A Norton Model of a Distribution Network for Harmonic Evaluation

This paper presents a Norton approach for modeling distribution networks where the system configuration is not fully known. Traditionally harmonic studies use complex distribution networks modeled by harmonic sources for specific frequencies. Although this approach has been proved to be adequate for some studies, this may not happen for other applications. When changing the operating condition of the supply-side system, the harmonic currents injected by the distribution network might change. This information is in this paper used to estimate a Norton model of the load-side distribution network. The estimated models can be used to analyze, for example, the effect of harmonic filters under different supply system configurations or operating conditions. The method of estimating the Norton models is illustrated on a test system, simulated on the well-known simulation program EMTDC. The performance of the estimated models is, for different configurations of the supply system, compared to the performance of the traditionally used constant current approach.

The harmonic transfer characteristics in the distribution network were analyzed from two aspects of harmonic current and harmonic voltage in this paper. The analysis results show that the harmonic current mainly flows into the upper system when the filter device is not input. The transmission coefficient is related to the transformer ratio when the harmonic current is injected into the upper system and transmitted from the low pressure side to the high pressure. The harmonic voltage of the higher-level power grid is mainly concentrated on the load equivalent impedance of the current-level power grid when passing through the distribution network. When the harmonic filter voltage of the upper-level power grid is mainly transmitted, the system is mainly concentrated on the system of the current-level power grid.

Here, the impacts of biogas power plants and photovoltaic (PV) power plants on the power quality in the distribution network are presented and analysed. The biogas power plant's influence on the current harmonics is explored, as a synchronous generator powers the biogas power plant, and the PV plants are connected to the distribution network via the inverter. All the analyses are based on actual measurements that are performed in the last 10 years. Results of the analysis of these numerous measurements were a challenge for investigation how large share of PV systems in distribution network with non‐linear loads influence on THD from one side and how biogas power plant with a synchronous generator can improve THD from the other side. In order to investigate interaction of the biomass power plant and PV power plants on the power quality in distribution network, computer model was developed in DIgSILENT Power Factory where PV power plant and household were modelled as a source of current harmonic injections and biomass power plant was modelled as synchronous machine, as a source of voltage harmonics. Analysis of harmonic behaviour in different scenarios is examined through the performed simulations.

Integration of large-scale renewable energy (RE) sources in particular, wind and solar energy into the grid introduces current and voltage harmonics due to power electronics devices as well as inverter connected into the RE sources. Ensuring adequate harmonics in the line currents of RE integrated power system is one of the biggest challenges today. Therefore, this study investigates the potential impacts in particular current and voltage harmonics causes due to large-scale integration of RE into the Berserker Street Feeder, Frenchville Substation under Rockhampton distribution network (DN), Central Queensland, Australia. From the model analyses, it has clearly evident that harmonics across the network increases with the increase of RE integration and causes uncertainties in the feeder as well as in the DN. This study also explores possible mitigation measures and it has seen that optimized STATCOM effectively reducing the adverse harmonic impacts of integrating large-scale RE into the DN.

Today, one of the important issues of power quality (PQ) in power systems is the current harmonics. Increasing expansion of nonlinear loads at different parts of the electric network makes harmonic distortion flow through the network. This causes the network to have background voltage and current harmonic distortion and even affect on the PQ of linear load performance. Therefore, it is important to determine the harmonic contribution between load and network in distribution networks and industrial centers. In this paper, a new procedure for determining the current harmonic contribution around a quiescent point at the PCC is presented using load modeling based on crossed-frequency admittance matrix. The novelty of this paper is that, firstly, instead of using the harmonic Norton model of load, which has often been used in papers related to the harmonic contribution, it uses the crossedfrequency admittance matrix model, which is closed to actual load model; and secondly, considering the fact that a linear load has a diagonal form for its crossed frequency admittance matrix, the separation of harmonic contribution is made.

Phasor-based assessment for harmonic sources in distribution networks

This paper develops a distributed voltage regulation scheme for high Photovoltaic (PV) penetrated distribution networks by utilizing battery energy storage (BES) units. In this study, multiple BES units form a multi-agent network, in which each BES unit acts as an individual agent and can communicate with its neighbors to perform the distribution network's voltage regulation in a cooperative manner. To cope with the uncertainties of PV power and load demand, a receding horizon-based approach is proposed, where BES control decisions for regulating bus voltages are updated with the update of the system's operational conditions (PV power, load, etc.). An efficient solving method - Distributed Alternating Direction Method of Multipliers Algorithm (D-ADMM) is applied to solve the proposed model under a colored network, where the BES agents with the same color synchronously update their states. Case studies are conducted on modified IEEE benchmark systems to validate the performance of the proposed method.

Comparison of harmonic emission in LV side of a large grid connected PV power plant

There is a certain amount of current harmonics in interior permanent magnet synchronous motor. In order to accurately control the current harmonic, this article focuses on three aspects: establishing current harmonic calculation model, extracting current harmonic, and designing current harmonic regulator. A current harmonic calculation model, which considers the effect of voltage harmonics induced by current controllers and digital control delay on the initial phase angle of voltage harmonics, is established for the first time. With the help of multiple synchronous rotating frame transformation, a current harmonic extraction method more suitable for motor drive field, which aims to extract the current harmonic characteristics of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis currents, is proposed. On this basis, the controlled quantity and control quantity are unified in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d–q</i> coordinate frame for analyzing the coupling relationship of current harmonic characteristics. And then a PI current harmonic regulator, including current harmonic decoupling and voltage harmonic initial phase angle compensation, is constructed, so as to realize the effective control of current harmonics. Finally, the effectiveness and superiority of the proposed method are verified by simulation and experiments.

Power converters generate current harmonics due to their nonlinear characteristic and switching function. Among these harmonics, 2-9kHz current harmonics have not been covered in the existing standards, so regulations are being developed by IEC standardisation committee SC77A, WG1. This paper explores the current harmonics generated by converters in the frequency range of 2-9kHz in a multi-converter distribution network. So, the impacts of different converter components such as a rectifier and an inverter switching function are analysed on 2-9kHz current harmonics using experimental and simulation data. Next, analytical equations for estimating 2-9kHz current harmonics in the multi-converter condition are developed. Moreover, the harmonic cancellation in the range of 2-9kHz is studied at the system level considering different operating power and load power factors for the converters. The outcome of these investigations can be utilised to estimate the current harmonic emissions in the frequency range of 2-9 kHz in a multi-converter network. This paper also provides valuable information for compatibility development in 2-9 kHz frequency range concerning IEC SC 77A standardisation committee at a system level.

In a distribution system, to compensate for the effect of voltage sag/swell and mitigate harmonic currents, a unified power quality conditioner (UPQC) is used. Conventionally, the power rating of UPQC is determined based on the expected sag/swell, linear and nonlinear loads. However, with the shift towards usage of electronic loads and electrical drives, the composition of nonlinear loads is increasing. Hence, the UPQC deployed earlier may not be capable of handling increased harmonic currents due to its limited current rating. While trying to compensate for both harmonic currents and voltage sag/swell, the shunt converter (of UPQC) may reach its current limit and would result in undesirable behaviour. To address this issue, a technique to prioritise sag/swell compensation over active filtering is proposed in this study. The remaining power capacity of UPQC (after sag/swell compensation) is utilised for harmonic filtering. Adaptive determination of harmonic compensation current also allows prioritising the various harmonics to be compensated. This helps in case passive tuned filters are installed for some harmonic frequencies for which UPQC need not respond. Detailed simulation studies are performed on MATLAB/SIMULINK platform and results are included in this study. A laboratory prototype of single phase UPQC is developed to validate the performance of the proposed scheme.

An improved controller design method for an inverter operating with extra functions to suppress current harmonics or voltage harmonics is proposed. When at least two of this-type inverters are operating in a micro-grid, they can collaboratively bear current harmonics and voltage harmonics suppression tasks respectively. This would benefit the micro-grid further wide applications. In this paper, firstly, the principles of harmonics comprehensive suppression are analyzed. Then, two kinds of inverter controllers with different harmonics suppression purposes are designed. Finally, the effectiveness of the above control strategies is verified by Matlab/Simulink simulation results. The total harmonic distortion (THD) values of current and voltage are obviously reduced when these two inverters are connected in parallel with the micro-grid.

DC-side harmonic analysis and DC filter design in hybrid HVDC transmission systems

Designing proper solid oxide fuel cell (SOFC) system configurations is essential for their high efficiency. The present study analyzes the performance improvement of the SOFC system with anode off-gas recirculation (AOGR). Two AOGR configurations are suggested. Depending on the heat flows of off gases, the configurations are called AOGR #1 and #2, respectively. Additionally, a reference system is examined for comparison. This study aims to numerically evaluate the characteristics and performance of each system under various operating conditions such as fuel and air utilization factors. The operating current density and steam to carbon ratio are fixed at 0.3 A/cm2 and 2.5, respectively. The results indicate that the system performance shows a large difference depending on the system configurations. The SOFC system with AOGR has better performance than the reference system under the operating conditions considered in this paper. However, it is also revealed that depending on the system configuration and operating conditions, AOGR can be effective or ineffective for system performance. Therefore, a deliberate operating strategy for AOGR systems needs to be developed based on the load conditions.

The test and analysis of the power quality of some 1000kV UHV transmission lines and 500kV EHV transmission lines is carried out. It is found that there is harmonic voltage excess problems when the power supply of the UHV and EHV voltage line is single-ended or single-loop, the problem basically disappeared after the operation mode change, different operating conditions, the harmonic current has not been greatly affected, indicating that the harmonic voltage changes mainly caused by the system harmonic impedance. With the analysis of MATLAB Simulink system model, it can be seen that there are specific harmonic voltage excess in the system under the specific operating mode, which results in serious distortion of the specific harmonic voltage. Since such phenomena are found in 500kV and 1000kV systems, it is suggested that the test evaluation work should be done under the typical mode of operation in 500kV, 1000kV Planning and construction process to prevent the occurrence of serious distortion and the regional harmonic current monitoring and suppression work should be done.