Reactive Power Exchange Research Articles

Developing an Integration of Smart-Inverter-Based Hosting-Capacity Enhancement in Dynamic Expansion Planning of PV-Penetrated LV Distribution Networks

With the penetration of distributed energy resources (DERs), new network challenges arise that limit the hosting capacity of the network, which consequently makes the current expansion-planning models inadequate. Smart inverters as a promising tool can be utilized to enhance the hosting capacity. Therefore, in response to technical, economic, and environmental challenges, as well as government support for renewable resources, especially domestic solar resources located at the point of consumption, this paper is an endeavor to propose a smart-inverter-based low-voltage (LV) distribution expansion-planning model. The proposed model is capable of dynamic planning, where multiple periods are considered over the planning horizon. In this model, a distribution company (DISCO), as the owner of the network, intends to minimize the planning and operational costs. Optimal loading of transformers is considered, which is utilized to operate the transformers efficiently. Here, to model the problem, a mixed-integer nonlinear programming (MINLP) model is utilized. Using the GAMS software, the decision variables of the problem, such as the site and size of the installation of distribution transformers, and their service areas specified by the LV lines over the planning years, and the reactive power generation/absorption of the smart inverters over the years, seasons, and hours are determined. To tackle the operational challenges such as voltage control in the points of common coupling (PCC) and the limitations in the hosting capacity of the network for the maximized penetration level of PV cells, a smart-inverter model with voltage control capability in PCC points is integrated into the expansion-planning problem. Then, a two-stage procedure is proposed to integrate the reactive power exchange capability of smart inverters in the distribution expansion planning. Based on the simulations of a residential district with PV penetration, results show that by a 14.7% share of PV energy generation, the loss cost of LV feeders is reduced by 28.3%. Also, it is observed that by optimally making use of the reactive power absorption capability of the smart inverters, the hosting capacity of the network is increased by 50%.

Reduction in Submodule Capacitance in FB- and HYB-MMCs With Variable DC Voltages

Full-bridge submodule (FBSM)-based modular multilevel converter (FB-MMC) or hybrid MMC (HYB-MMC) composed of FBSMs and half-bridge submodules are necessary in many applications. However, the arm energy fluctuation during reactive power exchange is very high, causing large sizes and high costs of submodule capacitors. Using the negative states of the FBSMs, the FB-, and HYB-MMCs can dynamically adjust the dc voltage, and this technique has been used in many commissioned projects. This study treats the dc voltage as a new dimension and proposes a new operation mode based on variable dc voltage for FB- or HYB-MMCs to reduce the arm energy fluctuations and submodule capacitance. The basic idea is to reduce the dc voltage in high reactive power and low active power conditions, thereby suppressing the arm energy fluctuations. Theoretical analysis shows that the energy fluctuation and submodule capacitance can be reduced by 30%–40%, and the valve cost can be reduced by 13%. Economic and technical comparison results show that the proposed approach can have similar reduction effects on the capacitance and valve cost, but has fewer technical limits. Simulation and experimental results verify the analysis.

Optimal routing and power management of electric vehicles in coupled power distribution and transportation systems

With the increasing penetration level of electric vehicles (EVs) in the transportation network, it is necessary to control the charging and discharging process of EVs to avoid operational issues in the power grid. Therefore, in this paper, a two-stage framework is presented for EVs optimal routing and their active and reactive power control in the distribution grid. In the first stage, the EVs mobility in the transportation system is taken into consideration. On this basis, EV daily trips in the transportation network are modelled using the trip chain method. Also, the Dijkstra algorithm is utilized for optimal routing with the shortest travel time between the origin and the destination node. Furthermore, the effects of ambient temperature, traffic congestion and road type on the EVs energy consumption are assessed. In the second stage, optimal active and reactive power exchange of EVs with the distribution grid is carried out. Thus, a mixed-integer linear programming (MILP) model is proposed that simultaneously considers EV owners’ and the distribution network operator's benefits as the optimization goals. The EV battery degradation cost due to the discharging of active power is also integrated into the objective function of the optimization problem. The proposed framework is implemented on a standard IEEE 33-bus system coupled with a 30-node transportation network. The simulation results show that the presented method can reduce the loss cost of the distribution grid and benefit EV owners compared to different charging strategies. In addition, with the battery technology development, the proposed framework can significantly improve the distribution system operation, decrease environmental issues, and reduce EV owner’s costs.

False Data Injection Impact on High RES Power Systems with Centralized Voltage Regulation Architecture

The increasing penetration of distributed generation (DG) across power distribution networks (DNs) is forcing distribution system operators (DSOs) to improve the voltage regulation capabilities of the system. The increase in power flows due to the installation of renewable plants in unexpected zones of the distribution grid can affect the voltage profile, even causing interruptions at the secondary substations (SSs) with the voltage limit violation. At the same time, widespread cyberattacks across critical infrastructure raise new challenges in security and reliability for DSOs. This paper analyzes the impact of false data injection related to residential and non-residential customers on a centralized voltage regulation system, in which the DG is required to adapt the reactive power exchange with the grid according to the voltage profile. The centralized system estimates the distribution grid state according to the field data and provides the DG plants with a reactive power request to avoid voltage violations. A preliminary false data analysis in the context of the energy sector is carried out to build up a false data generator algorithm. Afterward, a configurable false data generator is developed and exploited. The false data injection is tested in the IEEE 118-bus system with an increasing DG penetration. The false data injection impact analysis highlights the need to increase the security framework of DSOs to avoid facing a relevant number of electricity interruptions.

Control of Converter for a Solar PV-BESS Powered Telecom Load With Real, Reactive and Harmonic Power Exchange With Grid

Control of Converter for a Solar PV-BESS Powered Telecom Load With Real, Reactive and Harmonic Power Exchange With Grid

Floating, Production, Storage, and Offloading Unit: A Case Study Using Variable Frequency Drives

The use of variable frequency drives (VFD) in floating, production, storage, and offloading (FPSO) units provides well-known benefits to the power system, such as: soft-starting of motors, increase of energy-efficiency, reduction of short-circuit currents, and enhancement of the load shedding capability. The VFDs with passive front-end rectifiers are the most commonly used for these applications due to their wider commercial availability and lower cost compared to VFDs with active front-end rectifiers. However, the latter can control the exchange of reactive power with the grid, which may contribute to voltage support and power factor regulation at the electrical system. This paper presents a case study considering the Production operating mode of an FPSO unit with two VFDs connected to the largest motors (i.e., water injection pumps). Simulations of the FPSO power system are evaluated using both active and passive VFDs, considering scenarios with analysis of steady-state, start of largest motors and symmetrical three-phase short-circuit.

Coordinated Control of Three Single–Phase BESS Inverters Using Local Measurements to Mitigate Voltage Unbalance

This article proposes a control algorithm for single-phase battery inverters to provide voltage unbalance compensation in distribution networks with high penetration of photovoltaic panels and electric vehicles. A typical distribution system is studied to quantify the impact of single-phase loads and generating units on voltage unbalance levels, and identify the conditions that lead to the highest voltage unbalance. Voltage unbalance compensation is then performed by regulating active and reactive power exchange between three single-phase battery energy storage units and the power grid. The proposed control strategy consists of an upper level control system to coordinate the three units by sending active and reactive power reference signals, and a Battery Energy Storage System (BESS) control system to regulate active and reactive power exchange between each single-phase inverter and the grid. Simulation results show that the effectiveness of this approach depends on the location where the energy storage units are installed. The positive impact of the proposed methodology on balancing the power flow and reducing the transformer zero-sequence current is also demonstrated. Finally, Hardware-In-The-Loop (HIL) experiments are carried out to provide validation of the control algorithm in real-time.

Decentralized TSO-DSO coordination for voltage regulation purposes based on renewable energy sources management – Sensitivity and robustness analyses

• A decentralized coordination between a Transmission System Operator (TSO) and a Distribution System Operator (DSO) implemented via a standardized Business Use Case (BUC) for scheduling and deploying optimal reactive power exchanges in the DSO's boundary for improved voltage control in the TSO's networks is proposed. • The interoperability between the TSO, the DSO, and other stakeholders is solved by designing and developing the BUC within the framework of the International Electrotechnical Commission (IEC) Common Information Model (CIM) family of standards IEC61970, IEC61968, and IEC62325. • The standardized BUC is demonstrated on real-world Slovenian TSO's and DSO's networks. On one hand, a sensitivity analysis is conducted to evaluate to which extent different Renewable Energy Sources (RES), such as capacitor banks and different Distributed Generators (DGs), viz. hydropower, photovoltaic (PV), and thermal (co-generation) units, impact on the high voltage (HV) for different network topologies, DGs’ operating scenarios, and capacitor banks’ sizes and locations. On the other hand, a robustness analysis is conducted to evaluate the capability of the proposed approach to regulate the HV voltage by managing the reactive power injected by these different RES in the different simulated scenarios. • The proposed data exchange mechanism based on the standardized BUC can successfully exchange data between the TSO, the DSO, and other stakeholders, such as the Significant Grid Users (SGUs) and Meter Operators, as a CIM Common Grid Model Exchange Standard (CGMES) format. • The proposed approach can manage DGs at the DSO's boundary toward contributing additional (positive or negative) reactive power to reduce the voltage deviations in the grid, improve the power quality at the DSO's boundary by reducing the flow of reactive power from the TSO's to the DSO's networks and vice versa, and keep the HV within safe values. The increasing penetration of Renewable Energy Sources (RES) in distribution networks has led traditional voltage regulation to their boundaries. In order to develop advanced techniques for voltage control in this new context, an adequate and real-time coordination and communication between Transmission System Operators (TSOs) and Distribution System Operators (DSOs) is needed. In this paper, a decentralized TSO-DSO coordination approach for scheduling and deploying optimal reactive power exchanges in the DSO's boundary for improved voltage control in the TSO's networks is proposed. The proposed approach is implemented via a standardized Business Use Case (BUC). The interoperability between the TSO, the DSO, and other stakeholders is solved by designing and developing the BUC within the framework of the International Electrotechnical Commission (IEC) Common Information Model (CIM) family of standards IEC61970, IEC61968, and IEC62325. In view of the lack of pilot tests in the field, the proposed standardized BUC is demonstrated on real-world Slovenian TSO's and DSO's networks. The simulation experiments presented in this paper are twofold. On one hand, the proposed data exchange mechanism based on the standardized BUC demonstrates the feasibility of successfully exchanging data between the TSO, the DSO, and other stakeholders, such as the Significant Grid Users (SGUs) and Meter Operators, as a CIM Common Grid Model Exchange Standard (CGMES) format. On the other hand, the capability of the proposed decentralized TSO-DSO coordination approach to regulate the High Voltage (HV) by managing the reactive power injected by different RES, such as capacitor banks and different Distributed Generators (DGs), viz. hydropower, Photovoltaic (PV), and thermal (co-generation) units, is validated via sensitivity and robustness analyses for different network topologies, DGs’ operating scenarios, and capacitor banks’ sizes and locations. The simulation results show that the proposed approach can manage DGs toward contributing additional (positive or negative) reactive power to reduce the voltage deviations in the grid, improve the power quality at the DSO's boundary by reducing the flow of reactive power from the TSO's to the DSO's networks and vice versa, and keep the HV voltage within safe values. Unfortunately, this is not the case for the capacitor banks, where the capability of the proposed approach to manage their injected reactive power to regulate the HV voltage is highly dependent on their sizes and location, being necessary to be studied on a case-by-case basis.

Design and evaluation of a multi-level reactive power market

The shift from conventional power plants on transmission level to distributed energy resources in the distribution grids requires procedures to enable efficient and economic reactive power exchange across different voltage levels. In this paper, we propose a multi-level reactive power market that enables reactive power provision from distributed energy resources to higher voltage levels. Each grid operator operates a local reactive power market and offers local reactive power potential, as an intermediary, to superordinate grid operators by passing on its aggregated cost curve and flexibility range. As a result, there is a low requirement of communication between the market participants and each grid operator is free to choose its local market rules as well as the optimization algorithm used. First results from a case study show that our decentralized market approach can realize an economically efficient multi-level reactive power provision that is close to a centrally computed optimal solution, without violating local grid constraints.

Load Transfer Optimization Considering Hot-Spot and Top-Oil Temperature Limits of Transformers

This paper proposes a load transfer optimization (LTO) model involving hot-spot temperature (HST) and top-oil temperature (TOT) limits of transformers. This work aims to address the overload problem and transformer HST and TOT excursions via LTO. First, the equivalent thermal model for the HST and TOT of transformers under external cooling conditions are discussed, and then the corresponding linearized HST and TOT constraints with varied thermal resistances are derived using the discretization method. Moreover, the proposed LTO model is formulated on the reconfigurable topology of high voltage distribution networks (HVDNs) in China. This proposed LTO model can achieve the optimal network reconfiguration scheme subject to temperature limits and system operation constraints, with the least switching operation cost and the maximum margins of reactive power exchange between HVDNs and transmission systems. Finally, the effectiveness of the proposed LTO model is validated by the modified <i>IEEE</i>-14 node system and the practical HVDNs in China.

Control strategy of controllable shunt reactors (CSR) in Zhangbei substation of Zhangbei-Xiongan UHV project

Based on the incremental control method of reactive power demand, this paper considers accessing the status information of low-voltage reactive power compensation devices and the reactive power exchange information of the medium-voltage side of the main transformer, so as to realize the reactive power balancing of the partition system, which is composed of the substation and half the length of substation outlets. Aiming at the actual situation of Zhangbei UHV substation interconnected with wind farms, this paper further considers the constraint of reactive power flow ride-through, so as to generate an inner-layer control strategy, which comprehensively considers the reactive power balancing of partition system and reactive power flow exchange. This inner-layer control strategy and the outer-layer control strategy of voltage boundary emergency control together constitute double-layer control strategy of controllable shunt reactors in Zhangbei UHV substation. This control strategy has passed the thematic review conference organized by the State Grid UHV Department, and has been applied to Zhangbei UHV substation.

Parameter setting method of coordinated control between synchronous condenser and DC system based on dynamic reactive power optimization

It is of great significance for improving the safety and stability of the system to realize the coordinated and optimal control between synchronous condenser and DC system in UHVDC converter station. However, the existing dynamic reactive power optimization methods emphasize that the two systems can be scheduled uniformly, which are independent in actual engineering. Therefore, an area-diagram strategy for synchronous condenser and a dividing-stage strategy of reactive power exchange for AC/DC system are proposed here. Then, the setting methods of control parameters for the two systems based on dynamic reactive power optimization are proposed. The work of this paper fully considers the abilities and requirements of the two systems. Taking an UHVDC converter station in Northeast China as an example, the effectiveness of the proposed method is analysed. The simulation results show that the proposed method can effectively reduce the operation times of reactive power compensation devices and improve the operation level of system voltage.

An investigation of Grid-Integrated Photovoltaic system intended for hybrid Moth Flame Optimization using ANFIS techniques

As the move towards Grid Integrated-Photovoltaic (GI-PV) system is proposed to improve the power quality development. A novel Adaptive Neuro-Fuzzy Inference System (ANFIS) based on improved Moth Flame Optimization (MFO) algorithm is described for grid integrated approach. The solar integration of Maximum Power Point (MPP) fed into modified Switched Boost Inverter (SBI) is presented, this GI-PV connected circuit has become prominent research in a recent scenario for energy demand. Proposed MFOA-ANFIS controller has generated the duty cycle pulses to each converter circuit. The benefit of grid-tied SBI is direct control outer-loop employed to obtain MFO-ANFIS techniques. To maintain a constant voltage DC-link is employed for inner-loop, this presence of constant DC-power to grid loads with support of MFO-ANFIS assists Proportional Integral Differential (PID) method. The results acquired by the simulation expressed that the proposed controller is addressed to maintain active and reactive power exchange, regulate DC bus-link voltages, grid voltage, and grid current. The effectiveness of the practical implication research is achieved by the output as represented as minimum grid harmonics, load current, and compensator current as verified in MATLAB/Simulink platform.

Modeling of electric springs and their multi-objective voltage control based on continuous genetic algorithm for unbalanced distribution networks

This work proposes a multi-objective voltage control for multiple Electric Springs (ESs) in active unbalanced distribution systems, where the goal is to provide a suitable simulation scheme to control the ESs in real-time applications. This multi-objective voltage control relies on a robust ES model, a modified backward/forward solution method (BFSM), and a continuous genetic algorithm (CGA). The proposed ES model is suitable for unbalanced conditions and can be implemented at any network location in single-, two-, and three-phase connections. Also, it considers the self and mutual effects of the distribution line (DL) and the power flow of the elements connected at the point of common coupling (PCC) to determine the ES output voltage. The ES model is adapted to the BFSM to quickly and reliably solve the power flow in the distribution network. Besides, the CGA simultaneously solves the mode of operation and the amount of reactive power exchange of the ESs to minimize power losses, voltage unbalance, and voltage deviation. The proposed voltage control is evaluated on an active distribution network based on a modified IEEE 13-bus for considering both load variations and power intermittences.

Model-Based Maximum Power Point Tracking Algorithm With Constant Power Generation Capability and Fast DC-Link Dynamics for Two-Stage PV Systems

In this paper, a model-based maximum power point tracking (MPPT) technique is presented for a two-stage grid-connected photovoltaic (PV) system, where the loci of the maximum power points (MPPs) is specified accurately based on a new formulation. In this formulation, the effect of both irradiance and temperature is taken into consideration, whereas the irradiance is estimated to reduce the cost of the system and enhance its reliability. Furthermore, constant power generation (CPG) is integrated with the developed MPPT method to facilitate other power regulation schemes in the PV system. The proposed methodology is compared with the well-known perturb and observe (P&O) method for evaluation. Additionally, a modified version of the P&O is included in the comparison for better assessment. The effect of different partial shading conditions on the system’s performance is also investigated. The DC-link PI controller is replaced with a new adaptive DC-link controller to enhance the transient behavior of the PV system. Moreover, the suggested DC-link controller removes the DC offsets, which appear in case of gradient increase or decrease in the input PV power. In contrast to the conventional PI controller, which has poor performance at such circumstances. The active and reactive power exchange with the grid is managed using a computationally efficient finite-set model predictive control (FS-MPC) algorithm. Furthermore, switching frequency minimization is added as a secondary objective using a weighting factorless procedure. The grid-voltage sensors are eliminated and estimated using an extended Kalman filter (EKF). The overall control strategy is evaluated using experimental implementation, hardware-in-the-loop (HIL), and matlab/simulink.

An Operating Mode Based on Variable DC Voltage and Current for Enlarging P–Q Capability Range of Unidirectional Current H-Bridge Submodule-Based MMC

Modular multilevel converters (MMCs) are widely utilized in dc applications. With the development of MMCs, many applications require the MMCs to have additional functions and characteristics, such as dc fault clearing capability and low submodule capacitance. A unidirectional current H-bridge submodule (UCH-SM) has been proposed, which uses low submodule capacitance and possesses dc fault clearing capability. However, the reactive power capability is limited during low active power because of the inadequate dc bias in arm currents caused by the low dc current. Although retaining a rated dc current even at zero active power has been proposed, it results in increasing the power losses and large capacitance. This study proposes a new operating mode based on variable dc voltage and variable dc current (VVVCM) for point-to-point dc systems. The basic idea is that both the dc voltage and current vary on the basis of active power and a lower limit is set for the dc current to provide adequate dc bias in the arm currents during reactive power exchange. Analysis shows that the UCH-MMC in VVVCM has an enlarged P&#x2013;Q capability range and still retains low capacitor usage. Simulation results verify the effectiveness of the proposed mode.

A robust model-free controller for a three-phase grid-connected photovoltaic system based on ultra-local model

In this paper, a robust model-free controller for a grid-connected photovoltaic (PV) system is designed. The system consists of a PV generator connected to a three-phase grid by a DC/AC converter. The control objectives of the overall system are to extract maximum power from the PV source, to control reactive power exchange and to improve the quality of the current injected into the grid. The model-free control technique is based on the use of an ultra-local model instead of the dynamic model of the overall system. The local model is continuously updated based on a numerical differentiator using only the input–output behavior of the controlled system. The model-free controller consists of a classical feedback controller and a compensator for the effects of internal parameter changes and external disturbances. Simulation results illustrate the efficiency of the controller for grid-connected PV systems.

An Emergency Active and Reactive Power Exchange Solution for Interconnected Microgrids

In this article, an important application of interconnected microgrids (IMGs) is presented in order to handle an emergency condition of individual MGs, which causes an intolerable voltage magnitude or frequency deviation. In this method, the active and reactive powers are exchanged via an interlinking back-to-back converter (BTBC) and in accordance with voltage magnitude and/or frequency differences of individual MGs. In order to increase the IMG reliability and overcome the communication challenges, only local voltages and frequencies of both ac sides of the BTBC are used to determine the required active and reactive powers to be exchanged. A generalized droop control is employed to consider the impact of voltage magnitude and frequency differences on exchanging active and reactive powers separately for any type of interlinking lines. In this emergency power exchange method, a logical control is employed to detect the emergency condition, which results in the IMG formation, as well as to detect the normal condition and form the individual MGs operation again. The real-time results, obtained from the OPAL-RT simulator, practically show advantages of the proposed method, including voltage and frequency supports, postponing the load shedding, plug-and-play capability, good power sharing between the IMGs, and method compatibility with the controls of individual MGs.

Compliance of Distribution System Reactive Flows with Transmission System Requirements

Transmission system operators (TSOs) often set requirements to distribution system operators (DSOs) regarding the exchange of reactive power on the interface between the two parts of the system they operate, typically High Voltage and Medium Voltage. The presence of increasing amounts of Distributed Energy Resources (DERs) at the distribution networks complicates the problem, but provides control opportunities in order to keep the exchange within the prescribed limits. Typical DER control methods, such as constant cosϕ or Q/V functions, cannot adequately address these limits, while power electronics interfaced DERs provide to DSOs reactive power control capabilities for complying more effectively with TSO requirements. This paper proposes an optimisation method to provide power set-points to DERs in order to control the hourly reactive power exchanges with the transmission network. The method is tested via simulations using real data from the distribution substation at the Sundom Smart Grid, in Finland, using the operating guidelines imposed by the Finnish TSO. Results show the advantages of the proposed method compared to traditional methods for reactive power compensation from DERs. The application of more advanced Model Predictive Control techniques is further explored.

Application-Oriented Reactive Power Management in German Distribution Systems Using Decentralized Energy Resources

Due to higher penetration of renewable energy sources, grid reinforcements, and the utilization of local voltage control strategies, a significant change in the reactive power behavior as well as an increased demand for additional reactive power flexibility in the German power system can be predicted. In this paper, an application-oriented reactive power management concept is proposed, which allows distribution system operators (DSO) to enable a certain amount of reactive power flexibility at the grid interfaces while supporting voltage imitations in the grid. To evaluate its feasibility, the proposed concept is applied for real medium voltage grids in the south of Germany and is investigated comprehensively in different case studies. The results prove the feasibility and reliability of the proposed concept, which allows the DSO to control the reactive power exchange at grid interfaces without causing undesired local voltage problems. In addition, it can be simply adjusted and widely applied in real distribution grids without requiring high investment costs for complex information and communication infrastructures. As a significant contribution, this study provides an ideal bridging solution for DSOs who are facing reactive power issues but have no detailed and advanced monitoring system for their grid. Moreover, the comprehensive investigations in this study are performed in close cooperation with a German DSO, based on a detailed grid model and real measurement data.