Distributed Voltage Control Research Articles

Evaluation of an enhanced power dispatch control scheme for multi-terminal HVDC grids using Monte-Carlo simulation

This paper presents a new multi-objective optimization technique for multi-terminal DC (MTDC) networks to enhance power dispatch control under stochastic operating conditions. The proposed method utilizes the DC voltage droop control concept to achieve distributed DC voltage control of the MTDC grid, while a centralized power dispatch controller monitors system operation and updates the voltage droop settings of the onshore converters in order for them to comply with scheduled power set-points dispatched by the transmission system operator, maintaining at the same time low system losses. The overall optimization problem is formulated and its effectiveness is assessed applying the Monte-Carlo simulation method. A five-terminal MTDC grid is selected as a study-case system, which corresponds to the planning for the future interconnection of Aegean Sea islands with significant wind potential to the mainland grid.

Voltage Stability and Reactive Power Sharing in Inverter-Based Microgrids With Consensus-Based Distributed Voltage Control

We propose a consensus-based distributed voltage control (DVC) that solves the problem of reactive power sharing in autonomous inverter-based microgrids with dominantly inductive power lines and arbitrary electrical topology. Opposed to other control strategies available thus far, the control presented here does guarantee a desired reactive power distribution in steady state while only requiring distributed communication among inverters, i.e., no central computing nor communication unit is needed. For inductive impedance loads and under the assumption of small phase angle differences between the output voltages of the inverters, we prove that the choice of the control parameters uniquely determines the corresponding equilibrium point of the closed-loop voltage and reactive power dynamics. In addition, for the case of uniform time constants of the power measurement filters, a necessary and sufficient condition for local exponential stability of that equilibrium point is given. The compatibility of the DVC with the usual frequency droop control for inverters is shown and the performance of the proposed DVC is compared with the usual voltage droop control via simulation of a microgrid based on the Conseil International des Grands Reseaux Electriques (CIGRE) benchmark medium voltage distribution network.

Centralized Support Distributed Voltage Control by Using End-Users as Reactive Power Support

New generation of loads and the power factor correction devices of these loads provides an opportunity for distribution system operators to use them as reactive power support. In this paper, the integration of these end-user reactive-power-capable devices is investigated to provide voltage support to the grid. Due to the limitation on the number of smart homes, at first, the effective locations for injecting the reactive power into the distribution system is determined (i.e., Q-C buses) and showed how reactive power resources connected at those buses can be controlled. For control purposes, centralized support distributed voltage control (CSDVC) algorithm is proposed and the distribution system is decomposed into different areas by using ${\varepsilon}$ -decomposition. Then Q-C buses, as the candidate buses, for providing reactive power are obtained. Optimum reactive power injection in every candidate bus in each area and tap position of the regulator, if necessary, is calculated by using genetic algorithm. The CSDVC is proposed to cover the inaccuracy of the loads forecasting and the control area centers faults, and also to provide secure reactive power control. The proposed approach is tested on the IEEE 33 and 69 buses distribution test systems.

A New Method for Online Sensitivity-Based Distributed Voltage Control and Short Circuit Analysis of Unbalanced Distribution Feeders

Reactive power support through shunt capacitors and distributed generation is used in conjunction with step voltage regulators (SVRs) for voltage control of distribution feeders. Distribution management system is responsible for coordination among distributed reactive power sources, such as capacitors, photovoltaic and wind power generators, and SVRs. In this paper, a novel method for calculating voltage sensitivity to active, reactive power injection, and SVR taps in unbalanced distribution feeders is developed. The method is based on ABCD parameters of network components which are used in forward/backward sweep of power-flow solution. The method can deal with network unbalance, network shunt admittances, single-phase sections, and ungrounded sections. The initial sensitivities are calculated in one power-flow forward sweep through a recursive way, and the online updating of sensitivities is very easy and straightforward so the method is suitable for online voltage control. A novel sensitivity-based online optimization method is developed for online coordinated voltage control with no prior information about the load. The same ABCD parameters are used for developing a novel method for calculating short circuit currents in unbalanced distribution feeders. The method can also handle unbalance, shunt elements, and single phase sections, and is applicable to all fault types. The developed method for voltage control and short circuit calculation are tested on the IEEE comprehensive test feeder and the results prove the effectiveness of the proposed algorithms.

Distributed Voltage Control with Electric Springs: Comparison with STATCOM

The concept of electric spring (ES) has been proposed recently as an effective means of distributed voltage control. The idea is to regulate the voltage across the critical (C) loads while allowing the noncritical (NC) impedance-type loads (e.g., water heaters) to vary their power consumption and thus contribute to demand-side response. In this paper, a comparison is made between distributed voltage control using ES against the traditional single point control with STATic COMpensator (STATCOM). For a given range of supply voltage variation, the total reactive capacity required for each option to produce the desired voltage regulation at the point of connection is compared. A simple case study with a single ES and STATCOM is presented first to show that the ES and STATCOM require comparable reactive power to achieve similar voltage regulation. Comparison between a STATCOM and ES is further substantiated through similar case studies on the IEEE 13-bus test feeder system and also on a part of the distribution network in Sha Lo Wan Bay, Hong Kong. In both cases, it turns out that a group of ESs achieves better total voltage regulation than STATCOM with less overall reactive power capacity. Dependence of the ES capability on proportion of critical and NC load is also shown.

Impact of Different Uncertainty Sources on a Three-Phase State Estimator for Distribution Networks

Distribution networks present their own features, significantly different from transmission systems features. For instance, loads are often unbalanced on the phases of the network. Moreover, the increasing amount of distributed generation, installed in an unplanned manner, is creating significant challenges. Such changing scenario imposes new operational requirements, such as distributed voltage control and demand side management. New performances are required for distribution system state estimators. In this paper, a study on the impact of different uncertainty sources on a state estimator designed for monitoring unbalanced distribution networks is presented. The impact of different measurement devices and levels of knowledge of the network behavior is analyzed and discussed using simulations performed on the 123-bus IEEE distribution network, which is commonly used as test network for this kind of study.

Dynamic Modeling of Electric Springs

The use of “Electric Springs” is a novel way of distributed voltage control while simultaneously achieving effective demand-side management through modulation of noncritical loads in response to the fluctuations in intermittent renewable energy sources (e.g., wind). The proof-of-concept has been successfully demonstrated on a simple 10-kVA test system hardware. However, to show the effectiveness of such electric springs when installed in large numbers across the power system, there is a need to develop simple and yet accurate simulation models for these electric springs which can be incorporated in large-scale power system simulation studies. This paper describes the dynamic simulation approach for electric springs which is appropriate for voltage and frequency control studies at the power system level. The proposed model is validated by comparing the simulation results against the experimental results. Close similarity between the simulation and experimental results gave us the confidence to use this electric spring model for investigating the effectiveness of their collective operation when distributed in large number across a power system. Effectiveness of an electric spring under unity and non-unity load power factors and different proportions of critical and noncritical loads is also demonstrated.

Maximising low voltage grid hosting capacity for PV and electric mobility by distributed voltage control

European low voltage grids host an increasing amount of energy from rooftop PV installations. Experience in Germany has shown that PV installations do not grow uniformly but with regional hot spots. Meanwhile it is widely agreed that low voltage grids should only be updated to accommodate some 90 % or less of available PV power in peak times in order to avoid extensive costs for grid reinforcement. The main limiting factor in rural distribution grids is the grid voltage. In the Austrian research project “DG DemoNet—Smart LV Grid” new approaches to deal with restricted grid capacities are analysed using active voltage control. In three pilot regions, the actual amount of installed PV is increased to reach physical limits. Voltage control schemes are developed, tested in simulations and then validated in the field. In one pilot region, electric mobility as additional future load in low voltage grids and its integration into the overall system is considered. This paper analyses the performance of the developed control approaches in simulation.

Power Flow Algorithms for Multi-Terminal VSC-HVDC With Droop Control

This paper addresses the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems. More realistic dc grid control strategies can thus be carefully considered in power flow analysis of ac/dc grids. Power flow methods for multi-terminal VSC-HVDC (MTDC) systems are analyzed for different types of dc voltage control techniques and the weaknesses of present methods are addressed. As distributed voltage control is likely to be adopted by practical dc grids, a new generalized algorithm is proposed to solve the power flow problems with various nonlinear voltage droops, and the method to incorporate this algorithm with ac power flow models is also developed. With five sets of voltage characteristics implemented, the proposed scheme is applied to a five-terminal test system and shows satisfactory performance. For a range of wind power variations and converter outages, post-contingency behaviors of the system under the five control scenarios are examined. The impact of these controls on the power flow solutions is assessed.

Distribution Voltage Control Considering the Impact of PV Generation on Tap Changers and Autonomous Regulators

The uptake of variable megawatts from photovoltaics (PV) challenges distribution system operation. The primary problem is significant voltage rise in the feeder that forces existing voltage control devices such as on-load tap-changers and line voltage regulators to operate continuously. The consequence is the deterioration of the operating life of the voltage control mechanism. Also, conventional non-coordinated reactive power control can result in the operation of the line regulator at its control limit (runaway condition). This paper proposes an optimal reactive power coordination strategy based on the load and irradiance forecast. The objective is to minimize the number of tap operations so as not to reduce the operating life of the tap control mechanism and avoid runaway. The proposed objective is achieved by coordinating various reactive power control options in the distribution network while satisfying constraints such as maximum power point tracking of PV and voltage limits of the feeder. The option of voltage support from PV plant is also considered. The problem is formulated as constrained optimization and solved through the interior point technique. The effectiveness of the approach is demonstrated in a realistic distribution network model.

Research on the Multilevel STATCOM based on the H-bridge Cascaded

This paper mainly studies the main circuit topology, the working principle, modulation technique and control strategy of the STATCOM based on H-bridge. The STATCOM direct power control algorithm based on the virtual flux is used to control the AC side reactive power compensation, and in accordance with the principle of conservation of power, the DC side voltage imbalance factors are analyzed, and a distributed DC voltage control algorithm is also introduced. The DC side control algorithm is divided into upper and lower control parts, of which the upper control section is aimed at the average of DC side capacitor voltage of the sub-module unit for the STATCOM device, the lower control part is mainly for each sub-module DC side capacitor voltage. Finally, the control strategy is verified in a three level chain STATCOM system with VME control box as its core controller, and the experimental results show that the algorithm is feasible.

Distributed voltage control strategy for LV networks with inverter-interfaced generators

Low voltage distribution networks are characterized by an ever growing diffusion of single and three phase distributed generators whose unregulated operation may deplete the power quality levels, in particular as regard voltage profiles and unbalances. This issue is at present under discussion by several national and international standardization bodies and the general trend is to require, for the new connections of generators to medium and low voltage grids, their participation to the reactive power network management. In this paper a novel strategy proposes to control the network voltage unbalance suitably for coordinating single and three-phase inverter interfaced embedded generators, concurrently with a local volt/var regulation action as foreseen by the new grid connection requirements. Simulations conducted on case study network representing a typical Italian 4-wire LV distribution system under different load/generation conditions, demonstrate that the coordinated action of single-phase and three-phase inverters may considerably reduce the degree of unbalance thus improving the network power quality levels.

A Novel Distributed Direct-Voltage Control Strategy for Grid Integration of Offshore Wind Energy Systems Through MTDC Network

Although HVDC transmission systems have been available since mid-1950s, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters, together with complex controls and need for fast telecommunication links, may have prevented the construction of multiterminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, this paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy—with distributed slack nodes—is proposed by means of a DC optimal power flow. The distributed voltage control (DVC) strategy is numerically illustrated by loss minimization in an MTDC network. Finally, dynamic simulations are performed to demonstrate the benefits of the DVC strategy.

Automatic Distributed Voltage Control Algorithm in Smart Grids Applications

The widespread use of distributed generation (DG), which is installed in medium-voltage distribution networks, impacts the future development of modern electrical systems that must evolve towards smart grids. A fundamental topic for smart grids is automatic distributed voltage control (ADVC). The voltage is now regulated at the MV busbar acting on the on-load tap changer of the HV/MV transformer. This method does not guarantee the correct voltage value in the network nodes when the distributed generators deliver their power. In contrast, the ADVC allows control of the voltage acting on a single generator; therefore, a better voltage profile can be obtained. In this paper, an approach based on sensitivity theory is shown to control the node voltages regulating the reactive power injected by the generators. After the theoretical analysis, a numerical example is presented to validate the theory. The proposed voltage regulation method has been developed in collaboration with Enel Distribuzione S.p.A. (the major Italian DSO), and it will be applied in the Smart Grids POI-P3 pilot project, which is financed by the Italian Economic Development Ministry. Before the real field application in the pilot project, a real-time digital simulation has been used to validate the algorithm presented. Moving in this direction, Enel Distribuzione S.p.A. built a new test center in Milan equipped with a real-time digital simulator (from RTDS Technologies).

A Novel Cooperative Protocol for Distributed Voltage Control in Active Distribution Systems

In this paper, a novel cooperative protocol has been proposed to provide a proper voltage control for multiple feeders having a transformer tap-changer (LTC), unbalanced load diversity (station with different feeder loads) and multiple distributed generation (DG) units in each feeder. The proposed cooperative protocol has been defined according to the distributed control technology, where LTC and DG units are considered as control agents. Two conflicting objectives have been defined for each control agent. The first objective aims to achieve the system requirements by minimizing the voltage deviation and the second objective aims to achieve the device requirements by reducing the tap operation and maximizing the energy capture for LTC and DG units, respectively. The interior structures of the control agents and the communication acts between them have been designed to achieve the best compromise between the two objectives of each control agent. The effectiveness of the proposed cooperative scheme has been verified via different case studies.

Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique

Installation of many distributed generations (DGs) could be detrimental to the power quality of utility grids. Microgrids facilitate effortless installation of DGs in conventional power systems. In recent years, dc microgrids have gained popularity because dc output sources such as photovoltaic systems, fuel cells, and batteries can be interconnected without ac/dc conversion, which contributes to total system efficiency. Moreover, high-quality power can be supplied continuously when voltage sags or blackouts occur in utility grids. We had already proposed a “low-voltage bipolar-type dc microgrid” and described its configuration, operation, and control scheme, through experiments. In the experiments, we used one energy storage unit with a dc/dc converter to maintain the dc-bus voltage under intentional islanding operation. However, dc microgrids should have two or more energy storage units for system redundancy. Therefore, we modified the system by adding another energy storage unit to our experimental system. Several kinds of droop controls have been proposed for parallel operations, some of which were applied for ac or dc microgrids. If a gain-scheduling control scheme is adopted to share the storage unit outputs, the storage energy would become unbalanced. This paper therefore presents a new voltage control that combines fuzzy control with gain-scheduling techniques to accomplish both power sharing and energy management. The experimental results show that the dc distribution voltages were within 340 V ± 5%, and the ratios of the stored energy were approximately equal, which implies that dc voltage regulation and stored energy balancing control can be realized simultaneously.

Operation and Power Flow Control of Multi-Terminal DC Networks for Grid Integration of Offshore Wind Farms Using Genetic Algorithms

For achieving the European renewable electricity targets, a significant contribution is foreseen to come from offshore wind energy. Considering the large scale of the future planned offshore wind farms and the increasing distances to shore, grid integration through a transnational DC network is desirable for several reasons. This article investigates a nine-node DC grid connecting three northern European countries — namely UK, The Netherlands and Germany. The power-flow control inside the multi-terminal DC grid based on voltage-source converters is achieved through a novel method, called distributed voltage control (DVC). In this method, an optimal power flow (OPF) is solved in order to minimize the transmission losses in the network. The main contribution of the paper is the utilization of a genetic algorithm (GA) to solve the OPF problem while maintaining an N-1 security constraint. After describing main DC network component models, several case studies illustrate the dynamic behavior of the proposed control method.

Advanced voltage control integrated in DMS

The paper deals with distribution network (DN) voltage control from the network operation point of view. The basic voltage control devices, under load and no load tap-changing transformers, are specially stressed. Distributed generators (DGs) and capacitor banks are also discussed. The classical principle of single line voltage drop compensation (LDC) is treated as the base voltage control. The optimal voltage control (OVC), which appeared 15 years ago, was developed to overcome some of LDC shortcomings. After the appearance of OVC, distribution management systems (DMSs) developed intensively and were applied immensely in distribution practice. This opportunity has been taken to integrate the distribution voltage control into DMS. The integration of the voltage control with Power flow, State estimation, Medium- and Short-term load forecast is of special interest for the paper. The high quality integration is provided by application of the same DN Data base and Mathematical model. In this way, the advanced voltage control has been developed. It ultimately overcomes all shortcomings of OVC (and LDC as well). This control is the subject of the paper and its properties are compared with LDC and OVC.

Investigation of alternative methods for distribution voltage control: Impact on maximum loadability

This paper reviews basic existing and alternative schemes for voltage and power factor control of bulk power delivery substations and compares the effectiveness of the different control methods in extending the loadability of a radial power system. Devices examined in different schemes include load tap changers (LTC), switched capacitor banks, Static Var Compensators (SVC) and StatCom. The possibility of controlling both the primary (transmission side) and secondary (distribution side) voltages is considered and is found to offer advantages when the control is introduced in the transmission side. Increased needs for reactive compensation can potentially be procured by devices with power electronic converters.

A distributed DC voltage control method for VSC MTDC systems

Abstract With the development of VSC HVDC transmission, the realization of the first VSC MTDC grid is coming within reach. An outstanding issue is DC voltage control in MTDC systems. The easiest way to maintain stable operation is to assign the task of DC voltage regulation to one converter. This paper discusses a control strategy for an extended VSC MTDC grid using a typical DC voltage control on one DC bus, combined with a DC voltage droop characteristic on the other DC buses. The impact of the proposed control structure on the stability of the AC and DC grid is investigated by numeric simulations using the MatDyn software package.