The load-controllable transformer-type adjustable reactor can rapidly and continuously adjust its inductance, offering excellent control accuracy and linearity while generating minimal harmonics. As such, it finds applications across various fields. However, the voltage on the DC side of the reactor’s inverter bridge significantly impacts its performance. Using DC power on the DC side enhances the reactor's performance but inevitably raises the cost of the system and restricts its applicability. Conversely, employing capacitors as the DC power supply reduces costs and broadens the reactor's range of use, yet it necessitates careful voltage management to ensure the reactor operates correctly. During startup, designing a charging circuit for the capacitor is essential to bring the reactor into operational readiness. This research aligns with the National Natural Science Foundation of China (project No. 50777066), focusing on the application of power electronics in power systems.
A key challenge lies in the primary equivalent reactance of the reactor, which heavily depends on the transformer's body parameters. Accurate control of the inductance requires determining these parameters, yet they often vary due to manufacturing inconsistencies and operational conditions, complicating the development of a robust control system.
To address these issues, we propose a control system that doesn’t rely on the transformer’s body parameters and can stabilize the capacitor voltage on the inverter bridge’s DC side without requiring a specialized charging circuit. This ensures the reactor operates efficiently under varying conditions.
When the transformer functions as a reactor, its primary equivalent resistance is much smaller than its equivalent reactance. Thus, the ratio of the primary fundamental voltage to the current amplitude represents the reactor's equivalent reactance (X). Since both voltage and current contain harmonic components, extracting the fundamental component of either voltage or current is crucial for real-time monitoring of the reactance value.
For fundamental component extraction, the system employs a resonant link (G1), an inertial link (G2), and a proportional link. These form a subsystem (Gb) that exhibits specific amplitude and phase-frequency characteristics. When the input signal frequency is 50Hz, Gb’s gain is 10dB with zero phase shift. At 49Hz, the gain drops slightly to 0.95-0.446dB, with a phase shift of 3.44°. At 51Hz, the gain increases slightly to 1.05. Even when the fundamental frequency shifts, Gb’s output can accurately track the fundamental component of the input signal.
To enhance the system’s ability to suppress the second harmonic while preserving other harmonics, a delay link (D) with a delay time of 10ms and a proportional link (K) with a proportional coefficient of 0.5 are incorporated. This setup effectively eliminates the second harmonic and DC component in the signal, ensuring that the second harmonic component in the output signal remains minimal, regardless of frequency shifts. This design ensures superior frequency selectivity, effectively attenuating the DC and higher harmonic components while allowing the fundamental frequency signal to pass, thereby improving the extraction of the fundamental component.
The control of the DC-side capacitor voltage of the inverter bridge is critical for the reactor's performance. The current source is achieved using a single-phase inverter bridge. When /sinwt, the control kksinwt, where the real part is the in-phase component. The transformer operates as a reactor when i2 is in phase or reverse phase with i1, and its reactance value depends on this relationship. Effective control of Ud is vital for the reactor's normal operation, as it determines the energy exchange between the current source and the capacitor.
During each power cycle, the energy absorbed and released by the current source determines the capacitor voltage change, which is equal to the average power consumed. If i2 has only the in-phase component, the average power consumed over a T is P1 = k1^2R2/ω^2. Due to the presence of R2, Ud will decrease when the transformer operates in the reactor state. Without proper control, this could impair the reactor’s performance. In practical systems, R2 is small, resulting in a gradual decrease in Ud.
The system also monitors the reactance value in real time using a PI regulator, with the error between the measured and given values serving as the input signal. The output signal of the PI regulator controls the in-phase and vertical components of i2, thereby regulating the inductance value. This approach effectively extracts the fundamental components of the system, eliminating harmonic influences and ensuring stable operation.
Simulations confirm the effectiveness of the proposed method. When the input signal is a square wave, the output is a 50Hz fundamental signal. Fourier analysis shows that the fundamental phasor in the input signal is 560.87e-10617, while the output signal’s phasor is 560.805e-20645. These results validate the system’s ability to track the given reactance value accurately. Adjustments to Ud during operation demonstrate the system’s adaptability and stability.
In conclusion, the proposed control system offers a reliable solution for managing the DC-side capacitor voltage, enhancing the reactor's performance while reducing costs and expanding its application range. This approach demonstrates strong potential for practical implementation in power electronic systems.
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