Connecting distributed generation (DG) to weak distribution networks can often cause voltage rise problems. Traditionally, these voltage rise problems have been mitigated by passive methods such as reinforcing the network. This can, however, lead to high connection costs of DG. The connection costs can in many cases be lowered if active voltage control methods are used instead of the passive approach. In this paper, two coordinated voltage control algorithms suitable for usage in distribution networks including several distributed energy resources are proposed and studied. The first algorithm uses control rules to determine its control actions and the second algorithm utilizes optimization. The operation of the implemented algorithms is, at first, studied using time domain simulations. Thereafter, the network effects and costs of both algorithms are compared using statistical distribution network planning and also practical implementation issues are discussed.

Coordinated Voltage Control in Distribution Networks Including Several Distributed Energy Resources

This paper presents a three-phase power-flow algorithm, in the sequence-component frame, for the microgrid (μgrid) and active distribution system (ADS) applications. The developed algorithm accommodates single-phase laterals, unbalanced loads and lines, and three/four-wire distribution lines. This paper also presents steady-state sequence-component frame models of distributed energy resource (DER) units for the developed power-flow approach under balanced/unbalanced conditions. The DER models represent the synchronous-generator based and the electronically-coupled DER units. Both constant power (PQ) and regulated-voltage (PV) modes of operation of DER units are considered. The application of the developed power-flow method for two study systems is presented. The study results are validated based on comparison with the detailed solution of the system differential equations in time domain, using the PSCAD/EMTDC software tool.

https://www.engr.colostate.edu/academic/ece/graduate_exams/grad_exams/Panwar.pdf

Unbalanced Model and Power-Flow Analysis of Microgrids and Active Distribution Systems

On waveform distortion in the frequency range of 2 kHz–150 kHz—Review and research challenges

This paper presents a new method to calculate the utility harmonic impedance and the harmonic voltage emission level at the point of common coupling (PCC). Since harmonic current at the PCC commonly exhibits negligible dependence on the utility side background harmonic, the variable item of the background harmonic in the deviation formulation can be eliminated by using the covariance characteristic of random vectors. And then, the utility harmonic impedance, the customer harmonic voltage emission level, and the background harmonic can be calculated in real time. Based on the analysis results of the simulation and the field test case, it shows that the proposed method is effective to reduce the calculation errors caused by the fluctuation of background harmonic, and satisfactory results can be achieved.

Assessing Utility Harmonic Impedance Based on the Covariance Characteristic of Random Vectors

Determination of the utility harmonic impedance based on measurements is a significant task for utility power-quality improvement and management. Compared to those well-established, accurate invasive methods, the noninvasive methods are more desirable since they work with natural variations of the loads connected to the point of common coupling (PCC), so that no intentional disturbance is needed. However, the accuracy of these methods has to be improved. In this context, this paper first points out that the critical problem of the noninvasive methods is how to select the measurements that can be used with confidence for utility harmonic impedance calculation. Then, this paper presents a new measurement technique which is based on the complex data-based least-square regression, combined with two techniques of data selection. Simulation and field test results show that the proposed noninvasive method is practical and robust so that it can be used with confidence to determine the utility harmonic impedances.

Utility Harmonic Impedance Measurement Based on Data Selection

A method to evaluate harmonic contributions at the point of common coupling is presented in this paper. The proposed approach is based on the harmonic current vector method where reference impedances are introduced. Resistance defined at fundamental frequency with measurements is used as the customer-side reference impedance. The presented method allows calculation of harmonic contributions without knowing the actual customer impedances. It also enables better evaluation of the customer and utility harmonic contributions in resonance conditions. The method is verified through a simulation study and with extensive field measurements. Long-term measurements are proposed for determination of harmonic contributions.

Harmonic Contributions Evaluation With the Harmonic Current Vector Method

Rapidly increasing share of distributed generation (DG) in distribution networks introduced the need for active distribution network operation. As current distribution networks were not designed to integrate the power generation, the DG introduced many technical challenges in sense of power quality, network planning, protection schemes, voltage stability.... This paper presents the advantages of active approach in distribution network operation. The paper focuses on a voltage quality problem and introduces the coordinated voltage control technique to increase the share of DG in distribution networks and at the same time supply the customers with the required voltage quality.

Improving power quality with coordinated voltage control in networks with dispersed generation

The definition and determination of harmonic emission levels in accordance with the new draft edition of the IEC Technical Report 61000-3-6 is presented in this paper. The paper deals with contributions to the voltage harmonic distortion and, in addition, discusses an alternative method for estimating customer harmonic emission levels at the point of common coupling. This method is based on the harmonic vector method where reference impedances are introduced. The results of the harmonic vector method are compared with the results of the method based on the definitions in the IEC Technical Report. The presented harmonic vector method with reference impedances allows calculation of harmonic emission levels without any switching maneuvers or without knowing the actual impedances.

Methods for estimating customer voltage harmonic emission levels

This paper analyzes the influence of distributed energy resources (DER) on distribution network voltage profile A general description of the impact of DER on voltage profile is given and three advanced voltage control techniques are discussed: central voltage control with multiple voltage measurements, reactive power control with sources that enable such operation and the use of an autotransformer as a voltage regulator The aim was to maximize and to facilitate DER integration The influence of DER on voltage profile and the effectiveness of the investigated solutions were evaluated by means of simulation in DIgSILENT The simulated network was an actual distribution network in Slovenia with a relatively high penetration of distributed generation Recommendations for voltage control in networks with DER penetration are given at the end

Voltage control in networks with distributed generation — A case study

Many techniques for detecting harmonic sources have been applied to determine the customer and the utility harmonic contributions. The paper proposes reference impedance based harmonic current vector method for determination of customer and utility harmonic contributions at the point of common coupling (PCC). The method allows calculation of harmonic contributions without knowing the actual customer impedances. It also enables better evaluation of the customer and utility harmonic contributions in resonance conditions. In the paper the method is verified with extensive field measurements. Long-term measurements are proposed for determination of harmonic contributions.

T. Pfajfar

Papers

Harmonic Contributions Evaluation With the Harmonic Current Vector Method

Improving power quality with coordinated voltage control in networks with dispersed generation

Methods for estimating customer voltage harmonic emission levels

Voltage control in networks with distributed generation — A case study

Harmonic current vector method with reference impedances - field measurement verification