Design and Implemenatation of Non-Isolated Three Port DC/DC Converter for Stand-Alone Renewable System Applications

Design and Implemenatation of Non-Isolated Three Port DC/DC Converter for Stand-Alone Renewable System Applications

Author name: Archana.

Department: Electrical and Electronics Engineering. College: The Oxford college of Engineering Bangalore. Visvesvaraya Technological University, Belgaum, Indian.

Abstract An integrated non-isolated three-port DC-DC converters (NI-TPCs) interfacing a renewable source (Photovoltaic Array) , a storage battery and a load is proposed for stand-alone renewable power system application. The NI-TPCs proposed are generated by introducing a bidirectional cell to the basic DC-DC converters. With the NI-TPC proposed, single stage conversion between any two of the three ports can be achieved.

A multi-carrier based PWM scheme is proposed and applied to the NI-TPC based upon analysis of the power relations among the three ports. Power management strategy with four regulators achieving maximum power point tracking (MPPT) control of input source, charging control of battery and voltage control of load is proposed for a stand- alone renewable power system.

The converters should operate in every operation mode and switch between different modes freely and seamlessly. The topology proposed in these has advantages of high efficiency and simple control. These advantages make the converter promising for medium and high power applications. This topology has high power density, low cost, lightweight and high reliability. Open loop and Closed loop of Three Port Converters (TPC) are designed in MATLAB/SIMULINK environment.

KeywordsPhotovoltaic Array, DC-DC Boost converter, Bidirectional converter,MPPT controller.

1. INTRODUCTION

   Renewable sources, such as solar, wind and tide, are intermittent in nature. Hence, a storage element, such as a battery or a super capacitor is required for the stand-alone renewable power system to improve the system dynamics and steady-state characteristics. Three-port converters (TPCs), as shown in Fig. 1, have been proposed with the advantages of less component count and fewer conversion stages instead of employing several independent two-port converters [1]-[3]. The TPC system has the advantages of lower cost, higher reliability and enhanced dynamic performance with centralized control. Due to the remarkable merits of the TPC ,a wide variety of topologies have been proposed for various applications, such as hybrid electric vehicles [4]-[7], fuel-cell and battery systems [8]- [9], aerospace power systems[10]-[11], micro-inverter with power decoupling [12] and PV systems with battery backup or hybrid energy storage systems

   [13]-[18] etc..

   The TPC topologies can be classified into two categories: non-isolated topologies and isolated topologies. A NI-TPC featuring compact design and high power density may be the better choice than an isolated one for applications where isolation is not required. Conventionally, for the non-isolated applications, the input source, battery and load are linked together via a common DC bus with a unidirectional DC converter (UDC) and a bidirectional DC converter (BDC) or two UDCs, as shown in Fig. 2. However, the major disadvantage is the low system efficiency due to multiple stage power conversion.

   A family of integrated non-isolated three-port converter achieving single stage conversion between any two of the three ports is proposed in this paper. Based on the analysis of operation modes and system power control requirement, a PWM scheme is proposed. Experimental results are presented to validate the proposed PWM scheme and control strategy.

2. TOPOLOGY AND ANALYSIS

   Fig. 3(a) shows the basic topology of the Boost converter, in which the power flow from the input source to the load is configured. To supply the load smoothly, a storage element like battery is required in a stand-alone

   renewable power system. When the input power is more than what the load needs, it will charge the battery. The charging is controlled by a power switch connected in series with the battery. And an extra diode is needed to prevent the reverse current from flowing, as shown in Fig. 3(b). On the other hand, the battery should discharge to the load when the input source can no longer support the load all alone. The discharging of battery is controlled by another switch connected with the battery in series. Since the input source cant be connected with the battery in parallel, another diode is needed, as shown in Fig.3(c).

   Based on the above analysis, a Boost TPC is derived by introducing two additional power flow paths into the Boost converter. The additional circuit in the Boost TPC can be regarded as a bidirectional cell as shown in Fig. 4. In the Boost converter, the inductor L and the power switch S1 is connected to the input source in series. Thus, the power switch S3 can be introduced into the converter and a power flow path is configured to bridge the battery and the load. The equivalent circuit is a Boost converter. On the other hand, another power switch S2 is introduced into the converter and a power flow path can be configured to bridge the input source and the battery. The equivalent circuit is also a Boost converter.

3. OPERATION MODE AND PULSE WIDTH MODULATION SCHEME

1. Operation Mode Analysis

   In the Boost TPC, the power flows through the input source, battery and load are denoted with Pin, Pb and Po, respectively. Assuming Pb is positive when the battery discharges and ignoring the power loss in the conversion, we have

   Pin + Pb =Po (1)

   There are three possible operation modes in the Boost TPC.

   1. Dual-output mode: The Boost TPC works in dual-output (DO) mode when pin > po, as shown in Fig. 5(a). The input source feeds the load and charges the battery with excess power at the same time. The power switch S3 is kept off. pin is regulated with the duty cycle of S1, d1, and po is regulated with the duty cycle of S2, d2.

      In steady state, applying the volt-second balance principle to L, we obtain that:

      (2)

   2. Dual-input mode: The Boost TPC works in dual-input (DI) mode when pin < po, as shown in Fig. 5(b). The input source doesnt have enough power and the battery discharges to supply the power gap of (po – pin). In this mode, the power switch S2 is kept off. pin is regulated with d1, same as that in DO mode. po is regulated with the duty cycle of S3, d3, instead of d2.

      In steady state, applying the volt-second balance principle to L, we obtain that:

      (3)

   3. Single-Input Single-Output mode: The Boost TPC works in single-input single-output (SISO) mode when pin = 0, as shown in Fig. 5(c). In this mode, the battery powers the load alone. S3 is kept on and S2 is kept off, thus the Boost TPC is simplified to a Boost converter.

   In steady state, applying the volt-second balance principle to L, we obtain that:

   (4)

   Figure 3. Generation procedure of Boost TPC: (a) the Boost converter,

   (b) charge circuit of battery, (c) discharging circuit of the battery.

   Figure 4. The Boost TPC with a bidirectional cell.

2. Pulse width modulation Analysis

   Because there are two ports under control in both DO mode and DI mode, two independent control variables are needed. Thus, two control voltages vc1 and vc2 should be utilized in the PWM scheme. The control voltage vc1 is used to control the input source port and the battery port. And vc2 is used to control the load port. Corresponding to three power switches in the Boost TPC topology, three independent saw tooth carrier waveforms, vt1, vt2 and vt3, are used fr modulation. Fig. 7 shows the proposed PWM scheme for the Boost TPC. The voltage ranges of vc1, vc2, vt1, vt2 and vt3 are 2VT~3VT, 0 ~2VT, 0~VT, VT ~2VT and 2VT~3VT, respectively. The three carrier waveforms are compared with vc1 and vc2 to generate the drive signal vGS1, vGS2 and vGS3, respectively. The proposed. PWM scheme is analyzed in detail in the following.

   1. DO mode: In this mode, vt1 is compared with vc1 to generate the drive signal vGS1. While vt3 is compared with vc2 to generate the drive signal vGS2, as shown in Fig. 7. The falling edge of vGS1 and the rising edge of vGS2 are regulated with vc1 and vc2, and the rising edge of vGS1 and the falling edge of vGS2 are fixed. We can obtain that:

      When the input power pin decreases, the duty cycle d1 will be regulated by the control voltage vc1 as (5), to achieve MPPT control of input source. The control voltage vc2 should increase, and d2 decreases as (6), to reduce the charging power, pb, of the battery, so that the output power po is maintained.

      Figure 6. Generation circuit of proposed PWM scheme.

      When pin = po, pb is equal to zero. The control voltage vc2 and the duty cycle d2 are regulated to be VT and 0, respectively. Then, vc2 increases higher than VT and d3 increases from 0. The Boost TPC turns into DI mode.

   2. DI mode: In this mode, vGS1 is regulated by vc1 as in DO mode. vt2 is compared with vc2 to generate the drive signal vGS3, as shown in Fig. 7. The falling edges of vGS1 and vGS3 are regulated with vc1 and vc2, respectively. We can obtain that:

      As pin continues decreasing, d1 is regulated by vc1 as (5), too. vc2 should increase, and d3 increases as (7), to increase the discharging power, pb, of the battery, so that the output power po is maintained.

      When pin = 0, pb is equal to po. vc2 and d3 are regulated to be 2VT and 1, respectively. Then, the Boost TPC turns into SISO mode.

      1. SISO mode: The key waveforms are shown in Fig. 7. In this mode, vc2 is equal to 2VT. vGS1 is regulated by vc1 as in DO mode. And the output voltage is controlled by d1 instead of d3.

         It can be seen that the drive signals in the proposed PWM scheme meet the needs of control in all of the three operation modes. When power of input source fluctuates, power flows are regulated by variations of duty cycles of the three power switches. The Boost TPC can operate in every mode and switch between different modes freely and seamlessly.

         Figure7. Key waveform of PWM modulator in three operation mode.

         Design param eters	Boost DC/DC converter
         	Parameters	Values
         	Input voltage Vin	40V
         	Output voltage Vo	100V
         	Switching frequency Fs	100kHZ
         	Inductance L	50uH
         	Output Power	500W

         Design param eters	Boost DC/DC converter
         	Parameters	Values
         	Input voltage Vin	40V
         	Output voltage Vo	100V
         	Switching frequency Fs	100kHZ
         	Inductance L	50uH
         	Output Power	500W

         1. Table

1. ANALYSIS OF CONTROL STRATEGY

   To make the PWM active, two control variables, vc1 and vc2, should be employed as analyzed above. And four PI regulators are needed to ensure MPPT at input source, maximum voltage/current charging at the battery side and the voltage control for the load, respectively. The control strategy proposed is illustrated in Fig. 8.

   1. DO mode: When both the charging current and voltage doesnt reach the limitations, the control variable vc1 is regulated by the input voltage regulator (IVR), vc_IVR, to achieve MPPT. And vc2 is regulated by the output voltage regulator (OVR), vc_OVR, to realize the voltage control of the load. On the other hand, when ib (or ub) reaches the limitation, vc1 will be regulated by the battery current regulator (BCR) or the battery voltage regulator (BVR), vc_BCR or vc_BVR, to achieve maximum current charging control or maximum voltage charging control, instead of MPPT. And vc2 is still regulated by vc_OVR to control the output voltage.

      Simulation Circuit:

      MATLAB simulation diagram of DC/DC converter with bidirectional cell is employed. Design parameter values are shown in table 1.

      Figure 9. Conventional circuit of Boost DC/DC converter with bidirectional cell.

   2. DI mode: The battery is discharged in this mode. The BCR and BVR outputs are saturated at the lower limit. Thus vc1 is controlled by vc_IVR, to achieve MPPT. vc2 is controlled by vc_OVR, to control the voltage of the load.

   3. SISO mode: When the input source voltage is lower than the minimum voltage vin_min, the input source should be cut off. The IVR is used to control the load port. vc1 is controlled by vc_IVR, to control the voltage of the load.

   It can be concluded that there are at most two regulators working at any time. vc1 is controlled by one of vc_IVR, vc_BCR and vc_BVR. vc2 is controlled by vc_OVR in DO mode and DI mode.

   Input voltage=40v

   Time in second

   Output DC voltage=100v

2. SIMULATION

   Simulation is employed using MATLAB/SIMULINK in this project.

   Design The Boost converter design as follow.

   Time in second

   Open loop block diagram of Three Port DC/DC Converter (TPC) without MPPT controller.

   Figure 10. Conventional circuit of TPC without MPPT controller

   Photovoltaic Array

   The power that one module can produce is not sufficient to meet the requirements of home or business. Most PV arrays use an inverter to convert the DC power into alternating current that can power the motors, loads, lights etc. The modules in a PV array are usually first connected in series to obtain the desired voltages; the individual modules are then connected in parallel to allow the system to produce more current.

   State Of Charge (SOC) voltage (40v) versus Time in second

   Input voltage (120v) versus Time in second

   Output voltage (230v) versus Time in second

   Closed loop block diagram of Three Port DC/DC converter with MPPT controller

   In this Project Incremental Conductance MPPT controller is used.

   Maximum Power Point Tracking, referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in manner that allows the modules to produce all the power they are capable of. MPPT is not a mechanical tracking system that physically moves the modules to make them point more directly at the sun. MPPT is a fully electronic system that varies the electrical operating point of the modules are able to deliver maximum available power. Additional power harvested from the modules is then made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different.

   Incremental conductance

   • In the incremental conductance method, the controller measures incremental changes in PV array current and voltage to predict the effect of a voltage change. This method requires more computation in the controller, but can track changing conditions more rapidly than the perturb and observe method (P&O).

   • Like the P&O algorithm, it can produce oscillations in power outputThis method utilizes the incremental conductance (dI/dV) of the photovoltaic array to compute the sign of the change in power with respect to voltage (dP/dV)

   • The incremental conductance method computes the maximum power point by comparison of the icremental conductance (I / V) to the array conductance (I / V). When these two are the same (I / V = I / V), the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the process is repeated

   Figure 11. Conventional circuit of TPC with MPPT controller

   Solar irradiance (1000w/m2) Photovoltaic array

   The IV and PV curves for various irradiance but a fixed temperature (250c) is shown below in Figure.

   PWM pulses given to the switches

   DC Output voltage

   AC Output voltage (230v)

   ACKNOWLEDGMENT

   I wish to express our deep sense of gratitude and indebtedness to Assistant Professor Nalina Kumari, Department of Electrical Engineering, and The Oxford College of Engineering (TOCE) Bangalore, for introducing the present topic and for their inspiring guidance, constructive criticism and valuable suggestion throughout this project work.

3. CONCLUSION

With a bidirectional cell introduced into the basic converters, a family of NI-TPCs is proposed in this project. The proposed NI-TPCs share the same PWM scheme and control strategy. A PWM scheme and a control strategy are also proposed for the NI-TPCs. The PWM scheme and control strategy achieve MPPT at input source, the charging control at the battery and the voltage control for the load. The analysis on the topologies and the PWM scheme is verified with a prototype of the Boost TPC, which has good dynamic performance in each operation mode and during mode transitions. The power efficiencies of all the three power flows paths in the Boost TPC are higher than 96%.

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