Open-loop Phase Synchronization Method of Positive and Negative Sequences for Three-phase Asymmetric Grid

Currently,a grid-connected inverter is required to inject a dual-sequence current during asymmetric grid sags,which means both positive and negative sequence phases should be tracked.This paper proposes an open-loop phase synchronization(OLS)method of positive and negative sequences for an asymmetric grid based on the moving average filter(MAF),which does not need to separate the positive and negative sequence fundamental components of grid voltage.As a benefit,there is no double-frequency oscillation in the estimated phases at offnominal frequencies since the positive and negative sequence phases are obtained simultaneously.The proposed method not only has the inherent advantages of OLS,but also further improves the dynamic response since the window length of MAF is only 1/6 of the fundamental period.The effectiveness of the proposed OLS is verified by experimental results.

Effects of Voltage Sag on the Performance of Induction Motor Based on a New Transient Sequence Component Method

Voltage sag is one of the most common power quality disturbances in industry,which causes huge inrush currents in stator windings of induction motors,and adversely impacts the motor secure operation.This paper firstly introduces a 2D Time-Stepping multi-slice finite element method(2D T-S multi-slice FEM)which is used for calculating the magnetic field distribution in induction motors under different sag events.Then the paper deduces the transient analytical expression of stator inrush current based on the classical theory of AC motors and presents a separation method for the positive,negative and zero sequence values based on instantaneous currents.With this method,the paper studies the influences of voltage sag amplitude,phase-angle jump and initial phase angle on the stator positive-and negative-sequence peak currents of 5.5 kW and 55 kW induction motors.This paper further proposes a motor protection method under voltage sag condition with the stator negative-sequence peak currents as the protection threshold,so that the protection false trip can be avoided effectively.Finally,the calculation and analysis results are validated by the comparison of calculated and measured stator peak value of the 5.5 kW induction motor.

Intelligent power factor compensation equipment of three phase low voltage loads

An intelligent power factor correction scheme is presented for three phase low power factor loads. This new scheme is able to perform individual phase sensing of parameters by monitoring at all times to sense a change in system parameters and affects individual phase correction by applying the exact amount of reactive components needed for each phase, and can also reduce negative sequence current caused by the load to improve system balance. An optimization criterion is used for the proper calculation of reactive power steps in a power compensation installation of capacitor banks. The criterion is enabled by sampling measurements performed on the electrical plant examined within specific interval of time.

Design of DSTATCOM Controller for Compensating Unbalances

In a three phase power system, the voltages at the generation side are in sinusoidal and equal in magnitude with 120? phase difference between the phases. However, at the load side voltages may become unbalanced due to unequal voltage magnitudes at the fundamental frequency, phase angle deviations or unequal distribution of single phase loads. The voltage unbalance is a major power quality issue, because a small unbalance in the phase voltages can cause a larger unbalance in the phase currents. A completely balanced three-phase three wire system contains only positive sequence components of voltage, current and impedance, whereas unbalanced system contains both positive and negative sequence components of voltages and currents. The negative sequence component of current in the unbalanced system increases the temperature and losses in the equipments. Hence, it is necessary to mitigate this problem by supplying the negative sequence current to the load at the load side and keep the source side balanced. This paper proposes the shunt connected, current injecting Distribution Static Synchronous Compensator (DSTATCOM) with appropriate controller to mitigate the unbalanced load current. The symmetrical components based Hysteresis Current Controller (HCC) is designed for DSTATCOM to diminish the unbalances in a three-phase three-wire system. The performance of the controller is studied by simulating the entire system in the MATLAB/Simulink environment. The DSTATCOM with HCC is found to be better than other controllers because it is suitable for compensating both balanced and unbalanced loads.

Phases-Controlled Coordinated Charging Method for Electric Vehicles

When private electric vehicles(EVs),which will be the main part of the EVs’cluster in the future,are plugged in power system by single phase power line,can result to three-phase unbalance problem of distribution network.In this work,a phased-controlled coordinated charging method was put forward to solve this problem.Firstly,the impacts of charging load to distribution network was analyzed based on the equivalent circuit;and then an architecture of the control method and its corresponding optimal control model were introduced.The optimal model is a multi-objective optimization model,which includes minimizing load variance of each phase and minimizing the power asymmetrical degree of three-phase load;lastly,three scenarios considering balance and unbalance cases were envisioned to verify the reasonableness of this control method based on IEEE-37 distribution network.Results show that the phased-controlled coordinated charging method can minimize the load variance as well as the negative sequence current.

Fault Ride-Through(FRT)Behavior in VSC-HVDC as Key Enabler of Transmission Systems Using SCADA Viewer Software

The world’s energy consumption and power generation demand will continue to rise.Furthermore,the bulk of the energy resources needed to satisfy the rising demand is far from the load centers.The aforementioned requires long-distance transmission systems and one way to accomplish this is to use high voltage direct current(HVDC)transmission systems.The main technical issues for HVDC transmission systems are loss of synchronism,variation of quadrature currents,amplitude,the inability of station 1(rectifier),and station 2(inverter)to either inject,or absorb active,or reactive power in the network in any circumstances(before a fault occurs,during having a fault in network and after a fault cleared),and the variations of power transfer capabilities.Additionally,faults impact power quality such as voltage dips and power line outage time.This paper presents a method of overcoming the aforementioned technical issues using voltage-source converter(VSC)based HVDC transmission systems with SCADA VIEWER software and dynamic grid simulator.The benefits include having a higher capacity transmission system and proposed best method for control of active and reactive power transfer capabilities.Simulation results obtained using MATLAB validated the experimental results from SCADA Viewer software.The results indicate that the station’s rectifier or inverter can either inject or absorb either active power or reactive power in any circumstance.Also,the reverse power flow under different modes of operation can ride through faults.At a 100.0%power transfer rate,the rectifier injected 775.0 W into the network.At a 0.0%power transfer rate,the rectifier injected 164.0 W into the network.At a-100.0%rated power,the rectifier injected 1264.0 W into the network and direction was also changed.

Static Var Compensator to Increase the Availability of a Colombian Oil Field

The paper describes the application of a static var compensator to improve the electrical system of the ACN （Cravo Norte Association） oil field in Colombia. The paper summarizes the application, including system aspects that require special control functions in the SVC （static var compensator） regulator. Several important benefits for the system operation are demonstrated, such as increased power transmission import over an existing 230 kV network, dynamic bus voltage stabilization for various load conditions, including system outages and load rejection, and reduction of variable speed drive shutdowns by up to 95%. Some relevant design features of the SVC are treated, as well.

Large Deviation Results for Generalized Compound Negative Binomial Risk Models

In this paper we extend and improve some results of the large deviation for random sums of random variables. Let {Xn;n 〉 1} be a sequence of non-negative, independent and identically distributed random variables with common heavy-tailed distribution function F and finite mean μ ∈R^＋, {N（n）; n ≥0} be a sequence of negative binomial distributed random variables with a parameter p C （0, 1）, n ≥ 0, let {M（n）; n ≥ 0} be a Poisson process with intensity λ 〉 0. Suppose {N（n）; n ≥ 0}, {Xn; n≥1} and {M（n）; n ≥ 0} are mutually independent. Write S（n） =N（n）∑i=1 Xi-cM（n）.Under the assumption F ∈ C, we prove some large deviation results. These results can be applied to certain problems in insurance and finance.

Coordinated Control of DFIG Converters to Comply with Reactive Current Requirements in Emerging Grid Codes

The doubly-fed induction generator(DFIG)is considered to provide a low-reactance path in the negative-sequence system and naturally comply with requirements on the negative-sequence reactive current in emerging grid codes.This paper shows otherwise and how the control strategy of converters plays a key role in the formation of the active and reactive current components.After investigating the existing control strategies from the perspective of grid code compliance and showing how they fail in addressing emerging requirements on the negative-sequence reactive current,we propose a new coordinated control strategy that complies with reactive current requirements in grid codes in the positive-and negative-sequence systems.The proposed method fully takes advantage of the current and voltage capacities of both the rotor-side converter(RSC)and grid-side converter(GSC),which enables the grid code compliance of the DFIG under unbalanced three-phase voltages due to asymmetrical faults.The mathematical investigations and proposed strategy are validated with detailed simulation models using the Electric Power Research Institute(EPRI)benchmark system.The derived mathematical expressions provide analytical clarifications on the response of the DFIG in the negative-sequence system from the grid perspective.