Bostjan Blazic

Improved D-StatCom control for operation with unbalanced currents and voltages

This paper presents a new D-StatCom control algorithm which enables separate control of positive- and negative-sequence currents and decoupled control of d- and q-axes current components. The algorithm is based on the developed mathematical model in the d-q coordinates for a D-StatCom operating under unbalanced conditions. The problem of dc-side voltage ripple and ac-side harmonics generation due to unbalanced voltages/currents is solved by using switching function modulation, enabling the use of a relatively small capacitor on the dc side. The control algorithm is also suitable for high-power applications using multipulse voltage-source converters. The algorithm is tested by means of detailed simulation using PSCAD, where the performance is validated in the case of severe network fault and in the case of unbalanced load compensation. The compensator voltages are generated by means of pulsewidth modulation.

A Proportional-Resonant Current Controller for Selective Harmonic Compensation in a Hybrid Active Power Filter

This paper deals with reactive power compensation and harmonics elimination in medium-voltage industrial networks using a hybrid active power filter. It proposes a hybrid filter as a combination of a three-phase, two-level, voltage-source converter connected in parallel with the inductor of a shunt, single-tuned, passive filter. This topological structure greatly decreases the voltage and current stress over the elements of the active filter. Since the topology is composed of a single-tuned branch, the control algorithm also has to ensure sufficient filtering at other harmonic frequencies. We propose using a proportional-resonant, multiloop controller. Since the controller is implemented in a synchronous-reference frame, it allows us to use half the number of resonators, compared with the solution using proportional-integral controllers in the harmonic-reference frame. Theoretical analyses and simulation results obtained from an actual industrial network model in PSCAD verify the viability and effectiveness of the proposed hybrid filter. In addition, the simulation results are validated by a comparison with the results obtained from a real-time digital simulator.

Harmonic Contributions Evaluation With the Harmonic Current Vector Method

A method to evaluate harmonic contributions at the point of common coupling is presented in this paper. The proposed approach is based on the harmonic current vector method where reference impedances are introduced. Resistance defined at fundamental frequency with measurements is used as the customer-side reference impedance. The presented method allows calculation of harmonic contributions without knowing the actual customer impedances. It also enables better evaluation of the customer and utility harmonic contributions in resonance conditions. The method is verified through a simulation study and with extensive field measurements. Long-term measurements are proposed for determination of harmonic contributions.

EV Charging Facilities and Their Application in LV Feeders With Photovoltaics

Low-voltage (LV) grid feeders with high penetration of photovoltaics (PVs) are often affected by voltage magnitude problems. To solve such issues, previous research has shown that reactive power methods, active power curtailment and grid reinforcement can be used for voltage support, yet showing several limits. We introduce the use of electric vehicle (EV) public charging stations with energy storage system (ESS) as a solution for voltage regulation in LV feeders with PV. A novel method is proposed to determine the ESS charging load required for voltage regulation and compare the results for the different locations in the feeder. With time-series simulations, we quantify the energy size required for a station ESS. A Belgian LV residential grid, modeled using real PV generation and load profiles, is used as case study. The method and simulation results show the effectiveness of using public EV charging facilities with the additional function of voltage regulation in feeders with PV.

Case studies on large PV plants: Harmonic distortion, unbalance and their effects

Some case studies on PV systems are reported with reference to harmonic distortion, unbalance and their effects on protection systems. The experiences are related to: i) laboratory experiments made on commercial LV inverters aimed to study their harmonic impact; ii) harmonics produced by an existing PV plant both at lower and higher frequencies; iii) voltage unbalances caused by high penetration of single-phase connected PV sources; and, finally, iv) untimely tripping of protection systems of overhead lines fed by large PV plants.

Methods for estimating customer voltage harmonic emission levels

The definition and determination of harmonic emission levels in accordance with the new draft edition of the IEC Technical Report 61000-3-6 is presented in this paper. The paper deals with contributions to the voltage harmonic distortion and, in addition, discusses an alternative method for estimating customer harmonic emission levels at the point of common coupling. This method is based on the harmonic vector method where reference impedances are introduced. The results of the harmonic vector method are compared with the results of the method based on the definitions in the IEC Technical Report. The presented harmonic vector method with reference impedances allows calculation of harmonic emission levels without any switching maneuvers or without knowing the actual impedances.

Voltage profile support in distribution networks — influence of the network R/X ratio

One of the problems emerging with a rising share of distributed generation (DG) is also the regulation of voltage levels in different operating conditions. Distribution network voltage profile can be controlled with the use of active compensators and also with converters which are used for connection of DG. Both devices are usually based on a voltage-source converter with a capacitor or DC source connected to the DC side. The paper presents the influence of the network R/X ratio on compensation effectiveness of shunt compensators. This ratio influences the ability to control voltage and must be taken into account at device design.

Voltage control in networks with distributed generation — A case study

This paper analyzes the influence of distributed energy resources (DER) on distribution network voltage profile. A general description of the impact of DER on voltage profile is given and three advanced voltage control techniques are discussed: central voltage control with multiple voltage measurements, reactive power control with sources that enable such operation and the use of an autotransformer as a voltage regulator. The aim was to maximize and to facilitate DER integration. The influence of DER on voltage profile and the effectiveness of the investigated solutions were evaluated by means of simulation in DIgSILENT. The simulated network was an actual distribution network in Slovenia with a relatively high penetration of distributed generation. Recommendations for voltage control in networks with DER penetration are given at the end.

STATCOM Control for Operation with Unbalanced Voltages

In view of STATCOM operation analysis, special attention must be paid to the operation with unbalanced network voltages. In this paper a control algorithm that enables STATCOM operation with unbalanced voltages is presented. The algorithm is based on the compensator mathematical model in the d-q coordinate system. In case of an unbalanced voltage sag the compensator unbalances the generated voltages to limit the negative sequence currents. The problem of low-order harmonics generation is solved with the use of appropriate switching function modulation that enables the use of a relatively small capacitor on the DC side. The proposed algorithm was tested by means of simulation of STATCOM operation in the PSCAD program. The algorithm shows a good transient response also in the case of relatively deep voltage sags

Comparison of calculated and measured flicker values for two different network topologies

Frequent customer complaints resulting from high flicker levels in part of the Slovenian high-voltage network have necessitated an in-depth study of the source of such high flicker levels and means of flicker mitigation. Flicker measurements in several network nodes point to a local arc furnace as the single largest source of high flicker in the area. Calculation of flicker levels in the area via load flow and current injection method for two different network topologies — with connected and disconnected 110 kV busbars near the arc furnace — prove to be in good agreement with measurements. The current injection method is further employed in two simulation cases of flicker propagation in the network in 2020 and in an additional simulation, performed after the assumed installation of a STATCOM device.