Unbalanced Supply Voltage Research Articles

Extension of the CPC power theory to four-wire power systems with non-sinusoidal and unbalanced voltages

A modification of the CPC power theory to four-wire power systems with non-sinusoidal and unbalanced voltage supply is presented. The reasons behind the introduced modification are thoroughly explained by referring to Buchholz's apparent power definition. By applying Cauchy-Schwarz-Bunyakovsky inequality it is proved that the differences between the corresponding power quantities given by the original and proposed CPC definitions depend on the presence of voltage unbalance. In the case of load compensation to unity power factor the presented theory gives the smallest possible line losses for the same active power transferred to the load. In order to determine where the proposed definition stands among other power definitions, quantitative differences between the corresponding power quantities for the cases of different harmonic and unbalance levels in power system voltages are investigated by means of numerous computer simulations. This analysis includes also the definitions given by the IEEE and DIN standards, and Conservative Power Theory. Finally, the proposed CPC theory is extended for the cases when the supplying line has neutral conductor which resistance differs from the resistance of phase conductors.

System‐level dynamic phasor models of hybrid AC/DC microgrids suitable for real‐time simulation and small signal analysis

This study develops a system-level dynamic phasor (DP) model of a hybrid AC/DC microgrid. A new definition of DP, based on the multiple frequency shift transformation, is proposed to extend the models of a variety of AC/DC and DC/DC converters. The comprehensive control system is considered in developing the DP model of microgrid. Furthermore, a computational framework for applying this DP model into system-level simulation and small signal analysis (SSA) is given. The DP-based simulation is tested under conditions of environmental disturbances and unbalanced supply voltages. The SSA is performed on the developed DP model to evaluate the voltage stability and harmonic modes of the microgrid. Besides, this DP model shows high potential when implemented in the real-time simulation with larger scale system.

Frequency-domain harmonic modeling and analysis for 12-pulse series-connected rectifier under unbalanced supply voltage

The paper proposes a frequency-domain harmonic model for 12-pulse series-connected thyristor-controlled rectifier under unbalanced supply voltage, considering the firing angle offset and commutation overlap angle caused by unbalanced voltage. The harmonic coupled admittance matrix of 12-pulse rectifier is derived based on positive-sequence and negative-sequence voltages for the frequency-domain harmonic model. The simplified model of 12-pulse rectifier that only considers the primary matrix elements is proposed by comparing the values and distribution of admittance matrix elements under positive-sequence and negative-sequence voltages and analyzing the harmonic production process of the AC current of rectifier. The application condition of different simplified models is investigated by analyzing the calculation errors changing with the firing angle or commutation overlap angle. The accuracy and efficiency of the proposed models is verified by simulation and experimentation.

Detection and Isolation of Interturn Faults in Inductors of LCL Filter for Marine Electric Propulsion System

This paper presents an online technique to detect and isolate the interturn fault in the inductors of the LCL filter integrated with the active front-end rectifier used for electric propulsion of marine vessels. The proposed online technique is based on the fundamental component of the core-leakage flux measured at the yoke center of the inductor. A comparative study of the grid currents and the core-leakage fluxes under interturn faults is carried out using the proposed analytical models which reveal high sensitivity of the core-leakage flux at the yoke center when compared to the grid currents. The theoretical analysis is validated experimentally on a laboratory-based 11.3-kW marine propulsion system test rig for 1-turn, 2-turn, and 4-turn faults under various loading and supply voltage unbalance conditions. An online fault notification and isolation framework has also been proposed and validated experimentally with two different techniques to increase the reliability for 1-turn fault detection. The methodology requires just a single, low-bandwidth, and inexpensive magnetic flux sensor in addition to the existing current sensors to detect as low as 1-turn fault out of 63 turns.

Performance study of three synchronisation algorithms in renewable energy grid connected system

In grid connected applications, the efficiency of systems is strongly affected by the employed phase locked loop (PLL) strategy. This paper presents a comparative study of three PLL structures applied to a grid-connected renewable energy system with a nominal power of 7 MW. To select the suitable structure dedicated to previously mentioned application, we will evaluate the performance of the PLL techniques under distorted and unbalanced supply voltages. Several simulations test with a disturbed three phase utility grid using MATLAB-Simulink/SimpowerSystem are presented to valid the theoretical study.

Discrimination between Supply Unbalance and Stator Short-Circuit of an Induction Motor Using Neural Network

The objective of this paper is to investigate the contribution and effectiveness of the use of a Neural Network (NN) Multi-Layer Perceptron (MLP) of feed-forward type technique to discriminate between the Supply Voltage Unbalance (SVU) and the stator Inter Turn Short-Circuit (ITSC) of an induction motor. The proposed approach in this paper is based on the exploitation of any useful information existing in both the current and the voltage quantities simultaneously provided by the various instantaneous powers such as the active and the reactive powers as well as the power factor. The merits of the proposed approach using the NN technique as an effective solution to the problem of discrimination is assessed by a simulation work and then validated by an experimental work conducted by the Diagnosis Group of the LDEE Laboratory.

Reliable stator fault detection based on the induction motor negative sequence current compensation

The objective of this work is the compensation of the negative sequence current of the faulty induction motor in the aim to increase the accuracy and the sensitivity of the incipient stator fault detection under different disturbances. This is because, the negative sequence current generated in a faulty machine is the superposition of the different negative sequences current generated by the fault and others disturbances, which are the inherent asymmetry in the machine, the unbalanced supply voltage and the sensor inaccuracy. Thus, this paper proposes an efficient experimental method that allows the compensation of the negative sequence current of the faulty machine, by subtracting the negative sequence current of the different disturbances from the total negative sequence current of the faulty machine to isolate the negative sequence current due to the fault. Thus, the negative sequence currents of the different disturbances are isolated using an original experimental technique. The efficiency of the proposed method is validated experimentally on a 1.1kW motor under inter-turns short circuit faults and phase to phase faults.

Adaptation of a traction DC power system for high-speed traffic

Ways to develop and improve traction power-supply systems and adapt them for high-speed highways are considered. One way to improve a traction power-supply system and make it more efficient is to increase the nominal voltage in the dc traction network up to 24 kV. This allows one to increase the throughput and carrying capacities of railways, increase the distance between substations, reduce the cross section of traction-network wires, and significantly reduce power losses in power-supply facilities. All this, alongside the absence of unbalanced supply voltage and inductive losses, as well as a significantly lower electromagnetic action on the infrastructure under dc conditions, results in a greater efficiency than in the case of any alternating- current traction power systems of alternating current. A mathematical model of dynamic electromagnetic processes in electrical circuits with semiconductor converters is fleshed out. Analysis and synthesis of electromagnetic processes and the main characteristics of multipulse converters, as well as a comprehensive technical and economic comparison, shows that 24-pulse rectifying circuits with a serial connection of threephase bridge are most efficient for these systems. These rectifiers provide an improvement in the quality of electrical energy not only for the primary power-supply system, but also for the traction network. They allow one to use simpler and more economical active-passive smoothing filters that provide the electromagnetic compatibility (EMC) of a new traction dc power system with the communication and railway automation facilities. In addition, the use of 24-pulse rectifiers can allow one to significantly reduce the consumption of reactive energy, reduce power losses in a traction network, and increase the efficiency of traction substations.

Algorithm for estimating derating of induction motors supplied with under/over unbalanced voltages using response surface methodology

One of the adverse effects of unbalanced three-phase voltages on the induction motors (IMs) is overheating of the windings. The IMs should be loaded at less than their rated power in case of unbalanced supply voltages to prevent this overheating. Recent studies have pointed out that the maximum allowable loading ratio or derating factor (DF) of the IMs have various values for several combinations of the magnitude and angle of the complex voltage unbalance factor (CVUF) operating with a combination of unbalanced over- and under-voltage cases. This means that determination of DF requires plenty of experimental efforts for all possible unbalanced voltage conditions. In this study, the effective root-mean-square voltage definition which is defined in IEEE Standard 1459 is combined with the CVUF for proper identification of over and under unbalanced voltage conditions. An algorithm based on response surface methodology is proposed to estimate a precise DF for a broad range of unbalanced supply voltages. The simulation results are presented to validate the effectiveness of the proposed algorithm. It is apparently figured out that the proposed algorithm has better accuracy compared with the conventional approach reported in National Equipment Manufacturer's Association Standard MG1.

The use of the current complex factor to determine the precise output energy of the induction motor

The Complex Voltage Unbalance Factor (CVUF) has been commonly used to assess the output torque of the induction motor. However, this approach has been criticized, as CVUF has been found to depend solely on the unbalanced supply voltage, while neglecting the machine parameters, although, as known, the machine parameters affect the performance of the induction motor during the unbalanced operation. This paper investigates a different method of calculating the Complex Current Unbalance Factor (CUF), using both voltage unbalance and machine parameters for a more accurate estimate of the output energy. This paper also analyzes the output torque as a function of CUF, while examining CVUF and the Complex Impedance Unbalance Factor (IUF) to reach to the most accurate unbalance factor. The paper provides a comparative analysis of the performance during the unbalanced operation between a cascaded set of induction motors and single large sized induction motor having the same rating and proves that the operation of a cascaded set of induction motors under balanced and unbalanced supply voltage is more efficient than of a single large sized induction motor.

An Analytical and Experimental Approach to Diagnose Unbalanced Voltage Supply

Induction motors are the workhorse component of many industries and are frequently integrated into equipment. Many industries faced one of the serious problems regarding its maintenance issues. According to Neale and Wowk, the maintenance expenditure can be up to 80% of the total cost of motor. According to the reliable study by Electric Power Research Institute, the inter-turn short-circuit faults approximately contribute 37% of the induction motor failures. In the literature, most of the techniques are reviewed that are used to diagnose the fault in stator winding and it is found that majority of motor failures are due to insulation breakdown. It is found the unbalanced voltage is one of the major sources of insulation failure. And most of the exiting techniques to diagnose this fault are off-line or sensor bases. In this work, noninvasive online method is proposed; first of all, an analytical expression is derived for the conductor segments in the stator winding that are responsible for the generation of magnetomotive force (MMF). Therefore, stator windings are partitioned down into two set of segments (slot conductors and end conductors). The slot conductors are of major interest because their axial arrangement in the slots provides a room to establish MMF in order to generate a torque. Further, in this paper, an expression for MMF is derived through winding function approach. It was found that besides the fundamental MMF, there exist waves with a different number of poles. This MMF will induce a voltage in a stator winding through rotor side. In addition, the effect of MMF is considered on the current spectrum of induction motor because the knowledge of current spectrum under faulty regimes is the point of interest to diagnose a motor fault in a noninvasive way. Moreover, a new series of rotor harmonic frequency component are introduced to diagnose unbalanced voltage supply. Finally, the proposed analytical models and new series of rotor harmonic sheds a light on stator current components to diagnose and distinguish between balanced and unbalanced voltage supply in a real-time scenario. To validate the proposed method, two experimental hardware setups were designed comprising of a three-phase induction motor, two-axis magnetic field sensor, 8821-2A variable power source, current transformer, FLUKE 435 II series power quality energy analyzer, and Pasco interface for data acquisition. The appearance and significant increase in the magnitude of a new series of harmonics under unbalanced voltage are the indication of unbalanced voltage supply. Thus, the experimental data clearly justify and validate the proposed analytical model with the results of hardware setup.

Approach to measure the degree of the unbalanced power in induction motor for energy‐efficient use

This study utilises the power unbalance factors (PUFs) to determine the output precisely of the induction motor (IM) subjected to unbalanced supply voltage, and use them to analyse motor performance [i.e. copper losses, motor efficiency, power factor, input active and reactive powers, electromagnetic torque and derating factor (DF)]. The analysis of IM is applied using the approach of symmetrical component. Furthermore, MATLAB/Simulink is used to examine the behaviour of IM during balanced and unbalanced operations. Results illustrate that the reactive PUF ( K Q ) provides accurate assessment for the machine performance. Three cases of the unbalanced condition were selected to provide various scenarios, which may IM subjected to them. The influence of cases on electromagnetic torque, rotor speed, stator and rotor current, active and reactive input powers has been investigated. DF is investigated and tuned to improve energy efficiency during unbalanced operations.

Diagnosis of broken rotor bar fault in open‐ and closed‐loop controlled wye‐connected induction motors using zero‐sequence voltage

This study proposes a method to detect and quantify broken rotor bar fault using zero-sequence voltage in a wye-connected squirrel-cage induction motor. The zero-sequence voltage is analysed and two fault severity factors are defined. The factors are almost independent of motor speed and load torque. Furthermore, it is proved that closed-loop control has little influence on the diagnosis results. Consequently, the method can detect broken rotor bar fault for induction motors in both open- and closed-loop drives. Moreover, the method can discriminate the broken bar fault from a low-frequency load oscillation, even when these two phenomena occur simultaneously. In addition, the influence of unbalanced voltage supply on the diagnosis results is negligible. The effectiveness and robustness of the proposed method have been validated by experimental results.

Detection of Winding Faults in Wound Rotor Induction Motor Using Loci of Direct and Quadrature Axes of Rotor Currents

In this paper, loci of direct and quadrature axis of rotor currents are used for online detection of winding fault on stator side as well as rotor side in three-phase wound rotor induction motor. The proposed technique is used for detecting load on induction motor and open circuit fault condition on stator side as well as rotor side. Detection of winding fault is affected by unbalanced supply voltage. In order to overcome this problem, FFT of modulus of direct and quadrature axis is used. The experimental results of rotor current analysis are validated with stator current analysis. Research works reported in the literature are based on stator currents analysis. The proposed technique of detecting winding faults and distinguishing between stator winding faults and unbalanced supply voltage using rotor current is not reported in the literature.

Harmonic control using matrix converter of unbalanced input voltage supplied from solar power system

Most of the renewable energy sources are not distributed directly to consumer. It gone through several section to get optimum output voltage and current with the almost zero distortion. Direct Matrix converters are well known of its better power quality and low power distortion to the reactive loads. Conversely, multilevel H bridge inverters are broadly used to attain a sinusoidal output voltage in solar photovoltaic application. Reactive power is subject to high importance for the operation of alternating current (AC) power systems. Moreover there is always a challenge to obtain the balance current control from an unbalanced supply voltage with minimum harmonic distortion for reactive load application from solar power system. The main objective of the proposed system is to maximize the power usage by maintaining the current and power factor nearby unity. There are many different optimization techniques are used so far in matrix converter, a novel method of optimization BAT algorithm is proposed for the power quality to get the best result. This work mainly deals with the compensation of reactive power used in Solar Photovoltaic Power System. Bidirectional energy flow is possible with matrix converter for battery charging. Simulation results are presented to confirm the appropriate operation of the system under different of operating conditions. Also, in the proposed method the percentage of the higher order harmonics can reduced less than 9% of matrix converter for solar photovoltaic application.

Reduction of Input Current Harmonic Distortions and Balancing of Output Voltages of the Vienna Rectifier Under Supply Voltage Disturbances

This paper proposes a modified proportional resonant (PR) based control scheme for a three-level Vienna rectifier that reduces the input current harmonic distortion [total harmonic distortion (THD)] for harmonic contaminated and unbalanced supply voltage conditions. Low-order current harmonics that are present in the source side currents can be selectively eliminated using this proposed control strategy. In addition, the complete control technique includes the proportional integral (PI) based voltage balancing technique to balance the output voltages of the Vienna rectifier. This method includes the advantages of both hysteresis-based current control (fast transient response) and PI-based vector control (fixed frequency operation) methods. Various computational blocks such as space vector modulation (SVM) block, current/voltage transformation blocks, etc., are not necessary to implement this control method. Therefore, this scheme is relatively easy to implement using inexpensive digital controllers. A 1-kW prototype of the rectifier is built and tested in the laboratory environment using the proposed control method. The converter provides a unity power factor operation with low-current THD at the input side, balances the capacitor voltages, and tracks the voltage reference at the output side.

Control of distributed static compensator using extended structure-enhanced phase-locked loop-based algorithm under nonideal AC mains

Summary In this paper, a 3-phase extended structure-enhanced phase-locked loop (ESEPLL)–based control algorithm is proposed for a 3-phase 4-wire DSTATCOM (distributed static compensator) at linear and/or nonlinear consumer loads. A major feature of this control algorithm includes accurate and adaptive estimation of instantaneous fundamental positive sequence components of load currents at nonideal AC mains. In a highly distorted and highly unbalanced grid supply voltages, conventional phase-locked loop technique fails to extract the required signals. In such situation, the ESEPLL technique is used for estimating the reference supply currents. The proposed scheme not only enhances the performance of DSTATCOM system but it also provides the instantaneous compensation of harmonics currents, reactive power, unbalancing, and neutral currents due to load behavior. A single unit of ESEPLL technique is used in a 3-phase system for desired performance with necessary mathematical analysis. The control algorithm is tested on DSTATCOM using DSP-d-SPACE1103 interface with MATLAB. The obtained test results on a developed prototype show that the proposed control technique succeeds to enhance the power quality at point of common interface under various loading conditions in full-scale voltage at distribution level.

Improvement of compensation method for non-active current components at mains supply voltage unbalance

The necessity and topicality of the problem of improving the efficiency of compensation for power consuming asymmetrical equipment negative influence on the low-voltage mains have been demonstrated. That is why the ways of improvement of the compensation system have been offered on the basis of the analysis of the processes of compensation for currents non-active components in a three-phase power supply system under the conditions of supply voltage unbalance. The analysis was performed in an analytical form in the frequency domain. An instantaneous power cross-vector theory was used to generate active and reactive power instantaneous components and also to determine compensation currents in the analyzed power supply system. Improvement of the method consists in exclusion of voltage asymmetrical components, caused by the mains distortions, from the algorithm of generation of compensation currents. It was realized with the use of the designed blocks of separation and balancing of the supply mains voltage signals. The latter improved the efficiency of operation of power supply systems and the speed of operation of the compensation system. The significance of the presented paper consists in the fact that the offered method provides correct operation of the compensation system under the conditions of unbalance of the supply mains voltages. This makes it possible to improve power indices of the power supply system, to reduce the value of reactive power, voltage drop at the mains resistances, unbalance coefficient and the coefficient of distortion of currents sinusoidality and supply mains voltages. In the future, the presented method will be applied to compensation for currents non-active components in industrial power supply systems to which nonlinear and asymmetric equipment, characterized by a sharply changed operation modes, is connected.

Fuzzy logic controlled dynamic voltage restorer for mitigation of various power quality problems

Dynamic voltage restorer (DVR) is used to protect sensitive loads from harmonic distortion, voltage sag/voltage swell, and unbalance in supply voltage economically. In this paper, fuzzy logic-based control scheme has been developed for the production of compensating voltages for controlling DVR. Linear as well as nonlinear loads can be controlled by this scheme. A three-phase programmable voltage source is taken for producing voltage sag/swell and voltage harmonic distortion and this voltage is compared with a reference voltage of same magnitude. At the time of any disturbance, an error signal is generated which is processed by controller based on fuzzy logic theory. The controller generates the gate pulses for PWM inverter energised by a DC source and inverter injects the missing voltage into the distribution line and tightly regulates the voltage at the load terminals. By extensive simulation studies in MATLAB/Simulink and SimPowerSystems environment, the above control strategy is tested. The simultion results for voltage harmonic distortion, voltage sag/swell, and voltage unbalanced mitigation using the proposed control algorithm prove the effectiveness of DVR.

Performance study of three synchronisation algorithms in renewable energy grid connected system

In grid connected applications, the efficiency of systems is strongly affected by the employed phase locked loop (PLL) strategy. This paper presents a comparative study of three PLL structures applied to a grid-connected renewable energy system with a nominal power of 7 MW. To select the suitable structure dedicated to previously mentioned application, we will evaluate the performance of the PLL techniques under distorted and unbalanced supply voltages. Several simulations test with a disturbed three phase utility grid using MATLAB-Simulink/SimpowerSystem are presented to valid the theoretical study.