Comparison of the performance of sensitivity-based voltage control algorithms in DG-integrated distribution systems

Conference ProceedingsThe integration of renewable energy generators in distribution grids has increased the complexity of the voltage control problem. Reactive power control (RPC) algorithms based on sensitivity analysis have been proposed in the literature for the management of the voltage problem. Sensitivity methods are computationally complex for practical real-time analysis and this has led to use of de-coupled and other simplified load flow models. However, algorithms based on decoupled models have been shown to be inefficient for analysis of distribution systems with low X/R ratio. This paper uses a simplified line modelling approach recently proposed in the literature to facilitate the development of computationally simple distributed, non-decoupled, load flow equations that completely capture the characteristics of the radial distribution feeder, removing the need to use the decoupled models. Results show that the simple algorithm based on this new line modelling approach gives better voltage control performance compared to the decoupled models

Virtual energy storage system for voltage control of distribution networks

—Increasing amount of Distributed Generation (DG) connected to distribution networks may lead to the voltage and thermal limits violation. This paper proposes a Virtual Energy Storage System (VESS) to provide voltage control in distribution networks in order to accommodate more DG. A VESS control scheme coordinating the demand response and the energy storage system was developed. The demand response control measures the voltage of the connected bus and changes the power consumption of the demand to eliminate voltage violations. The response of energy storage systems was used to compensate for the uncertainty of demand response. The voltage control of energy storage system is a droop control with droop gain values determined by voltage sensitivity factors. The control strategy of the VESS was applied to a medium-voltage network and results show that the control of VESS not only facilitates the accommodation of higher DG capacity in the distribution network without voltage violations or network reinforcements but also prolongs the lifetime of transformer on-load tap changer

Optimization of the Steady Voltage Profile in Distribution Systems by Coordinating the Controls of Distributed Generations

International audienceDistributed generation (DG) is more and more installed and integrated in the electric power systems, mainly in distribution networks. The conventional method of voltage control with on-load tap changer transformers is not efficient enough to deal with the growth of integration of distributed generators. The DG power regulation will contribute to optimize the steady voltage profile in network and thus to rise the maximal DG installation capacities of network. The work reported in this paper shows that the coordination of DG power regulation can realize a voltage control of distribution network. The coupling between active power and voltage magnitude must be taken into account according to the our results

Virtual energy storage for frequency and voltage control

The secure and economic operation of the future power system is facing major challenges. These challenges are driven by the increase of the penetration of converter connected and distributed renewable generation and electrified demand. In this thesis, a new smart energy management paradigm, i.e. a Virtual Energy Storage System (VESS), to address these challenges was studied. A VESS aggregates energy storage and flexible demand units into a single entity which performs similarly to a large-capacity conventional energy storage system. A VESS mitigates the uncertainty of the response from flexible demand through coordination with a minimum capacity of costly energy storage systems. Mathematical models of four components of a VESS were developed. Specifically, models of two types of energy storage, i.e. flywheel energy storage and battery energy storage, were developed. Thermodynamic models of two types of flexible demand units, i.e. domestic refrigerators and industrial bitumen tanks, were developed. These models were validated against the performance of similar equipment from the literature. Aggregated models, representing a population of units, for each of flywheels, batteries, refrigerators and bitumen tanks were developed. These aggregated models represent a randomly diversified population of units. These aggregated models were used to establish the frequency and the voltage control schemes of a VESS. A frequency control scheme of the VESS was designed. The control scheme provides low, high and continuous frequency response services to the system operator. The centralised control scheme coordinates models of refrigerators and units of the flywheel energy storage system. Following frequency deviations, the local frequency controllers of refrigerators changed their power consumption. The local frequency controllers of the flywheel units cover the power mismatch between the change in refrigerators power consumption and the required response from the VESS. The required response from the VESS is determined by a droop control. Case studies were conducted to evaluate the frequency control scheme by connecting the VESS to a simplified GB power system. Results showed that the response from the frequency control scheme of the VESS was similar to that of only flywheel energy storage. Based on an economic evaluation, the VESS is estimated to obtain approximately 50% higher return compared with the case II that only uses flywheel energy storage system. These revenues are based on providing frequency response services to the system operator. A voltage control scheme of the VESS was also designed. The control scheme facilitates the integration of distributed renewable energy generation by enhancing the voltage control of the distribution network. The control scheme coordinates models of bitumen tanks and battery energy storage system through different time delay settings of their voltage controllers. The local voltage controllers of bitumen tanks alter their power consumption following significant voltage deviations. If voltage violations continue, the distributed voltage controller of the battery energy storage system charges or discharges the battery using a droop setting obtained from voltage sensitivity factors. A case study was undertaken to assess the voltage control scheme by connecting the VESS, solar panels and wind farms to a UK Generic Distribution System (UKGDS) network. Results showed that the voltage control scheme made a significant improvement in the voltage and reduced tap changing actions of the on-load tap changing transformer and the voltage regulator by approximately 30 % compared with the base case where no VESS was used. Based on an economic evaluation, The VESS is an efficient solution to accommodate distributed renewable energy generation compared with network reinforcement

Active Voltage Control in Distribution Networks Including Distributed Energy Resources

The structure and control methods of existing distribution networks are planned assuming unidirectional power flows. The amount of generation connected to distribution networks is, however, constantly increasing which changes the operational and planning principles of distribution networks radically. Distributed generation (DG) affects power flows and fault currents in the distribution network and its effect on network operation can be positive or negative depending on the size, type, location and time variation of the generator. In weak distribution networks, voltage rise is usually the factor that limits the network’s hosting capacity for DG. At present, voltage rise is usually mitigated either by increasing the conductor size or by connecting the generator to a dedicated feeder. These passive approaches maintain the current network operational principles but can lead to high DG connection costs. The voltage rise can be mitigated also using active voltage control methods that change the operational principles of the network radically but can, in many cases, lead to significantly smaller total costs of the distribution network than the passive approach. The active voltage control methods can utilize active resources such as DG in their control and also the control principles of existing voltage control equipment such as the main transformer tap changer can be altered. Although active voltage control can often decrease the distribution network total costs and its effect on voltage quality can also be positive, the number of real distribution network implementations is still very low and the distribution network operators (DNOs) do not consider active voltage control as a real option in distribution network planning. Some work is, hence, still needed to enable widespread utilization of active voltage control. This thesis aims at overcoming some of the barriers that are, at present, preventing active voltage control from becoming business as usual for the DNOs. In this thesis, active voltage control methods that can be easily implemented to real distribution networks are developed. The developed methods are, at first, tested using time domain simulations. Operation of one coordinated voltage control (CVC) method is tested also using real time simulations and finally a real distribution network demonstration is conducted. The conducted simulations and demonstrations verify that the developed voltage control methods can be implemented relatively easily and that they are able to keep all network voltages between acceptable limits as long as an adequate amount of controllable resources is available. The developed methods control the substation voltage based on voltages in the whole distribution network and also reactive and real powers of distributed energy resources (DERs) are utilized in some of the developed CVC methods. All types of DERs capable of reactive or real power control can be utilized in the control. The distribution network planning tools and procedures used currently are not capable of taking active voltage control into account. DG interconnection planning is based only on two extreme loading conditions (maximum generation/minimum load and minimum generation/maximum load) and network effects and costs of alternative voltage control methods cannot be compared. In this thesis, the distribution network planning procedure is developed to enable comparison of different voltage control strategies. The statistical distribution network planning method is introduced and its usage is demonstrated in example cases. In statistical distribution network planning, load flow is calculated for every hour of the year using statistical-based hourly load and production curves. When the outputs of hourly load flows (e.g. annual losses, transmission charges and curtailed generation) are combined with investment costs the total costs of alternative voltage control strategies can be compared and the most cost-effective approach can be selected. The example calculations show that the most suitable voltage control strategy varies depending on the network and DG characteristics. The studies of this thesis aim at making the introduction of active voltage control as easy as possible to the DNOs. The developed CVC methods are such that they can be implemented as a part of the existing distribution management systems and they can utilize the already existing data transfer infrastructure of SCADA. The developed planning procedure can be implemented as a part of the existing network information systems. Hence, the currently used network planning and operational tools do not need to be replaced but only enhanced

Active Voltage Control in Distribution Networks Including Distributed Energy Resources

The structure and control methods of existing distribution networks are planned assuming unidirectional power flows. The amount of generation connected to distribution networks is, however, constantly increasing which changes the operational and planning principles of distribution networks radically. Distributed generation (DG) affects power flows and fault currents in the distribution network and its effect on network operation can be positive or negative depending on the size, type, location and time variation of the generator. In weak distribution networks, voltage rise is usually the factor that limits the network’s hosting capacity for DG. At present, voltage rise is usually mitigated either by increasing the conductor size or by connecting the generator to a dedicated feeder. These passive approaches maintain the current network operational principles but can lead to high DG connection costs. The voltage rise can be mitigated also using active voltage control methods that change the operational principles of the network radically but can, in many cases, lead to significantly smaller total costs of the distribution network than the passive approach. The active voltage control methods can utilize active resources such as DG in their control and also the control principles of existing voltage control equipment such as the main transformer tap changer can be altered. Although active voltage control can often decrease the distribution network total costs and its effect on voltage quality can also be positive, the number of real distribution network implementations is still very low and the distribution network operators (DNOs) do not consider active voltage control as a real option in distribution network planning. Some work is, hence, still needed to enable widespread utilization of active voltage control. This thesis aims at overcoming some of the barriers that are, at present, preventing active voltage control from becoming business as usual for the DNOs. In this thesis, active voltage control methods that can be easily implemented to real distribution networks are developed. The developed methods are, at first, tested using time domain simulations. Operation of one coordinated voltage control (CVC) method is tested also using real time simulations and finally a real distribution network demonstration is conducted. The conducted simulations and demonstrations verify that the developed voltage control methods can be implemented relatively easily and that they are able to keep all network voltages between acceptable limits as long as an adequate amount of controllable resources is available. The developed methods control the substation voltage based on voltages in the whole distribution network and also reactive and real powers of distributed energy resources (DERs) are utilized in some of the developed CVC methods. All types of DERs capable of reactive or real power control can be utilized in the control. The distribution network planning tools and procedures used currently are not capable of taking active voltage control into account. DG interconnection planning is based only on two extreme loading conditions (maximum generation/minimum load and minimum generation/maximum load) and network effects and costs of alternative voltage control methods cannot be compared. In this thesis, the distribution network planning procedure is developed to enable comparison of different voltage control strategies. The statistical distribution network planning method is introduced and its usage is demonstrated in example cases. In statistical distribution network planning, load flow is calculated for every hour of the year using statistical-based hourly load and production curves. When the outputs of hourly load flows (e.g. annual losses, transmission charges and curtailed generation) are combined with investment costs the total costs of alternative voltage control strategies can be compared and the most cost-effective approach can be selected. The example calculations show that the most suitable voltage control strategy varies depending on the network and DG characteristics. The studies of this thesis aim at making the introduction of active voltage control as easy as possible to the DNOs. The developed CVC methods are such that they can be implemented as a part of the existing distribution management systems and they can utilize the already existing data transfer infrastructure of SCADA. The developed planning procedure can be implemented as a part of the existing network information systems. Hence, the currently used network planning and operational tools do not need to be replaced but only enhanced