Analysis of Automatic and Fast Switching Scheme for Closed Loop Point of the Power Grid in the Baotou Area

The influence of the access of different power source unloop points on the power grid is summarized. Transient simulation analysis is carried out on voltage fluctuation, power flow fluctuation, short-circuit current, and voltage unbalance degree after the power grid is closed. The feasibility of the fast-switching device configured on the distribution network side is demonstrated, and the possible problems of the fast-switching device in the operation process and their influence on the power grid are analyzed. Ultimately, it lays a theoretical foundation for researching automatic and fast-switching devices under specific operating modes.


Introduction
With the rapid growth of loads and power sources, the construction speed of the power grid often cannot meet the needs in time.The fast closing operations and the fast reverse closing operations of 110 kV buses and 220 kV buses occur usually when there are partially overloaded transmission lines, maintenance modes, and specific operating modes [1].Reasonable stratification and zoning of the power supply can be achieved by setting the open loop point of the electromagnetic ring network.On this basis, a completely automatic and fast switching device can be configured to achieve mutual backup between adjacent partitions.The automatic and fast switching device plays an important role in ensuring the safe and stable operation of the power system and avoiding the occurrence of largescale and long-term power outages [2][3].
The power grid mostly presents the pattern of closed-loop structure and open-loop operation with the rapid development of the local power grid [4].When the power grid is under maintenance or a fault occurs, the loop closing and the power exchange operation are needed to improve the reliability of the power supply and avoid load loss [5].As the amplitude and phase angle of bus voltage on both sides of the power grid are generally different before closing the loop, the voltage shock will be generated at the closed loop point when the loop is closed, which will cause system disturbance.After the loop is closed, the closed loop point is supplied by both sides and the short-circuit current increases compared with the loop before being closed, which results in the risk that the short-circuit current exceeds the limitation.In addition, the system will also be disturbed during the power exchange operation.After the power exchange, the system recovers stability after a transient process and the power flow is redistributed.There is the risk that the power flow overloads and the voltage exceeds the limitation.
We take the 220 kV power grid in the Changzhou area as an example, and the incoming power supply of some 110 kV substations in the Changzhou area comes from different subdivisions.This paper proposes the use of a fast switching device for the programmed operation to solve the influence of circulation.Besides, this paper discusses the impact of key factors on loop closing during the operation process of the fast switching device, such as operation mode, the permissible time of closing the loop, the conditions before closing the loop, mode discrimination after closing the loop, and other circumstances.
It can be seen that the operation reliability of the distribution network can be improved by closing the loop and exchanging power when the power grid is under maintenance or a fault occurs.However, the system will be disturbed by the loop closing and the power exchange operation.So, it is necessary to study the impact of the loop closing and the power exchange scheme on the power flow of the power grid and bus voltage through transient simulation.It is also necessary to verify the short-circuit current level during the loop closing and the power exchange process.This paper takes the power grid in the Baotou area as an example to conduct analysis and research on the automatic fast-cutting and fast-throwing scheme at closed-loop points.

Overview of Baotou power grid
The power grid in the Baotou area is supported by six 500 kV substations, including the Gaoxin substation, the Baobei substation, the Weijun substation, the Chunkunshan substation, Bailing substation, and Meiligeng substation, shown in Figure 1.Baotou power grid is formed by four independent 220 kV power supply areas connected at 500 kV and is an important link in the west-toeast power transmission channel.
After the 220 kV Gaoma double circuit and 220 kV Zhangzhao double circuit of the Baotou power grid operate in an open loop, the 220 kV system in the Baotou area is divided into eastern regions and western regions for power supply.There are twelve 110 kV substations with two power supplies coming from the eastern regions and western regions respectively.In the case of an unbalanced startup method of units in eastern regions and western regions, the electromagnetic loop closing of some 110 kV substations may be limited.The operation mode arrangement of scheduling and accident handling are affected seriously, and the reliable power supply of the main substation in the Baotou urban area is affected, which may lead to a power outage in the urban area and important users.

Problems and solutions of Baotou power grid
At present, the power grid in the Baotou area is in the construction and improvement stage of the 500 kV power grid.The electromagnetic ring network of the 500 kV power grid and 220 kV power grid is in open-loop operation.After the open-loop operation of the electromagnetic ring network of the 220 kV power grid, the two power sources of twelve 110 kV substations have changed to come from the eastern regions and western regions respectively.In the case of an unbalanced start-up method of units in eastern regions and western regions, the electromagnetic loop closing of some 110 kV substations Baobei Substation Bailing Substation

Chunkunshan Substation
Weijun Substation Meiligeng Substation Gaoxin Substation may be limited.The operation mode arrangement of scheduling and accident handling are affected seriously, and the reliable power supply of the main substation in the Baotou urban area is affected, which may lead to a power outage in the urban area and important users.
After 220 kV Zhangzhao double circuits operate in an open loop, the 220 kV Zhaomiao substation becomes a terminal substation.Besides, the 220 kV Gubei substation and Wansheng substation are located in the wind power gathering area in the north of Baotou.220 kV Gubei substation and the Wansheng substation operate separately to avoid the operation of the electromagnetic ring network of the 500 kV power grid and 220 kV power grid, but the operation mode is weak.
Reasonable stratification and zoning of the power supply can be realized by setting the open loop point of the electromagnetic ring network.On this basis, a completely automatic and fast switching device can be configured to achieve mutual backup between adjacent partitions.The automatic and fast switching device plays an important role in ensuring the safe and stable operation of the power system and avoiding the occurrence of large-scale and long-term power outages.The automatic and fast switching devices of the closed loop point are installed in the Zhaomiao substation, a new energy gathering substation including the Gubei substation and Wansheng substation, to put into 220 kV Zhangzhao double circuit transmission lines automatically after Shazhao double circuit transmission lines loss power.In addition, it also can improve the power supply reliability of 120 MW load in the urban area of the Zhaomiao substation and improve that of the Moni substation powered by the Zhaomiao substation planned in the future.
Therefore, this paper analyzes the influence and feasibility of adopting the automatic and fast switching technology of closed-loop points in new energy-gathering substations, including the Gubei substation and Wansheng substation on new energy station in detail.The above analysis aims to achieve emergency backup for the 110 kV closed-loop point of the district power supply and first stop and then reverses when the operation in normal operation mode is adjusted.

Simulation analysis and verification of the closed-loop switching scheme in the power grid of the Baotou area
The system is modeled by power system simulation software based on the actual operation of the Baotou power grid.Through simulation calculations, the impact of applying fast backup operation on the power grid is analyzed in those areas, such as the possible 110 kV closed-loop point of the zonal power supply, 220 kV power grid unloop point, and wind power consolidation and split operation substation including 220 kV Gubei substation and Wansheng substation.Besides, the feasibility of applying automatic devices is also analyzed.The impact on new energy stations should be analyzed with emphasis.
The simulation calculation is completed by PSD Power Tools of China Electric Power Research Institute, mainly including the PSD-BPA power flow calculation and analysis program, PSD-BPA transient stability calculation and analysis program, and PSD-SCCPC power system short-circuits current program.The generator adopts a detailed model that takes into account sub-transient potential changes and considers the dynamic characteristics of the exciter, prime mover, and governor.The load of each regional power grid adopts a comprehensive load model of 60% motor and 40% constant impedance.

Dynamic simulation and verification of power flow and bus voltage
Dynamic simulation and verification are carried out for the following substation closed loop switching schemes for the power grid in the Baotou area.Two scenarios are considered in each substation, which are that the load of bus I is powered by bus II, and the load of bus II is powered by bus I.The simulation process is as follows: the tie switch is closed at 1.00 s and the feeder switch is open after 1.08 s.
There are 21 110 kV transmission lines and 15 ways of the electromagnetic loop.We take the fast switching analysis of the 110 kV Hexi substation as an example, and the buses of the 110 kV Hexi substation operate respectively.Bus I of Hexi substation is powered by a bus of 220 kV Zhaomiao substation and bus II of Hexi substation is powered by a bus of 220 kV Zhangjiaying substation.The typical power flow is shown in Figure 2. substation and Hexi substation changes from 47.43 MW to -55.62 MW abruptly.The power of bus II in Zhangjiaying substation and Hexi substation changes from 1.68 MW to 111.09 MW abruptly.The feeder switch of bus I of the Hexi substation is open at 1.08 s and bus I of the Hexi substation is powered by Zhangjiaying substation instead.The power flow of the system changes abruptly.The power of bus I in the Zhaomiao substation and Hexi substation changes abruptly and recovers to 0 MW.The power of bus II in Zhangjiaying substation and Hexi substation changes abruptly and recovers to 49.27 MW.

Simulation and verification of short-circuit current
During the activation process of automatic and fast switching of the bus in 110 kV Hexi substation, the short-circuit current of the relevant bus is verified according to three-phase short-circuit and singlephase short-circuit respectively.According to the calculation of the three-phase short-circuit current, the short-circuit currents of the bus I and bus II in 110 kV Hexi substation are 8.49 kA and 7.24 kA respectively, before the operation of the closed-loop switching device in 110 kV Hexi substation.The short-circuit current of the Hexi substation is 15.60 kA after the tie switch is closed.The short-circuit current of the Hexi substation is 7.26 kA after the feeder switch of the bus I action.The short-circuit current of the Hexi substation is 8.50 kA after the feeder switch of bus II actions.According to the calculation of single-phase short-circuit current, the short-circuit currents of the bus I and bus II in 110 kV Hexi substation are 9.71 kA and 7.61 kA respectively before the operation of the closed-loop switching device in 110 kV Hexi substation.The short-circuit current of the Hexi substation is 17.20 kA after the tie switch is closed.The short-circuit current of the Hexi substation is 7.70 kA after the feeder switch of the bus I action.The short-circuit current of the Hexi substation is 9.20 kA after the feeder switch of bus II actions.In summary, during the activation process of automatic and fast switching, the maximum short-circuit current of bus I in the Hexi substation is 17.20 kA, and the maximum short-circuit current of bus II in the Hexi substation is 17.20 kA.The short-circuit currents do not exceed the switch-breaking capacity.

Simulation and verification of the maximum allowable unbalance degree of buses on both sides before closing the loop in the new energy station
The voltage fluctuation caused by loop closing operation is difficult to cause the wind turbine to enter the low voltage ride through and high voltage ride through, which can be disregarded.We focus on observing whether loop closing operation will cause the bus voltage of the new energy station to exceed the allowable range under the condition of considering the voltage and reactive power adjustment ability of the new energy station.It is required to improve the control ability of voltage and reactive power in the new energy station or adjacent stations if the bus voltage of the new energy station exceeds the allowable range.We take the Baoli wind power station as an example, and the power flow is shown in Figure 4 when the Baoli wind power station generates maximum power.

Conclusion
This paper is based on the actual operation of the Baotou power grid.Through simulation calculations, the impact of fast backup operation on the power grid is analyzed in those areas, such as the possible 110 kV closed-loop point of the zonal power supply, 220 kV Zhangzhao double circuits, Shazhao double circuits, Gubei substation, and Wansheng substation.This paper sets the closing and loop closing control strategy through simulation analysis, analyzes the influence of loop closing impact, and formulates the chain action control logic.Through the results of simulation analysis, it can be found that power flow transfer and bus voltage fluctuation caused by loop closing operation under the normal operation mode of the power grid is small, which will not affect the safe and stable operation of the power grid.The research in this paper provides basic data for the development of automatic and fast-switching technology under specific operating conditions.

Figure 1 .
Figure 1.Structure of Baotou power grid

Figure 2 .
Figure 2. Typical power flow of 110 kV Hexi substation The tie switch between bus I and bus II of Hexi substation is closed at 1.00 s.The feeder switch of bus I of the Hexi substation is open at 1.08 s.The changes in related bus voltage and transmission line current are shown in Figure 3.The simulation results are listed in Tables1 and 2, respectively.

Figure 4 .
Figure 4.The power flow when Baoli wind power station generates maximum power The change of related bus voltage is shown in Figure 5 (a) when the load of bus II is transferred to bus I in the 110 kV Xiashihao substation.The maximum voltage fluctuation of the Baoli wind power station is 1.09 kV, which meets the relevant regulations of "Technical Guidelines for Power System Voltage and Reactive Power".The change of the bus voltage in the Baoli wind power station is shown in Figure 5 (b) when a load of bus I is transferred to bus II in the 220 kV Gubei substation.The maximum voltage fluctuation of Baoli wind power station is 12.14 kV and the fluctuation range of bus voltage is from 0.98 p.u. to 1.03 p.u.It can be seen that the loop-closing operation mainly affects the voltage at the closed loop point and the voltage fluctuation at the point of common coupling of the wind turbine is small.

Table 1 .
Changes in bus voltage before and after closing the loop in the 110 kV Hexi substation

Table 2 .
Changes of power before and after closing the loop in the 110 kV Hexi substation