01-11-2016, 03:37 PM
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ABSTRACT
A multi-mode operation for the three-phase PV power system with low-voltage ride-through (LVRT) capability is proposed. With the proposed multi-mode control strategy, the active power from the PV arrays can be continuously extracted by the interleaved boost converter during LVRT while the maximum power point tracking (MPPT) operation can be quickly achieved after the grid fault clearance. Also, the multichannel boost converter with interleaved operation can increase power conversion efficiency while decreasing the input current ripple. On the other hand, the maximum current limitation control (MCLC) of the three-phase inverter can provide maximum reactive power under rated current amplitude during the voltage sag period as well as output the demanded reactive/rated current ratio to meet different LVRT codes. A three-phase 5kVA prototype PV converter is built and tested to verify the performance of the proposed multi-mode operation strategy and the LVRT capability.
PROBLEM STATEMENT
The renewable energy source, such as the Photovoltaic (PV) power system, has been rapidly developed in order to reduce the fossil fuel usage and carbon dioxide emission [1]-[8]. However, the unpredictable environmental conditions, such as irradiation or temperature, bring negative impacts to output characteristics of PV arrays. Therefore, developing an effective maximum power point tracking (MPPT) converter to improve the efficiency of PV power system is necessary.
INTRODUCTION
The renewable energy source, such as the Photovoltaic (PV) power system, has been rapidly developed in order to reduce the fossil fuel usage and carbon dioxide emission [1]-[8]. However, the unpredictable environmental conditions, such as irradiation or temperature, bring negative impacts to output characteristics of PV arrays. Therefore, developing an effective maximum power point tracking (MPPT) converter to improve the efficiency of PV power system is necessary.
Usually, the boost converter is used as the front-end MPPT converter for PV power systems because of its input current linearity [9]-[13]. Moreover, the interleaving technique, which can increase the input power rating but decrease the current ripple, has been adopted for PWM converters [14]- [17]. For an m-phase interleaved boost converter, the current of each phase is only 1/m times of the total current. In other words, the current stresses and the power losses of the power switches can be reduced. Also, the total current ripple can be reduced significantly because of the cancellation between different phase currents. It can improve the accuracy of the MPPT as well as increase the total output power of the PV system. Eventually, the boost converter with interleaved operation is adopted for high power system to achieve high power conversion efficiency and better MPPT performance. Recently, many countries have announced their own grid codes for the distributed generator to meet the low-voltage ride-through (LVRT) capability in order to increase the quality and stability of the power system [18]-[20]. The relationship between the grid voltage sag and the fault operation time of different national grid codes are shown in Fig. The area above the curve indicates that the distributed generators should remain grid-connected operation. Moreover, the E.ON code, which is presented by Germany, further requires the distribute generators to supply the demanded reactive/rated current ratio between 90% and 50% of grid voltage sag, as shown in Fig. 2 [21]. As soon as the grid voltage is lower than 50%, the distribute generators should be able to output 100% reactive current only. Usually, the PV power system consists of a boost converter and a three-phase inverter is adopted to achieve LVRT capability in many literatures [22]-[27]. The sequence separation and detection methods for LVRT during unbalance voltage sag are proposed in [22]. The maximum current limitation control (MCLC) for the three-phase inverter is circuit damage from severe voltage fault, a dc-link voltage adjustment method for the inverter is proposed in [24]. Different control schemes are proposed to achieve better performance of the power qualities of the PV inverters under unbalanced grid voltage sags [25]-[27]. Although those control strategies are effective for inverters, the operation of the front-stage boost converter is not considered and the power flow balancing between the MPPT and the inverter is not discussed. The concept of multi-mode operation for PV power system is presented in [28]. Based on the quantity of the generated PV power and the allowable maximum active output power of the inverter, the PV power system can be operated in the MPPT mode, the constant power control (CPC) mode, and the short-circuit current (SCC) mode. However, detail descriptions and corresponding mathematical equations of the multi-mode operation are not exposed. Therefore, an interleaved boost converter with improved multi-mode operation for the PV power system is proposed in this paper while the corresponding mathematical equations of different operation modes will be derived thoroughly in this paper. The interleaved boost converter can remain MPPT operation during the slight voltage sag. As soon as the output power of the PV array is greater than the allowable power of the three-phase inverter, the CPC Mode will be triggered with a proposed power losses compensation strategy. If a severe grid fault occurs, the SCC Mode will be triggered and the smooth transition concept with the derived incremental duty cycle is adopted to prevent the circuit damage. Moreover, the MCLC strategy for the three-phase inverter is developed in this paper. The operational principle and mathematical derivation of the proposed PV power system are described. Finally, experimental results of a 5kVA prototype are used to verify the performance of the proposed control strategy.
PROPOSED TECHNIQUE & WORKING PRINCIPLE
The proposed PV power system consists of an interleaved boost converter with the MPPT function and a three-phase inverter with the MCLC strategy. The three-channel interleaved boost converter with low input current ripple can achieve better MPPT accuracy as well as increase the PV system’s total output power. Under low insolation condition, one or two channels can be shut down to avoid unnecessary power losses at low PV power output. During the voltage sag period, the three-phase inverter can provide maximum available reactive power by adopting the MCLC strategy. where Sinv,max represents the rated complex power of the three phase inverter, Irated is the rated current and Ireactive is the required reactive current of the three-phase inverter. During the LVRT operation, when the inverter’s allowable active output power, which is calculated by (1), is smaller than its present output power, the MPPT function should be abandoned and input power of the interleaved boost converter should be reduced to maintain power flow balance and prevent the circuit damage. In order to achieve power flow management during LVRT as well as to shorten the MPPT recovery period after the grid fault is cleared, a multi-mode operation strategy is proposed. Based on the quantity of the generated PV power and the allowable maximum active output power of the inverter, the PV power system can be operated in the MPPT mode, the constant power control (CPC) mode, and the short-circuit current (SCC) mode.
DISADVANTAGES OF EXISTING SYSTEM
• The response time of an existing system is low.
• Due to the use of PWM technique maximum harmonics occurs at the outputside
• So the system efficiency reduces and the PF of the system is poor.
• Due to this the voltage regulation is low.
APPLICATION:
Domestic application.
Grid application.
ADVANTAGES OF PROPOSED SYSTEM
• High performance.
• lower Grid faults and power control tracking.
• Power Conversion Efficiency of the system is high and decreasing the input current ripple.
SOFTWARE TOOL:
• MATLAB/SIMULINK
HARDWARE USED:
• Controller
• Driver unit
• PV panel
EXPECTED RESULT:
Increasing the power conversion efficiency, while decreasing the input current ripple. And also we can minimize the grid faults and we can do the power control tracking. An improved multi mode control strategy for the 3 phase PV power system with LVRT capability. PV power system can operate in the MPPT mode , CPC mode, & the SSC mode to achieve better performance.