Leakage Current Elimination of Grid-Connected PV Panels Using an Improved Non-Isolated DCAC Heric Converter Mohammad Maal Andish * , Tohid Jalilzadeh, Rasoul Shalchi Alishah, Mehran Sabahi. Department of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran. 1.
[email protected] 2.
[email protected] 3.
[email protected]@gmail.com 4.
[email protected]@yahoo.com
Abstract In this paper, an improved non-isolated DC-AC HERIC converter is presented to eliminate the leakage current of grid-connected PV panels. The improved topology comprises of high voltage gain four-output boost converter and a nine-level HERIC converter at the output side. The high voltage gain boost converter can provide equal voltage levels for used nine-level HERIC converter considering maximum power point tracking. The nine-level converter can provide nine-stepped voltage waveform at the output voltage waveform. In the proposed transformerless structure, there are parasitic capacitors between grid and PV panel which leads to the leakage current flowing through parasitic capacitors to the system. It causes safety problem, injection of harmonic current into the grid, increasing losses, and efficiency reduction. To eliminate the leakage current of proposed topology, the HERIC topology is used at the output side. In order to control the current of nine-level HERIC converter, model predictive control method is utilized. To confirm the operation of presented structure and theoretical analysis, PSCAD/EMTDC software is used.
Key words: Grid-connected, Heric Transformerless topology, Leakage Current.
Topology,
Model
Predictive
Controller,
1. Introduction Nowadays, photovoltaic system as a renewable energy source is more attractive due to the predictable shortage of conventional energy sources. There are some challenges in PV systems as follows: The value of generated voltage by PV panels is low and is changed by ambient temperature and solar radiation. Harmonic distortion of injected current should be low. Capability of used converter to track maximum power point.
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Reduction of leakage current of PV panel parasitic capacitors. Increasing efficiency of PV panel. In low power grid-connected PV panels, transformerless inverters are better than isolated topologies because of high-efficiency, small size, lower weight and cost [1, 2]. However, the non-isolated converters suffer from several problems. One of the most important problems is electrical connection of between grid and PV panel which lead to a leakage current flowing through inverter to ground. This leakage current causes losses, electromagnetic interface, and injection of additional harmonics to the grid [3]. The amplitude of leakage current is related to the used converter, switching technique, the resonant circuit formed by PV panel capacitance, AC filter, and grid. In general, the PV system consists of dc-dc converter and dc-ac converter. The utilized dc-dc converter can increase the value of produced voltage by PV panel. Moreover, it can track the maximum power point by adjusting the duty cycle of the converter’s switch. A dc-ac converter is used to convert the generated DC current by PV arrays to AC current to deliver the power to the grid or AC load [4]. For this aim, multilevel converters has a good performance as a dc-ac converter due to the lower value of total harmonic distortion, low voltage stress on switches, and lower switching frequency [5]. The most important multilevel converter topology is full-bridge converter which requires the least number of power electronic components. However, the voltage across parasitic capacitors of full-bridge converter consists of voltage ripple with high frequency which leads to the following leakage current substantially. In safety standard, the value of leakage current should be lower than 300mA. To satisfy this standard, some structures have been proposed which can provide a constant voltage across the parasitic capacitor and minimize the leakage current [6]. The proposed topology in [7] can reduce the leakage current by the use of additional switch in full-bridge converter which has been called H5. However, there are three switches in current path for positive and negative levels which increases the conduction losses. In [8] and [9], the H6 and FB-DCBP structures have been presented which can reduce the leakage current and thermal losses. However, there are four switches in the current path of positive and negative levels which lead to the high conduction losses and low efficiency. In [10], a new structure have been added to the full-bridge topology which reduces leakage current and the conduction losses is lower than FB-DCBP topology. The main disadvantage of this topology is utilizing a large number of power diodes which increases the manufacturing cost. In this paper, an improved topology consisting of a four-output dc-dc converter, 9level HERIC converter is presented. The input current of proposed topology is continuous and the high voltage gin can be obtained in lower values of the duty cycle of the switch. Also, the proposed topology can eliminate the leakage current of PV panel. To confirm the performance of proposed topology, simulation results are presented using PSCAD/EMTDC software.
2. Proposed Non-Isolated DC-AC Heric Converter Topology To investigate the ability of proposed topology in eliminating the leakage current, two topologies are studied in this section. The proposed first structure is unable to eliminate leakage current. However, the proposed second topology can overcome to this problem.
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1-2. First Proposed Topology Figure 1 indicates the general structure of first proposed topology which is named non-isolated DC-AC boost converter. As shown in this figure, the presented structure comprises of PV panel, four-output boost converter, and nine-level converter. To clearly discuss on the proposed first topology, each section is investigated separately. Figure 2 shows the circuit of four-output boost converter which has been proposed in [11]. Compared to the conventional boost converter, this topology can provide similar voltages on each capacitor (Vc). In other words, the voltage gain of proposed topology is four times of conventional boost converter. The voltage on each output capacitor (Vc) and the voltage gain of four-output boost converter (Vd) are obtained by (1) and (2), respectively: VC 1 Vin 1 D
(1)
Vd 4 V in 1 D
(2)
Where D is duty cycle of the switch S. this switch can be controlled to track the maximum power point using the related algorithms.
Figure 1: The general structure of first proposed topology (non-isolated DC-AC boost converter)
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Figure 2: The proposed circuit of four-output boost converter in [11]
According to Figure 1, the four-output boost converter has been connected the ninelevel converter. As shown in figure 3, this structure comprises of three bidirectional switches and a full-bridge converter. Each bidirectional switch comprises of four power diodes along with an IGBT. The switches of full-bridge converter are unidirectional. The input capacitors of nine-level converter are adjusted by the duty cycle of the switch S in the four-output dc-dc converter section [12]. The shown nine-level converter in figure 3 can produce nine levels at output voltage waveform ( 4Vc , 3Vc , 2Vc , Vc , 0 , Vc , 2Vc , 3Vc , 4Vc ). The switching state of this topology to generate nine levels at output
voltage is indicated in table 1. In this table, 0 and 1 indicate ON and OFF states of switches, respectively.
Figure 3: The proposed circuit of nine-level converter in [12]
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Table 1: Switching States of nine-level converter Switches States VAB T1 T2 T3 S1 S2 S3 S4 +4Vc 0 0 0 1 0 0 1 +3Vc 1 0 0 0 0 0 1 +2Vc 0 1 0 0 0 0 1 +Vc 0 0 1 0 0 0 1 0 0 0 1 0 1 1 0 0 0 0 0 1 0 1 -Vc 1 0 0 0 0 1 0 -2Vc 0 1 0 0 0 1 0 -3Vc 0 0 1 0 0 1 0 -4Vc 0 0 0 0 1 1 0
In table 1, when the switches S 2 and S 4 are turned ON, the leakage current (ileakage ) flows
through parasitic capacitor of panel ( C 0 ), filters, full-bridge, and grid
which is shown in figure 4. In an isolated topology, the used transformer cuts off the leakage current path and its amplitude will be low. However, the amplitude of leakage current in the proposed non-isolated topology is high. The value of leakage current should be limited to reduce losses, electromagnetic interface, and injection of additional harmonics to the grid [13-15].
Figure 4: The leakage current path of first proposed topology during ON-state of the switches S 2 and S 4
To eliminate the leakage current, the voltage across parasitic capacitor of PV panel ( C0 ) should be constant in all switching states. In other words, the switches S 2 and S 4 should not to be turned on simultaneously. To solve this problem, we have to make a current path in zero level which separates the grid from the PV panel. Then, the proposed second topology is proposed.
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1-3. Second Proposed Topology To solve the problem of leakage current in the first proposed topology, the second topology is presented which is called non-isolated DC-AC HERIC converter which is shown in figure 5. As indicated in this figure, two IGBTs H1, H2 along with anti-parallel diodes D1 and D2 are added to the full-bridge converter. The switching state of this topology to produce nine levels at output voltage is provided in table 2.
Figure 5: The general structure of second proposed topology (non-isolated DC-AC HERIC converter) Table 2: Switching States of nine-level HERIC converter VAB T1 T2 T3 S1 S2 S3 S4 H1 DH1 H2 DH2 +4Vc +3Vc +2Vc +Vc 0 -Vc -2Vc -3Vc -4Vc
0 1 0 0 0 0 1 0 0 0
0 0 1 0 0 0 0 1 0 0
0 0 0 1 0 0 0 0 1 0
1 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 1 1 1 1
1 1 1 1 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0
0 0 0 0 1 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0
As shown in table 2, the IGBT H2 and diode D1 or the IGBT H1 and diode D2 are turned on during zero level. To produce zero level which isolates the grid from the PV panel, figure 6(a) indicates the switching state when voltage (Vg) is positive and figure 6(b) shows the switching state when the grid voltage is negative. Moreover, it prevent reactive power exchange between inductive filters ( Lf 1(2) ) and input capacitors ( C in ) in zero level which increase the efficiency.
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(a)
(b)
Figure 6: The switching state proposed non-isolated DC-AC HERIC converter to produce zero level for (a)V g 0 , (b) V g 0
3. Model Predictive Controller The maximum number of pages which contain the text of the article and all of its components such as figures and tables are 14 pages. Model Predictive Control (MPC) is a controller method which uses the model of the system for the prediction of the future behavior of the controlled variables considering a prediction horizon N. For MPC, the used algorithm is executed again every sampling period and only the first magnitude of the optimal sequence is used to the studied system at instant k. In general, the most usual cost function of MPC is: p 2 * g i ( X i X i ) i
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(3)
Where X i * and X i represent the reference command and the predicted magnitude for variable X i , respectively. Also, i is the weighting factor and the index I is the number of controlled variables. Figure 7 indicates the block diagram of MPC method which a power converter has been used along with a load. The studied power converter has J switching states. In MPC, the variable X pursuits the reference X * and consists of the below steps: 1. Measure the controlled variables. 2. Apply the optimal switching states which have been calculated at in the previous sampling period. 3. For each switching state, predict the behavior of variable X using the mathematical model at sampling period. 4. Evaluate the cost function or error for each prediction as follows: g X * X
p
(4)
5. Select the switching state ( S opt ) that minimize the cost function and store it for using at the next sampling period. Figure 8 indicates the time diagram of the execution of the MPC method.
Figure 7: MPC block diagram
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Figure 8: Time diagram of the execution of the MPC algorithm
Now, the MPC method is applied for the proposed topology. We suppose that f and fs are the grid and switching frequencies, respectively. since f S f , we can suppose that switching period of grid voltage (V g ) can be constant. To analyze the MPC method for proposed topology, we consider the output section of proposed topology consisting of the VAB, Rf and Lf as a filter elements, and the grid with curren is as shown in figure 9.
Figure 9: Nine level Heric converter for MPC analysis
By applying KVL for the circuit as shown in Figure 9, we have: Lf
i s R f i s V g V AB t
(5)
Eq. (6) can be rewritten for a switching period as follows: Lf
i s (k 1) i s (k ) R f i s (k ) V g (k ) V AB (k ) Ts
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(6)
Eq. (7) can be simplified as follows: i s (k 1)
Ts R (V AB (k ) V g (k )) f T s i s (k ) i s (k ) Lf Lf
(7)
By knowing V g and i s at time k, the grid current can be estimated at k+1. The main aim of applying MPC method in the proposed topology is the prediction of the behavior of grid current for any generated voltage levels by proposed inverter (V AB ). The magnitudes of V AB are the voltage levels according to table 2. If the reference current grid to be a sinusoidal current as follows: is ref I mref sin t
(8)
Then, error function of each value of predicted current is defined as follows: e j (i s (k 1) V
jV c i s ref (k 1)) AB
2
(9)
At time k, the switching vector is selected in which that the cost function or the related error to its vector should be minimum compared to other switching vectors.
4. Simulation Results The proposed non-isolated DC-AC HERIC converter is simulated using PSCAD/EMTDC software. A PV panel with maximum generated voltage and current of 75V and 31A is used in simulation. The characteristic of PV panel and proposed converter is provided in table 3. By applying MPC method in the proposed topology, the grid and reference currents are indicated in figure 10. It is clear that the injected current is able to track the reference current. Based on figure 11, it is clear that both grid voltage and injected current are in phase. It means that the power factor is unity which causes the active power is injected to the grid. It is clear that the injected current is almost sinusoidal. Based on figure 12, it is obvious that the total harmonic distortion (THD) of injected current is 3.25%.
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Table 3: The characteristic of PV panel and proposed converter Parameter Value Open circuit voltage (V oc ) 90 V Short circuit current ( I sc ) 62 A PV The parasitic resistance 1Ω ( R0 ) Parasitic capacitors ( C 0 ) 0.1 µF input voltage (V in ) 75 V Switching frequency ( f s ) 5 kHz Voltage network/ 50Hz/220Vrms Frequency( f /V g ) Converter Input inductance ( L ) 0.001 H Equivalent series 0.005 Ω resistance( Resr ) Output capacitance ( C ) 2200 µF
Figure 10: Grid and injected current waveforms
Figure 11: Grid voltage and injected current waveforms
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Figure 12: THD waveform of injected current
Figure 13 indicates the waveforms of generated voltage by PV panel (VPV), the voltage of each capacitor (Vc), and input voltage of nine-level converter(Vd) which their maximum values are 75V, 90V, and 380V, respectively. Figure 14 indicates the output voltage waveform of proposed converter which the generated levels are clear. The maximum output voltage is 380V. Figure 15 shows the leakage current waveform without using Heric converter (or first proposed topology). As shown in this figure, the amplitude of leakage current is 0.95A. Figure 16 shows the leakage current waveform of the second proposed topology using Heric converter. Based on this figure, it is obvious that the maximum value of leakage current is 0.16A. It means that using a Heric converter causes the voltage across parasitic capacitors will be constant and the amplitude of leakage current reduces substantially.
Figure 13: The waveforms of generated voltage by PV panel (VPV), the voltage of each capacitor (Vc), and input voltage of nine-level converter (Vd)
Figure 14: The output voltage waveform of proposed converter
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Figure 15: The leakage current waveform of proposed non-isolated DC-AC HERIC converter (first proposed topology)
Figure 16: The leakage current waveform of proposed non-isolated DC-AC HERIC converter (second proposed topology)
5. Conclusion In this paper, an improved non-isolated DC-AC converter was presented for photovoltaic applications. The proposed converter can increase the level of generated voltage by PV panel using a four-output DC-DC boost converter. Also, a 9-level converter is used to reduce the harmonic distortion of injected current to the grid. The switches of 9-level converter were controlled by MPC method. It was shown that injected current and reference current are in phase. To eliminate the leakage current of PV panels, Heric converter is used for isolating of PV panel from grid. Capability of proposed topology is confirmed by simulation results.
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