Converter Current Limits Research Articles

On the sizing of PV inverters with reactive power capability to regulate power factor: A reliability approach

The multifunctional operation of photovoltaic (PV) inverters (M-PVI) providing ancillary services to the grid has been widely studied in the literature in the last years. The provision of reactive power by the inverters can be used for grid voltage regulation, support during faults and to regulate the installation power factor (PF). However, the capability of the M-PVI to control reactive power depends on the converter current limits. Therefore, the correct sizing of the M-PVI is a key step in the design for this operation. This paper aims to determine the minimum inverter rated power when applied to regulate the installation PF considering a real load mission profile as a case study. Additionally, the trade-off between the reactive power provision and the effects on the lifetime of the PV inverter components will be carried out using Monte Carlo simulations.

A new perspective on wind integrated optimal power flow considering turbine characteristics, wind correlation and generator reactive limits

The operation of power system is facing newer challenges due to inherent uncertainty in wind generation. The current study fills the gap in the available literature by combining the nature of uncertainty of wind power in terms of varying wind speed correlation, turbine characteristics and generator reactive power limits. Wind speed correlation is modelled using Weibull distribution and Gaussian copula; the turbine characteristics considered are rated power and turbine performance index (TPI); the generators considered are doubly fed induction generators (DFIG) whose reactive power limits are varied based on converter current limits and grid voltage conditions. The above variations are combined to simulate a large number of dependent wind power scenarios which are then incorporated as two wind farms replacing the conventional generators at bus 11 and bus 13 in a modified IEEE 30 bus 6 generator system. Optimal power flow solution is obtained for each of the scenarios and the impact on generation cost, emission, losses, capacity factor and voltage profile is investigated. Another novelty in this work is computation of voltage violation index (VVI) for varying correlation and turbine characteristics when the system is subjected to contingency states of load increase, line outage and their combination.

Hardware‐in‐the‐loop tests on distance protection considering VSC fault‐ride‐through control strategies

The short-circuit response of a voltage source converter (VSC) can vary from each other significantly with different control strategies. This study investigates the performance of distance protection under unbalanced faults considering two control strategies during fault-ride-through using hardware-in-the-loop tests. Variations on fault type, converter current limit, fault resistance and pre-fault power flow conditions are also examined. The tests reveal that the error in measured impedance caused by fault resistance will be enlarged in a converter-dominated power system. The first control strategy tends to cause overreach problem while the second can raise both overreach and underreach problems. The indeterminacy associated with the second strategy is jointly affected by control parameters, converter current limit, fault resistance and pre-fault power flow conditions. The findings of this study can help to stipulate grid requirements under unbalanced faults.

A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators are revising the grid-connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of the future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage-source converters (VSCs) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSCs under unbalanced faults for short-circuit power provision are reviewed in two groups, namely, power-characteristic-oriented and voltage-support-oriented control strategy, respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.

Transmission support using Wind Farm controls during voltage stability emergencies

This paper investigates the effect of reactive support by Wind Farms (WF) on the maximum power transfer, and thus the voltage stability limit of a transmission system. An insecure snapshot in a weak area of the Hellenic Interconnected System is examined, as well as a simpler test system. MV feeder characteristics and substation controls (feeder resistance and reactance, switched capacitors, LTC controls) are modelled in detail. Most importantly, the converter current limits, as well as the overvoltage limitations on the MV feeder, are taken into consideration in assessing the effect of the WF support.

A Novel Transient Control Strategy for VSC-HVDC Connecting Offshore Wind Power Plant

This paper proposes a novel steady-state and transient management scheme for voltage source converter based high voltage direct current (VSC-HVDC) connecting permanent magnet synchronous generator (PMSG)-based offshore wind power plant (WPP). The proposed control arrangement aims to fully utilize the HVDC converters' normal loading capabilities during steady-state operation. Furthermore, it targets the employment of the available converters' overloading capabilities to enhance the fault ride through (FRT) performance during faults. The positive and negative sequence components are controlled to 1) neutralize dc-link voltage ripples due to asymmetrical grid faults, 2) inject reactive current support for the grid voltage, and 3) deliver the maximum possible active power during different faults. Novel mathematical and time-variant representations of the positive and negative sequence components are introduced to adapt the converter current limits for full utilization of the converter limit. Comprehensive simulation studies using PSCAD/EMTDC are presented to verify the functioning of the proposed control strategy.

Dynamic Contribution of DFIG-Based Wind Plants to System Frequency Disturbances

The paper investigates contribution of doubly fed induction generator (DFIG) to system frequency responses. Impact of different governor settings and system inertia are investigated. Three distinct cases are simulated in order to illustrate the influence of DFIG penetration on frequency regulation. Provision of inertial response by DFIG through artificial speed coupling is also presented. The effects of the inertial response on the machine behavior and its significance for frequency regulation are discussed. The influence of converter current limits and auxiliary loop parameters on the inertial response are illustrated and a novel control algorithm is developed for extracting maximum energy from the turbine in a stable manner. The results of the study are illustrated on the example of an isolated power system consisting of a diesel generator and a DFIG.

Comparison of Different Frequency Controllers for a VSC-HVDC Supplied System

This paper studies three different frequency controllers and their effects on the voltage disturbance ride-through capability of a VSC-HVDC supplied industrial system. The idea of implementing frequency controller is to improve the power quality of industrial plants by slightly decreasing the VSC output voltage frequency since industrial processes are more sensitive to voltage drops than frequency deviations. The first two controllers, frequency controllers I and II, are fixed frequency controllers and the third one, frequency controller III, is a PI frequency controller. In order to compare three different controllers, a system with a simplified VSC-HVDC and different load types is simulated in PSCAD/EMTDC. Simulation results show that with frequency controller III, the VSC-HVDC supplied industrial plant can avoid a voltage collapse by decreasing frequency during or after disturbances. Furthermore, with an increase of the converter current limit, the possibility of mitigating voltage dips increases. For frequency controllers I and II, the extent of the disturbance ride-through capability depends on the current limit of the VSC-HVDC. A higher current limit results in a higher ride-through capability. The effect of the dc capacitor on improving the system voltage disturbance tolerance is also investigated during and after disturbances when the VSC-HVDC uses frequency controller I. The system voltage disturbance ride-through capability increases with an increase of the dc capacitance or the current limit.

P -γ characteristics for the unified power flow controller-analysis inclusive of equipment ratings and line limits

The paper presents a direct and systematic method for determining the entire operating range of a UPFC in the presence of equipment and system operating limits. The method is inclusive of all limits: series converter voltage and current limits, shunt converter current limit, and voltage limits at the equipment terminals. The formulation is general and permits calculation of P-/spl delta/ curves for a UPFC installed at any point along the transmission line. Equipment and system operating limits are shown to significantly impact the P-/spl delta/ curves of the UPFC. The methodology presented in the paper provides system planners a means to realistically quantify potential benefits of a UPFC installation for a transmission system.