Three-phase Unbalanced Conditions Research Articles

Operation Optimization Adapting to Active Distribution Networks Under Three-Phase Unbalanced Conditions: Parametric Linear Relaxation Programming

Asymmetric phase distributed generation, which is widely used in distribution systems, aggravates the severity of the intrinsic three-phase unbalanced condition of distribution systems. To overcome these problems, operation optimization formulas of the three-phase unbalanced active distribution network (ADN) are established considering soft open point (SOP) and belong to a nonconvex quadratic programming (NQP) problem. However, in conventional methods, either approximations that are not valid for an unbalanced distribution system are used or convex relaxation methods are applied to primitive nonlinear optimization models. However, these methods do not guarantee a feasible optimization solution. Therefore, determining the solution to the NQP problem is challenging. In this article, we propose a parametric linear relaxation algorithm to obtain the optimal feasible solution by solving the operation optimization problem of the three-phase unbalanced ADN with the SOP. The parametric linear relaxation technique is constructed based on the properties of quadratic functions and the mean value theorem, and the NQP is converted to a series of parametric linear relaxation programming problems. The results revealed that the proposed algorithm can achieve the optimal and feasible solution and considerably reduce the computation time compared with an equivalent NLP optimization model for the same distribution system.

New phase‐changing soft open point and impacts on optimising unbalanced power distribution networks

Three-phase unbalanced conditions in distribution networks are conventionally caused by load imbalance, asymmetrical fault conditions of transformers and impedances of three phases. The uneven integration of single-phase distributed generation (DG) worsens the imbalance situation. These unbalanced conditions result in financial losses, inefficient utilisation of assets and security risks to the network infrastructure. In this study, a phase-changing soft open point (PC-SOP) is proposed as a new way of connecting soft open points (SOPs) to balance the power flows among three phases by controlling active power and reactive power. Then an operational strategy based on PC-SOPs is presented for three-phase four-wire unbalanced systems. By optimising the regulation of SOPs, optimal energy storage systems dispatch and DG curtailment, the proposed strategy can reduce power losses and three-phase imbalance. Second-order cone programming (SOCP) relaxation is utilised to convert the original non-convex and non-linear model into an SOCP model which can be solved efficiently by commercial solvers. Case studies are conducted on a modified IEEE 34-node three-phase four-wire system and the IEEE 123-node test feeder to verify the effectiveness, efficiency and scalability of the proposed PC-SOP concept and its operational strategy.

Enhanced harmonic state estimation in unbalanced three-phase electrical grids based on the Kalman filter and physical scale-down implementation

A time-domain methodology is proposed based on Kalman filter, to determine the harmonic state estimation in three-phase electrical grids. Time-domain methodologies based on the Kalman filter need an initial solution to begin estimating the harmonic state in electrical grids. Previous methods have found this solution using simulated electrical grids; thus, the measurements taken from the simulations are synchronised only with the simulated grid model. In practical power networks, measurements taken from the grid need to be synchronised with the theoretical grid model. Synchronization between the model and measurements using the Kalman filter is proposed. This paper also proposes an alternative method to obtain the periodic steady-state solution in power grids. The method adequately processes the information given by a limited number of measurements to find the required solution. An experimental case study for a three-phase unbalanced condition with a nonlinear load and a limited number of measurements is used to validate the proposed methodology, i.e. practical data has been used instead of synthetic data. Hence, the results are validated by direct comparison of the state estimation response against the actual data recorded from the experimental laboratory implementation. The results show that using the proposed methodology, the time-domain harmonic state estimation can be efficiently applied to the analysis of practical power grids.

Optimal Operation of Soft Open Points in Active Distribution Networks Under Three-Phase Unbalanced Conditions

The asymmetric integration of distributed generators (DGs) exacerbates the three-phase unbalanced condition in distribution systems. The serious unbalanced operation causes inefficient utilization of network assets and security risks in the system. Soft open point (SOP) is a flexible power electronic device which can achieve accurate active and reactive power flow control to balance the power flow among phases. This paper proposes an SOPs-based operation strategy for unbalanced active distribution networks. By regulating the operation of SOPs, the strategy can reduce power losses and simultaneously mitigate the three-phase unbalance of the upper-level grid. Semidefinite programming (SDP) relaxation is advocated to convert the original non-convex, nonlinear optimization model into an SDP formulation, which can be efficiently solved to meet the requirement of rapid adjustment. Case studies are conducted on the modified IEEE 33-node and IEEE 123-node distribution system to verify the effectiveness and efficiency of the proposed strategy.

Research on adaptability of unbalanced three-phase pv after asynchronous networking

After the asynchronous network of Yunnan Power Grid and China Southern Power Grid, the characteristics of small capacity and poor anti-disturbance capability of Yunnan Power Grid are highlighted. Changes in the power grid environment make the problem of three-phase voltage imbalance more likely to occur and may deteriorate. This paper studies the ability of photovoltaic three-phase unbalanced adaptability after asynchronous networking. A double-synchronized reference coordinate model for PV in the three-phase unbalanced condition was established. The influence of the voltage negative-sequence component of the grid on the DC voltage, output current and power of the inverter was studied. A scheme for suppressing oscillation by adding a notch filter is proposed. In Matlab/Simulink, the whole process simulation of wind turbine adaptability under three-phase unbalanced change is realized. The simulation results verify the correctness of the theoretical analysis and the effectiveness of the proposed solution.

Study on closed loop operation of low voltage distribution network under three-phase unbalanced condition

The closed loop operation to achieve power load can greatly improve the power supply reliability of low-voltage distribution network, but because of the low voltage distribution network asymmetry and the difference of voltage loop, closed loop operation will not change the balance of system, generating loop impulse current, affect the safe operation of power grid. So this paper considers the low voltage distribution network closed loop current in the load transfer method, and through the simulation and analysis of the distribution network load rate and the unbalance of the loop current and loop distribution after unbalance effect, verify the closed loop operation of distribution network considering the unbalanced degree of necessity.

Modal estimation in distributed synchronous generation systems under three-phase unbalanced conditions

Underdamped electromechanical oscillations (EMO), which are a typical problem of transmission systems, can also appear in power distribution systems (PDS) where synchronous generators are present. The tools to study EMO in bulk power systems are well established. Nevertheless, distributed synchronous generators have a different behavior from the ones connected to balanced three-phase bulk power systems. In these cases, some well-consolidated tools used to study EMO in bulk power systems are not adequate, since in distributed generation systems the load fluctuation is proportionally much higher and, therefore, it is not possible to define a good approximation for an equilibrium point. In this paper, a set of appropriate tools, which does not depend on the system model, is utilized to assess the presence of EMO in unbalanced PDS with synchronous generators. The obtained results show that the measurement-based methods which use ambient and ringdown responses can be applied to estimate the parameters of EMO in unbalanced PDS with high accuracy.

A Method Based on Combination Operation of Detecting Harmonic in Unbalanced Power System

Aiming at defects of traditional harmonic detection in three-phase unbalanced condition, an improved ip-iq detecting method is proposed based on instantaneous reactive power theory. When power grid voltage unbalanced or voltage distortion, through coordinate transformation, trigonometric transforming and combination operation of the three-phase voltage, the fundamental positive sequence voltage is extracted, and the average filter string into the low pass filter (LPF) to reduce the ripples in fundamental positive sequence current. The improved method is more accurate and quick than traditional ip-iq detecting method when extracting positive sequence fundamental voltage, detecting the negative sequence current of power grid and harmonic reactive current, significantly to reduce THD of fundamental positive sequence current and the detection error caused by unbalance and distortion voltage. Simulative results verify the correctness and validity of theoretical analysis.

Implementation of Single Phase Locked Loop Based on FPGA and its Application in SVC

Whereas three-phase phase locked loop could not get accurate phase position under three-phase unbalanced condition of the power grid, the design of single phase locked loop is implemented in the principle of single phase locked loop, based on FPGA technology. the paper explains design difficulties of single phase locked loop in detail, puts forward adaptive sampling scheme using single phase locked loop under variable frequency, increases accuracy of SVC sampling system. And tests response speed of phase locked loop via Real Time Digital Simulator for Power Systems (RTDS), through final verification in Fushun Lishizhai SVC Project, the design could meet system requirement for voltage phase accuracy.

Convergence problem in forward/backward sweep power flow method caused by non-positive-sequence impedance of distributed generators and its solution

Abstract A variety of distributed generators (DGs) are integrated in distribution system which is usually operated under three-phase unbalanced conditions. The zero and negative sequence impedances of DGs may vary within a large range. In this paper, the convergence problem caused by the zero and negative sequence impedances of DGs in forward/backward sweep three-phase power flow is found through numerical experiments. The reason of this phenomenon is explained and an impedance compensation method is proposed to solve this problem.

A new approach for minimum-torque-ripple maximum-efficiency control of BLDC motor

Torque-ripple control of the brushless DC motor has been the main issue of the servo drive systems in which the speed fluctuation, vibration, and acoustic noise should be minimized. Most methods for suppressing the torque ripples require Fourier series analysis and either iterative or least-mean-square minimization. In this paper, a novel approach to achieve ripple-free torque control with maximum efficiency based on the d-q-0 reference frame is presented. The proposed method optimizes the reference phase current waveforms which include the case of three-phase unbalanced conditions. As a result, the proposed approach provides a simple way to obtain optimal motor excitation currents. The validity and practical applications of the proposed control scheme are verified through the simulations and experimental results.