Gu Herong

An improved droop controller for grid-connected voltage source inverter in microgrid

This paper addresses the low-frequency relative stability problem of the grid-connected voltage source inverter (VSI) based distributed generation (DG) unit in microgrid, in which the conventional active power-frequency (P-ω) and reactive power-voltage (Q-E) droop controllers are commonly used to share the loads. In the sense of the characteristic equation and eigenvalues, the relative stability and the transient response are remarkably affected by the change of the droop gains and/or stable operating points, resulting in the poor damping and eventually instability. An improved P-ω and Q-E droop scheme based on the static droop characteristics combined with a derivative term is presented to preserve the system stability and obtain an improved damping performance. Subsequently, the dynamic performance of the system can be adjusted without affecting the static droop gains to damp the oscillatory modes of the power sharing controllers. Finally, the desired transient and steady-state responses can be obtained.

Real-time DC injection measurement technique for transformerless PV systems

DC injection suppression is crucial for the transformerless PV systems. IEEE Std.929-2000 specifies that the PV systems shall not inject the dc current greater than 0.5% of the full rated output current. DC injection measurement is not only useful for evaluating whether a transformerless PV system meets IEEE standards, but also for the effective dc suppression with injecting an equal but opposite dc component via a proper control scheme. In practice, however, it is difficult to extract the very low-level (0.5% Irated) dc currents in the presence of high levels of ac currents in transformerless PV systems. In order to overcome the limitation, this paper presents a real-time dc injection measurement technique. Some major issues such as the basic operating principle, frequency excursion effect mitigation and discrete-domain implementation of the proposed method are discussed. Theoretical analysis and performance evaluation results verify the effectiveness of the proposed dc injection measurement technique.

Two-degree-of-freedom current regulation of grid-connected inverters in microgrid

Microgrid is an effective way to integrate the distributed energy resources into the utility networks. Conventionally, the voltage-source inverters in microgrid with sinusoidal current regulation are used in grid-connected mode. In this paper, a two-degree-of-freedom nonsinusoidal current regulation (NCR) is proposed for both the distributed power generation and power quality enhancement. The equivalent Thevenin-Norton circuit is provided to clarify the concept that NCR with virtual admittance leads to the power quality improvement. It is demonstrated that without any harmonic current sensor or additional compensating device, the harmonics of the voltages at the point of common coupling and currents in microgrid can be mitigated significantly. Time-domain performance evaluation results confirm the concept.

Grid-connected inverters interface control with unified constant-frequency integration control

A new control approach based on unified constant-frequency integration control (UCI) for current tracing is presented for photovoltaic grid-connected inverters. The working theory is analyzed and the control equation is deduced. The control approach is realized by a digital controller DSP. Different from the normal SPWM and PLL (phase locked loop) control approach in grid-connected inverters interface control, this control approach has the merit of automatic phase tracking capability, simplified control and high output power factor. And it can regulate the output in accordance with the dc voltage and grid voltage automatically

Frequency-adaptive sinusoidal signal integrator for grid-connected inverters in weak grids

Sinusoidal signal integrator has the capability of controlling the ac current with zero steady-state error. However, their performances significantly degrade in case of the frequency variations with the steady-state error. So far, no literature has addressed the quantitative analysis of this steady-state error. Therefore, this paper reveals, for the first time, a theoretical guidance to get the accurate quantitative analysis for the error evaluation. Meanwhile, a frequency-adaptive sinusoidal signal integrator is proposed aiming at eliminating this steady-state error. The proposed strategy is suitable for grid-connected inverter applications in weak grids where the grid frequency may experience fluctuations. Detailed analysis and results verify the effectiveness of the proposed strategy.

Analysis and Design of a Novel Dual Secondary Winding and Dual Power Bridge High Frequency Link Inverter

A novel dual secondary winding and dual power bridge high frequency link (HLF) inverter is discussed in this paper. Single-stage power conversion, standard half-bridge connection of devices, soft-switching for all the power devices (ZVS or ZCS), low conduction loss, simple bipolar combined phase-shifted control, and high efficiency are among the salient features of the HLF inverter. The principles of circuit operation, PWM control and synthesis, topological extension are discussed. The theoretical analysis is presented and the realization of power stage and close loop control is described. The experimental results are given to validate the effectiveness of analytical result and operation principle of the circuit

Two-degree-of-freedom PI digital control of bi-directional voltage mode high frequency link inverter with active clamp

This paper presents a novel modulation and a two-degree-of-freedom (2-DOF) PI digital controller based on a bi-directional voltage mode high frequency link inverter with active clamp. A novel modulation strategy is applied by modulating the triangle carrier wave and two inverse sinusoidal modulation waves. It is no need of checking the pole of the output voltage to switch the drive signals, and it is able to weaken the distortion of the output on zero passage. This 2-DOF PI controller has two parameters to be designed separately and can obtain good dynamic responses both in the command tracking and load disturbance regulation characteristic. In this paper, the 2-DOF PI digital controller for the inverter system is designed and implemented to obtain high dynamic and static performance. The parameters of the controller are found by model building, theoretical deduction according to the desired specifications. The simulation and experimental results are provided to verify the proposed modulation and digital control strategy