Fanqiang Gao

Single-Loop Digital Control of High-Power 400-Hz Ground Power Unit for Airplanes

In this paper, the influence of one-sample delay for sampling and computation in digital control on the bandwidth of the inner current loop of a 400-Hz ground power unit (GPU) is analyzed first. The results show that it is difficult and even impossible for high-power 400-Hz GPUs to maintain low total harmonic distortion content in the output voltage with the conventional proportional-integral-based double-loop control. To improve the performance, resonant controllers with parallel structure which are widely used in active power filters are applied to the single-loop control of the 400-Hz GPU. The parameter design criterion for the parallel resonant controllers is discussed in the discrete time domain. Meanwhile, adoption of proportional gain in the single-loop control is investigated. The results show that it can improve the performance little and may cause instability problems. Comparisons between different control methods for the 400-Hz GPU are also made, and the single-loop control method in this paper seems to be the most suitable one in terms of simplicity and performance. Experiments on a 16-b fixed-point DSP-controlled 90-kVA 400-Hz GPU prototype show satisfactory results of the single-loop method feeding linear/nonlinear and balanced/unbalanced loads.

A Novel Single-Phase Five-Level Inverter With Coupled Inductors

In this paper, a novel single-phase five-level inverter is proposed using coupled inductors. This inverter can output five-level voltage with only one dc source. No split of the dc voltage capacitor is needed, totally avoiding the voltage balancing problem in conventional multilevel inverters. The level of the output voltage is only half of the dc-link voltage in all conditions, leading to much reduced dv/dt . This inverter is based on the widely used three-arm power module and the voltage stresses on all the power switches are the same, making it very easy to construct. Operation mechanism of this inverter is analyzed and the possible switching patterns are investigated. Based on these analyses, a novel optimized modulation scheme is presented. With this modulation method, no dc components exist in the inductor currents, which is very helpful for minimization of the inductors. Simulation and experimental results show the validity of the proposed inverter together with the optimized modulation scheme.

Power Module Capacitor Voltage Balancing Method for a ±350-kV/1000-MW Modular Multilevel Converter

This letter focuses on the power module (PM) capacitor voltage balancing of a

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

\pm 350

A new single-phase five-level inverter with no problem of voltage balancing

-kV/1000-MW modular multilevel converter (MMC). For this MMC, the current flowing through the IGBTs will exceed the nominal current of the adopted commercial high-voltage (3.3 kV or higher) IGBT products, i.e., 1500 A. As for the PM voltage balancing of this MMC, the switching frequency must be kept low to reduce the losses and the accurate calculation of the PM switching frequency is of great importance to the design of the cooling system. This letter presented a low-switching frequency PM balancing method by introducing the balancing adjusting number (BAN). The PM switching frequency calculation method is also presented with different BANs. Computer simulation and the real-time control hardware in the loop test results based on RT-LAB show the effectiveness of the proposed PM balancing method and the accuracy of the switching frequency calculation method under the rated power.

Prototype of smart energy router for distribution DC grid

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.

Small-signal stability analysis of parallel-connected inverters based on time-varying phasor

In this paper, a new single-phase five-level inverter is presented using coupled inductors. This inverter can output five-level voltage with only one dc source and no split of the dc voltage capacitor is needed, totally avoiding the voltage balancing problem. What is more, as this inverter is based on the widely used three-arm power module, it is very easy to construct. Operation mechanism of this inverter is analyzed and the possible switching patterns are investigated. Based on these analyses, the rules for optimizing the modulation schemes are also presented. Following these rules, no dc component will exist in the inductor currents, which is very helpful for minimization of the inductors. Simulation and experimental results show validity of the proposed inverter together with the optimized modulation scheme.

Wireless parallel operation of 400-Hz high-power inverters in ground power units for airplanes

Smart energy router (SER), also called as power electronics transformer (PET) or solid state transformer (SST), will serve as a critical component in the next-generation electric power system. Taking advantage of modular multilevel converter (MMC) and input series output parallel (ISOP) topologies, new type of smart energy router has been developed. Presented architecture using two stages with AC/DC conversion in the medium-voltage (MV) side, and ISOP type DC/DC conversion in the low-voltage (LV) side, thus allows interaction with DC grid and local renewable sources, etc. Compared to existing topologies, proposed SER uses less power switches and high-frequency transformers. The details related to the 2-MVA SER prototype, including the electrical design of the circuits and control system is presented in this paper. Simulation and experimental results on this prototype with distribution DC grid show validity of this SER.

A simplified loss model of modular multilevel converter

The droop method has been widely used in the parallel operation of inverters. The coefficients of the droop controller influence the transient response and stability of the wireless paralleled inverters system. However, the conventional phasor representation of sinusoidal signals is constrained by the quasi-stationary assumption, which is generally acceptable for the steady-state or slow-acting analysis. Whatever, this paper presents a small-signal model based on time-varying phasor for parallel connected inverters. Compared with the conventional model, the proposed model can reflect the transient characteristics with more details for the paralleled inverters. Thus, more accurate range for the selection of the droop controller parameters can also be obtained. Simulation and experimental results show validity of the proposed model.

Power module voltage balancing method for a ±350 kV/ 1000 MW modular multilevel converter

This paper deals with parallel operation of 400Hz inverters in ground power units (GPU) for airplanes. For high-power 400-Hz inverters, the modulation frequency ratio is fairly low, which causes difficulties to the parallel operation. To reach low THD content in output voltage, resonant controllers are used for control of each 400-Hz inverter in the paralleled GPUs. With resonant controllers, very small phase difference between the paralleled inverters may cause huge circulating current because the output impedance is rather low. As is known, the conventional droop method has to trade off between sharing effect and voltage regulation, and effective load sharing cannot be achieved if the initial phases of the paralleled inverters are unknown. This paper proposes a novel method for parallel operation of 400-Hz inverters. Compared with the conventional droop method, the proposed method can improve the load sharing significantly together with improved dynamic response. What is more, satisfactory sharing of the power can also be achieved even under low line impedance while maintaining good frequency regulation (<1Hz). Simulation and experimental results show validity and effectiveness of the proposed method.