Three-Phase Multilevel Inverter with Reduced Number of Active Power Semiconductor Switches for Solar PV Modules Mohd Aizuddin Yusof, Muzaidi Othman, Sze Sing Lee, M. A. Roslan and J. H. Leong School of Electrical System Engineering, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia
[email protected] Abstract—This paper presents a three-phase switched-battery multilevel (SBM) inverter for solar photovoltaic (PV) applications. The proposed inverter requires less number of power MOSFETs and gate drivers, and therefore, it is expected to be more compact and reliable than the conventional cascaded H-bridge multilevel (CHBM) inverter. For example, sixty units of power MOSFETs are required to construct a three-phase 11level CHBM inverter, whilst a SBM inverter with the same number of levels needs only 27 units. The switching losses of the SBM inverter are expected to be lower than that in conventional PWM-controlled inverters because the power MOSFETs in the SBM inverter are switched at a much lower frequency. In addition, the proposed inverter has an integrated charge mode operation which is very suitable for solar PV applications. The performance of an 11-level SBM inverter has been evaluated using PSIM software and the simulation results confirm that the proposed inverter is capable of producing low total harmonic distortion (THD) AC voltages without the need of bulky filters. Keywords— Photovoltaic, switched-battery, multilevel inverter, total harmonic distortion
three-phase
I. INTRODUCTION An enormous amount of electricity is consumed by data centers every year and the consumption is projected to double by 2020 [1]. Given that the world electricity generation is primarily dependant on fossil fuels such as coal, oil, natural gas, etc. [2], two problems are deemed inevitable. First, the growth of data center may not be sustainable as the sources of fossil fuels are limited and expected to deplete before the next century [3]. Second, burning fossil fuels to generate electricity will increase the concentration of atmospheric carbon dioxide (CO2) which is one of the greenhouse gasses (GHG) that causes global warming. To alleviate these problems, the green data center concept has been promoted and one of the strategies to achieve the “green” status is to power the data center using renewable sources of energy. One of the most promising sources of renewable energy for data centers is the sun’s energy as it is clean and widely available [4]. Sun’s energy can be harnessed using solar photovoltaic (PV) modules which convert the solar radiation into direct current (DC) electricity. It is common that the DC electricity is converted into alternating current (AC) form so
that it is compatible with existing power grid systems and most of the standard electrical appliances. Various types of inverters have been proposed in the past to provide the DC-to-AC conversion and multilevel inverter is one of them. In general, multilevel inverters can be grouped into three different classes: diode-clamp, flying-capacitor and cascaded multilevel inverters [5]. The use of diode-clamp multilevel inverter in solar PV systems has been demonstrated in [6][7][8]. For an m-level diode-clamp multilevel inverter, (m– 1) DC-link capacitors are required. The discharging rates of these capacitors are different and hence unbalanced capacitor voltages are resulted. This problem becomes more critical at higher number of levels. Flying-capacitor multilevel inverter has several drawbacks. For example, it requires a large number of bulk capacitors, i.e. a total of ((m–1) + (m–1)(m–2)/2) units. Besides, the control of the inverter can be very complicated whilst the switching frequency and switching losses are high [9]. Cascaded H-bridge multilevel (CHBM) inverter, as its name implies, consists of several H-bridge units connected in series. While CHBM inverter is among the most popular inverter topology in PV systems [10][11][12][13], it is, however, requires separate DC source and charge controller for each H-bridge unit. The multilevel inverters presented in [10][11][12][13] are for single-phase systems. For applications such as large data centers that require higher capacity of electric power, threephase supply system is always desired [14][15]. In this paper, a 3-phase multilevel inverter for solar PV modules is proposed. As will be shown in next section, the proposed topology has less number of active switching devices and hence the cost of the inverter is expected to be lower. In Section II, the operation of the proposed inverter is described, whilst the simulation results of an 11-level of the proposed topology are given and discussed in Section III. II. PROPOSED THREE-PHASE MULTILEVEL INVERTER A PV system with 3-phase CHBM inverter is shown in Fig. 1 [14]. For an m-level of 3-phase CHBM inverter, each phase leg consists of (m–1)/2 H-bridge units. Since the batteries connected to each H-bridge unit must be isolated from the batteries of other H-bridge units, a separate maximum power
PV
PV
PV
MPPT
MPPT
MPPT
H BRIDGE
H BRIDGE
H BRIDGE
PHASE B
PHASE C
PHASE A
N
n 3-Phase Load
Fig. 1. Three-phase PV system with cascaded H-bridge multilevel inverter.
Fig. 2. Three-phase PV system with switched-battery multilevel inverter.
point tracking (MPPT) charge controller is required for every H-bridge unit to charge its storage batteries independently. Hence, a PV system that utilizes the CHBM inverter topology can be very costly and bulky due to the need of high number of components.
required to construct a 3-phase SBM inverter. Alternatively, the required total number of power MOSFETs can also be expressed in term of m as follows
Fig. 2 shows the block diagram of a PV system that employs the proposed 3-phase multilevel inverter. Each phase leg of the inverter consists of a switched-battery multilevel (SBM) inverter module. It is worth noting that regardless of the levels of the inverter, each phase leg requires only a single MPPT charge controller. Therefore, whenever high number of levels is required, the cost saving offered by a PV system employing the 3-phase SBM inverter can be attractive compared to that with the conventional CHBM inverter. The schematic circuit diagram of phase A of the SBM inverter is depicted in Fig. 3(a). It is composed of a single-pole double-throw (SPDT) relay, cascaded switched-battery cells and an H-bridge inverter, whilst each switched-battery cell contains a diode, a power MOSFET, a single-pole single-throw (SPST) relay, and storage batteries. With a N number of switched-battery cells being cascaded in each phase, the number of levels of the SBM inverter topology can be determined as follows m = 2N + 1
(1)
As can be seen from Fig. 3(a), the number of power MOSFETs in each phase is essentially equal to the number of switched-battery cells plus four additional units used in the Hbridge inverter. Hence, a total of (3N+12) power MOSFETs is
§ m −1 · + 4¸ © 2 ¹
Total number of power MOSFETs = ¨
×3
(2)
Another advantage of the proposed SBM inverter is that it has a higher power MOSFET utilization factor than that of CHBM inverter. For example, an 11-level of 3-phase CHBM inverter requires 60 units of power MOSFET, whilst the SBM inverter with the same number of levels requires only 27 units. Hence, the cost of the proposed SBM inverter with high mlevel is expected to be much lower compared to CHBM inverter. The SBM inverter in a PV system can be configured to work either in charge mode or inverter mode. As shown in Fig. 3(b), the operation in charge mode can be enabled by switching the SPDT relay to the MPPT charge controller while closing all the SPST relays. This circuit configuration allows the storage batteries of all cells to be connected in parallel and charged simultaneously by the PV modules via the MPPT charge controller. It should be noted that all power MOSFETs are in off state during the battery charging process. Once the storage batteries are fully charged, the SBM inverter can be reconfigured to operate in inverter mode as shown in Fig. 3(c). When all power MOSFETs in switched-battery cells are off, the DC bus voltage (Vi) at the input of the H-bridge is zero. Different non-zero levels of DC bus voltage can be generated
PV
MPPT Inverter Mode SSPDT
SSPST1 BAT1
SSPST2 BAT2 VBAT
SSPSTN BATN Vi Si2
Si1
Si4
Si3
N (a)
Vout (b)
Load (c)
Fig. 3. Schematic circuit diagram of phase A of SBM inverter. The configuration of the circuit in charge mode and inverter mode are shown in (b) and (c), respectively.
by switching on one or more of the power MOSFETs. For example, when S1 and S2 are turned on, BAT1 and BAT2 will be connected in series and the effective DC bus voltage produced by these series connected storage batteries is simply equal to 2VBAT. For a SBM inverter with N switchedbattery cells, the maximum DC bus voltage that can be generated is NVBAT. Therefore, by switching the power MOSFETs (S1, S2, S3… SN) in a specific sequential pattern, a varying staircase DC bus voltage as shown in Fig. 4 can be obtained. An output AC voltage as shown in Fig. 5 can be
synthesized from the staircase DC bus voltage as follows. During the first half cycle, i.e. the interval between 0 to ʌ, switches Si1 and Si4 are turned on and the output of the Hbridge inverter follows the staircase DC bus voltage. For the second half cycle from ʌ to 2ʌ, Si2 and Si3 are turned on and the output voltage seen by the load is an inverted version of the staircase DC bus voltage. All switching states during inverter mode are summarized in Table 1. Note that the same switching states are also used to generate the output voltages of phases B and C.
TABLE I.
SWITCHING STATES DURING INVERTER MODE.
Binary logic: 0 = on & 1 = off
Fig. 4. Input DC bus voltage (Vi) of H-bridge inverter.
VdcBusB
VdcBusA
V
VdcBusC
V
Vab
V
V
Vbc
V V Vca
VphA
VphB
VphC
Fig. 6. PSIM schematic circuit diagram of 3-phase 11-level SBM inverter. Fig. 5. Output AC voltage (Vout) of H-bridge inverter.
III. SIMULATION RESULTS The operation of the proposed 3-phase SBM inverter has been simulated using PSIM software. Fig. 6 shows the equivalent schematic circuit diagram of an 11-level SBM inverter. Note that the SBM inverter is configured to operate in inverter mode, and hence, the PV modules, MPPT charge controllers, SPDT relays and SPST relays are not included in the circuit diagram. In the simulation study, all storage batteries are assumed fully charged and the voltages of the storage batteries are selected so that an output phase voltage of 240 VRMS is produced. As shown in Fig. 6, a 3-phase Yconnected load in connected to the SBM inverter. One of the
output terminals of each phase leg is connected to the 3phase load, whilst another output terminal is short-circuited to the same output terminals of other phase legs to form a neutral point at the inverter side. Each power MOSFET in the simulation circuit is driven using a switch gating block. The switching patterns applied to phase B and phase C are similar to that applied to phase A, except that they are phase shifted by 120° and 240°, respectively. Fig. 7 shows the staircase DC bus voltages of phases A, B, and C, whilst the phase voltages and line-toline voltages are shown in Figs. 8 and 9, respectively. Both the phase and line-to-line voltage waveforms, which are compared in Fig. 10, are almost sinusoidal and have a low harmonic content as confirmed in Figs. 11 and 12. These
simulation results show that the proposed 3-phase SBM inverter is capable of producing sinusoidal voltage waveforms without the need of bulky filters as those required in a two-level PWM inverter. As can be seen from Figs. 8 and 9, the fundamental frequency of the output AC waveforms is 50 Hz. While the power MOSFETs in the Hbridge are switched at fundamental frequency, those in switched-battery cells are turned on and off twice in every fundamental switching cycle. These 50 and 100 Hz switching frequencies are very low compared to that employed in two-level PWM inverter, and hence, the switching losses are expected to be lower than conventional PWM-controlled inverters.
VphA
VphB
VphC
400
200
0
-200
-400 0.04
0.045
0.05 Time (s)
0.055
0.06
Fig. 8. Output phase voltage waveform of 11-level SBM inverter VdcBusA 400
Vab
300
600
200
400
Vbc
Vca
200
100
0
0 -200
VdcBusB
-400
400
-600
300
0.04
0.045
0.05 Time (s)
0.055
0.06
200
Fig. 9. Output line to line voltage of 11-level SBM inverter 100 0
VphA
VdcBusC
600
400
400
300
200
200
0
100
-200
0
-400
0.04
Vab
0.045
0.05 Time (s)
0.055
0.06
-600 0.04
Fig. 7. Staircase DC bus voltages of 11-level SBM inverter.
0.045
0.05 Time (s)
0.055
0.06
Fig. 10. Comparison of output phase and line-to-line voltages of 11-level SBM inverter.
ACKNOWLEDGMENT
400
This work was supported by the Ministry of Education Malaysia through the Fundamental Research Grant Scheme (FRGS/1/2014).
Voltage (V)
300
200
REFERENCES [1]
100
0 0
10
20 30 Harmonic Number
40
50
Fig. 11. FFT of phase voltage waveform (THD is approximately 7.93%).
[2]
[3]
[4] 600
Voltage (V)
500
[5]
400
[6] 300 200
[7] 100 0 0
10
20 30 Harmonic Number
40
50
Fig. 12. FFT of line-to-line voltage waveform (THD is approximately 7.93%).
IV. CONCLUSION A 3-phase switched-battery multilevel (SBM) inverter for solar PV modules has been proposed. It offers several advantages over the conventional CHBM inverter. For example, it requires less number of power MOSFETs and gate driving units, and therefore, the inverter is expected to be more compact and reliable. The performance of an 11level SBM inverter has been simulated using PSIM software. As confirmed by the simulation results, the proposed inverter is capable of producing low THD AC voltages without the need of bulky filters. Since the power MOSFETs in the proposed inverter are either switched at fundamental or twice the fundamental frequency, the switching losses are expected to be lower than conventional PWM-controlled inverters. In addition, the proposed inverter has an integrated charge mode operation which makes it very suitable for solar PV applications.
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