Flicker Mitigation by Optimization of Voltage Control

Legacy protection schemes face new challenges as Inverter Interfaced Distribution Generation (IIDG) significantly changes the transient fault response of the distribution grid. The performance of the protection near the IIDG side is adversely affected by Low Voltage Ride Through (LVRT) and the negative sequence current suppression control characteristics of the IIDG. The operational characteristics of the protection are very different from those of the legacy protection schemes used in the distribution grid. Traditional overcurrent protection schemes cannot meet the requirements of selectivity and sensitivity. This paper analyses the influence of the IIDG on the protection schemes used in the distribution grid. Based on the positive sequence voltage polarization impedance criterion this study proposes a polarizing impedance criterion based on the fault component of positive sequence voltage, which can adaptively follow the fault resistance variation to satisfy the requirements of grid operation. The simulation results show that: (a) the proposed criterion is immune to the adverse effects of the transient characteristics of the IIDG; and, (b) it can adaptively follow the change of fault resistance making it suitable for application in short distribution lines. Using specialized System-on-Chip technology, a new distance protection device has been developed and tested on an industrial site. Simulation results and field tests showed that the new distance protection meets the requirements of the distribution grid with IIDGs.

The emerging trend for electric utilities to install wind-based generation on a large scale into distribution networks, employing a doubly fed induction generator or full-converter turbine technologies is raising issues for distribution planning engineers with regard to flicker among other power-quality (PQ) issues. This paper investigates dynamic volt/var control, a viable flicker mitigation technique which exploits the reactive power capabilities of modern power-electronic interfaced wind turbine generators. Dynamic volt/var control essentially mitigates flicker by solely dispatching reactive power based on voltage fluctuations within the flicker frequency band. This technique can be incorporated as part of a wind farm multiobjective reactive power-control scheme, which facilitates regulating: 1) the voltage level at the point of common coupling (PCC), employing slow-acting voltage control or power factor control and 2) the flicker level at the PCC with fast-acting dynamic volt/var control. This two-level approach can reduce the wind farm flicker level up to 80% and maintains the voltage level at the PCC well within acceptable bands. The results presented in this paper focus on a typical 25-kV medium-voltage distribution feeder integrating wind-based generation on a large scale. The performance of the two-level controller was tested for varying levels of penetration and wind speed conditions. The applicable standards and utility grid-code requirements are considered throughout.

In response to the poor reliability in identifying fault direction in distribution networks with Inverter Interfaced Distributed Generators (IIDGs), considering the control strategy of low-voltage ride-through, a fault direction criterion based on post-fault positive-sequence steady-state components is proposed. Firstly, the output steady-state characteristics of IIDGs considering the low-voltage ride-through capability are analyzed during grid failure, and the applicability of existing directional elements in a distribution network with IIDGs connected dispersively is demonstrated. Subsequently, for the typical structure of an active distribution grid operating under flexible modes, the positive-sequence voltage and current are examined in various fault scenarios, and a reliable direction criterion is suggested based on the difference in post-fault positive-sequence impedance angles on different sides of the lines that are suitable whether on the grid side or the IIDG side. Lastly, the reliability of the proposed direction criterion is verified by simulation and the results indicate that the fault direction can be correctly determined, whereas phase-to-phase and three-phase short circuit faults occur in different scenarios, independent of the penetration and grid-connected positions of IIDGs, fault location, and transition resistance. It is suitable for fault direction discrimination of an IIDGs multi-point grid-connected system under a flexible operation mode.

The voltage flicker emission levels caused by electric arc furnace operation can cause objectionable disturbances in residential, incandescent lamp based light sources. Installation of a newly designed SPLC (smart predictive line controller) beta unit in the arc furnace power supply system of Co-Steel Lasco (46 kV, 100 MVA) reduced the flicker levels at the source. The paper presents the SPLC system description, details of installation, current and voltage fluctuation profiles, flicker measurements and operational benefits.

Modelling of three-phase electric arc furnace for estimation of voltage flicker in power transmission network

Rapid load variations can cause abrupt changes in the utility voltage, so-called voltage flicker. The voltage flicker may result in light flickering and, in extreme cases, damage to electronic equipment. Electrified railroads are just one example where such rapid load variation occurs as trains accelerate, decelerate, and encounter and leave grades. For balanced loads, the voltage flicker is easily determined using per-phase analysis. AC electrification system substations operating at a commercial frequency, however, are supplied from only two phases of utility three-phase transmission system. In order to calculate the voltage flicker for such an unbalanced system, symmetrical component method needs to be used. In this paper, a procedure is developed for evaluating the effects of short-time traction load variation onto utility system. Applying the symmetrical component method, voltage flicker equations are developed for loads connected to A-B, B-C, and C-A phases of a three-phase utility system. Using a specially-developed software simulating the train and electrification system performance, loads at the traction power substation transformers are calculated in one-second intervals. Subsequently, voltages at the utility busbars are calculated for each interval, and the voltage variation from interval to interval is expressed in percent. The calculated voltage flicker is then compared to the utility accepted limits. Based on this comparison, the capability of the utility power system to support the traction loads can be assessed and the suitability of the proposed line taps for the traction power substations evaluated.

This paper deals with mitigation of voltage flicker using an intelligent dynamic voltage restorer (DVR). Voltage flicker is produced in the distribution system due to an arc furnace which is a highly nonlinear load in nature. The control scheme of the proposed DVR is based on a neural network (NN) controller whose weights are trained using hybrid of particle swarm optimization (PSO) and gradient descent (GD). A comparative analysis is done for three different controllers: proportional integral (PI), NN with GD, and NN with hybrid of PSO and GD. Simulated results based on peak overshoot and maximum percentage total harmonic distortion (THD) of load voltage shows the superiority of the proposed NN controller.

In the present paper we perform, for the first time, an analysis of the average sensitivity of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source to external actions. The sensitivity of this OTDR can be defined in a conventional manner via average SNR at its output, which in turn is defined by the average useful signal power and the average intensity noise power in the OTDR spatial channels in the bandwidth defined by the OTDR sampling frequency. The average intensity noise power is considered in detail in a previous paper. In the current paper we examine the average useful signal power at the output of a phase-OTDR.The analysis of the average useful signal power of a phase-OTDR is based on the study of a fiber scattered-light interferometer (FSLI) which is treated as a constituent part of a phase- OTDR. In the analysis, one of the conventional phase-OTDR schemes with a rectangular dual-pulse probe signal is considered. The FSLI which corresponds to this OTDR scheme has two scattering fiber segments with additional time delay, introduced between backscattered fields. The average useful signal power and the resulting average SNR at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments, and by the additional time delay between the scattering fiber segments. The average useful signal power characteristic of the corresponding phase-OTDR is determined by analogous parameters: the source coherence, the time durations of the parts constituting the dual-pulse, and the time interval which separates these parts.In the paper an expression for the average useful signal power of a phase-OTDR is theoretically derived and experimentally verified. Based on the found average useful signal power of a phase-OTDR and the average intensity noise power, derived in the previous paper, the average SNR of a phase-OTDR is defined. Setting the average signal SNR to 1, at a defined spectral band the minimum detectable external action amplitude for our particular phase-OTDR setup is determined. We also derive a simple relation for the average useful signal power and the average SNR which results when making the assumption that the laser source coherence is high. The results of the paper can serve as the basis for further development of the concept of phase-OTDR sensitivity.

See article on page 1244 > No man can be a pure specialist without being in the strict sense an idiot George Bernard Shaw In this issue, Asseburg and colleagues...

Measurement of magnitude and frequency ofthe voltage flicker is very important for monitoring and controlling voltage flicker efficiently to improve the network power quality. This paper presents two new methods for measurement of flicker signal parameters using Shuffled Frog Leaping Algorithm (SFLA) and Imperialist Competitive Algorithm (ICA). This paper estimates fundamental voltage and flicker magnitudes and frequencies with proposed methods. The goal is to minimize the error ofthe estimated magnitudes and frequencies via a designed fitness function. At first we introduce voltage flicker and its measuring techniques. Then voltage flicker model is analyzed. At the next part a review of SFLA and ICA is presented these methods are applied to a test voltage signal and the results are given and analyzed.

One must be able to accurately measure voltage flicker levels and its frequency. This paper introduces a new application of the least error squares (LES) state estimation algorithm to measure voltage flicker and its frequency. We use a linear model of the flicker voltage magnitude, its phase angle and frequency deviation, and the system voltage magnitude and its phase angle. The proposed algorithm uses digitized samples of the voltage signal. Effects of data size, sampling frequency, and number of samples on the estimated parameters are studied.

The Multi-Zonal Medium Voltage DC (MVDC) Shipboard Power System (SPS) architecture, proposed by the U.S. Navy for their future combatant system, consists of several voltage source converters (VSCs). The proposed architecture is tightly-coupled, power-limited and its performance needs to be evaluated for security, reliability, and survivability. Following system damage or a fault, the current flow pattern in the DC network may change, which may result in the failure of VSCs due to overvoltage developed across them in certain operating conditions. For a given MVDC system, DC voltage reference setting for one of the VSCs operating in the voltage regulator mode, and the optimal power reference settings of the remaining VSCs in the power dispatcher mode have to be pre-determined. These settings and control modes of VSCs are needed to maintain the DC voltage within desired margins (usually 5% around the nominal DC voltage), both in “pre-fault” and “post-fault outage” conditions. The problem has been formulated as an optimization problem with three different objective functions. Computational intelligence techniques have been applied for solving the optimization problem. These include the genetic algorithm (GA) and biogeography based optimization (BBO) methods. The results have been compared with a conventional Lagrange multiplier based method.

With recent advances in technology, utilities can expect to see increasing amounts of distributed generation (DG) on their distribution systems. This DG can have positive impacts such as voltage support and deferred capital investments, but can also have negative impacts on protection coordination, voltage regulation, voltage flicker and short circuit levels. Reliability is an area where DG can have both a positive and a negative impact. This paper describes these situations and develops methods to analyze them using predictive reliability assessment tools.

The paper shows, with the help of numerical simulation and hardware experiments, that the tracking response of a NLA-based second order PLL would be affected in the presence of additional loop time delay. The inclusion of a phase modulator in the system structure can reduce the adverse effects to some extent, but the design parameters should have to be properly chosen to get a stable system.

Flickers and interharmonics have an inherent relationship. The magnitude of a voltage can fluctuate if it contains interharmonics. This paper investigates the amount of voltage fluctuation caused by two interharmonic components since interharmonics are often produced in pairs. The impact of the interharmonics on the flicker frequency and flicker magnitudes (expressed in the form of RMS and peak value fluctuations) is quantified. Analytical solutions to assess the impact are developed. One of the interesting findings of this work is that the two interharmonic components can be equivalenced into one interharmonic component that yields the same level of voltage fluctuation.