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Grid-Connected Three-Input PV/FC/Battery Power System with Active Power Filter Capability S. H. Hosseini, Member, IEEE, F. Nejabatkhah, Member, IEEE, and S. Danyali, and S. A. Kh. Mozaffari Niapour, Member, IEEE
Abstract-- This paper develops a grid connected hybrid PV/FC/Battery power system proposed by authors. This system integrates photovoltaic (PV) array, fuel cell (FC) stack and battery as input power sources in a unified structure by means of a new three-input DC-DC boost converter which supplies a gridconnected inverter. In this structure, each switching cycle of the proposed boost converter is divided into five switching periods in comparison with the conventional structure. These switching periods introduce five different duty ratios for the proposed boost converter. Because the summation of these duty ratios should be equaled to one in the prior paper, achieving a high level output voltage at the DC-link is not possible. Therefore, this paper tries to presents some modifications in order to cancel this limitation of the duty ratios and control them independently. Consequently, a high level output voltage is achieved in addition to tracking the maximum power of the PV source, setting the power of FC source, and charging or discharging the battery. Utilizing a unified structure and improving the control strategy of the proposed three-input DC-DC converter facilitate power management of the input sources in order to supply a gridconnected residential load. All the system possible power operation modes are defined and managed by the power management control scheme. The proposed system is also able to compensate both reactive and harmonic current components drawn by nonlinear loads as active filter functionality. Index Terms-- Photovoltaic system, Fuel cell, Hybrid power integration, Power quality, Active filter
I. INTRODUCTION
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s the conventional energy sources are decreasing fast with a corresponding rise in cost and continuously growing energy demand, considerable attention is being paid to new energy sources. Among various types of these energy sources, photovoltaic (PV) energy appears quite attractive for electricity generation because of its noiseless, pollution free, scale flexibility, rather simple operation, and little maintenance. Although the PV power is present throughout
S. H. Hosseini is with the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]). F. Nejabatkhah is M.S. student in the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]). S. Danyali is Ph.D. student in the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]). S. A. Kh. Mozaffari Niapour is M.S. student in the Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran (e-mail:
[email protected]).
the day, it depends on sun irradiation level, ambient temperature, and unpredictable shadows, making it an unreliable intermittent power source. Therefore, a PV-based power system requires to be supplemented by other alternative energy sources to ensure a reliable power supply. Fuel cells (FCs) are emerging as a promising supplementary power sources because of their merits of cleanness, high efficiency, and high reliability [1]. Because of slow response characteristics of FC, fast step increase or decrease in demand power cannot be followed by these systems. So, batteries are utilized to improve dynamic response characteristic of these systems. Therefore, combining these energy sources introduces hybrid distributed generation system (HDGS). Such a system can be utilized as a stand-alone or grid connected system to supply critical loads. In grid connected systems, contributing HDGS to compensate grid power quality issues such as reactive and harmonic currents can enhance the system functionality. Compared to single-sourced systems, the hybrid power systems have the potential to provide higher quality, more reliable and efficient power to customers. In general, hybrid power systems employ diverse power electronic conversion stages. In these systems with a storage element, the bidirectional power flow capability is a key feature at the storage port. Further, the input power sources should enjoy the ability of supplying the load individually and simultaneously. In recent years, HDGSs have been developed by some literature [2-4]. Using of multi-winding transformer to hybrid DG sources [2], separated inverters for the input sources [3] and separated dc boost converters for each DG sources [4] introduce different structures of power electronic converters. The main disadvantages of these non-unified topologies are highly costs and losses, complicated control system, lower efficiency and inferior time response. Because the primary costs of HDGSs are extremely high, reducing of the cost and increasing the efficiency of these systems is essential. This purpose can be achieved by reducing the stages of the power converters. Multi-input converters as unified structure converters have been considered in renewable energy applications [5], [6]. A grid connected multi-input inverter has been proposed in [5] to combine PV and wind power sources.
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D1
+
D2
L1
VPV
T1
T2 K
PV
_ VBat
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T7
L3
T3
IL
IFC
+ Fuel Cell
+
C
T4
Gate Signals Generator (PWM)
D1 D2 D3 D4
VO
VFC
+ Grid
VG
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T4
T1 T2 T3
IHDGS IG Nonlinear Load
IPV
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Control System & Power Management
Iref
T8
Hysteresis Current Controller
Fig.1. Circuit diagram of proposed HDGS.
The structure composes of a buck/buck-boost fused multiinput DC-DC converter and a full-bridge DC-AC inverter. In [6], a DC-AC bidirectional multi-input converter utilizing a High-Frequency Isolating Link (HFIL) transformer, two half bridge boost converters at the input ports and a bidirectional cycloconverter at the output port was proposed for clean energy application. The converter control system is designed based on Input-Output feedback Linearization. This paper develops a new hybrid power system, consisting PV, FC and battery sources, proposed by authors in [7]. That paper presents a novel multi-input DC-DC boost converter which combines three DC input sources in a unified structure. In this structure each switching cycle of the proposed boost converter is divided into five switching periods in comparison with the conventional structure. These switching periods introduce five different duty ratios for the proposed boost converter. Supplying the inverter with PV/FC/Battery, and also battery charging are possible in one switching cycle. By utilizing these duty ratios all possible power operation modes of the converter are defined. In the previous control scheme, all duty ratios of the proposed converter are utilized in each switching cycle with a restriction that the summation of them should be equaled to one. This restriction leads to have low values for the duty ratios. As a result, small duty ratios cause not to achieve high level output voltage. In this paper, these duty ratios are controlled independently without any restriction. This development of the control system is accomplished by choosing one or two duty ratios related to each operation mode. All the possible system power operation modes are defined and managed by the power management control scheme. Additionally, the system is contributed to operate as an active power filter in order to compensate reactive and harmonic currents produced by the nonlinear loads. Such a functionality of the system can locally supply undesired load current components while providing the active load power. The proposed system has been simulated by PSCAD/EMTDC software for different operation modes.
II. PROPOSED HDGS SYSTEM Fig. 1 shows the circuit diagram of the proposed HDGS. The system consists of the three-input DC-DC boost converter and a full bridge DC/AC inverter connected to the grid. Also, Fig. 2 shows the possible switching states and their related duty ratios. As introduced in the prior paper and shown in the figure, all these possible switching states are embedded in a one switching period of the converter. This performance of the converter introduces a limitation for duty ratios so as their summation should be equaled to 1. This limitation consequently causes not to boost the input voltages enough in order to reach a desired value at the DC link. To solve this problem, this paper proposes some modifications as follows: At first note that if the battery is not utilized its current relay K should not conduct while in the two modes of the charging or discharging the battery it should conduct. Assume that the battery is not considered to be utilized in the converter. So, the PV and the FC should supply the load. Therefore, from the converter topology, the switch T3 can be entirely turned on in the switching period and the switch T1 can be frequently turned on and off with the duty ratio D1 to adjust the drawn power of the PV source. In a similar way, switch T4 is entirely turned on and the switch T2 is frequently turned on and off with the duty ratio D2 to adjust the power of the FC source. If the battery is needed to be discharged by the PV current to supply the load, the switch T3 should be entirely turned off and the switch T4 is entirely turned on. So, a current path to discharge the battery is provided. In this state, frequently turning on and off of the switch T1 and T2 with the duty ratios D3 and D2 can regulate powers of the PV and the FC sources respectively while the battery is discharged. In this state the battery necessitates to be charged, a path for the FC current to charge the battery is provided
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when the switch T3 is entirely turned on and the switch T4 is turned off in the switching period. In this state two duty ratios D1 and D3 are utilized to regulate powers of the PV and FC sources while battery charging is performed. All the introduced duty ratios D1, D2, D3, and D4 can be independently controlled without any limitation and reach to 1 value (0 < Dx < 1). The proposed boost converter is responsible to extract maximum power of the PV source (MPPT), set the power of the FC source and charge/discharge of the battery by adjusting these duty ratios. Depending on demand power, PV available power, FC maximum deliverable power, and battery charging/discharging necessity, proper operation mode of the system is determined by power management scheme of the control system. Table I shows possible power operation modes of the system. According to this table, in each operation mode, only one or two duty ratios of the proposed converter are chosen to achieve the power management targets. The inverter also roles as a local active filter for a nonlinear load. Therefore, at the AC side of the proposed system, the produced active power of the proposed HDGS along with reactive and harmonic component of the load current are injected to the load and grid by hysteresis current control scheme. System control basis, calculation of total reference current and power management control scheme are described in the next section.
these duty ratios, so the switching signals of the proposed boost converter are achieved. TABLE I VARIOUS POWER OPERATION MODES OF SYSTEM Power Management Operation Mode Only PV Only FC PV+ FC PV+ Battery Discharging PV+ FC+ Battery Discharging FC+ Battery Charging PV+ FC+ Battery Charging
Chosen Duty Ratio D1 D2 D1 and D2 D4 D2 and D4 D3 D1 and D3
In the AC side of the proposed system, in order to inject input DC powers along with the reactive and harmonic components of the load current to the grid, the control system requires an accurate reference current waveform. So the inverter of the HDGS produces a current as close as possible to the reference waveform. This reference current waveform is calculated as follows and injected to the grid by hysteresis current control scheme. If the grid voltage and load current are considered as follow, Vac = Vm sin(ωt )
(1)
I L (t ) = I m sin(ωt ) + I h+ q
The load compensation current can be calculated as (2): I h+q = I L (t ) −
2 PL Sin (ω t ) Vm
(2)
Where the (Ih+q) is defined as current components that remain after subtracting the active current component (ImSin(ωt)) from the iL(t) and PL is the average value of the load power. Moreover, the amplitude of the active current (I1m) of the total reference current is obtained from the DC link controller as follow:
∫
I1m = K p (VDCref − VDC ) + K i (VDCref − VDC )dt Fig. 2. Switching states of proposed converter.
(3)
So, the total reference current is obtained as (4): III. CONTROL SYSTEM BASIS AND REFERENCE CURRENT CALCULATION
The operation principle of the control system is based on the injection of active and compensating currents. Achieving this goal is performed by adjusting the chosen duty ratios to set the HDGS input sources on their reference power values. The control system can be divided into three different sections, proposed boost converter duty ratios controller, inverter controller and reference current calculator, and the power management control scheme. These control sections are shown in Fig. 3. As shown in the figure, after determining operation mode of the system by the power management control scheme, proper duty ratios are chosen according to the Table I. Moreover, adjusting the power of the FC and accomplishing the MPPT of the PV source cause to regulate
I ref = I h + q + I1 (t ) = [ I L (t ) −
2 PL Sin (ω t )] + I1m Sin (ω t ) Vm
(4)
As Table I shows, seven possible operation modes can be defined for the converter. Considering maximum available PV power, maximum deliverable FC power, load power and battery charging necessity, proper operation mode is determined by power management control. According this unit if (PPV>PL) the first operation mode is chosen, and the spare power of the PV is injected to the grid. In the condition that (PPV
