Pinned Mid-Points Multilevel Inverter (PMP): Three-Phase Topology with High Voltage Levels and One Bidirectional Switch Hani Vahedi, Student Member IEEE, Salem Rahmani, Kamal Al-Haddad, Fellow Member, IEEE École de Technologie Supérieure, GREPCI, Montreal, Canada
[email protected],
[email protected],
[email protected] inverter with PWM switching technique, (n-1) carrier waves are necessitated to produce the firing pulses required to command the switches [5, 6]. Some modifications have been done on NPC in the literature. For instance, active NPC has been introduced to make the components losses equal while increase the number of active switches [7]. As well, the anti-parallel IGBTs have replaced the clamping diodes of NPC [8]. However, the number of semiconductor switches didn’t change. Optimizing the numbers of switches used in multilevel inverters reduces the required gate driver’s boards and consequently, the manufacturing cost will become cheaper. A three-level three-phase BNPC has been invented in 1987 using bidirectional switch (BS) which is made of a diode bridge and an IGBT [9]. Afterwards, a multilevel inverter based on BNPC generating higher levels has been introduced which employs more BSs [10]. In this paper, a new multilevel inverter topology has been introduced which uses only one BS in each phase which connects the neutral point to the output. In the proposed PMP topology, each middle point of DC sources have been pinned to the inverter leg via an IGBT switch. In comparison with a NPC, the clamping diodes are removed and the number of switches has been decreased. Against BNPC, in PMP inverter the higher levels are achieved just by adding normal IGBTs which are available in various voltages and currents ratings in market. In next section, the conventional NPC is described. The modified half-bridge inverter employed in BNPC and the BNPC are explained in section three. Afterwards, the proposed PMP multilevel inverter as well as the switching states is described in section four. The simulation of a five-level PMP inverter has been performed in Matlab/SimPowerSystems and the results are shown and compared in section five.
Abstract- High power applications need high efficiency devices to produce lower power losses and harmonics while meeting the limitation of voltage and current. Multilevel inverters generate smoother and higher voltage at the output with lower harmonics. They can deliver high power while using medium-voltage switches. In this paper a Pinned MidPoints (PMP) multilevel inverter topology is introduced and studied which is derived from a Bidirectional Neutral Point Clamped (BNPC) three-level inverter. The proposed PMP multilevel inverter has fewer switches and clamping diodes than the Cascaded H-bridge (CHB) and Neutral Point Clamped (NPC) inverters, moreover it has less bidirectional switches in comparison with the BNPC. Moreover, it can be extended to three-phase inverter same as a NPC only using three legs and common DC link. A five-level inverter using proposed topology is validated by Matlab/SimPowerSystems. It shows the appropriate results of voltage and current as well as their THD%. Keywords: Multilevel Inverter, Neutral Point Clamped, Bidirectional Switch, Pinned Mid-Points, High Power Energy Conversion
I.
INTRODUCTION
Increasing world energy demand leads to considering the high electric energy generation and transmission. Using renewable energy resources and higher energy conversion to produce required electric power needs associated equipment like high power switching inverters. While the output of a conventional three-phase two-level inverter has large amounts of harmonics, as well as the switches have to suffer high voltages, the power losses is increased significantly in high power applications. Therefore the efficiency decreases remarkably. Moreover, power rating limitation of commercialized switches makes it difficult to build such inverter economically [1, 2]. The multilevel inverters are designed by configuring several switches and DC supplies to generate various voltage levels at the output. Thus, the output is more similar to the sinusoidal wave with lower harmonic contents. Besides, the voltage stress of the switches diminishes. As a result, such inverters using mediumvoltage switches can deliver high power to the consumers. Cascaded H-Bridge [3] and NPC [4] inverters are some popular examples of multilevel inverter which are widely used in industries and power networks. In an n-level
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II. THREE-LEVEL NEUTRAL POINT CLAMPED INVERTER As shown in figure 1, the three-level NPC, which was first introduced by Nabae [11], contains four switches in each leg and two diodes (D1a, D2a) clamped to the middle point of the capacitors. Table 1 shows the switching pattern and output voltage for one leg of the mentioned three-level NPC. It should be mentioned that the switches
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voltages (+Vdc and –Vdc) at the output. Table 2 shows the switching states of a half-bridge inverter. The lower switch is turned ON and OFF in conversely of upper switch. Figure 4 shows the output voltage of Half-bridge inverter.
S3a and S4a work in complementary of the S1a and S2a, respectively. So there are just two separate pulses to command those four switches. As it can be seen in table 1, the NPC adds a zero voltage level to the output that makes the output voltage threelevel instead of the two-level of conventional inverters. The clamped diodes are used to produce zero voltage. Figure 2 shows the output voltage for one cycle of mentioned NPC to produce a sinusoidal waveform using PWM. Compared with 2-level inverters, the zero level makes the output waveform smoother and more similar to the reference sinusoidal wave, with lower THD.
Fig. 3: Half-Bridge Single Phase Inverter TABLE II SWITCHING STATES AND OUTPUTS OF HALF-BRIDGE INVERTER Switching States Output Voltage Van S1 S2 1 0 +Vdc 0 1 -Vdc 200
150
100
Voltage (V)
50
Fig. 1: Three-Phase Three-Level NPC 200
-50
150
-100
100
-150
50 Voltage (V)
0
-200
0.018
0.02
0.022
0.024
0.026
0.028
0.03
0.032
Time (s)
Fig. 4: Output voltage of a Half-Bridge inverter
0
Comparing figures 1 and 3 shows that the difference between one leg of the NPC and the Half-bridge inverter is only about two switches and two clamping diodes. As mentioned earlier, these two switches and two diodes are responsible in producing the zero voltage level at the output, that is the act of connecting the point a to the neutral point n. This feature can achieve on half-bridge inverter using a BS which is shown in figure 5. The BS is commercialized with part-number of FIO5012BD [12]. It contains four diodes and one IGBT integrated together in one element. The current can flow from both nodes 1 or 2 to the other node. Such kind of BS is mainly used in Vienna Rectifier [13]. The five pins of that switch in figure 5 have been assigned to the input, output, collector, emitter and gate, respectively. Figure 6 shows the half-bridge inverter equipped with the mentioned BS [9, 10]. Using this configuration leads to generate the zero voltage level at the output without any DC source short circuit. The output voltage levels of this
-50
-100
-150
-200
0.018
0.02
0.022
0.024
0.026
0.028
0.03
0.032
Time (s)
Fig. 2: Output voltage of Phase A of a 3-level NPC TABLE I SWITCHING STATES AND OUTPUTS OF 3-LEVEL NPC
S1a 1 0 0
Switching States S2a S3a 1 0 1 1 0 1
S4a 0 0 1
Output Voltage Van +Vdc 0 -Vdc
III. THREE-LEVEL BIDIRECTIONAL NEUTRAL POINT CLAMPED Half-bridge single-phase inverter with two DC sources which is illustrated in figure 3 can produce two level
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modified half-bridge are listed in table 3. It is clear from the table that the output voltage waveform is exactly same as figure 2 for a single phase modified half-bridge.
application for multilevel inverters is in three-phase system where high power is delivered usually. The inherent advantage of the conventional half-bridge is the capability of connecting three legs with common DC link. This characteristic is extended into modified half-bridges to propose a new topology called BNPC [9]. The threephase three-level BNPC includes three legs of modified half-bridge and a common leg for DC sources that is illustrated in figure 8.
2
1 BS
Vdc
Fig. 5: Bidirectional Switch (BS) or Bidirectional IGBT
BS1a S1a
S1b BS1b
S1c a b
Vdc
S1
n
a
BS1
Vdc
BS1c
n
+ V_an
c
S2
Vdc
S2a
S2b
S2c
Fig. 6: Modified Half-Bridge Single Phase Inverter Fig. 8: Three-Phase Three-Level BNPC
TABLE III SWITCHING STATES AND OUTPUTS OF THREE-LEVEL MODIFIED HALF-
The switching state of proposed BNPC is same as table 3 for each phase. The output voltage is a three-level like figure 2 same as a NPC. It should be noted that no pairs of S1a, S2a and BS1a are turned ON simultaneously to prevent a short circuit on DC sources. They all act in complementary manner. When the S1a is ON the two other switches in one leg are OFF and the output is +Vdc. If the switch S2a is turn ON to produce the –Vdc, the switches S1a and BS1a are turn OFF. BS1a generates the zero voltage level at the output when the other switches S1a and S2a are both OFF. This BS plays an important role in BNPC topology. Two middle switches and two clamping diodes in NPC have been replaced by only one BS to build the BNPC multilevel inverter. The BNPC multilevel inverter has all advantages of NPC against conventional two-level voltage source inverter (VSI) like three levels at the output, lower harmonic in output voltage, lower voltage stress, lower common mode voltage etc. [4, 9, 11]. Besides, the most important superiority of BNPC versus NPC is the reduced numbers of components including switches and diodes which is explained as the following. As it is obvious from the figure 8, there are just 9 switches required to make a three-phase three-level inverter while producing the same voltage levels with conventional NPC which needs 12 switches and 6 clamping diodes, as well in CHB, 12 switches are necessary to generate these voltage levels in three-phase case. The less switches in BNPC, the lower power losses, higher efficiency, cheaper manufacturing cost, less associated components to fire the switches like gate drives.
BRIDGE INVERTER
S1 1 0 0
Switching States S2 0 0 1
S11a
BS1 0 1 0
Output Voltage Van +Vdc 0 -Vdc
S31a + V _1
Vdc1 S21a
S41a
Fig. 7: H-Bridge inverter to be used in CHB
Comparing the modified half-bridge and H-bridge shown in figure 7 gives the fact that in modified halfbridge three switches and two DC sources can generate three voltage levels while the H-bridge uses four switches and one DC source to make the same voltage levels. Although the difference between single phase modified half-bridge and H-bridge is just one switch and one DC source while the sum of components are equal, the main advantage of modified half-bridge is the ability to be used as a three-leg three-phase inverter. Therefore, the total number of switches and DC sources in three-phase applications is reduced significantly using the modified half-bridge. Regarding the above-mentioned details, the modified half-bridge can produce three voltage levels at the output. The modified one is a single-phase inverter while the main
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normal IGBTs are enough. Figure 10 shows a five-level PMP that clears the above-mentioned details.
These are the advantages of this topology among lots of cited structures [9]. A higher level inverter based on BNPC has been introduced in 2006 [10] that uses more BSs to connect the middle points of DC sources to the output. Figure 9 shows a five-level mentioned inverter.
Vdc
BS1a
a
Vdc
S5a
S2a a
BS3a
+ V_an
S5c
S1c
BS1aS2a
BS1bS2b
BS1cS2c
S3a
S3b
S3c
Vdc
S3a
S6a
Vdc Vdc
S1b
n
BS2a
n
Vdc
S5b
S1a
Vdc Vdc
S1a
c
b
S6b
S4a
S6c
S4b
S4c
S4a Fig. 10: Three-Phase 5-level PMP Fig. 9: Single-Phase 5-level BNPC
As it is clear in this figure 6 IGBT and 1 bidirectional IGBT (or BS) is used in each phase to generate five-level PMP. Table 4 compares the number of required components in three-phase NPC, CHB, BNPC and proposed PMP.
IV. PROPOSED PINNED MID-POINTS MULTILEVEL INVERTER It can be concluded that more levels can be achieved in BNPC topology just by adding DC sources and switches like an NPC but the clamping switches and diodes are reduced while showing the good results. Extending the three-level BNPC to more levels need more BSs which is clear in figure 9. These BSs are more expensive than normal IGBT switches as well as the voltage and current rating for such BSs are more limited than the variety of ratings for normal IGBTs. The main role of the BS in BNPC is producing a proper path for load current to flow under the control of the switching pattern. Assume if the BS2a in figure 9 was replaced by a normal IGBT like S1a. Therefore, the load current would flow in half-cycle by switching action, and in next half-cycle through the anti-parallel diode without any control on it due to change in polarity. This situation causes undesired current flow which makes non-smooth sinusoidal current at the output. Considering these facts, it can be concluded that for other mid-points (except neutral point n) between DC sources, the BSs are not required because there are not any direct connection between midpoints and the load. These connections have more switches between the mid-point and the load. Therefore the undesirable load current which can flow via the antiparallel diodes uncontrollably can be controlled by other switches. To be concluded, only one BS is required to clamp the output to the neutral point. For other branches just the
TABLE IV NUMBER OF COMPONENTS IN 3Φ MULTILEVEL INVERTERS Level Topology DC Source IGBT BS Clamped Diodes CHB 3 12 0 0 3 NPC 2 12 0 6 BNPC 2 6 3 0 PMP 2 6 3 0 CHB 6 24 0 0 5 NPC 4 24 0 18 BNPC 4 12 9 0 PMP 4 18 3 0
n
CHB
n −1 3 2
6( n − 1)
0
0
NPC
n −1
6( n − 1)
0
6(n − 2)
BNPC
n −1
3( n − 1)
3( n − 2)
0
PMP
n −1
6( n − 2)
3
0
It is obvious from the table that the proposed PMP topology has less components including switches and DC sources than the CHB. Besides, due to removing clamping diodes in PMP, it has less switches and diodes compared to the NPC. In comparison with the BNPC, it is clear that the number of BSs used in BNPC is much more than PMP. The PMP inverter just uses three BSs for three legs regardless of the number of levels. It means that the proposed topology has not limitations in using BSs with various ratings because each switch suffers different
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flowed in S1a, S5a, BS1a, S6a and S4a have been illustrated in figure 14 which are symmetric and the neutral current is the lower one.
voltage and current magnitude. The BSs used in pinning the neutral point in PMP are turned on only when the output voltage is zero and their current are lower than the other switches. Therefore, low current BSs are suitable in this case. The performed comparisons clarify the effective advantages of the proposed PMP multilevel inverter both in efficiency and commercial aspects. While the main issue in using capacitors as DC link in NPC is the voltage balancing problem, the proposed PMP also faces the same problems. Therefore, the voltage control methods introduced for NPC are useful for the PMP too. In this paper this issue has been ignored by employing isolated DC supplies as DC link. The switching pattern of the single-phase five-level PMP shown in figure 10 is listed in table 5.
200 150 100
Voltage (V)
50 0 -50 -100 -150 -200
0.03
0.04
0.05 Time (Sec)
0.06
0.05 Time (Sec)
0.06
0.07
0.08
0.07
0.08
Fig. 11: 5 level Phase a Voltage (Van)
TABLE V SWITCHING STATES OF SINGLE-PHASE FIVE-LEVEL PMP
Switching States
0.02
400 300
Van
BS1a
S1a
S2a
S3a
S4a
S5a
S6a
0
1
1
0
0
0
0
+2Vdc
0
0
1
0
0
1
0
+Vdc
1
0
0
0
0
0
0
0
0
0
0
1
0
0
1
-Vdc
0
0
0
1
1
0
0
-2Vdc
200
Voltage (V)
100 0 -100 -200 -300
In order to produce the switching pulses using PWM technique, four carrier waves are required. These carrier waves are compared with a sinusoidal reference and the appropriate pulses are generated and sent to the switches to make the five-level voltage at the output. As it is deduced from table 5, each voltage level is generated by two switches and the zero level is achieved only by turning ON the BS. The low frequency switching is an advantage of the introduced topology leads to low switching losses. Since the proposed PMP topology is similar to the NPC, the SVM and SHE modulation can be applied which can be combined with evolutionary algorithms to improve and optimize the ability in reducing the losses and THD.
-400
0.02
0.03
0.04
Fig. 12: 9 level line-line voltage (Vab) 6
4
Current (A)
2
0
-2
-4
-6
V. SIMULATION RESULTS
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Time (Sec)
Fig. 13: Three-Phase load currents
Simulation of a three-phase five-level PMP illustrated in figure 10 has been performed in Matlab/SimPowerSystems. Each phase is connected to a RL load consists of a 30Ω resistor and a 20mH. The switching frequency is set to 2 kHz and the output voltage frequency is 60Hz. The DC voltage is 100V for each source (Vdc=100 V). Figures 11, 12 and 13 show the output phase voltage and line to line voltage and current of five-level PMP, respectively. The THD percentages of the voltages and currents waveforms are listed in table 6. Finally, to prove the capability of proposed PMP topology in three-phase implementation, the currents
10
S1a
5 0 10
S5a
Current (A)
5 0 5
BS1a
0 -5 0 -5
S6a
-10 0 -5 -10
S4a 0.02
0.03
0.04
0.05 Time (Sec)
0.06
Fig. 14: Switches currents
106
0.07
0.08
TABLE VI HARMONIC ANALYSIS OF OUTPUT VOLTAGE AND CURRENT Harmonic Order THD 3rd 5th 7th 0.36% 0.32% 0.24% 10.88% Van 0.36% 0.32% 0.24% 10.86% Vbn 0.35% 0.33% 0.23% 10.86% Vcn 0.00% 0.32% 0.24% 8.72% Vab 0.00% 0.32% 0.24% 8.70% Vbc 0.00% 0.33% 0.23% 8.72% Vca 0.00% 0.21% 0.12% 0.39% ia 0.00% 0.20% 0.12% 0.39% ib 0.00% 0.21% 0.12% 0.39% ic
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[2]
[3]
The illustrated results prove the efficiency of the proposed PMP topology for multilevel inverters. The output phase voltage has five levels and the sinusoidal current has lower harmonics due to the smoother voltage output. It has acceptable results in three-phase application. Reducing components has the direct effect on manufacturing cost. The new proposed PMP multilevel inverter is economical and has shown same results of voltage and current as the conventional NPC, therefore it can be a proper alternative to be used in industries like machine drives and energy conversion.
[4]
[5]
[6]
[7] VI. CONCLUSION Multilevel inverters have found their way to the high power system applications such as high power machine drives and high energy conversion using renewable energy resources like photovoltaic and wind. The three-level NPC is a popular multilevel inverter which is widely used in industries. This paper proposed a new PMP multilevel inverter that can be built in three-phase form with less switches and no clamping diodes. The proposed topology has shown appropriate results in output voltage and current. Lower THD, lower voltage stress, lower common mode voltage, higher efficiency, high power application and smoother output voltage and current are the advantages of multilevel inverters and also for the PMP. The main advantage of PMP which distinguishes this topology among lots of introduced structures is the reduced components including DC sources, switches and diodes that minimize the manufacturing cost. The simulation results validate the above characteristics.
[8]
[9] [10]
[11]
[12]
[13]
ACKNOWLEDGMENT
The authors gratefully thank the Canada Research Chair in Energy Conversion and Power Electronics at the École de Technologie Supérieure for their financial support.
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