International Journal of Emerging Electric Power Systems

Control Strategy for VSC-HVDC Power Quality Improvement under Unbalanced Grid Conditions

Xiaoyun Wei et al. — Wed, 16 Nov 2011 17:43:12 PST

This paper presents a novel unified dynamic model and control strategy to improve the power quality for VSC based high voltage direct current transmission system (VSC-HVDC) under unbalanced grid conditions. The unified models present the dynamic behavior of VSC-HVDC in the unique positive synchronously rotating reference (dq-p) frame with respect to both the positive- and negative-sequence components. Based on the unified model, a strategy to either eliminate the dc-link ripple or achieve the balanced currents, along with a rather low level harmonics in each grid is introduced by resorting to the resonant integrator and filter based scheme in the two cascaded control loops. The resonant integrator and filter scheme enables effective control of the positive- and negative-sequence currents, and avoid the sequence component decomposition. The simulation studies in PSCAD/EMTDC verify the improved performance of VSC-HVDC system regarding the power quality and the ride-through capability.

A Solution to CT Saturation by Gregory Newton's Backward Interpolation

Sukanta Das et al. — Wed, 16 Nov 2011 17:43:08 PST

Saturation problem in steel core Current Transformers (CTs) causes dreadful effects in protection systems as true replica of the primary current is not always reflected in CT secondary side under abnormal operating conditions. This is why detection and production of exact replica of primary current in the CT secondary side is very crucial. This paper presents a discrete wavelet transform based application using ‘Daubechies3’ as mother wavelet for the detection of saturation. The compensation of saturated signal is done by Gregory Newton's Backward Interpolation (GNBI). The whole scheme of detection of CT saturation and compensation of saturated CT secondary current has been successfully tested on various saturated signals simulated in MATLAB/SIMULINK with various case studies. Finally, the scheme is validated in real time using DS1104 controller board of dSPACE. The results obtained are very encouraging.

A New Digital Down Conductor Detection Scheme for Overhead Electrical Power Distribution Systems

Bhavesh Bhalja et al. — Fri, 14 Oct 2011 11:24:53 PDT

Detection of down conductor faults in an overhead electrical power distribution system is extremely difficult by the conventional over current protection devices. This paper presents a new digital down conductor detection scheme which is based on the combination (AND logic) of the ratio of the negative sequence current to the positive sequence current and the value of under current during an actual down conductor condition. Real time implementation of the proposed scheme has been carried out on an existing Indian 11 kV, 3 –Phase, 50 Hz power distribution system with the help of digital over current relay. At the end, a comparative evaluation of the proposed scheme with the scheme modeled in PSCAD/EMTDC software package is also presented. Both practical and simulation results indicate the effectiveness of the proposed scheme.

A Novel Steady State Modeling and Analysis of Six-phase Self-excited Induction Generators for Renewable Energy Generation

S. Singaravelu et al. — Fri, 14 Oct 2011 11:24:51 PDT

This paper presents a simple and generalized steady state model and analysis of six-phase self-excited induction generators. The developed matrix equation is formulated using nodal admittance method based on inspection. This model does not involve any lengthy derivations of nonlinear equations which are followed so far. Also this model is flexible such that inclusion or elimination of any equivalent circuit elements can be carried out easily. Moreover, this model can be used to find any combination of unknown quantities of the equivalent circuit. To determine the steady state performance of six-phase self-excited induction generators, applications of genetic algorithms have been proposed. In addition, the details of winding scheme of the six-phase induction generator which is used as prototype model for the experimental study is also presented. The experimental and theoretical results are found to be in close agreement, which validates the proposed method.

DC Ripple Reinjection: A Review

Bhaba Das et al. — Fri, 14 Oct 2011 11:24:48 PDT

This paper presents a comprehensive review of power quality improvements made by applying the concept of DC ripple reinjection in current source series connected AC-DC rectifier for HVDC transmission. Recent advances in applying the reinjection concept, their suitability and selection is discussed. The main aim is to present a state of the art on the DC ripple reinjection technique over the years.

RTDS Hardware Implementation and Simulation of 3-ph 4-wire SHAF for Mitigation of Current Harmonics with p-q and I_d-I_q Control Strategies using Fuzzy Logic Controller

Suresh Mikkili et al. — Fri, 14 Oct 2011 11:24:45 PDT

Control strategies for extracting the three-phase reference currents for shunt active power filters are compared, evaluating their performance under different source voltage conditions in MATLAB/Simulink environment and also with Real Time Digital Simulator (RTDS) Hardware. When the supply voltages are balanced and sinusoidal, then all control strategies converge to the same compensation characteristics. However, when the supply voltages are distorted and/or un-balanced sinusoidal, these control strategies result in different degrees of compensation in harmonics. The p-q control strategy unable to yield an adequate solution when source voltages are not ideal. Extensive Simulations are carried out with Fuzzy Controller for both p-q and I_d-I_q control strategies for different voltage conditions and adequate results were presented. The 3-ph 4-wire SHAF system is also implemented on a RTDS Hardware to further verify its effectiveness. The detailed simulation and RTDS Hardware results are included.

A New Phase Detection Method by Using Kalman Filter

Hayato Yamauchi et al. — Fri, 14 Oct 2011 11:24:40 PDT

This paper proposes a phase detection method for harmonics and unbalanced voltage conditions. The proposed method uses harmonics and unbalanced voltage compensation circuit in addition to basic PLL (Phase Locked Loop) circuit. In the harmonic compensation circuit, the harmonic voltage component are eliminated from the input voltages using specific harmonic detection method. Besides, frequency information of power system used in the specific harmonic detection method is estimated by an extended complex Kalman filter. In the unbalanced voltage compensation circuit, the input voltage is normalized after the calculated positive sequence component. By means of the proposed method, excellent phase detection performance can be achieved under harmonics and unbalanced voltage conditions. Moreover, detection of positive sequence voltage component becomes possible under voltage drop conditions due to faults in power systems. The effectiveness of the proposed method is verified by simulation results.

High Efficiency Drive for Micro-Turbine Generator Based on Current Phase and Revolving Speed Optimizations

Naomitsu Urasaki et al. — Tue, 30 Aug 2011 10:04:23 PDT

This paper presents a high efficiency drive strategy for micro-turbine generator (MTG) in which permanent magnet synchronous generator (PMSG) is used. PMSG operates at maximum efficiency point by optimizing the phase of armature current of PMSG; i.e., an optimum d-axis current in the rotor reference frame is analytically derived from mathematical model of PMSG taking iron loss into account. In addition, in order to use turbine output power effectively, compressor input power is minimized by optimizing revolving speed of PMSG. Simulation results confirm validity of the proposed method.

Evaluation of Hottest-Spot Temperature Models using Field Measured Transformer Data

Oluwaseun Amoda et al. — Tue, 30 Aug 2011 10:04:20 PDT

The acceptability of two hottest-spot temperature models is assessed in this paper. The first model, contained in the IEEE loading guide, is shown not to accurately account for the effects of the top-oil temperature (TOT) variation on the hottest-spot temperature. A new model which accounts for TOT variation is derived. The original and modified models are linearized and fitted to field data using linear regression to obtain optimal parameter estimates. Comparison of the parameter estimates and prediction simulations show that even though the original model is not structurally accurate, it, as well as the modified model, are acceptable for prediction purposes. The method of nonlinear regression is also used in an attempt to find better parameter estimates for the nonlinear modified model. It is shown that parameter estimates for the nonlinear model are inferior to those obtained for the linear models.

Harmonics Identification with Artificial Neural Networks: Application to Active Power Filtering

Ngac Ky Nguyen et al. — Tue, 30 Aug 2011 10:04:16 PDT

This study proposes several high precision selective harmonics compensation schemes for an active power filter. Harmonic currents are identified and on-line tracked by novel Adaline-based architectures which work in different reference-frames resulting from specific currents or powers decompositions. Adalines are linear and adaptive neural networks which present an appropriate structure to fit and learn a weighted sum of terms. Sinusoidal signals with a frequency multiple of the fundamental frequency are synthesized and used as inputs. Therefore, the amplitude of each harmonic term can be extracted separately from the Adaline weights adjusted with a recursive LMS (Least Mean Squares) algorithm. A first method is based on the modified instantaneous powers, a second method optimizes the active currents, and a third method relies on estimated fundamental currents synchronized with the direct voltage components. By tracking the fluctuating harmonic terms, the Adalines learning process allows the compensation schemes to be well suited for on-line adaptive compensation. Digital implementations of the identification schemes are performed and their effectiveness is verified by experiments.

Operation Characteristics of Zone 3 Distance Protection in Wind Power Systems with Fixed-Speed Induction Generators

Shenghu Li — Mon, 01 Aug 2011 15:11:41 PDT

The induction generators (IGs) are basic to wind energy conversion. They produce the active power and consume the reactive power, with the voltage characteristics fragile compared with that of the synchronous generators and doubly-fed IGs. In the stressed system states, they may intensify var imbalance, yielding undesirable operation of zone 3 impedance relays.

In this paper, the operation characteristics of the zone 3 relays in the wind power systems is studied. With the theoretical and load flow analysis, it is proved that the equivalent impedance of the IGs lies in the 2^nd quadrature, possibly seen as the backward faults by the mho relays, i.e. the apparent impedance enters into the protection region from the left side. The undesirable operation may be caused by more wind power, larger load, less var compensation, and larger torque angle.

Multi-Objective Day-Ahead Real Power Market Clearing with Voltage Dependent Load Models

SurenderReddy Salkuti et al. — Mon, 01 Aug 2011 15:11:37 PDT

In this paper, we investigate the influence of voltage dependent load models on day ahead real power market clearing (DA-RPMC). The investigations clearly bring out the unsuitability of conventional single objectives such as production cost minimization (PCM) (or social welfare maximization (SWM)), due to reduction of load served. Hence, the multi-objective optimization is essential in this context. The paper proposes several objectives such as production cost minimization (PCM) (or social welfare maximization (SWM)), load served maximization (LSM), and Voltage Stability Enhancement Index (VSEI); which can be judiciously combined as per the needs of the operating condition. Multi-objective Strength Pareto Evolutionary Algorithm (SPEA) has been used to solve the DA-RPMC problem. The effectiveness of the proposed approach is tested on IEEE 30 bus system and the detailed simulation studies have been carried out by considering different operating conditions with voltage dependent load modeling.

Maintenance of Three-phase Load Voltage during Single Phase Auto Reclosing in Medium Voltage Radial Distribution Lines

Kartik Prasad Basu et al. — Thu, 21 Jul 2011 19:05:34 PDT

Most faults in medium voltage (MV) distribution lines are temporary line to ground (LG) faults. Three-phase auto reclosing (TPAR) is commonly used to remove this fault with temporary disconnection of all the phases. Multi-shot single-phase auto reclosing (SPAR) may also be used to remove the LG fault. But it produces highly unbalanced and low voltage across the load during the reclosure dead time. It is proposed to connect a zigzag winding grounding transformer at the load bus to maintain the 3-phase load voltage when one phase opens during the SPAR. With low value of grounding resistance the 3-phase voltage during the SPAR dead time becomes approximately balanced. Directional over current relays may be used for the protection. Analysis of a MV radial distribution system having a zigzag transformer connected to the remotest load bus is presented with the computation of voltages during the dead time of SPAR.

A Novel Approach to Tackle Miscoordination of Protective Device in Radial Distribution Network during DG Interconnections

Bhavesh Bhalja et al. — Thu, 21 Jul 2011 19:05:31 PDT

Due to incorporation of Distributed Generation (DG), the traditional protection scheme for electric power distribution system lost its radial nature and behaves more like multifeed transmission system. Hence, there is a need to develop a new protection scheme for electric power distribution system which remains stable in all conditions. This paper presents a new directional protection scheme for distribution system containing DG. Authors have developed a laboratory prototype of the three-phase radial distribution system containing DG. The proposed scheme has also been simulated using the PSCAD/EMTDC software package with fault data generated by modeling the distribution systems. The proposed directional protection scheme has been tested for various types of faults in different sections of radial distribution network along with DG. At the end, a comparative evaluation of the results obtained using the developed laboratory prototype has been carried out with the simulation results obtained using PSCAD. It has been observed that the proposed scheme has the ability to isolate the faulted section without disturbing the healthy section in the presence of DG.

Comparison of Virtual Flow Approach with Proportional Sharing Methods for Tracing of Network Power Flows

Surendra S. et al. — Thu, 21 Jul 2011 19:05:29 PDT

Fair allocation of charges based on transmission facility availed by the entities present in the open access system is the need of the hour. In order to achieve transparency and fairness from the point of participants involved, it is necessary to get contributions from each generator (source) to loads, flows in the network and losses for the system. This paper attempts to evaluate real power flow in the lines of the network due to individual sources and its contribution to each load using the virtual flow methodology which is fairly simple and straightforward. The proposed method is compared against two well known tracing techniques adapted from proportional sharing principle. Virtual tracing method involves the network topology, parameters and the voltage gradients between buses as determined from a solved power flow/on-line state estimator. It is believed that this is more realistic in nature. Counter flow components are easily determined and loop flows are handled without any difficulty. Case studies on a sample 5 bus and a practical 11 bus Indian southern grid equivalent 400kV networks are presented.

Rolling Window Based Dynamic Transmission Network Expansion Planning

Ashu Verma et al. — Thu, 21 Jul 2011 19:05:26 PDT

Dynamic transmission network expansion planning (DTNEP) decides when (which year) and where (which R.O.W.), the new lines are to be built so that the overall cost of the plan is minimum and the system is secure. Generally, this is done for a fixed time horizon window (3/5 years) at a time. DTNEP has been proved to be better than the incremental approach in which optimal investment plans are found out for each year separately. In DTNEP, 1st year and last/terminal years are given different treatment, and there is a complete disconnect between two DTNEP windows. Also DTNEP assumes that the conditions during optimization like the earlier year generation, transmission expansion plans get\ executed exactly as per plan &/or the load forecast for the entire period remains unchanged. However, it is well known that this is generally not true. In view of above, this paper presents a rolling window based approach which treats every year of investment in exactly the same fashion. It also has an inherent capability to take care of such changes. It involves successive, overlapping DTNEP for 3/5 years planning horizon. A harmony search algorithm is used for the solution of the problem. Results for two test systems have been obtained to demonstrate the potential of the algorithm.

Influence of Nanofillers on the Surface Electrical Discharge Resistance of Epoxy Insulation Used in HV Power Apparatus

P. Preetha et al. — Thu, 30 Jun 2011 14:36:34 PDT

Resistance to surface electrical discharge was investigated for epoxy alumina and epoxy silica nanocomposites. Epoxy alumina nanocomposites with 0.1, 1, 5, 10 and 15 wt% nanoalumina fillers as well as epoxy silica nanocomposites with 0.1, 1 and 5 wt% nanosilica fillers were prepared in the laboratory using direct dispersion method. Experiments were conducted at 10kV ac voltage for different durations using IEC (b) type electrodes. The degradation of the sample surfaces were analyzed using SEM techniques, surface roughness measurements and FTIR spectroscopy. The effect of interface on the material properties were investigated by DSC analysis. It was observed that the resistance to surface degradation improved for both epoxy alumina and epoxy silica nanocomposites as compared to the performance of unfilled epoxy. Also epoxy alumina nanocomposites showed better resistance to degradation as compared to epoxy silica nanocomposites for all the filler concentrations considered. The enhancement in degradation resistance for epoxy nanocomposites is attributed to the surface modifications of the nanocomposites when exposed to discharges and the nature of the interface in the region around the nanoparticles. The filler concentration and thereby the interparticle distance also has a significant effect on the discharge degradation resistance.

Robust Distribution Power Flow

Kasi Viswanadha Raju G et al. — Tue, 21 Jun 2011 06:26:17 PDT

A power system analyst wants to be absolutely sure, whether the divergence of power flow method is really due to infeasible operation, or failure of the numerical method, while performing power flow studies. Because of the special structure and characteristics of a distribution system, Newton based methods are not very well suited. Forward/backward sweep methods are well suited for these systems, which exploit the structural advantage of distribution systems. An efficient power flow is required for the analysis of ill-conditioned or highly stressed systems, which are more common in unbalanced systems. In this paper, a robust non-divergent power flow is developed for distribution system using optimal multiplier. It is based on Newton-like power flow derived from conventional forward backward sweep distribution power flow. Two versions of the power flow methods (i) normal variable matrix and (ii) constant matrix have been investigated. Results for three sample systems, with different loading conditions, demonstrate the potential of the proposed algorithm.

Wind Power Extraction from DFIG Wind Turbines Using Stator-Voltage and Stator-Flux Oriented Frames

Shuhui Li et al. — Tue, 21 Jun 2011 06:26:13 PDT

This paper investigates and compares wind power extraction from a DFIG (doubly-fed induction generator) wind turbine using stator-voltage and stator-flux oriented frames. The paper analyzes how wind power extraction control of a DFIG wind turbine is converted to speed control, and then how speed and reactive power control of the wind turbine is converted to generator current control using the two different orientation frames. The paper also investigates what are the differences in developing wind power extraction control strategies of a DFIG wind turbine using the two different orientation frames. Simulation study is conducted for a 1.5MW DFIG wind turbine, in which the turbine driving torque is modeled by considering typical wind turbine aerodynamic characteristics. The study shows that the performance of DFIG wind power extraction is similar using both stator-voltage and stator-flux oriented frames. But, it is found that a conventional wind power extraction approach using the stator-flux oriented frame could deteriorate the power quality of the DFIG system while it is more stable to estimate the position of the stator-flux space vector by simply adding -90 degree to the stator-voltage space vector.

H∞ Observer Based Sensor-less Control Strategy for Interior Permanent Magnet Synchronous Motor

Naomitsu Urasaki et al. — Tue, 21 Jun 2011 06:26:09 PDT

Nowadays a sensor-less vector control system for a motor drive becomes promising to avoid a position sensor. A back electro-motive-force or flux observer is used for sensor-less drive of permanent magnet synchronous motors from middle to high speed range. Thus, the estimation accuracy of the observer influences sensor-less drive performance. This paper proposes a sensor-less vector control system using the H∞ flux observer in which the observer gain is designed based on the H∞ control theory. This observer is insensitive to high frequency disturbances such as voltage disturbance due to non-linearity of PWM voltage source inverter, measurement noise of current sensors, and modeling errors. So, the total performance of a sensor-less control system is enhanced. The detailed configuration of the H∞ flux observer is described in this paper. Simulation results are compared with the conventional full-order flux observer-based sensor-less system.