the DC-link voltage within 50 milliseconds. Here MPPT control is integrated at the DC-link reference voltage. The MPPT is designed to be a slow acting steady state control to avoid unnecessary power flickering. PV data sheets indicate that for any changes in sun radiation, the DC voltage does not need to
be changed to get the maximum power. However, the DC voltage has to be varied with varying cell temperature. Therefore, the cell temperature was taken as an input to the MPPT control to vary the DC-link voltage reference accordingly. This control technique is very stable when compared to the conventional MPPT controls. This control is designed to accommodate all in one module. As a result, it avoids the needs of additional DC/DC inverter module, which conventionally used to keep the PV output DC voltage at constant.
The modulation index is varied according to the reference DC-link voltage variation to prevent any effect on the response of the AC terminal voltage control. Here the reference voltage is used to calculate the modulation index. It is to slow down the variation in modulation index compared to the DC-link voltage regulation. This is satisfied because the MPPT control is slower than the DC-link voltage regulation control.
Figure 4 shows the three phase voltage vector Phase Lock Loop (PLL) used to obtain the phase angle of the PV terminal voltage vector (SPLd. The voltage vector angle is added with the DC-link voltage regulator output angle shift (Dpv) to get the inverter internal voltage vector angle (Spv). The internal voltage vector magnitude (VPVmag) and the angle (Spv) are used to calculate the three phase instantaneous voltages of the PV inverter unit. The conventional sine-triangular Pulse Width Modulation (PWM) technique is used to produce the firing pulses to the Insulated Gate Bipolar Transistor (lGBT) switches of the inverter.
B. Control circuits of the diesel generator
(Omeas TM initial
T Mech
Figure 5. Simple governor Droop control of the diesel generator.
VOref
V Omeas V Dref Figure 6. Simple automatic voltage regulator control of the diesel generator.
The diesel generator is employed with a simple droop governor control and PI based A VR control. Figure 5 shows droop governor controller. The purpose of the droop controller is to balance the load to the generation during slow changes either in demand or generation. However, any power mismatch due to fast changes will affect the energy in the magnetic field of the network thus the grid voltage. Therefore, mismatch due to fast power variation is addressed indirectly through the A VR control by adjusting the generator terminal voltage. Simplified A VR controller employed at diesel generator is shown in Figure 6.
V. SIMULATION RESULTS
The network with three generator technologies of PV, wind and diesel shown in Figure 1 was modeled in PSCADIEMTDC with the controls discussed in Section III. The PV cells are represented by a DC current source injecting to the DC-link of the inverter ci rcui t.
Figure 7. Simulation results of the proposed system when it was operated at islanded mode.
The simulations were carried out to check the operation of the system with proposed controllers in islanding and grid connected modes. The simulation results are shown in Figures 7 to 9 for the islanding operation. The measured parameters
and directions are indicated in Figure 1. Figures 7 shows variations of power from main grid, diesel generator, loads, PV and wind power plants.
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Figure 8. Simulation results of the proposed system when it was operated at islanded mode.
Figure 8 shows the variations on internal voltage magnitude (Vpvmag) and phase angle (DeltaPV) of the PV inverter, DClink voltage, AC rms voltages at the network bus bars and line power flows. The main grid was disconnected at 4 seconds. The intermittent nature was introduced at the wind farm input torque at 6 seconds. Then the intermittent nature was
introduced at 9 seconds at the PV plant inverter DC link as current injection. The results show that the PV inverter DC-link voltage is maintained at its set value and fluctuation at the PV bus voltage is very small and acceptable.
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Figure 9. Steady state simulation results of the proposed system when it was operated at islanded mode with all intermittent nature inputs.
Figure 9 shows important waveforms at steady state operation of the islanded system. Here all the intermittent nature inputs are engaged. This confirms that the proposed control maintains the DC-link voltage at its set value while minimizes the effect on PV AC terminal voltage. The grid connected performance was better than this.
VI. CONCLUSIONS
A PV-Wind-Diesel hybrid system is used in the proposed MG. Droop voltage control and DC-link voltage regulation together with simple pre-defined maximum power point traction control are explained for PV system inverter. Simple droop governor and AVR controls are discussed for diesel generator. Operations of the proposed system were studied for islanded mode with intermittent nature of wind and PV power plants. The results confirm the smooth operation of MG. Further it proves that affects due to intermittent nature of renewable plants are minimized at the PV DC-link voltage and its AC terminal voltage.
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