Introduction
Power-Electronic Transformers (PETs), as a novel type of power transformer, have attracted increasing attention from researchers both domestically and internationally. They are transformer devices that incorporate power electronic converters and achieve magnetic coupling through a high-frequency transformer. While performing conventional functions of voltage transformation, isolation, and energy transmission, PETs can also act as power quality controllers, making them a multifunctional new type of transformer. Their application in power distribution systems can achieve both voltage reduction and ensure power quality.
Parallel operation of two or more transformer servo systems (PETs) is an important operating mode for transformers and has significant research value. However, current research on PETs, both domestically and internationally, mainly focuses on their topology and control strategies, while research on their application in power systems and their operating characteristics is relatively weak. This paper employs a master-slave control scheme to solve the parallel current sharing problem on the AC output side of parallel PETs; and conducts simulation studies on the dynamic processes of parallel PETs experiencing step load changes and nonlinear loads. This paper studies the control strategy for load distribution using parallel PETs in power distribution and performs dynamic simulations of typical operations.
1. Basic structure and control strategy of PET
The basic topology of PET (Polyelectric Transformer) is divided into AC-AC-AC converter and AC-DC-AC-DC-AC dual DC converter. The former has a simple structure but low controllability; the latter has a complex structure, a sophisticated control strategy, and is more practical. A typical AC-DC-AC-DC-AC dual DC topology is shown in Figure 1.
The voltage-source PWM rectifier circuit on the primary side of the PET transformer employs decoupled voltage and current dual closed-loop control. Regardless of whether the transformer load is inductive or capacitive, as long as it is within a certain range, the power factor of the power grid can approach 1. The single-phase inverter circuit on the primary side achieves high-frequency inversion using open-loop control. To reduce the size and weight of the transformer, high-permeability magnetic cores such as ferrite are used. The secondary rectifier circuit is used to achieve high-frequency rectification. Since bidirectional energy flow is not considered for distribution transformers, an uncontrolled rectifier circuit is used. To output a constant voltage and constant frequency AC voltage, the PET secondary inverter circuit uses voltage closed-loop control.
2. PET Parallel Operation Control Principle
Parallel operation of two or more PET inverters is an effective way to improve system reliability and expand capacity, but research on parallel operation of PET inverters is not yet in-depth. The working principle of PET secondary inverters is the same as that of UPS inverters, while there are relatively rich results on the parallel operation of multiple UPS units [5-7], which can be used as a reference when studying the parallel operation of PET inverters.
The proposed parallel control methods for PETs mainly include: centralized control, master-slave control, distributed logic control, and control without interconnection lines[2]. This paper mainly studies the parallel operation of two PETs without interconnection lines. Figure 2 is a structural diagram of a parallel system of two PETs, which are originally connected to the same common bus.
To avoid circulating current in the parallel transformers, the frequency, amplitude, and phase of the secondary voltage of each PET unit must be kept consistent. To achieve a stable distribution of active and reactive loads among the parallel transformers, each PET unit should have active droop and reactive droop characteristics. The control structure diagram of a PET secondary inverter with droop characteristics is shown in Figure 3.
The frequency, amplitude, and phase of the PET secondary voltage depend on the sinusoidal modulation signal of the inverter's PWM pulse. The characteristics of the sinusoidal modulation signal are related to the frequency setpoint f0, the phase setpoint ρ0, and the amplitude setpoint. f0 = 50Hz is taken to ensure the rated frequency. ρ0 corresponds to the initial phase angle of the voltage when the active load P0 is reached (generally taken as 0, with an active compensation coefficient Kp > 0), thus forming the active droop characteristic ρ = ρ0 - KpP (1).
U0 corresponds to the voltage amplitude when the reactive load Q=0. By introducing a reactive compensation coefficient KQ＞0, a reactive droop characteristic U=U0-KQQ (2) can be formed.
For each PET operating in parallel, the values of ρ0 and U0 should be the same. Due to the introduction of active and reactive power compensation, when the load changes, each PET operating in parallel will automatically adjust the phase angle and amplitude of its output voltage, and automatically realize the stable power distribution among the transformers. In order to reasonably distribute the load according to the transformer capacity, the per-unit values of Kp and KQ of each PET based on its own capacity should be equal, generally taken as 0.01 to 0.05.
Frequency droop characteristics are used to distribute active power among parallel PETs and inverters. Obviously, under this control method, the supply frequency cannot be maintained at 50Hz under different loads; however, to ensure frequency quality, the frequency droop coefficient must be very small, which is not conducive to the stable distribution of active load among parallel PETs. In contrast, this paper uses initial phase angle droop characteristics, which can maintain constant frequency supply and allow for the selection of a reasonable droop coefficient as needed, achieving a stable and reasonable distribution of active load. The output voltage frequency of each PET participating in parallel must be equal to 50Hz to ensure normal operation. In Figure 3, this can be achieved by using closed-loop PI control for the frequency.
The parameters of parallel-operated PETs may not be entirely consistent, most commonly due to differences in the inductance parameters of the current-limiting reactors or connecting lines. The voltage measurement point in Figure 3 is intentionally located on the common busbar to ensure a stable and reasonable power distribution among the parallel PETs, even with inconsistent PET parameters. This cannot be guaranteed if the voltage measurement point is located at the output terminal of each PET.
3. Simulation Analysis
This paper uses Matlab 6.5/Simulink to build a simulation model and simulates the parallel operation of two PETs with the same parameters. The main system parameters are: PET2 rated capacity 10kVA, rated voltage 240/110V; PET2 rated capacity 10kVA, rated voltage 240/110V; system frequency 50Hz; high-frequency transformer frequency 1000Hz; IGBT switching frequency 9000Hz; KP and KQ are both taken as per unit value 0.01; frequency setpoint f0 is taken as 50Hz; phase setpoint ρ0 is taken as 0; amplitude setpoint U0 is taken as per unit value 1.0.
3.1 Two PET units are put into parallel operation simultaneously (Case 1)
At 1.0s, the two PET units were switched on from no-load operation to parallel operation on the low-voltage side, undertaking a comprehensive load with a power factor of 0.8. The relevant variable waveforms are shown in Figures 4-6. As can be seen from the figures, the waveforms of the corresponding variables for the two PET units are consistent. After parallel operation, the load current they bear is equal, achieving current sharing control and stable distribution of active and reactive loads, while maintaining a constant frequency.
3.2 PET2 is added to parallel operation (Case 2)
PET1 operates under load, and PET2 is switched on from no-load status at 1.0s, with the two PETs operating in parallel. The relevant waveforms are shown in Figures 7 and 8. As can be seen from the figures, after PET1 switches from stand-alone operation to parallel operation, its load current, active power, and reactive power all decrease. The decreased load is then absorbed by PET2. Ultimately, current sharing control and stable distribution of active and reactive power loads are achieved between the two parallel PETs, exhibiting good dynamic response performance.
4. Conclusion
This paper establishes a PET control strategy and model based on the active and reactive power droop characteristic equations, and conducts a simulation study on the dynamic process of PET parallel operation based on this model. Simulation results show that the proposed control strategy can achieve stable distribution of active and reactive power loads while maintaining the rated power supply frequency, and exhibits good dynamic characteristics.
