Longcheng Tan

A new digital control method for high performance 400 Hz ground power unit

In digital control, the one-sample delay for computation which is often ignored in 50/60 Hz cases has great impact on the performance of a 400 Hz ground power unit (GPU). Taking the delay into account, a detailed proof that it is very hard and even impossible for 400 Hz GPU to maintain a low THD content in the output voltage using the conventional inductor, capacitor or load current feedback based single-loop or double-loop control is given. A new primary-plus-resonant-controller method suitable for fixed-point DSP without modifying the existing single-loop or double-loop control topology is presented. With the new approach, the computation delay is not compensated, which means that complex computation will be avoided. Meanwhile, the relationships between the discrete realization method in other literatures and the zero order hold method for resonant controllers have also been discussed. Two types of tests on a 90-kVA 400 Hz GPU prototype based-on a 16-bit fixed-point DSP show satisfactory performance of the new method feeding linear / nonlinear load.

High performance controller with effective voltage balance regulation for a cascade STATCOM with star configuration under unbalanced conditions

A high performance controller with effective voltage balance regulation is proposed for a cascade STATtic synchronous COMpensator(STATCOM) with star configuration. Zero-sequence voltages are injected to the output voltages of STATCOM in order to compensate the unblanced load currents, which can also decouple the star-connected three-phase STATCOM system into three single-phase systems. Additionally, an effective voltage balance control method and a robust current tracking technique are proposed to guarantee all capacitor voltages balanced and to ensure good current tracking ability. Moreover, a quick and accurate current extraction method is proposed to obtain the negative and reactive currents of unblanced loads to improve the systems dynamic response. The proposed control method is validated on a 10kV/±1Mvar cascade star-connected STATCOM industrial prototype. All the experimental results verify that with the proposed control methods the STATCOM can compensate unblanced load currents with quick dynamic response while keeping all capacitor voltages balanced.

An Overmodulation Method for Space Vector PWM Current Source Inverters

Operating current source inverters (CSI) in the overmodulation range extends their current and power ranges as the voltage source inverters do. This paper presents a simple overmodulation algorithm for space vector pulse width modulation (PWM) suitable for CSI. The proposed algorithm views the operation area as a whole, and allows smooth transitions through the entire operating range up to six-step operation. In addition, harmonic components of the output current is analyzed. When the technique is applied to the CSI-based uninterruptible power supplies (UPS), quicker dynamic response and higher output power are obtained, and a smooth operation during transition between the linear control range and the six-step mode is also demonstrated through simulation results.

High performance controller for voltage-controlled current source inverter with nonlinear loads

This paper presents an advanced voltage control method for voltage-controlled current source inverter(VC-CSI) with nonlinear loads. The proposed technique adopts the proportional sinusoidal signal integrator (PSSI) controller to achieve a rather good voltage tracking ability under nonlinear load conditions, which can be implemented directly in the stationary αϟ frame with no need of the rotating coordinate transformation. This method can selectively eliminate the harmonics of the output voltage and does not require accurate circuit parameters, which can improve the control systems flexibility and robustness. Both the control principle of the proposed technique for the whole system and the influence of the PSSI controllers parameters on the control performance have been discussed in detail. Finally, all these analyses and the effectiveness of the proposed control method are firstly validated by simulation, and then are verified further in a 5kVA laboratory prototype.

The optimized strategy for input current harmonic of low switching frequency PWM rectifier

The industry application of High-power medium-voltage drives converter is an appealing spot in Power electronics technology in recent years. This paper presents an optimized control strategy for input current harmonic of rectifier of three-level NPC back-to-back converter under low switching frequency conditions. A modified P-N SVPWM method includes asymmetrical regular sampling, even-order harmonic elimination and natural DC bus balancing is applied for optimize the THD performance of input currents. And a modified proportional sinusoidal signal integrator method based on control scheme in two phase stationary reference frame which is different from ordinary direct-quadrature (DQ) rotating reference frame is proposed. It provides a specific input current harmonic elimination performance especially considerable in low switching frequency circumstance. Simulation and experimental results based on a rectifier of a 75kVA three-level NPC back-to-back converter prototype with a 400Hz device switching frequency constraint show satisfactory performance.

An improved control method for PWM current source rectifier with active damping function

The pulse width modulation (PWM) current source rectifier requires an LC filter to filter out the line current harmonics and to assist the commutation of switching devices, however, the use of the filter may affect the input power factor and cause LC resonances as well. In order to overcome the two drawbacks the LC filter brings, an improved control method for PWM current source rectifier with active damping function is proposed in this paper. The proposed power factor control method is based on the source voltage oriented d-q rotating frame, which is parameters insensitive and can make the rectifier get the maximum achievable power factor. Meanwhile, in order to suppress the LC resonance and to reduce the line current total harmonic distortion (THD) without the physical damping resistor, the active damping technique is adopted, which is realized together with the power factor control, directly through detecting the capacitor voltage. All these analyses and the effectiveness of the control method are firstly demonstrated by simulation results, and are verified further in a 5kVA laboratory prototype.

Improved NP-SVM suitable for multi-sampling used in high power NPC rectifiers without even order harmonics

In order to eliminate the even order harmonic currents of the NPC rectifiers, the modified SVM (short for NP-SVM) is proposed in the past. However, when the NP-SVM is used in the multi-sampling system, there will be direct jumps between N and P which can result in narrower pulse width, higher dv/dt, even the destruction of devices, which is very undesirable. In this paper, the reasons of direct NP jumps generation are briefly analyzed at first, then an improved NP-SVM(INP-SVM) suitable for multi-sampling system is proposed to overcome the problems in high power three level NPC rectifiers. Finally, all these analyses and the effectiveness of the proposed NP-SVM are firstly validated by simulation, and then are verified further in a 75kVA laboratory prototype.

A new PWM strategy for three-level Active NPC converter

Three-level neutral point clamped voltage source converter (NPC VSC) is widely used in high-power high-voltage applications. The main drawback of unequal loss distribution among the semiconductors of NPC VSC is overcomed by using active NPC (ANPC) VSC. The ANPC VSC can provide more commutations than NPC VSC and increase the maximum output power or the converter output frequency. In this paper, based on the conventional PWM strategy, a new PWM strategy for Three-level ANPC is proposed. Simulation results show that this new PWM strategy can provide more equal distribution of losses among the switches than the existing PWM strategies for Three-level ANPC VSC.

Three-level NPC back-to-back converter with IGCT for high power AC drive

This paper presents a three-level neutral point clamped (NPC) back-to-back converter with IGCT for high power ac drive. The power circuit topology of the three-level NPC back-to-back converter and the hardware control system based VMEBUS and TMS320F28335 DSP are introduced. Voltage-Oriented Control (VOC) with feed-forward decoupling control for the front-end PWM rectifier and rotor field oriented control (FOC) for the back-end inverter are adopted respectively. Meanwhile, power feed-forward control and neutral point potential balancing control method are introduced. Experimental results show that the three-level NPC back-to-back converter with IGCT exhibits excellent performance with the proposed control scheme.

The improved voltage balancing control for high power three-level NPC rectifiers under standby and light load conditions

The voltage balancing control (VBC) ability of the three-level neutral point clamped (NPC) rectifier under standby and light load conditions is weakened because the phase current direction changes very quickly and the self-balancing ability of conventional space vector modulation (SVM) is not ensured. In order to improve the VBC ability, current prediction method together with the multi-sampling technique is proposed to obtain the current polarity information of next PWM carrier period so as to regulate the time assignment of small vectors correctly. Meanwhile, in order to improve the self-balancing ability of SVM for relieving the burden of VBC, the synchronized NP-SVM is adopted and one synchronization method is proposed in this paper. All these analyses and the effectiveness of the proposed method are firstly verified by simulation results, and are validated further in a 75kVA laboratory prototype.