Flexible closed-loop control device and control method for distribution network based on hybrid phase shifting transformer

With the increasingly complex structure of the distribution network and the increasing demand for power supply quality from the user side, relying solely on the original grid layout for power supply can no longer meet existing needs. Closed loop power supply can achieve uninterrupted load to improve power supply reliability. This article proposes a flexible loop closing device based on HPST for the case of large voltage phasor differences on both sides of the busbar loop closing point in the distribution network and proposes corresponding flexible loop closing and load transfer strategies based on the characteristics of HPST. Finally, a simulation of loop closure and power control was conducted based on the 10 kV distribution network loop closure scenario, verifying the feasibility of the HPST flexible loop closure device and the effectiveness of the control strategy.


Introduction
With the massive access to new energy and power electronic equipment in recent years, the quality of power supply in distribution networks has been seriously challenged.Distribution networks are directly oriented towards users, and controlling and ensuring their safe operation is the key to ensuring the quality of power supply.The distribution network is designed with a closed-loop structure and operates with an open-loop power supply, which greatly improves the reliability of the power supply [1][2].However, when equipment such as transformers and transmission lines are overhauled or fail, for important loads with demanding power supply quality, the open-loop power supply cannot avoid the impact of power outages caused by switching operations [3].If the closed loop is forced, there may be a large voltage difference and phase difference at the connection closing point of the distribution network.When the closed-loop structure operates directly, there may be a large impact current, causing greater risks such as equipment overload, protection device tripping, and more serious power outage accidents [4].
Currently, domestic research on distribution network loop-closing devices is still in the theoretical stage, and there is less research on loop-closing scenarios where the voltage amplitude and phase difference at the loop-closing point are large.In [5], a loop-closing device is proposed based on a phase shifting transformer, which can adjust the voltage phasor difference on both sides of the loop closing point, but the voltage regulating winding and the phase regulating winding are coupled to each other.In [6], the use of dual VSC loop closing devices can quickly transfer uninterrupted load, but the high cost and large floor space of power electronic equipment limit its practical application.In [7], a 2 loop closing device is proposed for a rotating power flow controller, which can continuously adjust the voltage phasor, but its adjustment accuracy depends on the accuracy of the rotating motor.Due to the inertia of the rotor, it is easy to cause current oscillations at the loop closing point.In [8], flexible DC equipment is proposed to flexibly control voltage phasor regulation.The above loop-closing devices are designed when the voltage at the loop-closing point is symmetrical.Due to the inflexibility and slow speed of traditional phase shifters in voltage and phase adjustment, they cannot cope with rapid changes in load.Due to the large footprint, high cost, and difficulty in later operation and maintenance of loop closing devices based on power electronics technology [9], a hybrid phase shifting transformer (HPST) loop closing device that combines the advantages of the two devices is expected to solve the flexible loop closing problem in complex scenarios.
This article focuses on typical closed-loop scenarios in medium voltage distribution networks.Firstly, the topology structure and working principle of HPST are elaborated.Based on this, the optimal capacity and control strategy for the electromagnetic and power electronics parts are proposed.Finally, simulation analysis is conducted on the closed-loop current situation by using this device, verifying the economy and feasibility of the proposed flexible closed-loop device.

Structure and principle of HPST ring closing device
At present, the network structure of China's medium voltage distribution network is mainly radial, with different connections from higher-level power sources, high and low voltage sides of the main transformer, as well as differences in the structure and load of the distribution network, resulting in arbitrary values of voltage on both sides of the closing point.If the loop is forcibly closed, the voltage phasor difference on both sides of the loop closing switch will quickly decrease to zero, and the steady-state and impulse current of the loop closing may cause protection malfunction.
At the closing point, we construct the closing equivalent circuit, as shown in Figure 1.When the parameters of the closing line and the voltage on both sides of the closing point are known, the steadystate current of the closing loop is determined by KVL, as shown in Equation (1).
where c I represents the steady-state current of the closed loop; 1 U and 2 U represents the voltage on both sides of the closed loop point; Z1 and Z2 represents the equivalent impedance of the line; HPST U and ZHPST represents the equivalent voltage and impedance of the HPST.

HPST structure and access method
The HPST loop closing device is shown in Figure 2, mainly composed of HPST, closing switch (QF), circuit breaker (Kb), voltage transformer (TV), current transformer (TA), and corresponding control The HPST topology is shown in Figure 3.It is composed of a combination of large-capacity Sen transformers and small-capacity UPFC devices.UPFC is designed to compensate for the ST regulation range and expansion and can achieve a controllable voltage source with amplitude UHPST (0≤UHPST≤UHPSTmax) and phase angle δHPST (0≤δHPST≤δHPSTmax), thereby achieving load transfer without impulse closing.

Working principle analysis
Before closing the loop, power supplies 1 # and 2 # operate in separate rows, supplying their respective loads separately.When a certain power supply or switch needs to be repaired and maintained, it is necessary to transfer this load from another power supply through the closing device for load transfer.Whether to achieve very small or even no impact is the key to effective closing.In the case of three-phase symmetry, the compensation voltage on the series side of each phase includes the compensation voltage generated by ST and the compensation voltage generated by UPFC.The compensation relationship is shown in Figure 4.The voltage phasors of the secondary winding on the ST side are stepa U , stepb U , and stepc U , respectively, so the equations for the transmission voltage on the ST series side are as follows [10]: where kx is the gear of phase x.

Capacity analysis
HPST is an equivalent voltage source in the line, and the maximum series compensation voltage calculated by the cosine theorem is based on the maximum phase angle difference that may occur on both sides of the tie line closing point.(3) If the maximum current passing through the connecting line is equal to Icmax on the load side to be transferred, the capacity of HPST will be: The HPST capacity is the sum of ST and UPFC capacities.It is necessary to achieve full coverage of voltage amplitude and phase angle compensation to ensure continuous and smooth adjustment of voltage, which cannot accelerate the response speed and expand capacity when the ST gear exceeds a certain number.Therefore, it is necessary to consider the optimal ratio of HPST and UPFC to reduce the cost of the entire device.
To further obtain the optimal capacity ratio between ST and UPFC, we set the ST level voltage Ustep as 1 p.u.Since the current flowing through the secondary side ST and UPFC series sidelines is I, the capacity ratio between them is the voltage ratio, which is: (5)

Control strategy for closed-loop power conversion
The series part is the core part of the entire loop-closing device.The compensation voltage of the series part needs to be adjusted to complete the loop closing, load transfer, and exit of the loop closing transformer.The loop closing and power transfer through a flexible loop closing transformer mainly consists of three steps: (1) Loop closing To reduce the closing current and the molecules of Equation ( 1), the voltage difference on both sides of the closing point is measured through a voltage transformer, and the closing is completed by compensating for the voltage through a flexible closing transformer.
(2) Load transfer Based on the equivalent model of the closed-loop transformer in Figure 1, the voltage equation in the d-q coordinate system on both sides of the closed-loop point is obtained.According to the load to be transferred, the HPST is adjusted for power transfer, with Pref = PL2 and Qref = QL2.The d-axis is used as the reference axis.At this time, the series line current is stored as follows: According to the instantaneous reactive power theory, we can obtain: The specific control strategy diagram of HPST is shown in Figure 5.According to the load to be transferred, the required reference value of series compensation voltage is obtained, and the compensation voltage required for UPFC and ST is determined based on their respective capacity ratios to complete the load transfer.
(3) Device exit After the load transfer is completed, we exit HPST step by step, close the bypass switch K1, and achieve the entire device-to-exit operation.

Simulation verification
To verify the correctness and effectiveness of the HPST flexible loop closing device proposed in this article, a corresponding simulation model was built in MATLAB/Simulink based on a typical loop closing and supply transfer scenario of a 10 kV distribution network in a certain region.
In the simulation, it is assumed that the 2nd power supply side line is undergoing maintenance, the voltage amplitude of power supply 1 is 10.5 kV, the initial phase angle is 0°, and the internal resistance is 0.23 Ω.The voltage amplitude of power supply 2 is 9.5 kV, the initial phase angle is -15°, and the internal resistance is 0.23 Ω.The active power of Load 1 is 1.1 MW, the reactive power is 0.4 Mvar, the active power of Load 2 is 4.7 MW, and the reactive power is 0.9 Mvar.Control parameters Δθ=0.5°,ΔU=0.1 kV.Parameters of closed-loop device of HPST are shown in Table 1.We perform simulation analysis according to the operation steps of closing the loop and transferring the power supply mentioned above.Before starting the adjustment, we measure that the voltage amplitude difference ΔU on both sides of QF4 is 2.87kV and phase angle difference Δθ is -20.5°.We adjust the ST tap and UPFC at 0.2s-1s, the voltage amplitude difference ΔU on both sides of QF4 decreases to 0.08kV, and phase angle difference Δθ reduces to -0.2°, as shown in figures 6 and 7. We adjust the voltage phasor difference on both sides of QF4 to basically zero before and after adjustment and have no impact on closing conditions.At Second 1, QF4 closes, and the maximum peak value of the closing impulse current flowing through the HPST is 10 A, as shown in Figure 8.Compared to the load current of 304 A, it can be seen as a nonimpact closing.The electrical connection between Load 2 and power supply 1 is established through HPST, but the active power PL2 and reactive power QL2 flowing through HPST are both very small.Load 2 is still powered by power supply 1.We adjust the HPST again between 1.2 and 1.6 seconds to reduce the output power of the power supply 2. At 1.8 seconds, the active power and reactive power of power supply 2 were 5.2 kW and 1.1 kvar, respectively.Most of the power was transferred through the HPST.At this time, QF3 was disconnected and power supply 2 exited.Figures 9 and 10 show the power changes of power supply 1 and 2.    After the power supply 2 is disconnected, the amplitude difference ΔU on both sides of the bypass switch K1 is 1.42 kV, and phase angle difference Δθ adjusts HPST by -10.1°at 2.0-2.4 seconds.The voltage amplitude difference ΔU on both sides of K1 decreases to 0.05 kV, and phase angle difference Δθ reduces to -0.3° and has no impact closing conditions, as shown in figures 11 and 12.At this point, K1 and HPST will exit as a whole.

Conclusion
To address the phase angle difference of active distribution network lines, a transformer loop closing device of an electromagnetic hybrid phase-shifting is proposed.Firstly, based on the structure and principle of the HPST device, a control strategy for loop closing and load transfer is proposed, which can flexibly adjust the voltage amplitude and phase, effectively reduce the loop closing impulse current, and perform load transfer to meet the demand of no power outage on the load side.In addition, in a 10 kV distribution network system, compared to traditional phase-shifting transformers and power electronic loop closing devices, HPST combines the advantages of both, taking into account voltage regulation range, accuracy, and faster response speed, which has good economy and promotion.

Figure 9 .
Figure 9. Active and reactive power of power supply 1.

Figure 10 .
Figure 10.Active and reactive power of power supply 2.