이경민
(Kyung-Min Lee)
1
박철원
(Chul-Won Park)
†
-
(Ph.D. Course, Dept. of Electrical Engineering, Gangneung-Wonju National University,
Korea)
Copyright © The Korean Institute of Illuminating and Electrical Engineers(KIIEE)
Key words
BESS, Controller, DC circuit breaker, LVDC, Modeling, PSCAD, PV
1. Introduction
Since Paris Climate Change Conference in 2015, the MOTIE (Ministry of Trade, Industry
and Energy) in South Korea has proposed the “Renewable Energy 3020” implementation
plan at the end of 2017. The plan includes to raise the level of renewable in its
energy mix to 20% by 2030 and will significantly expand solar and WP (Wind Power)
to 95%. Meanwhile, LVDC (Low Voltage Direct Current) distribution network, one type
of DC micro grid, has many advantages such as connecting the DC output of a RES (Renewable
Energy System) directly to a digital DC load, reducing the power conversion step,
and increasing energy efficiency. There are many differences between LVDC distribution
and LVAC (Low Voltage Alternative Current) distribution. In order to commercialize
and expand the LVDC distribution network, DC protection system must be provided through
DC faults current analysis (1-3).
In the last few years, research papers about power converter and control technology
for modeling PV (Photovoltaic), BESS, WP generation, components of LVDC distribution
network, have been published. In addition, various methods have been proposed to solve
the DC protection problem (4-8). Recently, in order to protect short circuit fault of variable LVDC distribution
network, studies are being conducted to apply directional overcurrent relay, distance
relay, and differential relay used in the AC protection technique to DC distribution
network protection (9).
For various tests of ± 750V DC distribution network, lately, KEPCO is trying to construct
a power testbed center in Gochang (14,16). The RES and DR (Distributed Resource) for the power test center are composed of
PV system, BESS, WPP, and Diesel generator system etc. However, the modeling and control
technologies of the PV system, BESS, WP of the ± 750V DC distribution network are
not perfect, and correction and supplementation is required. In addition, precise
control and various interconnection rules are required when linking to the power system
(10-15).
This paper is part of a research project on the development of a DC circuit breaker
with protective function to be installed in this power testbed center. The modeling
and controller of PV and BESS, which are the core components of LVDC distribution
network used for the power testbed, are designed and its performance is verified by
simulation using a power system transient analysis tool, PSCAD software. The simulation
results show the modeling and dynamic behavior from controllers of PV and BESS system.
2. LVDC Distribution Network Modeling
2.1 Introduction
The LVDC distribution network consists of PV array with DC/DC converter, BESS with
bidirectional DC/DC converter, grid power converter, DC and AC loads as shown in Fig. 1. A voltage 22.9kV of AC electric power system is converted into ± 750V DC distribution
network through grid power converter of converter station. (2-3, 8).
Fig. 1. Systematic diagram of LVDC distribution network
2.2 Grid Power Converter
Fig. 2 shows systematic diagram of three phase AC/DC converter. The bidirectional three
phase AC/DC converter in the converter station controls the phase and magnitude of
the current using a PWM scheme. The converter can control the current close to the
sinusoidal wave and control the power bidirectionally. When switching at high speed,
a passive filter, such as a reactor and a capacitor on the back of the AC/DC converter,
is needed on the supply side to eliminate the harmful effects of harmonics. The AC/DC
converter is composed of 6-pulse type using IGBT. In order to maintain the DC-Link
voltage, the capacity of the DC-Link capacitor was selected to be 18400$\mu F$ by
equation (1).
Fig. 2. Systematic diagram of three phase AC/DC converter
2.3 PV System
2.3.1 PV Cell
Fig. 3 shows an equivalent circuit model of a solar cell, which consists of a current source
in parallel with a diode and in series with a series resistor (5, 7- 8, 15).
Fig. 3. Equivalent circuit mode of PV cell
The relation between the array terminal current and voltage is explained in (15). Parameters of the PV module is obtained from the basic specifications provided in
the PSCAD library. The output current (I) of the PV module consisted of series and
parallel cells is given as,
Actually, the output current of the solar array is calculated by (number of series)
× (number of parallel) solar cell modules.
2.3.2 DC/DC Converter with MPPT
Since the PV array outputs DC voltage, DC/DC boost converter was used, and they were
connected to LVDC distribution network through voltage control. In order to capture
the maximum power, MPPT (Maximum Power Point Tracking) which depends on panels temperature
and on irradiance conditions is applied for photovoltaic system. In this study, P&O
(Perturb & Observe) technique for MPPT, which is easy to implement through a simple
algorithm with few parameters, was used (7-8).
2.4 BESS
2.4.1 Battery Model
The secondary battery includes a lead acid battery, a lithium ion battery, a lithium
polymer battery, and a nickel cadmium battery etc. In this study, dynamic battery
model for LiFePO4 battery of Li-ion type battery has best characteristic which is
coupled with Shepherd model and the Thevenin battery model with the additional parallel
RC circuit is proposed, as shown in Fig. 4. (11-12).
As shown in Fig. 4, the proposed battery model consists of the open circuit voltage ($V_{OCV}$), battery
constant voltage ($E_{O}$), polarization voltage ($K$), battery capacity ($Q$), battery
current ($i_{batt}$), exponential zone amplitude ($A$), exponential zone time constant
inverse ($B$), internal resistance ($R_{i}$), discharging current ($i$).
Fig. 4. Dynamic battery model
The open circuit voltage can be represented by several variables, polar voltage, and
constant voltage as the following(11-12). It was used for battery modeling through the following.
equation (4) shows the voltage drop during the exponential zone.
equation (5) is a variable representing the charge in the end of the exponential zone.
equation (6) shows the pole voltage.
equation (7) shows the constant voltage of the battery.
2.4.2 Bidirectional DC/DC Converter with Control of Battery Model
The bidirectional DC/DC converter of this proposed battery model is power controlled
through two IGBTs and then linked to the DC feeder. The DC Link capacitance of the
DC/DC converter for ESS is designed in a manner similar to the DC/DC converter of
PV.
3. Modeling and Controller Design
The modeled LVDC distribution network using PSCAD software is shown in Fig. 5. We can see that considering LVDC distribution network which is composed of a AC/DC
converter of converter station, a PV array with DC/DC converter, a BESS with DC/DC
converter, and loads, as shown in Fig. 5. All of converter have IGBTs as switching devices. All models and controls of the
PV array and BESS were modeled and simulated using the PSCAD software internal library.
Fig. 5. Schematic diagram of AC grid connected LVDC distribution network
3.1 Modeling and Control of Grid Power Converter
Fig. 6. Bidirectional three phase AC/DC converter using PSCAD
In a three phase AC/DC converter design specification, the rated capacity is 500kVA,
the grid voltage is 380$V_{R M{S}}$, the DC-link voltage is 1500$V_{DC}$, the DC-link
capacitor is 18400$\mu F$, and the switching frequency is 2.5$k Hz$, respectively.
Figure 6 shows the bidirectional three phase AC/DC converter modeling using PSCAD. As shown
in Fig. 6, AC voltage is input and converted to DC voltage of bipolar ± 750V through PWM control
using switching of IGBT. Also, the voltage must be kept constant through the DC Link
capacitor.
Fig. 7 shows the voltage output waveform of a bidirectional three phase AC/DC converter
designed. From Fig. 7, it can be seen that when the bidirectional three phase AC/DC converter is operated
at 0.5s, the output voltage is maintained constant at 1500V (bipolar ± 750V) at 1s.
Fig. 7. Voltage output waveform
3.2 Modeling and MPPT Control of PV Array
Fig. 8 shows the PV array model using PSCAD software. An ideal circuit is constructed, and
the value input to the PV array model is derived by taking into consideration various
variables.
Fig. 9 shows the characteristic curves of the modeled PV array. Simulation conditions are
552V for open-circuit voltage, 120A for short-circuit current, 1200$W/m^{2}$, for
solar radiation, and 28℃ for temperature. From Fig. 9(a), it can be seen that the PV array shows an open circuit voltage and no current flows
during no load state. From Fig. 9(b), it can be seen that the MPPT control is performed at 441.6V, which is about 80%
of the open circuit voltage, in order to produce the maximum output during given conditions.
Fig. 10 shows a single phase DC/DC boost converter model using PSCAD. It receives the voltage
from the PV side and boosts the voltage to the distribution network.
Fig. 11 shows the output voltage waveform of the DC/DC boost converter of the PV. From Fig. 11, it can be seen that when the PV array is operated at 2s, the input voltage is controlled
according to the reference voltage by MPPT control. Also, the system voltage is measured
to 553V before the PV is operated. It can be seen that the input voltage is traced
according to the MPPT reference voltage at 2s and converges to 448V at 9s. At this
time, the output voltage of the PV module becomes 750V.
Fig. 9. Characteristic curves
Fig. 10. DC/DC boost converter
Fig. 11. Output voltage waveform of DC/DC boost converter
3.3 Modeling and Control of BESS
The ESS consists of a proposed battery and a bidirectional DC/DC converter. It emits
voltage according to the SOC and can be charged and discharged using a bidirectional
single phase DC/DC converter. Fig. 12 shows the proposed ESS model using PSCAD.
Fig. 13 shows the battery characteristic curve. It can be verified by the characteristic
curve according to battery discharge. As shown in Fig. 13, the specification of the battery is 740V, and the current is about 50A. In addition,
since the output time until the complete discharge is about 18s, it has a capacity
of about 666kWh by $740V\times 50A\times 18h$.
Fig. 13. Battery characteristic curve
Fig. 14 shows a bidirectional single phase DC/DC converter model using PSCAD. This single
phase DC/DC converter is controlled through two IGBTs, is controlled through PWM power
control. This DC/DC converter is similar to DC/DC converter design of the PV, but
buck operation should be taken into account.
Fig. 15 shows the PWM power control model using PSCAD. In Fig. 15, PWM power control is applied to the IGBT according to the input power to control
the voltage by operating Buck mode and Boost mode.
Fig. 14. Bidirectional DC/DC converter
Fig. 15. PWM power control
Fig. 16 shows the several waveform output from the ESS. As shown in Fig. 16, when the ESS operates at 3s, we can see that the output capacity is 50kW, the current
is about 35.6A, and the voltage is 1.5kV.
Fig. 16. Several waveform of ESS
4. Conclusion
In this paper, the modeling and controller of PV and BESS, which are the core components
of LVDC distribution network used for Gochang testbed, were designed and its performance
was verified by simulation using PSCAD software. The PV with this DC/DC converter
was fully modeled using PSCAD, and the BESS modeling with bidirectional DC/DC converter
using PSCAD had been performed, and verified to show the improved dynamic transient
characteristics. The results of this paper can be contributed to faults analysis,
protection solution planning, and short-circuit capacity calculation of DC circuit
breakers on the LVDC distribution network.
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Biography
He was born in Korea in 1990. He received his B.S. and M.S. degrees in Electrical
Engineering from Gangneung-Wonju National University, Wonju, Korea, in 2014 and 2017.
At present, he is working on his Ph.D in the Department of Electrical Engineering
at Gangneung-Wonju National University. He is a teaching assistant at Gangneung-Wonju
National University, since 2018. His research interests include Smartgrid, LVDC, Microgrid,
RES, PMU, AI application of power system, power system modeling & control, and power
system protection. He is a member of the KIEE, KIIEE, and IEEE.
Tel : (033)760-8796, Fax : (033)760-8781
E-mail : point2529@naver.com
He was born in Korea in 1961. He received his B.S., M.S. and Ph.D. degrees in Electrical
Engineering from Sungkyunkwan University, Seoul, Korea, in 1988, 1990, and 1996, respectively.
From 1989 to 1993 he was an associate researcher at Lucky GoldStar Industrial Systems.
From 1993 to 1996, he was a senior researcher at PROCOM system and lecturer at S.K.K.
University. At present, he is a professor in the Department of Electrical Engineering
at Gangneung-Wonju National University, since 1997. Currently he is the President
of the Faculty Council. His research interests include IED, SAS, Hybrid AC-DC power
grid, RES, PMU, AI application to power grid, power grid modeling & control, and computer
application in power grid. He is a member of the KIEE, KIIEE, KIPE, and IEEE. He is
president of PSPES since 2018. Dr. Park was awarded the Paper Prize of KIEE in 2010,
the Paper Prize of the KOFST in 2017, and an Academic Prize of KIIEE in 2018.
Tel : (033)760-8786, 640-2972, Lab : (033)760-8796
Fax : (033)760-8781
E-mail : cwpark1@gwnu.ac.kr