D A Robinson

Synchronized flicker measurement for flicker transfer evaluation in power systems

Voltage fluctuations caused by rapidly changing loads, such as arc furnaces, can propagate to different parts of a power system. Although the flicker level at its origin can be high, levels that are measured at other sites are subject to attenuation, a process that is influenced by fault levels, transformer impedances, line impedances, and composition of the connected loads. This paper presents the methodology, measurement results, and data analysis in relation to synchronized flicker measurements carried out in a high-voltage (HV)/medium-voltage (MV) power system which contains an arc furnace supplied by a dedicated feeder connected to the HV busbar. The flicker transfer coefficients derived from measurement results clearly indicate that flicker transfer from the arc furnace site to the upstream HV busbar is governed by the fault levels at the two locations. However, the transfer of flicker from the upstream HV busbar to other downstream busbars is dependent on the downstream load composition. These flicker transfer coefficients are vital in the application of methodologies described in many reports and standards in relation to establishing planning levels at various voltage levels and in the allocation of flicker emission to customers.

Simulation of HTS saturable core-type FCLs for MV distribution systems

The design principles and performance characteristics of a prototype high-temperature superconductor saturable magnetic core-type fault current limiter are described. These are based on a distribution network service provider feasibility specification that included the footprint and regulatory requirements for limiting fault currents. Time-domain simulations using PSCAD/EMTDC are given to illustrate specific applications and the transient behavior of the different distribution system configurations are investigated.

Microgrids of Commercial Buildings: Strategies to Manage Mode Transfer From Grid Connected to Islanded Mode

Microgrid systems located within commercial premises are becoming increasingly popular and their dynamic behavior is still uncharted territory in modern power networks. Improved understanding in design and operation is required for the electricity utility and building services design sectors. This paper evaluates the design requirements for a commercial building microgrid system to facilitate seamless mode transition considering an actual commercial building microgrid system. A dynamic simulation model of the proposed microgrid system is established (utilizing DIgSILENT Power Factory) to aid the development of planning and operational philosophy for the practical system. An economic operational criterion is developed for the microgrid to incorporate selective mode transition in different time intervals and demand scenarios. In addition, a multi-droop control strategy has been developed to mitigate voltage and frequency variations during mode transition. Different system conditions considering variability in load and generation are analyzed to examine the responses of associated microgrid network parameters (i.e., voltage and frequency) with the proposed mode transition strategy during planned and unplanned islanding conditions. It has been demonstrated that despite having a rigorous mode transition strategy, control of certain loads such as direct online (DOL) and variable-speed-drive (VSD) driven motor loads is vital for ensuring seamless mode-transition, in particular for unplanned islanding conditions.

Flicker Attenuation—Part I: Response of Three-Phase Induction Motors to Regular Voltage Fluctuations

Voltage fluctuations leading to lamp flicker that originate in one place in a power system tend to propagate to other parts of the network with some level of attenuation depending on the network impedances and the loads connected. Numerous subsynchronous-type frequency components exist in these voltage fluctuations that are responsible for lamp flicker. The rudimentary theory and the experimental measurements support the idea that industrial load bases, which contain a large percentage of mains-connected induction motors, tend to attenuate flicker better compared to residential load bases having mainly passive loads. This paper reports on the response of three-phase induction motors of several sizes when subjected to low-frequency voltage fluctuations: 1) the case where a balanced single-frequency component is superimposed on the mains voltage and 2) the case where the mains voltage is sinusoidally amplitude modulated, a scheme that is frequently used in the flicker-related work. Small-signal models are presented that will enable systematic understanding of the behavior which is verified using large signal models.

Flicker Attenuation—Part II: Transfer Coefficients for Regular Voltage Fluctuations in Radial Power Systems With Induction Motor Loads

Electromagnetic compatibility standards and guidelines used for planning of flicker allocation at various busbars of a power system requires some knowledge of the manner in which voltage fluctuations and hence the flicker propagate and how various loads respond to them. The current rudimentary approach taken for the determination of the load response to voltage fluctuations is based on network impedance values and hypothetical dynamic impedance values of the connected loads. Practical results and recently developed theory on the subsynchronous behavior of induction machines suggest that where there is a large base of induction machine exists on a power system the flicker attenuation is significant. This paper reports on a methodology to include the influence of induction machine behavior in flicker propagation and attenuation studies. The work is based on small-signal models to describe induction machine and system behavior which are utilized for the development of a flicker transfer coefficient. A systematic approach for the practical application of the methodology for flicker related work is included.

Identification of Load Power Quality Characteristics using Data Mining

The rapid increase in computer technology and the availability of large scale power quality monitoring data should now motivate distribution network service providers to attempt to extract information that may otherwise remain hidden within the recorded data. Such information may be critical for identification and diagnoses of power quality disturbance problems, prediction of system abnormalities or failure, and alarming of critical system situations. Data mining tools are an obvious candidate for assisting in such analysis of large scale power quality monitoring data. This paper describes a method of applying unsupervised and supervised learning strategies of data mining in power quality data analysis. Firstly underlying classes in harmonic data from medium and low voltage (MV/LV) distribution systems were identified using clustering. Secondly the link analysis is used to merge the obtained clusters into supergroups. The characteristics of these super-groups are discovered using various algorithms for classification techniques. Finally the a priori algorithm of association rules is used to find the correlation between the harmonic currents and voltages at different sites (substation, residential, commercial and industrial) for the interconnected supergroups

How should power quality be reported

Abstract Regulators are increasingly asking utilities to conduct “quality control” surveys on selected parts of their power system to establish the level of power quality. This activity leads can lead to a vast amount of data and its reporting needs to be carefully thought out if desired insights are to be achieved. Three styles are identified as serving most purposes - Site, Network and Utility reporting. These are designed to give respectively all the data of one site, summary data for each monitored site in the network, and average values across the utility’s system. The data analysis to be used in reporting must be oriented to the parameters used by standards to limit PQ disturbance levels. However, a review of standards reveals some impractical and inconsistent procedures and some deviation is necessary.

Establishment of typical harmonic voltage levels in radial distribution systems

This paper reports on a simple method to identify problem areas with relation to harmonic distortion within radial distribution systems at the planning stage. The purpose of the work is to produce a simple method of establishing harmonic distortion levels throughout a study distribution system when only limited system data and load information is available. The preliminary work presented in this paper is aimed at establishing useful planning guidelines for utilities to help with the analysis of harmonics in medium voltage (MV) distribution systems during the design phase. Harmonic analysis during the design of a distribution system is seen as an important step in controlling harmonic distortion levels.

Sag testing of dairy farm milking equipment

Sags are widely reported as being one of the worst power quality problems and are particularly common in rural areas. Automated milking equipment on dairy farms are thus very susceptible to malfunction by sags. In assessing the relative cost/benefits of sag mitigation in the dairy or the supply network it is important to know the dairy equipment immunity level. It is shown how a harmonic generator can be used to determine this information. Testing revealed that the motor contactor was a critical component as it was unable to ride through a 40% sag longer than about 0.5 seconds. The milking equipment without contactor gave unsatisfactory performance for 50% sags longer than 1 second and for interruptions exceeding 0.5 second.

Dynamic modelling, simulation and control of a commercial building microgrid

Microgrids are becoming popular in commercial buildings due to the recent drive to achieve net-zero energy targets and improve energy efficiency and reliability in building installations. The commercial building microgrids depict unique characteristics due to generation mix and various types of loads connected to the commercial building installations. Therefore, the commercial building microgrids require special consideration during the operation and control stages. Dynamic models of a microgrid enable researchers to investigate their operational performance and control techniques before the control schemes are actually being implemented in the microgrid system. It is essential to develop more accurate dynamic simulation models of a microgrid in order to verify the fidelity and the effectiveness of control schemes. This chapter presents a dynamic modelling of a commercial building microgrid system and conducts a stability analysis of the microgrid under various operating conditions.