Abstract:Currently, the dominant trend in new energy power supply systems is the heterogeneous inverters-paralleled system (HIPS), which is a combination of grid-following (GFL) and grid-forming (GFM) inverters. The dynamic characteristics of different inverters in HIPS and the differences between GFL and GFM inverters undoubtedly increase the difficulty of the stability analysis and coordinated control. This paper establishes an interactive admittance matrix model of HIPS, fully considers the interactive effects among different inverters, and explores the multi-dimensional resonance characteristics of HIPS by utilizing the modal analysis method. To achieve the coordinated control and oscillation suppression among different inverters, a frequency-divided compensation strategy is proposed, which divides the operation modes of HIPS into three categories, i.e., GFM, GFL, and hybrid modes. Specifically, the frequency division boundary is determined based on the resonance characteristics of GFL and GFM inverters, with the operation modes of HIPS being dynamically adjusted according to the harmonic power ratio. Finally, the simulation and experimental results demonstrate that the HIPS can flexibly adjust the operation modes to adapt to the complex conditions after adopting the frequency-divided compensation strategy and suppressing the oscillation frequency ratio to less than 2%, ensuring the safe and reliable operation of HIPS.

Abstract:The continuous power supply system, which eliminates the neutral section and realizes safe and reliable operation, shows a development trend in suburban railways. However, the access of a power quality compensator (PQC) may alter the impedance characteristics of the system and introduce additional harmonics with a broader frequency band, potentially increasing the risk of resonance. Accordingly, in this paper, an analytical method is first adopted in conjunction with a field test to construct a simplified harmonic model for an actual continuous suburban line. A modal scanning algorithm is then used to analyze the effects of the controller and filter in the PQC on the harmonic resonance of the suburban railway continuous power supply system. Based on the improved particle swarm optimization algorithm, a multi-objective optimization design for PQC is proposed that can suppress harmonic resonance, filter the harmonics, and reduce the cost while preserving the stability of the control system. Finally, a real case study based on the field test demonstrates the effectiveness of the proposed design.

Abstract:On-shore wind parks are typically connected to the high-voltage (HV) transmission system through a bulk transformer. However, wind generators may be connected directly at a medium-voltage (MV) level, such as a utility-owned primary distribution network, if the network is capable of sustaining the power flow and ensuring adequate power quality for its users. This paper presents the findings of a comprehensive study on the management of resonance in a utility-owned wind park in Costa Rica. The wind park is connected directly to the MV primary distribution network and has no shunt capacitor for power factor correction. The results demonstrate that such configuration has a higher immunity to resonances, as the total grid equivalent impedance perceived by the wind park is typically dominated by the absent HV/MV transformer and shunt capacitor bank. Moreover, the capacitance provided by the underground feeders of the wind park did not result in natural oscillation frequencies in the range of typical harmonic distortions observed in MV distribution networks that violated power quality standards.

Abstract:The mutual impedance between doubly-fed induction generator (DFIG) system and weak grid may cause a resonance, which yields to undesirable distortions and harmonics. The equivalent impedance of DFIG systems is high, which creates high-frequency resonance (HFR) in interaction with weak grids. Although several studies are conducted to mitigate HFRs, more improvements are needed in terms of damping and phase-margin. Accordingly, an active damping control strategy based on virtual admittance is proposed, which properly mitigates the disturbances. The proposed strategy is accurate as it considers the dynamic high-frequency model of DFIG system to effectively reduce the HFR. The performance of the proposed strategy is verified by using different case studies on a 2 MW DFIG system with time-domain simulations in MATLAB/Simulink environment.

Abstract:Stray currents from DC metro systems intrude into the grounded neutrals of large power transformers, posing a major threat to the differential relay protection of transformer. In this paper, the performance of harmonic blocking based differential relay protection considering neutral stray currents (NSCs) from DC metro systems is thoroughly investigated. The findings reveal that relays may fail to clear internal faults in some scenarios because they are blocked due to NSC-induced harmonic currents. To improve the reliability of differential relay protection, a method for preventing incorrect operation is proposed using a skewness-based criterion to detect the presence of NSCs. Then, the relay is unblocked when an internal fault is simultaneously detected by the novel internal fault detection block. The proposed method is resistant to current transformer saturation and accounts for NSC fluctuations. Various time-domain simulations conducted in PSCAD/EMTDC verify the effectiveness of the proposed method.

Abstract:The equivalent impedance parameters of loads have been widely used to identify and locate the harmonic sources. However, the existing calculation methods suffer from outliers caused by the zero-crossing of the denominator. These outliers can result in inaccuracy and unreliability of harmonic source location. To address this issue, this paper proposes an innovative method of equivalent impedance parameter calculation of three-phase symmetrical loads that avoid outliers. The correctness and effectiveness of the proposed method are verified by simulations on Simulink using actual monitoring data. The results show that the proposed method is not only simple and easy to implement but also highly accurate.

Abstract:This study presents a harmonic transfer function (HTF) based single-input single-output (SISO) impedance modeling method. The method converts an HTF from phase domain to sequence domain and then transforms it into an SISO impedance while preserving the frequency coupling information of different sequences and different harmonics. Applications of this method to a line-commutated converter based high-voltage direct current (LCC-HVDC) system are presented. The results demonstrate the accuracy of the derived SISO impedance, and a truncation-order selection is suggested. The case study shows that the proposed method facilitates simpler impedance measurements and associated stability analysis.

Abstract:The effects of nonlinear loads on voltage quality represent an emerging concern for islanded microgrids. Existing research works have mainly focused on harmonic power sharing among multiple inverters, which ignores the diversity of different inverters to mitigate harmonics from nonlinear loads. As a result, the voltage quality of microgrids cannot be effectively improved. To address this issue, this study proposes an adaptive harmonic virtual impedance (HVI) control for improving voltage quality of microgrids. Based on the premise that no inverter is overloaded, the main objective of the proposed control is to maximize harmonic power absorption by shaping the lowest output impedances of inverters. To achieve this, the proposed control is utilized to adjust the HVI of each inverter based on its operation conditions. In addition, the evaluation based on Monte Carlo harmonic power flow is designed to assess the performance of the proposed control in practice. Finally, comparative studies and control-in-the-loop experiments are conducted.

Abstract:Considering the majority of electrical equipment utilized in society is driven by DC, integrating a DC system can significantly enhance the efficiency and reliability of power systems by implementing the integration of diverse loads, renewable energy sources (RESs), and energy storage systems (ESSs). In this paper, the integration of multiple DC zero-carbon buildings (DC-ZCBs) is proposed to achieve the unbalanced voltage suppression of the bipolar DC microgrid (DCMG). The photovoltaic (PV) technology, loads, and DC electric springs (DC-ESs) are adopted as a unified entity to achieve the zero-carbon emission of the building. Firstly, a new configuration of PV and DC-ESs is introduced. The energy management of PV, ESS, and load are fully considered in this new configuration, which can reduce the capacity of the ESS. Subsequently, a distributed cooperative control strategy for DC-ESs based on the modulus voltage is presented, which is implemented with integration of the new configuration into the bipolar DCMG. The proposed approach addresses the issues of unbalanced voltage to improve the operating efficiency and power quality of the bipolar DCMG. The simulation is conducted in MATLAB/Simulink platform to confirm the effectiveness of the proposed approach.