The impedance interactions between weak grid and LCL-filtered converter can bring severe harmonic resonance. Recently, frequency domain passivity theory has been applied to converter damping design. However, it is usually not easy to achieve full frequency passivity. What’s more, traditional passivity based damping design is not enough for system robustness. This paper first proposes a relaxed passivity based active damping design for capacitor current feedback. Then the system stability under grid impedance variation is also analyzed. It has been found that the wide variation of grid impedance can decrease system stability margin. As a result, critical oscillation can even happen in the passive region of converter output admittance, causing power quality problems. In order to further improve system stability, this paper proposes an enhanced passivity based impedance shaping method, which is based on hybrid damping and combined active damping. Effects of damping methods and damping parameters are also investigated. A step by step damping design procedure is formulated, which is easy to implement. Simulation and experimental results validate the effectiveness of proposed method.

In this paper, a novel passive filter installed at PWM inverter output terminals is proposed with an objective of eliminating the common-mode and differential-mode voltage generated by PWM inverter simultaneously. For determining the parameters of filter, the filter transfer function is utilized to achieve a desirable filtering performance. The detailed design procedure of the filter parameters is given. The validity and effectiveness of proposed filter are supported by the simulation and experimental results carried out on 380 V/3 kW induction motor system. Further investigations also demonstrate that the proposed filter permits motor to be connected with inverter through a long cable. The side-effects induced by common-mode and differential-mode voltage such as shaft voltage, bearing current and leakage current are mitigated effectively

In this paper, a sensorless rotor position estimation method based on the field current harmonic for a load-commutated inverter (LCI)-fed electrically excited synchronous motor (EESM) operating at zero or low speed is proposed. A detailed theoretical analysis and mathematical deduction are presented. The field current harmonic is used instead of the extra high-frequency signal injection. A reasonable harmonic frequency is selected, and a realizable field current control method is given. To simplify the complex signal processing and improve the measurement accuracy, a specific bandpass filter and a normalized quadrature phase-locked loop are designed. The phase change caused by the filters is compensated properly. The proposed approach is demonstrated through the simulation and experiment with a 15-kW LCI-fed EESM drive platform, confirming its feasibility and accuracy.

An envelope-prediction-based sensorless rotor position observation scheme is proposed for a load commutated inverter (LCI)-fed electrically excited synchronous motor (EESM) at zero and low speed. The proposed rotor position observation scheme does not need to utilize the full controlled power device, nor to modify the LCI drive topology. An interval trigger mode for the exciter control is proposed to generate the appropriate fixed-frequency injected signal. The bandpass filters (BPFs) are optimized to extract the target signal, and the observation error due to the BPF is analyzed and compensated. A specific signal sampling method and an envelope-prediction algorithm are presented to simplify calculation and improve the observation accuracy. A normalized phase-locked loop is designed such that the observer has a better observational performance. Finally, the proposed rotor position observation scheme is verified by a 15-kW LCI-fed EESM experimental platform.

This paper addresses the sensorless speed and position control of induction motors using high frequency injection at zero and low frequencies. A novel algorithm is presented which allows the rejection of saturation and nonlinear inverter effects without the need for an off-line pre-commissioning process. The method is based on a set of synchronous filters to identify the disturbance waveforms, and a memory algorithm, which refines the quality of the disturbance waveforms as the motors operational history is increased. The algorithm is entirely sensorless. Experimental results show sensorless low frequency operation with and without the memory algorithm