Reactive Power Injection Capability Research Articles

A bi-level optimization method for voltage control in distribution networks using batteries and smart inverters with high wind and photovoltaic penetrations

In electrical distribution systems, the voltage quality problem considers the most restrictive issue that hinders high photovoltaic (PV) and wind integration. Therefore, this study aims at providing a reliable control method for the voltage problem in distribution networks embedded with high PV and wind sources. The proposed voltage management strategy consists of two controlling stages. The first stage uses battery energy storage systems (BESS), while the second stage employs the smart PV inverters' reactive power injection capability. For BESS operation, a bi-level optimization method based on metaheuristic optimization algorithms (MOA) is developed to regulate the voltage levels by governing the batteries charging/discharging rates. The bi-level optimization problem aims to maximize PV and wind energy exploitation while improving the voltage profile. In this study, a detailed comparison is conducted between three MOA: Social Spider Optimization, Particle Swarm Optimization, and Cuckoo Search Optimization. In addition, considering the uncertainty associated with PV system generation, a PV generation forecast is embedded in the voltage control strategy to reinforce the decision-making process of BESS operation. The efficacy of the proposed method is deployed on a modified IEEE-34 bus test feeder with real-world load and PV and wind power generation data. Experimental results show the effectiveness of the proposed method and indicate that proper coordination between the BESS and smart PV inverters is beneficial for distribution system operations that seamlessly integrate PV and wind energy. Overall, the proposed voltage management strategy can be used to control the voltage to any desired range at different locations.

Reactive Power Injection Capability of Buck–Boost Inverter With Unfolding Circuit

New emerging power inverter topologies are aiming at high-power density and efficiency with reliable performance. The recently proposed family of single-phase single-stage buck–boost inverters with output unfolding circuits have promising features for application in different fields. Nevertheless, the incapacity of injecting reactive power by the classical unfolding inverters has been assumed as its main drawback. Thus, our focus is on the reactive power injection capability of the single-stage buck–boost inverters with unfolding circuits. This attribute enables the converter to provide some extra ancillary services for friendly integration of the renewable energy resources and energy storage systems, providing support to the smart grid/microgrid, thus contributing to its power quality welfare. By means of a comprehensive analysis and implementation of proper closed-loop model predictive control, together with an adequate selection of the output capacitor, the single-inductor unfolding buck–boost inverter is capable of delivering reactive power to the utility grid. We used the simulation and experimental tests to validate the theoretical aspects.

A Fault-Tolerant Method for Cascaded H-Bridge-Based Photovoltaic Inverters With Improved Active and Reactive Power Injection Capability in Postfault Condition

This article proposes a fault-tolerant strategy for the cascaded H-bridge-based photovoltaic (PV) inverters with a single failure in power switches. In the introduced method, the faulty leg is bypassed after a switch failure, and the faulty cell (FC) continues to operate with the remaining healthy leg. After bypassing the faulty leg, one voltage level is missed at the ac side of the FC, and if other cells operate normally, a dc offset will appear at the ac terminal voltage, leading to dc current injection to the grid. To tackle this problem, in one of the healthy cells, which is called halved H-bridge cell (HHC), the leg complementary to the faulty leg in the FC is bypassed. In this faulty condition, FC and HHC cannot operate in maximum power point. Therefore, the control system is modified to get the maximum possible active power from the existing PV arrays. Using the proposed configuration and the modified control system, all PVs are kept in use, and consequently, the drawn active power increases compared to the conventional bypassing methods and existing partially bypassing methods. Moreover, in the postfault condition, the ability of the system to inject reactive power to the grid is improved since the suggested method synthesizes higher voltage than the alternative methods at the ac side of the inverter. Mathematical equations describing the system behavior in the postfault condition are derived, and the improvement in active and reactive power injection capability is analytically proved. The effectiveness of the proposed method is verified through simulations and experiments.

Smart transformer-based medium voltage grid support by means of active power control

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid, due to the spread of non-dispatchable generation sources. This paper introduces a Smart Transformer (ST)-based Medium Voltage (MV) grid support by means of active power control in the ST-fed Low Voltage (LV) grid. The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer, which needs tens of seconds. This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid, controlling the ST output voltage. The last feature has two main effects: the first is to reduce the active power withdrawn from MV grid, and consequently the MV voltage drop caused by the active current component. At the same time, higher reactive power injection capability in the MV converter is unlocked, due to the lower active power demand. As result, the ST increases the voltage support in MV grid. The analysis and simulation results carried out in this paper show improvements compared to similar solutions, i.e. the only reactive power compensation. The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.

Control and Performance Analysis of a Single-Stage Utility-Scale Grid-Connected PV System

For utility-scale photovoltaic (PV) systems, the control objectives, such as maximum power point tracking, synchronization with grid, current control, and harmonic reduction in output current, are realized in single stage for high efficiency and simple power converter topology. This paper considers a high-power three-phase single-stage PV system, which is connected to a distribution network, with a modified control strategy, which includes compensation for grid voltage dip and reactive power injection capability. To regulate the dc-link voltage, a modified voltage controller using feedback linearization scheme with feedforward PV current signal is presented. The real and reactive powers are controlled by using $dq$ components of the grid current. A small-signal stability/eigenvalue analysis of a grid-connected PV system with the complete linearized model is performed to assess the robustness of the controller and the decoupling character of the grid-connected PV system. The dynamic performance is evaluated on a real-time digital simulator.

Modified particle swarm optimisation‐based maximum power point tracking controller for single‐stage utility‐scale photovoltaic system with reactive power injection capability

In this study, the control objectives such as maximum power point tracking (MPPT), synchronisation with grid and current control are realised for single-stage utility-scale photovoltaic (PV) system which is also capable of injecting the reactive power into the grid. For utility-scale PV system, the single-stage scheme is used for high efficiency and simple power converter topology. The proposed MPPT approach utilising a modified particle swarm optimisation method is interrelated with other control schemes. The MPP changes with variation in solar irradiation and temperature. Also, the PV power characteristic of large system is characterised by multiple peaks under partial shaded condition. The conventional methods such as hill climbing and incremental conductance methods do not work properly in these conditions. The proposed method is suitable to all conditions with reduced steady-state fluctuations. The overall system is simulated in PSCAD and the effectiveness of the proposed method is tested on various conditions.

이중여자 유도형 풍력발전기 기반 풍력단지의 계통 연계점 전압제어

Wake effects cause wind turbine generators (WTGs) within a wind power plant (WPP) to produce different levels of active power and subsequent reactive power capabilities. Further, the impedance between a WTG and the point of interconnection (POI)—which depends on the distance between them—impacts the WPP’s reactive power injection capability at the POI. This paper proposes a voltage control scheme for a WPP based on the available reactive current of the doubly-fed induction generators (DFIGs) and its impacts on the POI to improve the reactive power injection capability of the WPP. In this paper, a design strategy for modifying the gain of DFIG controller is suggested and the comprehensive properties of these control gains are investigated. In the proposed scheme, the WPP controller, which operates in a voltage control mode, sends the command signal to the DFIGs based on the voltage difference at the POI. The DFIG controllers, which operate in a voltage control mode, employ a proportional controller with a limiter. The gain of the proportional controller is adjusted depending on the available reactive current of the DFIG and the series impedance between the DFIG and the POI. The performance of the proposed scheme is validated for various disturbances such as a reactive load connection and grid fault using an EMTP-RV simulator. Simulation results demonstrate that the proposed scheme promptly recovers the POI voltage by injecting more reactive power after a disturbance than the conventional scheme.

Reactive Power Compensation in Single-Phase Operation of Microgrid

A coordinated control of distributed generators (DG) and distribution static compensator (DSTATCOM) in a microgrid is proposed in this paper. With high penetration of distributed sources and single-phase operation of the system, voltage unbalance can often go beyond the acceptable limit. With the feeders geographically spread out, it is not always possible to achieve reactive compensation at optimum location with the three-phase devices. In this paper, a simple control strategy for DSTATCOM with communication in loop is proposed. The proposed reactive compensation technique is based on the voltage sag and the power flow in the line. The power flow and the voltage at different locations of the feeders are communicated to the DSTATCOM to modulate the reactive compensation. The single-phase DSTATCOM compensates for the reactive power deficiency in the phase while the DGs supply “maximum available active power.” During reactive power limit of the DG, the “maximum available active power” is fixed to a value lower than maximum active power to increase reactive power injection capability of the DGs. A primary control loop based on local measurement in the DSTATCOM always ensures a part of reactive compensation in case of communication failure. It is shown that the proposed method can always ensure to achieve acceptable voltage regulation. The data traffic analysis of the communication scheme and closed-loop simulation of power network and communication network are presented to validate the proposed method.

IMPROVING VOLTAGE STABILITY BY UTILIZING REACTIVE POWER INJECTION CAPABILITY OF VARIABLE SPEED WIND TURBINES

The impact that wind turbines have on the voltage stability is investigated in this paper. In particular, the effect of utilizing the reactive power injection capability of modern wind energy converters is investigated. It is found that large wind farms, integrated into the transmission level with their control properly modified, can increase the voltage stability of the power grid substantially, as well as moderately increase the steady-state power transfer limit. For a high wind speed situation in which the wind turbine converter is fully utilized, it is found that it is worth reducing the active power production from the wind turbine to make room for reactive power injection from a voltage stability point of view. An interesting observation is that a modern variable speed wind turbine constantly operating at maximum power factor does not provide much voltage stability improvement compared with a traditional fixed-speed system under its usual operating condition, i.e., at lower wind speeds. The finding is that the worst case to handle, from a voltage stability point of view, is the case where there is a high load demand, irrespective of the wind speed situation.