Content uploaded by Abu Bakar Siddique
Author content
All content in this area was uploaded by Abu Bakar Siddique on Sep 06, 2021
Content may be subject to copyright.
Proceedings of the 3rd International Conference on Sustainable Energy Technologies (ICSET 2021)
Peshawar, Pakistan, 10 August 2021
1
Performance Assessment of Transformer-less Grid Connected HERIC
Inverter Topology with Proportional Resonant Current Control
Abu Bakar Siddique 1 *, Hassan Abdullah Khalid 1, Muhammad Waqas Nazar 1
1U.S-Pakistan Center for Advance Studies in Energy (USPCAS-E), National University of Sciences and
Technology (NUST), Islamabad, Pakistan
*Corresponding author
Email: imabubakar1@gmail.com
ABSTRACT
“Transformer-less inverter topologies are preferred to use in grid connected photovoltaic (PV)
systems because of their compact footprint, greater output, and affordability. To meet the requirements of
leakage current in VDE-4105 standard various transformer-less inverter solutions have appeared in the
research done so far. This paper presents a detailed performance analysis of a single-phase transformer-less
highly efficient and reliable inverter concept (HERIC) topology together with a proportional resonant (PR)
current controller. Moreover, a comparison with the traditional PI current controller is also carried out based
on the output current harmonics and total harmonic distortion. For the evaluation of the results, simulations
are performed in PSIM.
KEYWORDS: Photovoltaic inverter, HERIC, Proportional resonant (PR) controller, Harmonic
distortion, Leakage current
1 INTRODUCTION
Renewable energy is regarded as a promising energy source to meet the rapidly growing demand of
electricity without leaving negative impact on the climate and environment. Out of all the available
renewable energy methods, solar power is the most significant energy source (Liu et al., 2019). Photovoltaic
(PV) converts light energy into unregulated DC voltage. Solar inverter converts DC voltage generated by
PV into desired magnitude of AC voltage, to be supplied to the load. Therefore, development of solar
inverter is becoming more and more valuable to provide clean energy (Clavadetscher & Nordmann, 2007).
In grid tied PV systems, the inverter should meet the perquisites of grid parameters such as voltage, current,
frequency, harmonics and leakage current (Goetz et al., 2019).
With today’s advancement in power electronics, single phase inverter topologies emerged as an
attractive solution for residential PV systems, categorized into isolated and non-isolated inverters. The
isolated inverters have galvanic isolation provided by a transformer and low leakage current (Khan et al.,
2020). However, inclusion of transformer reduces the efficiency and increases the weight, cost and volume.
On the other hand, non-isolated inverters provide higher efficiency, lower cost and smaller size than isolated
inverter because of the absence of a line frequency transformer. (Zeb et al., 2018) However, non-isolated
inverters can cause leakage current to flow between the actual earth of the grid and parasitic capacitances
of the poles of the panel.
For leakage current suppression in single phase transformer-less inverter, peak frequency oscillations
in the common mode voltage
must be removed (Kafle et al., 2017). To attain so, various transformer-
less inverter topologies have been proposed. One of such topologies is HERIC which stands for highly
efficient and reliable inverter concept inverter topology (Ma et al., 2015). It is composed of a full H bridge
and a branch of two back-to-back switches in parallel alongside the output filter of the inverter. It has a
2
three-stage output voltage, high productivity and low current drop compared to the other transformer less
inverter topologies.
Current controllers are made part of the inverter system to maintain high quality of output current
that can be supplied to any level of electrical system either linear or nonlinear. (Zhang et al., 2014) Current
controllers can be classified as proportional-integral (PI), hysteresis and proportional-resonant (PR) current
controllers. Proportional-integral (PI) controller is being used in commercial transformer less inverter
(Parvez et al., 2016). However, it has a few drawbacks. 1) Inability to track sinusoidal reference. 2) Poor
disturbance rejection capability. Hysteresis current control overcomes the issues of conventional PI
controller and offers unconditioned stability, robustness and good accuracy (Sezen et al., 2014). However,
it lacks variable switching frequency which results in production of comprehensive band harmonic
spectrum. While proportional resonant (PR) controller is the best way to get zero steady state error (Ul
Islam et al., 2018). It offers infinite gain at resonant frequency, better sinusoidal reference tracking and
disturbance rejection.
In this paper, a detailed performance analysis and design of HERIC inverter topology with
proportional resonant current controller is presented. In Section 2 of this paper, HERIC inverter topology
is presented. Then, its modes of operation and switching strategy is presented in the section 3. The design
and mathematical model of proportional resonant controller is discussed in section 4 and simulation results
are presented in section 5 of this paper. Finally, section 6 provides concluding remarks.
2 HERIC INVERTER TOPOLOGY
To keep the advantages of transformer-less inverter topologies and to lessen the flow of current loss
between the ground and parasitic capacitance of the PV array, the conventional H-bridge inverter topology
is altered as illustrated in Figure 1. HERIC inverter topology includes a full H bridge and a branch of two
back-to-back switches in parallel alongside the output filter of the inverter as illustrated in Figure 2. The
two extra switches operate at the grid frequency and provide isolation between DC and AC side of the
inverter resulting in high output and low current loss. It also helps in the formation of third voltage level in
inverter output voltage . These two extra switches and cut off the connection between the PV
array and the grid when zero vector is implemented which is called the AC decoupling.
S1
S6
L1
L2
Vgrid
S5
S2S4
S3
Vpv
Cdc
Figure 1: Modification in conventional H-bridge inverter topology.
S1
S2S4
S3
S6
L1
L2
Vgrid
Vpv
S5
Cdc
A
B
Figure 2: Derived HERIC inverter topology.
3
3 OPERATION OF HERIC INVERTER TOPOLOGY
Based on the modulation strategy for the HERIC inverter topology as illustrated in Figure 3, operating
modes are categorized as follows:
3.1 Operating mode for
In the positive half cycle of the grid voltage, switch is on while the switches and are switched
at the same time at switching frequency. During this state, current will be held through - and would
return through and provide positive voltage to the load as illustrated in Figure 4 (a). When zero vector is
applied, to are not used and current goes through and , this is referred to as the he is freewheeling
period. During this period PV is disconnected from the gird as illustrated in Figure 4 (b).
3.2 Operating mode for
In the negative half cycle of the grid voltage, is turned on keeping the off while and are
switched simultaneously on the switching frequency. During this state current will be held through –
and would return through and provide negative voltage to the load as illustrated in Figure 5 (a). Similarly,
when zero vector occurs, to are not used and current goes through and , this is referred to as the
Figure 3: Modulation strategy for HERIC inverter topology.
S1
S2S4
S3
S6
L1
L2
Vgrid
PV
Array
S5
Cdc
A
B
S1
S2S4
S3
S6
L1
L2
Vgrid
PV
Array
S5
Cdc
A
B
(a)
(b)
Figure 4: Operating mode for
.
4
freewheeling period. During this period PV is disconnected from the gird as illustrated in Figure 5 (b).
Hence unipolar output voltage is achieved, and high frequency fluctuations are eliminated from the DC end
of the inverter resulting in high efficiency and low leakage current.
4 CURRENT CONTROL SCHEME
Current controllers are utilized to maintain high-quality and low harmonic distortion in the output
current waveform. The traditional PI controller is not able to spot a sinusoidal reference with zero steady
state error. Hence, a method based on proportional resonant (PR) controller is employed. The PR
controller can significantly reduce the calculation complexity and control process while realizing a
similar frequency response characterize to PI current controller (Zammit et al., 2017).
Figure 6 illustrates the block diagram of a single-phase HERIC inverter and current control loop
to handle the inverter output current. A current error signal is obtained from the comparison of reference
current and grid current. Then, the current error signal is sent to the current controller and output is taken
in the form of modulating signal which will be employed to generate PWM switching signals.
4.1 Proportional resonant (PR) controller
The traditional PI controller is not able to track a sinusoidal reference without steady state error, a
proportional resonant (PR) controller based current control scheme is used as illustrated in figure 7. PR
Figure 6: Block diagram of grid connected PV system with current controller.
S1
S2S4
S3
S6
L1
L2
Vgrid
PV
Array
S5
Cdc
A
B
S1
S2S4
S3
S6
L1
L2
Vgrid
PV
Array
S5
Cdc
A
B
(a)
(b)
Figure 5: Operating mode for
.
5
controller is efficient at tracking sinusoidal reference due to its enormous gain at fundamental frequency.
The Laplace transform of the ideal PR controller is defined below:
where, is the proportional gain, is the resonant frequency and is the resonant gain.
Ideal PR controller represented by equation (1) leads to instability due to infinite gain. To overcome the
stability problem, a non-ideal PR controller can be realized by introducing damping as shown in equation
(2) below.
Where, is the cut off frequency. The non-ideal PR controller provides finite gain at the AC
frequency and small steady state error.
5 SIMULATION RESULTS
For the evaluation of results, HERIC inverter topology with PR current controller as illustrated in
Figure 7 is simulated in PSIM. The simulated circuit is designed for a single-phase transformer-less grid
connected system. The parameters and components of the simulation are listed in Table 1.
Table 1: Parameters for simulation.
Parameters
Symbols
Values
Input voltage
400V
Grid voltage
220V
Grid frequency
60Hz
Switching frequency
20kHz
Parasitic capacitance
20uC
Proportional gain
0.9
Resonant gain
1000
Vpv
Cdc
L1
L2
Vgrid
Cos
Sine
PWM
DC
AC
PI
Filter
PLLPR control
Figure 7: HERIC inverter with phase locked loop (PLL) and PR controller.
6
The output current of HERIC inverter topology by using PI and PR controller is shown in figure 8
and Fast Fourier Transform (FFT) analysis is carried out to compare the results of both current controller
schemes. When using PI current controller, FFT analysis yields the THD value equal to 7.53%. Whereas,
using PR current controller FFT analysis yields the total harmonic distortion (THD) value equal to 5.65%
and results in lower harmonic components compared to the conventional PI current controller as illustrated
in figure 9. The simulation results shows that PR current controller can suppress the current harmonics
better than PI current controller.
To test the robustness of PR controller against grid current fluctuations, a step is provided in the grid
current. Figure 10 illustrate the response of PR controller against the gird current and calculated error. PR
controller was able to effectively track the sinusoidal signal even after the step at o.5s offering very low
error signal.
Moreover, figure 11 (a) shows common mode voltage of HERIC inverter topology. The common
mode voltage of HERIC inverter topology remains constant in each switching state and has no high
frequency fluctuations. Thus, the HERIC inverter topology can significantly reduce the leakage current
issue. Figure 11 (b) illustrate the leakage current of the simulated topology which is below 20 mA and
within the limits of VDE-4105 standard.
(a)
(b)
Figure 8: (a) Inverter output current using PR controller. (b) Inverter output current using PI controller.
(a)
(b)
Figure 9: (a) Low harmonic spectrum of inverter output current using PR controller. (b) Low harmonic
spectrum of inverter output current using PI current controller.
7
Figure 10: Step response of PR controller.
6 CONCLUSION
In this paper, the performance of single-phase grid connected transformer less HERIC inverter
topology with PR current controller is analyzed and compared to traditional PI current controller by
simulation in terms of inverter output current and total harmonic distortion. Moreover, step response of PR
controller, leakage current and common mode voltage of HERIC inverter topology is also presented. The
results demonstrated that HERIC inverter with PR controller is superior to traditional PI current controller
in harmonic suppression, better step response and very low total harmonic distortion.”
REFERENCES
Clavadetscher, L., & Nordmann, T. (2007). Cost and performance trends in grid-connected photovoltaic
systems and case studies. Nuclear Safety.
Goetz, S. M., Wang, C., Li, Z., Murphy, D. L. K., & Peterchev, A. V. (2019). Concept of a distributed
photovoltaic multilevel inverter with cascaded double H-bridge topology. International Journal of
Electrical Power and Energy Systems, 110(March), 667–678.
https://doi.org/10.1016/j.ijepes.2019.03.054
Kafle, Y. R., Town, G. E., Guochun, X., & Gautam, S. (2017). Performance comparison of single-phase
transformerless PV inverter systems. Conference Proceedings - IEEE Applied Power Electronics
Conference and Exposition - APEC, 3589–3593. https://doi.org/10.1109/APEC.2017.7931213
(a)
(b)
Figure 11: (a) Common mode voltage of HERIC inverter topology. (b) Leakage current of HERIC
inverter topology.
8
Khan, A. A., Eberle, W., Wang, L., Khan, U. A., Ahmed, S., & Agamy, M. (2020). Novel Multilevel
Inverters Based on AC–AC Converter for Photovoltaic Applications. IEEE Journal of Emerging and
Selected Topics in Power Electronics, 9(3), 3384–3393. https://doi.org/10.1109/jestpe.2020.3007632
Liu, W., Niazi, K. A. K., Kerekes, T., & Yang, Y. (2019). A review on transformerless step-up single-phase
inverters with different DC-link voltage for photovoltaic applications. Energies, 12(19), 1–17.
https://doi.org/10.3390/en12193626
López, Ó., Teodorescu, R., Freijedo, F., & Dovalgandoy, J. (2007). Leakage current evaluation of a
singlephase transformerless PV inverter connected to the grid. Conference Proceedings - IEEE
Applied Power Electronics Conference and Exposition - APEC, 907–912.
https://doi.org/10.1109/APEX.2007.357623
Ma, L., Kerekes, T., Tedorescu, R., Tang, F., Jin, X., Tong, Y., & Liang, J. (2015). Leakage current analysis
of single-phase transformer-less grid-connected PV inverters. IECON 2015 - 41st Annual Conference
of the IEEE Industrial Electronics Society, 887–892. https://doi.org/10.1109/IECON.2015.7392211
Parvez, M., Elias, M. F. M., & Rahim, N. A. (2016). Performance analysis of PR current controller for
single-phase inverters. IET Conference Publications, 2016(CP688), 1–8.
https://doi.org/10.1049/cp.2016.1311
Sezen, S., Aktas, A., Ucar, M., & Ozdemir, E. (2014). A three-phase three-level NPC inverter based grid-
connected photovoltaic system with active power filtering. 16th International Power Electronics and
Motion Control Conference and Exposition, PEMC 2014, February 2017, 1331–1335.
https://doi.org/10.1109/EPEPEMC.2014.6980697
Ul Islam, S., Zeb, K., Ud Din, W., Khan, I., Ishfaq, M., Busarello, T. D. C., & Kim, H. J. (2018). Design
of a proportional resonant controller with resonant harmonic compensator and fault ride trough
strategies for a grid-connected photovoltaic system. Electronics (Switzerland), 7(12).
https://doi.org/10.3390/electronics7120451
Zammit, D., Spiteri Staines, C., Apap, M., & Licari, J. (2017). Design of PR current control with selective
harmonic compensators using Matlab. Journal of Electrical Systems and Information Technology,
4(3), 347–358. https://doi.org/10.1016/j.jesit.2017.01.003
Zeb, K., Khan, I., Uddin, W., Khan, M. A., Sathishkumar, P., Busarello, T. D. C., Ahmad, I., & Kim, H. J.
(2018). A review on recent advances and future trends of transformerless inverter structures for single-
phase grid-connected photovoltaic systems. Energies, 11(8), 1–33.
https://doi.org/10.3390/en11081968
Zhang, N., Tang, H., & Yao, C. (2014). A systematic method for designing a PR controller and active
damping of the LCL filter for single-phase grid-connected PV inverters. Energies, 7(6), 3934–3954.
https://doi.org/10.3390/en7063934
