Effects Of Voltage Unbalance Research Articles

Coherence‐based integrated detection and assessment algorithm for voltages and currents unbalance/disturbance in three‐phase synchronous generators: A validation of simulation results

AbstractThe phenomenon of imbalance is a variation of voltage and current waveforms from a perfect sinusoidal waveform, which is considered a crucial challenge in power systems. Uneven loads and disturbances result in three‐phase voltage/current unbalance that leads to a temperature rise and low output curves at synchronous machines. Thus, the unbalance/fault must be recognized and avoided. This article suggests a methodology to detect and evaluate three‐phase voltage and current unbalance/fault using a coherence algorithm. The suggested scheme develops integrated protection functions that can be divided into three modules: voltage unbalance, current unbalance, and power factor unbalance. Therefore, it can be useful in mitigation techniques applied to compensate the voltage unbalance effects. The present protection introduces a new proposal of closed‐tripping curves based on the values range of nine coherence factors. To investigate the protection performance, a segment of power grid with real parameters’ information is simulated utilizing ATP platform. Several fault types, such as shunt and series faults, are conducted on the simulated system to analyze the results and monitor the system status using the coherence statistic processed in MATLAB software. The results show that the approach is smart, swift, precise, reliable and stable. Moreover, the algorithm can be practically implemented, and represents a base for digital unbalance/fault detectors and relays because its performance is superior in detecting Power Quality Disturbances (PQDs).

Rotor Winding Short-Circuit-Fault Protection Method for VSPSGM Combining the Stator and Rotor Currents

Rotor winding short circuit faults are common faults for variable-speed pumped-storage generator-motors (VSPSGM). At present, the exciting rotor fault protection of VSPSGM is simple and has low sensitivity. It can only act when the instantaneous value of the rotor phase current reaches three times the rated current. Therefore, it is difficult to cover some rotor winding short-circuit faults with weak fault characteristics. It is urgent to study a novel rotor winding short-circuit-fault protection method for VSPSGM. In this paper, a protection method that combines the stator and rotor currents with different frequencies is proposed. The characteristics of the stator and rotor currents before and after the fault is analyzed by using Clark transformation. On this basis, a specific protection criterion is constructed based on the discrete integral operation, which is easy to implement and not affected by the change of rotor speed. Then, the calculation method of the protection setting is proposed, considering the effect of unbalanced voltage and sensor measurement error. Simulation results show that the proposed method can reliably realize the protection of rotor winding faults. It has faster protection action speed than other methods in the same field. The protection coverage rate is over 90%.

Effects of voltage unbalance and harmonics on drive systems with induction motor

Due to the significant share of the induction motors (IMs) in global electricity consumption, the study of the IM in different power supply regimes is the aim of this work. In practice, these electrical receivers are not supplied from ideal voltage systems, therefore the power quality (PQ) is an important aspect in analysing the operation of this type of motors. We proposed a methodology for analysing the effects of unbalanced distorted voltages not only on the technical parameters of the IM but also on the power supply as well as on the driven mechanical system. We identified some dependencies, in analytical forms, between the sizes of interest and the considered PQ disturbances. The results showed that the supply voltage disturbances have significant effects on the induction motor, both electrically and mechanically. These effects are amplified if the IM is running at loads below 50% of the rated load.

An Alternative Methodology for Quantifying Voltage Unbalance Based on the Effects of the Temperatures and Efficiency of Induction Motors

Many unbalanced voltage conditions, which produce different effects on electric equipment, are associated with the same voltage unbalance factor. This fact hinders the precise association between the values of this index and the effects of voltage unbalance. In the present study, an alternative methodology for quantifying voltage unbalance based on the effects of this phenomenon on the temperature and efficiency of three-phase induction motors is proposed. The application of the suggested methodology results in an index that contemplates the combined effects of the modules and angles of the positive and negative sequence voltages. Aiming to apply this methodology and evaluate its performance, we selected a comprehensive set of balanced and unbalanced voltage conditions and used it on a three-phase induction motor. The results show that the index obtained, when compared to the traditional voltage unbalance factors, reduces the uncertainties inherent to the quantification process of this phenomenon. In other words, it allows a better association between the values and effects of the voltage unbalance. For these reasons, this index is considered as a potential alternative that can be employed in industrial utilities and standards that deal with voltage unbalance.

Enabling Grid-Feeding Converters With a Dissonant-Resonant Controller for Negative-Sequence Voltage Elimination

The mitigation of the adverse effects of voltage unbalance in equipment and power quality can be performed by the power electronic converters that interface distributed generators to the grid. Inspired in a resonant controller, this article presents a dissonant-resonant controller for negative-sequence voltage elimination for a grid-feeding converter connected to the grid. The controller eliminates the negative-sequence voltage at the converter output with a regulable precision, it does not require knowing the grid impedance for successful operation, and it can be a good candidate for parallel operation because it operates not like an integrator, but like an “untuned” integrator. Using the stationary $\alpha \beta$ frame, a closed-loop model is developed in a complex space vector built from the complexification of the stationary components. This allows extracting stability conditions for safe closed-loop operation as well as deriving design guidelines for the controller parameters. Numerical and experimental results show the ability of the proposed controller to meet its design goals, thus, corroborating the theoretical approach.

Application of ETAPTM eTraXTM software package for digital simulation of distribution network that feeds an AC traction power supply system

Voltage unbalance in power systems feeding AC traction power systems is a worldwide known problem. One of the main aspects of this problem is the negative effect of voltage unbalance on motor loads causing operating problems and economic damage. It is necessary to perform unbalanced power flow studies and calculate voltage unbalance indexes to assess this negative effect of voltage unbalance and the develop of compensating measures during the design stage. Usually, calculations of the AC traction power supply system and the distribution network feeding it are carried out separately during the design of new lines of railways electrified by alternating current (AC) and reconstructing of existing ones. This approach is a source of deviations in the power flow studies because the lack of consideration of mutual influences between the traction power system and the distribution power system. In this paper, we compare a digital model in which the traction power supply system and the distribution network are modeled separately with a model that considers mutual influences between the traction and external power supply systems, including power flows through the traction network caused by the distribution network (transit currents). For digital modeling of these processes, authors used ETAPTM software with eTraXTM package. It allows to run unbalanced power flow studies when the generation and load are being changed over time, including train movement. During a separate simulation of the traction power system and distribution network, traction network was modelled using the equivalent sources connected to traction substation buses and the distribution network was modeled taking into account the fact that traction load was given from the simulation of the traction power system. The traction load was considered as lumped loads connected to the traction substation buses.At the same time, in both cases, the unbalanced power flow study was carried out by the phase domain method. Based on the results of two models comparison, it was concluded that the combined model containing a traction power supply system and distribution network, is more effective in terms of improving the accuracy of assessment voltage unbalance in accordance with current regulatory and technical documents on power quality.

Analysis of Voltage Unbalance and Energy Loss in Residential Low Voltage Distribution Systems with Rooftop Photovoltaic Systems

This paper investigates effects of voltage unbalance and energy losses due to the connection of rooftop photovoltaic systems in a low voltage distribution system of a housing estate, which has light loads during daytime. The paper presents a case study of a real distribution power system of housing estate in Thailand. Voltage unbalance and energy losses were simulated by using system characteristic and load data from GIS database of PEA with the DIgSILENT Power Factory program. The key findings of our analysis are as follows. Firstly, the number of installable 1-phase rooftop PV systems varies directly with load density. Secondly, the number of installed 1-phase rooftop PV systems can be increased if the installation locations are closer to the transformer. For 3-phase rooftop PV systems, their installations do not have any effects on the voltage unbalance. Furthermore, system energy loss relates to the load density and PV system installation locations in the same way as the voltage unbalance. The key implication of our study is that the installation of 1-phase rooftop PV system should be granted based on a careful consideration of the installation location and the load density.

Theoretical and Experimental Investigation of the Supply Voltage Unbalance Effects on Squirrel Cage Induction Motors

Voltage unbalance is one of the power quality problems causing a lot of ill effects on induction motors. These are namely: Overheating, line-current unbalance, derating, torque pulsation, and inefficiency. The objective of the present work is to assess and quantify the effects of voltage unbalance on induction motors using both theory and experimentation. The analytical study is performed using the complex voltage unbalance factor model of this problem. The work deals specifically with the influence of the variations in the complex angle of the voltage unbalance factor on the stator and rotor currents, stator and rotor copper losses and total losses. The experimental investigation is carried out using an apparatus that measures currents, torque and power losses.

An analytical approach for calculating critical voltage-sag removal time of induction motors using manufacturer technical data

A critical voltage-sag removal time (CVSRT), reflecting a duration from the beginning of voltage sags to the instant when speed of an induction motor reduces to the critical value, is important for maintaining the motor’s stable running. The CVSRT can be usually determined by time-domain dynamic simulation that always requires a full set of induction motor parameters. This approach is quite tedious since the simulation must be repeated until the correct CVSRT is found. Also, the motor equivalent-circuit parameter is often not readily available, particularly with the on-site motors. Thus, this paper presents an alternative analytical approach to determine the CVSRT under balanced and unbalanced sag conditions. A motor slip-torque curve mainly required for CVSRT calculation is analytically computed using manufacturer technical data instead of the equivalent-circuit parameters. The slip-torque curve is modified to take effects of voltage unbalance and feeder cable impedance into account. The obtained slip-torque curves are validated against the published data. The CVSRTs straightforwardly computed from the proposed analytical approach are compared against those from the time-domain simulation. A good agreement between them is observed in satisfactory manner.

Negative Sequence Voltage Compensating for Unbalanced Standalone Brushless Doubly-Fed Induction Generator

During the unbalance operation of a standalone brushless doubly-fed induction generator (BDFIG), the negative sequence component causes significant unbalanced effect on the voltage and current of the power winding (PW). In this paper, a new control strategy is proposed to minimize the voltage unbalance effect of a standalone BDFIG under unbalanced loads. The proposed control strategy consists of two parts. The first part represents the conventional direct voltage control strategy to control the frequency and magnitude of the positive sequence PW voltage. The second part is the proposed strategy to minimize the negative sequence component in the output voltage of PW with a negative sequence compensator in the machine side converter. The dual second-order generalized integrator is used to extract negative-sequence component of the PW voltage. Proportional integral controllers are used to obtain reference of the control winding (CW) current which compensate the negative sequence component of the PW voltage. The proportional-integral-resonant controller is applied to regulate the CW current in positive reference frame. A prototype 30 kVA BDFIG test platform is developed in the laboratory to verify the proposed strategy.

Effect of Voltage Unbalance and Stator InterTurn Short Circuit on the Characteristic of Three Phase Induction Motor

Effect of Voltage Unbalance and Stator InterTurn Short Circuit on the Characteristic of Three Phase Induction Motor

Unbalanced Voltages Impacts on the Energy Performance of Induction Motors

<p class="Abstract">This paper presents the results of a study about the effects of unbalanced voltages on the energy performance of three-phase induction motors. The principal contribution of this paper is that presents a detailed analysis of the influence of positive and negative sequence voltage components and the angle between them on several characteristics such as: line currents, losses, efficiency and power factor under different voltage unbalanced conditions. A three-phase induction motor of 3 HP was used as a case study. The results of the investigation show that the positive sequence voltage must be considered together with the voltage unbalance factor (VUF) or percent voltage unbalance (PVU) index to evaluate the performance of the induction motor. It is also shown that the behavior of the motor load influences on the positive sequence parameters next to the voltage, while in the case of negative sequence only influences the negative sequence voltage.</p>

Effect of voltage unbalance and distortion on the loss characteristics of three‐phase cage induction motor

Voltage unbalance and distortion become one of the most common power quality problems in industrial utilities, which cause significant increase of the electromagnetic losses in induction motors. For energy saving of the induction motor system, there is a need for a detailed research of the loss characteristics of induction motor under unbalanced and distorted supply voltage. For accurate and fast prediction of the induction motor losses, this study proposes a method to identify rotor harmonics using the data from one supply cycle. The slip-frequency rotor harmonics are determined by using the space-time symmetrical characteristics of the induction motor. And based on a numerical method, the higher order harmonics of rotor flux density or current density are determined from data within one supply cycle. The electromagnetic losses are then calculated from those harmonics. With this method, the electromagnetic losses of a 5.5 kW induction motor under unbalanced and distorted supply voltage are calculated and analyzed. The proposed method and the analysis results are validated by comparing the predicted and measured motor losses of the 5.5 kW induction motor under different supply conditions.

Quantitative assessment of three phase supply voltage unbalance effects on induction motors

In this work, the performance of the induction motor under unbalanced three phase supply voltage conditions is assessed. This study compares the effects of scalar and complex modeling of voltage unbalance on the induction motor performance parameters such as torque, current, losses, efficiency and power factor. Different scenarios of unbalanced phase voltage magnitudes and angles are considered. It is found that the variation of the phase angles creates an unbalance and has an important effect on the induction motor performances which seems to be underestimated in the works reported in literature.

IJIREEICE - Electrical, Electronics, Instrumentation and Control

3 Abstract: This paper presents greater value to control a doubly fed induction generator (DFIG) - based wind power complete persons living time system with series grid side changer (SGSC) under unbalanced network conditions. The behaviours of the DFIG system with SGSC during voltage unbalance are described by injecting a series control voltage Produced from the SGSC. To balance the stator voltage against effects of voltage unbalance upon the DFIG such as stator and rotor current unbalances electromagnetic torque and power pulsations can be removed , and then the Conventional vector control strategy for the rotor-side converter remains in full force under unbalanced conditions. Mean while, three control targets for the parallel grid-side converter (PGSC) are identified, including eliminating the oscillations in the total active power or reactive power, or eliminating negative-sequence current injected to the grid. Furthermore, a precise current reference Generation strategy for the PGSC has been proposed for The PGSC to further improve the operation performance of the whole system. The simulation results of a mw DFIG based wind turbine 12with SGSC and testing results on a laboratory13scale based on experience trick prices under small unchanging state grid voltage unbalance.

The effect of fast changes in voltage unbalance on vibration characteristics of asynchronous motors

The effect of voltage unbalance in electrical networks that supply electrified traction loads on the operating regimes of large asynchronous motors (AMs) is considered. Based on the method of symmetrical components and analysis of hodographs of magnetic fields of AMs with a short squirrel-cage rotor, the influence of various types of voltage unbalance (continuous and intermittent) on its vibration characteristics is studied. It is shown that an increase in magnetic induction in the air gap of an AM causes an increase in radial vibration generating forces in the case of voltage unbalance. The obtained results indicate that alternating voltage unbalance can be riskier for AM operation than is constant voltage unbalance, in addition to having increased levels of AM vibrations. It is concluded that the use of new parameters of electric-energy quality allowing determination of instantaneous values of unbalance amplitude and the angle between symmetrical components of voltage of positive and negative phase-sequence is reasonable. The relevance of this research is confirmed by typical cases of AM shutoff at an oil-pumping station due to tripping of vibration-protection sensors.

Enhanced Control of a DFIG-Based Wind-Power Generation System With Series Grid-Side Converter Under Unbalanced Grid Voltage Conditions

This paper presents an enhanced control method for a doubly fed induction generator (DFIG)-based wind-power generation system with series grid-side converter (SGSC) under unbalanced grid voltage conditions. The behaviors of the DFIG system with SGSC during network unbalance are described. By injecting a series control voltage generated from the SGSC to balance the stator voltage, the adverse effects of voltage unbalance upon the DFIG, such as stator and rotor current unbalances, electromagnetic torque, and power pulsations, can be removed, and then the conventional vector control strategy for the rotor-side converter remains in full force under unbalanced conditions. Meanwhile, three control targets for the parallel grid-side converter (PGSC) are identified, including eliminating the oscillations in the total active power or reactive power, or eliminating negative-sequence current injected to the grid. Furthermore, a precise current reference generation strategy for the PGSC has been proposed for the PGSC to further improve the operation performance of the whole system. Finally, the proposed coordinated control strategy for the DFIG system with SGSC has been validated by the simulation results of a 2-MW-DFIG-based wind turbine with SGSC and experimental results on a laboratory-scale experimental rig under small steady-state grid voltage unbalance.

Analysis of voltage unbalance effects on induction motors with open and closed slots

This paper aims to complement studies concerning the influence of voltage unbalance on the performance of induction motors. We use sequence equivalent circuits to determine the increase of losses in the induction motor. We take into account the dependence of the rotor negative-sequence reactance with the load state and the increase in rotor resistance with the negative-sequence currents. Variations in the negative-sequence impedance are related with the structural characteristics of the rotor. We analyze motors with open and closed rotor slots, because the impedance of rotors with closed slots grows considerably when the load is less than rated, producing lower negative-sequence currents and lower losses. Increased rotor impedance in closed slot motors protects these motors against problems due to unbalanced supply voltage. For both type of rotors, we analyze motor derating factor based on voltage unbalance and increases in total losses and rotor losses.

Operation and control of a grid-connected DFIG-based wind turbine with series grid-side converter during network unbalance

This paper proposes a control scheme of a grid-connected doubly-fed induction generator (DFIG) wind turbine with series grid-side converter (SGSC) to improve the control and operation performance of DFIG system during network unbalance. The behaviors of DFIG system with SGSC under unbalanced grid voltage conditions are described. The SGSC is controlled to inject voltage in series to balance the stator voltage. Therefore, the adverse effects of voltage unbalance upon the DFIG such as large stator and rotor current unbalances, electromagnetic torque and power pulsations are removed and the conventional vector control strategy for the rotor-side converter (RSC) remains in full force under unbalanced conditions. Meanwhile, three selective control targets for the parallel grid-side converter (PGSC), such as eliminating the oscillations in total active or reactive power, or no negative-sequence current injected to the grid are identified and compared. Besides, the proportional resonant controllers in the stationary reference frame are designed for both the SGSC and PGSC to further improve the dynamic performance of the whole system. Finally, the ratings and losses of the SGSC and the injected transformer are discussed and the effectiveness of the proposed control scheme is verified by the simulation results of a 2 MW DFIG-based wind turbine with SGSC under steady state and small transient grid voltage unbalance.

A Power-Quality Index to Assess the Impact of Voltage Harmonic Distortions and Unbalance to Three-Phase Induction Motors

This paper discusses the need to simultaneously monitor voltage unbalance and harmonic distortions in addition to root-mean-square voltage values. An alternative way to obtain the parameters related to voltage unbalance at fundamental frequency as well as voltage harmonic distortions is here proposed, which is based on the representation of instantaneous values at the dq axes and at the instantaneous Euclidean norm. A new power-quality (PQ) index is then proposed to combine the effects of voltage unbalance and harmonic distortions. This new index is easily implemented into existing electronic power meters. This PQ index is determined from the analysis of temperature rise in induction motor windings, which were tested for long periods of time. This paper also shows that these voltage disturbances, which are harmful to the lifetime expectancy of motors, can be measured by alternative ways in relation to conventional methods. Although this paper deals with induction motors only, the results show the relevance for further studies on other pieces of equipment.