Unbalanced Distribution Networks Research Articles

Optimal parameterization of Kalman filter based three-phase dynamic state estimator for active distribution networks

This paper presents a new method for the assessment of the process noise covariance matrix for three-phase dynamic state estimation in unbalanced active distribution networks which operate under normal conditions. The assessment is done in order to minimize the estimation error. The proposed assessment method, based on minimization of a particular cost function, enables the a priori assessment of covariance matrix by extracting information from previously observed measurements, without the need to simulate the true state of the system. The method was applied on two commonly used Kalman filter based estimation algorithms in nonlinear systems: Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF). A comparative analysis was performed between two different cost functions based on average root mean square of innovations and maximum likelihood technique. Also, the importance of determining initial state vector and its error covariance matrix needed for the initialization of dynamic state estimation was examined, as well as the ability of UKF and EKF to handle measurement nonlinearities. The analysis was carried out and the proposed method was verified on modified IEEE 13- and 37-bus distribution test systems.

Zero-sequence voltage trajectory analysis for unbalanced distribution networks on single-line-to-ground fault condition

Faulty phase recognition under single-line-to-ground (SLG) fault condition is critical for some arc suppression methods in medium voltage (MV) networks. Traditional methods usually assume that the distributed parameters of the network are symmetrical, thus are unsuitable for the asymmetrical networks, especially when the ground-fault resistance is relatively high. In this paper, the trajectory of the zero-sequence voltage with the change of ground-fault conductance is analyzed in detail. The magnitude and phase angle variation rules of the voltage under different asymmetry ratios are presented. Then, the ranges of their unique variation rules on SLG fault for each phases are separately and strictly discussed both for the under-compensated and over-compensated grounding conditions. Furthermore, a faulty phase recognition method is proposed based on the variation rules of zero-sequence voltage on faulty condition. Simulation results verify the concluded variation rules and validate the effectiveness of the proposed fault phase recognition method, which ensures exact recognition of faulty phase in asymmetrical network and high-resistance ground fault condition.

Optimized Dispatch of Energy Storage Systems in Unbalanced Distribution Networks

This paper presents a method to achieve optimal active and reactive power contributions from each energy storage system in an unbalanced distribution network to minimize power loss, while ensuring network current and voltage constraints are satisfied. By modeling loads as either constant current or constant impedance, the ac optimal power flow is transformed into a noniterative convex optimization problem. The application of capacity constraints, voltage constraints, and energy storage constraints in an unbalanced three-phase four-wire system is considered, addressing specific issues pertaining to unbalanced networks such as voltage unbalance and neutral voltage displacement. The proposed method is then used to demonstrate optimized dispatch of energy storage systems in a suitable four-wire unbalanced distribution test network. The contribution of losses in the neutral wire to the total losses is also determined for a test system under a range of operating conditions and various neutral earthing systems, highlighting the importance of considering this in a typical unbalanced distribution network.

Robust Capacity Assessment of Distributed Generation in Unbalanced Distribution Networks Incorporating ANM Techniques

To settle a large-scale integration of renewable distributed generations (DGs), it requires to assess the maximal DG hosting capacity of active distribution networks (ADNs). For fully exploiting the ability of ADNs to accommodate DG, this paper proposes a robust comprehensive DG capacity assessment method considering three-phase power flow modelling and active network management (ANM) techniques. The two-stage adjustable robust optimization is employed to tackle the uncertainties of load demands and DG outputs. With our method, system planners can obtain the maximum penetration level of DGs with their optimal sizing and sitting decisions. Meanwhile, the robust optimal ANM schemes can be generated for each operation time period, including network reconfiguration, on-load-tap-changers regulation, and reactive power compensation. In addition, a three-step optimization algorithm is proposed to enhance the accuracy of DG capacity assessment results. The optimality and robustness of our method are validated via numerical tests on an unbalanced IEEE 33-bus distribution system.

The impact of harmonics compensation ancillary services of photovoltaic microgeneration in low voltage distribution networks

PhotoVoltaic (PV) microgeneration (μG) located close to the end-users is gradually increasing and is expected to increase more in the future. However, a high μG penetration level may cause overvoltage issues in the Low Voltage (LV) distribution grid. On the other hand, these μG PV generators are usually placed in residential houses, composed by nonlinear loads that generate harmonics. The use of the harmonics compensation capabilities provided by modern PV systems can be an added value, as this function improves the voltage profile and reduces losses on the LV distribution network. This paper proposes an additional contribution to the subject by studying the impact of the use of PV systems harmonics compensation features to mitigate the harmonics produced by residential nonlinear loads. The assessment tool is a suitable algorithm created to compute an unbalanced three-phase power flow in the presence of harmonics. This is based on the Common Object Model (COM) interface of two software tools, MATLAB and OpenDSS. Three case-studies are proposed: harmonic compensation disabled, harmonic compensation enabled and reactive power control. The last one is introduced for comparison purposes. The assessment is made for a typical summer day, on a highly unbalanced test radial LV distribution network with high μG penetration and harmonic pollution. The obtained results allow the conclusion that the PV system harmonic compensation capability is able to attenuate the harmonics, therefore improving the voltage profile, and reducing the power losses and voltage drop on the LV distribution network.

Distributed optimal residential demand response considering operational constraints of unbalanced distribution networks

As a typical approach to demand response (DR), direct load control (DLC) enables a load service entity (LSE) to adjust the electricity usage of residential customers for peak shaving during a DLC event. Households are connected in low-voltage distribution networks, which are always three-phase unbalanced. However, existing work has not considered the detailed operational constraints of three-phase distribution networks, which may lead to decisions that deviate from reality or are even infeasible in practice. Moreover, centralised control may cause privacy and communication issues. This study proposes a distributed residential DLC method that considers the operational constraints of three-phase unbalanced distribution networks and privacy of residential customers. Numerical tests on IEEE benchmark systems demonstrate effectiveness of the method. The proposed distributed method can converge within 17 iterations in IEEE 123-bus distribution system, which demonstrates scalability of the proposed algorithm.

Assessment of overvoltage mitigation techniques in low‐voltage distribution networks with high penetration of photovoltaic microgeneration

Nowadays, a strong concern to decrease greenhouse gas emissions is encouraging the implementation of renewable energy sources closer to end-users, in low-voltage (LV) distribution networks. Due to the expected high microgeneration (μG) penetration level, several problems are likely to arise, such as overvoltages and reverse power flow. This study presents a review of the several techniques used to deal with these problems. These are compared in terms of their capacity to smooth the voltage profile and avoid reverse power flow. An unbalanced three-phase power flow algorithm, based on current summation method for radial distribution networks, is proposed. A study based on a highly unbalanced test radial LV distribution network for a typical summer day, with a high μG penetration, is performed. The voltage profile, active power flow in the service transformer, and power losses on the network are the monitored electrical quantities. The obtained results indicate that self-consumption with storage is the recommended solution to eliminate overvoltages, to avoid reverse power flow and allow for a decreasing in the power losses. Nevertheless, the economic viability of this solution must be carefully assessed, because the profitability of the project is not straightforward at the current time.

Adaptive robust optimal reactive power dispatch in unbalanced distribution networks with high penetration of distributed generation

Due to the increasing penetration of the uncertain single-phase distributed generations (DGs), the current distribution networks are becoming more uncertain, unbalanced, and complicated than ever before, which brings great challenges for distribution network operators. This study proposes an adaptive robust optimal reactive power dispatch approach for the unbalanced distribution networks (U-DNs) considering uncertainty caused by DGs. Leveraging the reactive power compensation from inverters of DGs, the purpose of the proposed method is to minimise power losses and maintain the voltage within regulatory limits. The feasible region of DGs is estimated and considered as a new constraint, aiming to guarantee the reliable operation of U-DNs. The optimal reactive power dispatch problem is then formulated as an adaptive robust optimisation problem based on semidefinite programming. The adaptive function, which derives the relationship between reactive power and active power outputs of DGs, is utilised to make the method more flexible and less conservative. The cutting plane algorithm is introduced to solve the proposed adaptive robust reactive power dispatch model efficiently. Moreover, case studies are separately conducted on the modified IEEE 13-bus and 123-bus test systems to demonstrate the effectiveness of the proposed method.

A new approach to voltage management in unbalanced low voltage networks using demand response and OLTC considering consumer preference

Voltage unbalance and magnitude violations under normal operating conditions have become main power quality problems in many low voltage (LV) distribution networks. Maintaining the voltage level in an LV network within the standard limits is the main constraining factor in increasing the network hosting ability for rooftop photovoltaic (PV). This study presents a new effective method for voltage management in unbalanced distribution networks through the implementation of optimal residential demand response (DR) and on-load tap changers (OLTCs). The proposed method minimises the compensation costs of voltage management (cost of DR and network loss), while prioritises the consumer consumption preferences for minimising their comfort level violations. A modified particle swarm optimisation algorithm (MPSO) is utilised to identify the optimal switching combination of household appliances and OLTC tap positions for the network voltage management. The proposed method is comprehensively examined on a real three-phase four-wire Australian LV network with considerable unbalanced and distributed generations. Several scenarios are investigated for improving the network voltage magnitude and unbalance considering individual and coordinated operations of DR and OLTCs (three phase tap control and independent phase tap control). Simulation results show that the coordinated approach of DR and OLTC, especially, DR integrated with OLTC independent phase tap control effectively improves the network voltage and increases the PV hosting capacity.

Optimal Placement and Sizing of Distributed Generation in an Unbalance Distribution System using Gray Wolf Optimization Method

The distributed generation sources (DGs) are becoming increasingly attractive due to introduction of small scale renewable energy sources. They can be integrated in to low voltage distribution networks, to reduce the burden on transmission and sub transmission network. However, the number of DGs, their placement, and sizing can influence the advantages from the distribution network operation point of view. Also, most of the time the planning is done considering the peak load demand only. However, the losses obtained at peak load, may not give the realistic picture. This paper demonstrates the application of a grey wolf optimisation method for obtaining the optimal size and location of DGs (solar photovoltaic-based) in an unbalanced distribution network. The method proposed in this paper provides a set of solutions from the point of view of voltage stability enhancement and loss minimisation. The utility can prioritise either voltage stability enhancement or loss minimisation or both to choose the best compromised solution. Moreover, the losses are calculated by considering the seasonal load and PV generation patterns during the year to simulate the real picture of distribution system. Results on 33 bus balanced and 25 bus unbalanced distribution system are taken to demonstrate the potential of the proposed algorithm.

Convergence of the Z-Bus Method for Three-Phase Distribution Load-Flow with ZIP Loads

This paper derives a set of sufficient conditions guaranteeing that the load-flow problem in unbalanced three-phase distribution networks with wye and delta constant-power, constant-current, and constant-impedance loads (ZIP loads) has a unique solution over a region that can be explicitly calculated from the network parameters. It is also proved that the well-known Z-Bus iterative method is a contraction over the defined region, and hence converges to the unique solution.

Research on Reactive Power Compensation Optimization of 35kV Three Phase Unbalanced Distribution Network System

In view of the actual situation of three-phase load unbalance in distribution power system, a three-phase load balancing compensation method for three-phase-and-three-wire systems in distribution network is proposed, and load model is established. Then I have deduced the mathematical model of three-phase load compensation in distribution network, and analyzed the energy loss of the two kinds of compensation method about the power supply and the dispersion compensation on the load spot. It is verified that the load end compensation can contribute more to the reduction of energy loss, under the premise of ensuring active load, possess more economical, this method has important significance for the solution in research on reactive power compensation optimization of three phase unbalanced distribution power network.

Dynamic reconfiguration of three-phase unbalanced distribution networks

The increasing penetration of distributed generations (DGs) and variable loads introduces significant power fluctuations to distribution networks, rendering conventional reconfiguration strategies ineffective. In the context of an active distribution network (ADN), remote control switches can be operated in real time through a centralized control scheme. Therein the distribution network topology can be configured in a flexible and dynamic manner capable of adapting to time-varying load demand and DG output. This paper presents a dynamic reconfiguration approach for a three-phase unbalanced distribution network. The ADN topology is optimized for the look-ahead time periods and is adaptive to the time-varying load demand and DG output while minimizing the daily power loss costs. To improve the calculation efficiency, several linearization methods are introduced to formulate the dynamic reconfiguration as a mixed-integer linear programming problem, which can be effectively solved using off-the-shelf solvers. The effectiveness of the proposed approach is verified by the test results obtained on a modified IEEE 34 node test feeder.

Optimally Allocation of Distributed Generators in Three-Phase Unbalanced Distribution Network

Increasing energy demand can be compensated with integration of distributed energy resources in the three-phase distribution system. Load flow analysis of the unbalanced three-phase distribution system requires a tool and algorithm to manage the multiple sources. In this study, Jaya algorithm is applied and interfaced with open source software openDSS to solve the unbalanced three-phase optimal power flow. Further, co-simulation framework is used to obtain the optimal allocation of two types of multiple distributed generators in unbalanced radial distribution system. The effectiveness of the approach is validated on IEEE 123 node distribution system. For a realistic study, mixes of all type of loads and configuration of the actual distribution system are considered. The results are compared with already published results obtained from established particle swarm optimization.

Effective Loss Minimization and Allocation of Unbalanced Distribution Network

An efficient distribution network must be able to supply power with good voltage profile. The main objective of the proposed work is to allocate losses of the unbalanced distribution network by the firefly algorithm in regulated and deregulated environments before and after loss minimization. Reconfiguration is one of the methods for loss reduction of unbalanced distribution network. Further, optimal placement of distributed generation and capacitor in the reconfigured unbalanced distribution network can further reduce the loss. The results of reconfigured unbalanced distribution network in regulated environment have already been reported. In this paper reconfiguration of an unbalanced distribution network in a deregulated environment is also carried out using an established Fuzzy Firefly algorithm. Loss sensitivity factor of unbalanced distribution networks is used to get the appropriate location of distributed generation and capacitor to be placed in the unbalanced distribution network. Their ratings have been found out by using bacteria foraging optimization algorithm (BFOA). The suggested loss allocation method using Firefly algorithm is implemented at first on 13 node unbalanced distribution network to check the performance of the proposed loss allocation method when compared to other available method. Finally the proposed method has been implemented on 25 node unbalanced distribution network. Both of the implementations are carried out under MATLAB environment.

Decomposed Newton algorithm-based three-phase power-flow for unbalanced radial distribution networks with distributed energy resources and electric vehicle demands

This paper proposes a three-phase power-flow algorithm using graph theory, injected current, and matrix decomposition techniques for unbalanced radial distribution networks. A decomposed Newton-Raphson (DNR) method is applied to solve the set of nonlinear power equations described in polar form. Unlike conventional Newton-Raphson-based methods, the proposed DNR algorithm does not involve calculating the lower and upper triangular matrix (LU) factorization, Gaussian elimination, and inversion of the full bus admittance matrix or Jacobian matrix and building the bus impedance matrix; it also requires less computation time and has high robustness with respect to the X/R ratio and load changes. The mathematical component models, such as three-phase conductors, transformers, automatic voltage regulators (AVRs), ZIP load demands, shunt capacitors/reactors, inverter-based distributed energy resources (DERs) and electric vehicle (EV) demands can easily be integrated into the proposed algorithm by using the injected current technique. Therefore, a three-phase power-flow problem can be decomposed into three single-phase power-flow problems with individual phase representation. To validate the performance and effectiveness of the proposed algorithm, four three-phase IEEE test systems and a practical Taiwan Power Company (Taipower) distribution system are used for comparisons. The results reveal that the proposed algorithm has good potential for improving the computational efficiency of optimal planning and design and also real-time power dispatch applications, even for ill-conditioned distribution networks.

Newton Power Flow Methods for Unbalanced Three-Phase Distribution Networks

Two mismatch functions (power or current) and three coordinates (polar, Cartesian and complex form) result in six versions of the Newton–Raphson method for the solution of power flow problems. In this paper, five new versions of the Newton power flow method developed for single-phase problems in our previous paper are extended to three-phase power flow problems. Mathematical models of the load, load connection, transformer, and distributed generation (DG) are presented. A three-phase power flow formulation is described for both power and current mismatch functions. Extended versions of the Newton power flow method are compared with the backward-forward sweep-based algorithm. Furthermore, the convergence behavior for different loading conditions, R / X ratios, and load models, is investigated by numerical experiments on balanced and unbalanced distribution networks. On the basis of these experiments, we conclude that two versions using the current mismatch function in polar and Cartesian coordinates perform the best for both balanced and unbalanced distribution networks.

Topological analysis of unbalanced distribution networks with single‐phase switching equipment and temporary elements

Summary A need to minimize the duration of outages in weakly meshed distribution networks leads to the intensive application of single-phase switching equipment and temporary elements. The result creates a wide range of allowed, but not typical, network topologies such as cross-phase jumpers, meshed islands, pseudo loops, and bidirectional branches, which cannot be processed in a simple/efficient way with existing algorithms for topological analyses. This paper proposes improvement of the traditional practice in distribution network modeling and topological analysis by using the phase granular adjacent matrix model with static and dynamic data, which provides a simple and efficient update of the mathematical model after switching and temporary element application. Additionally, the proposed algorithm provides a wide range of topological information such as network-layered structure, tracing, islands, energization, and active network phases. The results of the proposed algorithm are shown in an 84-bus test and 502,113-bus real-world schemes of unbalanced distribution networks.

Linear three-phase power flow for unbalanced active distribution networks with PV nodes

High penetration of distributed renewable energy promotes the development of an active distribution network (ADN). The power flow calculation is the basis of ADN analysis. This paper proposes an approximate linear three-phase power flow model for an ADN with the consideration of the ZIP model of the loads and PV nodes. The proposed method is not limited to radial topology and can handle high R/X ratio branches. Case studies on the IEEE 37-node distribution network show a high accuracy and the proposed method is applicable to practical uses such as linear or convex optimal power flow of the ADN.

Volt-VAr Control and Energy Storage Device Operation to Improve the Electric Vehicle Charging Coordination in Unbalanced Distribution Networks

In this paper, a new approach is presented to solve the electric vehicle charging coordination (EVCC) problem considering Volt-VAr control, energy storage device (ESD) operation and dispatchable distributed generation (DG) available in three-phase unbalanced electrical distribution networks (EDNs). Dynamic scheduling for the EVCC is proposed through a step-by-step methodology, which solves a mixed integer linear programming (MILP) problem for the whole time period. The objective is to minimize the total cost of energy purchased from the substation and DG units, the cost of energy curtailment on electric vehicles, the cost of energy injected from the ESDs, and the cost of energy curtailment on the ESDs. The Volt-VAr control considers the management of on-load tap changers, voltage regulators, and switchable capacitors installed along the grid. Furthermore, the formulation takes into account the voltage dependence of the loads, while the steady-state operation of the unbalanced distribution systems is modeled using linear constraints. The proposed model was tested in a 178-node three-phase unbalanced EDN considering a one-day time period.