=Paper=
{{Paper
|id=Vol-3734/invited2
|storemode=property
|title=Research on control strategy of flexible interconnection with multi-source
cooperative in various applications
|pdfUrl=https://ceur-ws.org/Vol-3734/paper2.pdf
|volume=Vol-3734
|authors=Ya'nan Wang,Yanqiang Wan,Yongjuan Wang,Zhilian Sun,Yaqian Wang
|dblpUrl=https://dblp.org/rec/conf/iccic/WangWWSW24
}}
==Research on control strategy of flexible interconnection with multi-source
cooperative in various applications==
https://ceur-ws.org/Vol-3734/paper2.pdf
Research on control strategy of flexible interconnection
with multi-source cooperative in various applications
Ya’nan Wang1, Yanqiang Wan1, Yongjuan Wang1, Zhilian Sun1 and Yaqian Wang2, ∗
1 Haibei Power Supply Company of State Grid Qinghai Electric Power Company, Qinghai, China.
2 Pinggao Group Co., Ltd, Zhenghzhou, Henan, China.
Abstract
In the face of the high requirements of end-user for power quality and the stable impact of
distributed energy resources on the power grid, the flexible interconnection system with multi-
source cooperation is proposed, including distributed photovoltaic (PV), hydrogen fuel cell and
lithium battery system. This template introduced the research and design of system collaborative
control strategy in different working conditions under the premise of high efficiency and
reliability. Then, a model of two-zone distribution network system were simulated to verify the
effectiveness of strategy, so as to realize the power support of the multi-source collaborative
flexible interconnection system to the distribution station area. It improves the power supply
reliability of the local power grid, effectively reduces the impact of distributed power on the
power grid, and improves the utilization rate of micro power supply.
Keywords
multi source-collaboration, flexible interconnection, distribution network
1. Background
With the promotion and popularization of electrical substitution, the importance of electric
energy has been further enhanced in life. At the same time, the demand for power quality
and reliability has been increasing at the power distribution network closing to the end-
user [1-3]. At present, the scale of distributed energy access in the distribution network is
becoming more and more extensive, in the meantime, the amount and utilization rate of
electric vehicle charging piles are also gradually rising. The above impacts of source and
load has become the development tendency of new power system. Based on the premise of
reliable distribution network, how to meet users' requirements for power quality has
become the key to the research of new power system.
In view of the current power supply situation of distribution area, combined with the
structural characteristics of distribution network, flexible interconnection and mutual
sharing between stations have become an effective solution to improve the power quality
ICCIC 2024: International Conference on Computer and Intelligent Control, June 29–30, 2024, Kuala Lumpur,
Malaysia
∗ Corresponding author.
343947676@qq.com (Ya‘nan Wang); 345066126@qq.com (Yanqiang Wan); wyiwyiwy@163.com (Yongjuan
Wang); 459181158@qq.com (Zhilian Sun); suniwest@163.com (Yaqian Wang)
0009-0004-3450-9611 (Ya‘nan Wang); 0009-0006-1009-7047 (Yanqiang Wan); 0009-0005-4920-8341
(Yongjuan Wang); 0009-0001-7153-9121 (Zhilian Sun); 0009-0007-1258-2457 (Yaqian Wang)
© 2024 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
CEUR
ceur-ws.org
Workshop ISSN 1613-0073
Proceedings
of distribution network [4]. Literature [5] proposed a power optimization cooperative
control strategy of flexible interconnection device with energy storage. Wang Chuyang
proposed a master-slave control strategy of the flexible DC interconnection system based
on the capacity margin of the main station, which optimized the operation mechanism of
the main station, and ensured the continuous regulation ability of the DC bus voltage [6].
However, most literatures only focus on the discussion and research of AC grid-connected
state of flexible interconnection system, and pay less attention to the technical research
under other working conditions such as off-grid, and don’t take full advantages of flexible
interconnection in distribution network with multi-source access conditions. Therefore,
based on the existing technical scheme of flexible interconnection, this paper fully considers
multi-source access and multiple application scenarios, then, it carries out modeling
simulation and control strategy design under multi-working conditions, finally, a more
comprehensive power collaborative optimization is disscussed.
2. System overiew
Flexible interconnection mainly connects two or more transformers together through the
control equipment with two-way power flow, which provides power support to each other,
and shares mutual capacity [7].
The mainstream topology is that the bidirectional ACDC converters are interconnected
by the DC bus. Although the DC side of the system is mostly reserved for the interface of
photovoltaic, energy storage and other micro power supplies, no further control strategy is
designed. When multiple types of micro power supply are connected to the flexible
interconnected system, the increasing schedulable elements lead to more complicated
coordinated control. Therefore, it is necessary to consider the optimal control of multi-
source system under power balance and voltage stability and seamless switching of
working modes.
2.1 Topology Structure
At present, the power supply connected to the distribution network mainly includes
distributed photovoltaic, hydrogen fuel cell, lithium battery energy storage system,
distributed wind generator system, diesel generator (DG) and other power generations or
storage systems. Considering the actual situation, several types of multi-source equipment
will be planned and configured according to the application scenario. In this paper,
distributed photovoltaic, hydrogen fuel cell and lithium battery energy storage systems are
selected to form a multi-source access flexible interconnection system. The specific
topology is as follows.
Figure 1: Flexible interconnection system with multiple sources.
2.2 Control Strategy
The general principles of control strategy are described briefly below.
First, when the power supply load is greater than the maximum power that the multi-
source flexible interconnection system can provide, it is necessary to consider the load
situation comprehensively. According to the load grade standard, the power supply of
important load is given priority along with the unimportant load is removed.
Second, aiming at the stability, economy and environmental operation of the system,
photovoltaic power generation works in the maximum power point tracking (MPPT) mode
to make full use of renewable energy sources. Then, the working mode of lithium battery
system and fuel cell are optimized and coordinated
The specific energy management strategy is as follows: In the premise of the stable
system, the photovoltaic power generation supplies power to the load firstly. During the
peak time of power generation, if the energy of PV is remained, the battery is charged; If the
power of PV doesn’t meet the load demand, the battery is discharged firstly, the hydrogen
energy storage is discharged next. The stable operation of the system is achieved by
controlling the working mode of the energy storage converters and AC/DC converters [8-
11].
The flexible interconnection system with multi-source can be divided into the following
three working conditions according to the interactive connection status with the power grid:
(1) both AC ports are off-grid; (2) one AC port is on-grid, another is off-grid; (3) Both AC
ports are on-grid.
3. Simulation Analysis
According to the accessing scale of distributed power sources, a system is modeled
including 50kW PV, 75kW lithium battery system, 50kW fuel cell system, two 50kW AC/DC
converters and 75kW diesel generator. The maximum power supply capacity of multi-
source flexible interconnection system is 100kW. The following simulation model is shown
in the following figure, in which the parameters of irradiation intensity are designed
according to the 24-hour variation trend under sunny conditions.
Figure 2: Simulation model.
3.1 Both AC Ports are Off-grid (BPOF)
When the two AC ports of an interconnected system are both off-grid, the AC load status and
the predicted load data of the distribution grid must be fully considered. When the load is
greater than the maximum power provided by the source of the interconnected system, the
power of important load must be provided firstly. At the same time, the contact switch is
closed, then the two ACDC converters are connected. Taking into account the cost of energy
storage and fuel cell system, direct photovoltaic power supply is given priority during the
photovoltaic output period, and energy storage and fuel cell are used to supplement the
insufficient photovoltaic output.
It is obvious from the simulation results that the photovoltaic is always in MPPT working
mode with the change of irradiation intensity, and the output power is consistent with the
change of irradiation intensity. Considering the response speed, the fuel cell is set to
constant voltage working mode, and the energy demand of the load is met by adjusting the
output power of battery. Before 10:40, the total load is 40kW, which is mainly powered by
fuel cell and photovoltaic. Between 10:40 and 13:20, the total load is 70kW, at which time
photovoltaic, fuel cell and battery power the load together. After 13:20, the total load
increases to 90kW. At this time, the photovoltaic, fuel cell and battery are still used to supply
power to the load, and the power of battery is adjusted to meet the normal energy demand
of the electricity load.
Figure 3: Flow chart of BPOF.
(a) Power of source
(b) Power of grid, load and ACDC
(c) Voltage and current of 1# grid
(d) Voltage and current of 2# grid
Figure 4: Simulation results of BPOF.
3.2 Single AC Port is On-grid (SPON)
In the working condition of SPON, one AC interface is in on-grid, and the other is in off-grid.
Both AC ports are connected with flexible interconnection system, so as to realize the
uninterrupted power supply of the system.
Figure 5: Flow chart of SPON.
(a) Power of source
(b) Power of grid, load and ACDC
(c) Voltage of DC line
Figure 6: Simulation results of SPON.
The simulation results show that when an AC port is off-grid, the system can adjust the
output according to different load scenarios to meet the load requirements. At the moment,
lithium battery and fuel cell are in constant power control state. Before 5:30, the grid-
connected transformer can meet the load demand of the two transformers, and the output
of the two ACDC converters are 0. From 5:30 to 14:00, a single on-grid transformer can’t
meet the load demand, battery and fuel cell are started to supply power, and the output
power of fuel cell is reduced during the period of high photovoltaic output, thereby reducing
the power supply cost.
3.3 Both AC Ports are On-grid (BPON)
The flexible interconnection system with multi-source is in the grid-connected state and
supplies power to two AC ports
1. When the load distribution is uneven between the two transformers, they can be
connected through the flexible interconnection system. At the moment, the contact
switch is disconnected, and the load balance between the two areas can be achieved
through the power flow control on DC bus. Power supply is distributed according to
the load. Because of the economical efficiency, battery isn’t work in this condition.
2. When the two transformers are in overload state, the contact switch is disconnected.
Photovoltaic, energy storage, fuel cell provide power as the emergency power supply
to relieve the load pressure of the overload transformer, so that the load ratio of the
transformer is kept below 80%.
3. In important scenarios, when the power grid need other power supply prepared, the
flexible interconnection system with multi-source is mainly used as backup power,
energy storage and fuel cell are in hot standby state. In the event of power cut in grid,
the system can supply power to critical loads.
Figure 7: Flow chart of BPON.
It can be seen from the simulation results that the flexible interconnection with multi-
source can adjust the power output to meet the load requirements while matching the
photovoltaic output power in different load scenarios. At the moment, lithium battery and
fuel cell are in constant power control state. Before 5:30, the photovoltaic output is very low,
but the load of station 1 exceeds the rated load of transformer by 80%, then part of the over-
load is transferred to the station 2 through ACDC converters. Between 5:30 and 8:00, the
load power increases, If the two transformers are overloaded during continuous operation,
the battery should be started firstly for supplementary power supply considering the cost.
From 8:00 to 14:00, the photovoltaic output power increases, the power supply of energy
storage battery decreases. After 14:00, the photovoltaic output power decreases, and the
energy storage battery power continues to rise.
(a) Power of source
(b) Power of grid, load and ACDC
Figure 8: Simulation results of BPON.
4. Summary
Based on the existing flexible interconnection technology scheme of the distribution
network, this paper fully considers multiple application scenarios. A hybrid AC-DC power
distribution topology of two AC ports with multiple sources was built, next, the working
modes and coordinated control strategies of different forms of power supply were
optimized in different scenarios, such as photovoltaic, energy storage, fuel cells and diesel
generators. Three typical working scenarios of dual AC ports off-grid state, single AC port
off-grid state and dual AC ports on-grid state were selected for modeling and simulation
calculation, then the effectiveness and feasibility of the multi-source cooperative flexible
interconnection system control strategy were verified.
References
[1] R. Garmabdari, M. Moghimi, F. Yang, E. Gray, J. Lu, “Multi-objective energy storage
capacity optimisation considering microgrid generation uncertainties,” International
Journal of Electrical Power & Energy Systems, vol. 119, pp. 105908, 2020.
[2] B. Li, M. Chen, H. Zhong, Z. Ma, D. Liu, G. He, “A review of long-term planning of new
power system with large share of renewable energy,” Proceedings of the CSEE, vol. 2,
pp. 555, 2023.
[3] Z. Liu, Z. Deng, G. He, H. Wang, X. Zhang, J. Lin, Y. Qi, X. Liang, “Challenges and
opportunities for carbon neutrality in China,” Nature Reviews Earth & Environment,
vol. 3, pp. 141-155, 2022.
[4] Q. Qi, Q. Jiang, Y. Xu, “Research Status and Development Prospect of Flexible
Interconnection for Smart Distribution Networks,” Power System Technology. vol. 012,
pp. 044, 2020.
[5] M. Shi, J. Zhang, X. Ge, J. Fei, J. Tan, “Power Optimization Cooperative Control Strategy
for Flexible Fast Interconnection Device with Energy Storage,” Energy Engineering, vol.
120, no. 8, pp. 1885-1897, 2023.
[6] C. Wang, Q. Zhang, L. Zhang, F. Li, “The master-slave control strategy of flexible DC
interconnection system considering the capacity margin of master station,” Electric
Power Engineering Technology, vol. 42, no. 3, pp. 81-91, 2023.
[7] Y. Kong, Y. Wang, Y. Li, Z. Zhao, Y. Guo, J. Zhong, “AC-DC Bidirectional Converter-based
Flexible Interconnection for Low Voltage Side in Power Systems,” Advances in
Electrical and Computer Engineering, vol. 24, pp. 81-90, 2024.
[8] X. Wei, X. Zhu, J. Ge, Q. Zhou, Z. Zhang, Z. Li, “Improved droop control strategy for
parallel operation of cascaded power electronic transformers,” Gaodianya Jishu/High
Voltage Engineering, vol. 47, pp. 1274-1282, 2021.
[9] K. Zhou, Q. Ge, P. Ge, Y. Li, B. Wang, “The research on the control strategy of PET under
unbalanced load,” Diangong Jishu Xuebao/Transactions of China Electrotechnical
Society, vol. 33, pp. 1499-156, 2018.
[10] S. Jeong, K. Kim, J. Kwon, B. Kwon, “High-Efficiency Three-Phase Bidirectional DC-AC
Converter for Energy Storage Systems,” IET Power Electronics, vol. 12, pp. 2031-2037,
2019.
[11] J. Lai, X. Yin, X. Yin, Z. Ullah, L. Jiang, Z. Wang, “Improved Comprehensive Control of
Modular Multilevel Converterunder AC/DC Grid Faults and Harmonic Operation
Conditions,” IEEE Transactions on Power Electronics, vol. 36, pp. 6537-6556, 2021.