Unbalanced Voltage Conditions Research Articles

Control of Grid Fed PV Generation Using Infinite Impulse Response Peak Filter in Distribution Network

An upsurge in the integration of renewable sources to the utility grid has resulted into escalating concerns regarding power quality (PQ) improvement. In order to provide efficient operation under weak grid conditions, the necessity of a competent control technique is dominant. Consequently, the implementation of an infinite impulse response (IIR) peak filter is used here for harmonics mitigation, power factor correction, and alleviating other PQ issues by providing distribution static compensator (DSTATCOM) capabilities. In addition, this control technique also works during conditions when solar photovoltaic (PV) power is not procurable. During day, the requirement of load is fed from the solar PV array and the stored grid power. On the other hand, at night when the power from solar PV array is unavailable, the requirements of load are fulfilled from the grid and DSTATCOM operation ensures power quality improvement. In order to validate this system, it is maintained according to the IEEE-519 standard. The considered weak grid conditions include load unbalancing, intermittent solar insolation, voltage sag, voltage swell, voltage distortion, and voltage unbalance conditions. The benefits of IIR peak filter include adaptive computation during filtering, thereby alleviating the computational complexity. Moreover, less memory space is required during execution guaranteeing fast performance during weak grid conditions. Simulated results are utilized to depict the behavior of the system during abnormal grid conditions. Moreover, its comparative performance with the existing control techniques, is also presented. For further substantiation and in order to observe reliable operation, a laboratory prototype is developed and tested during weak grid conditions such as voltage distortion, voltage unbalance, voltage sag, voltage swell, and load unbalancing.

Assessing Voltage Unbalance Conditions in IE2, IE3 and IE4 Classes Induction Motors

Brazil has started an energy transition process through more efficient electric motors. The new legislation determining the IE3 Class as the minimum efficiency level in commercial electric motors became valid in August 2019. Currently, four induction motor classes are defined by the IEC 60034-30-1, with the IE4 class being the most efficient, and in the same way, proposals to achieve IE5 efficiency class are available in the market. Considering the upcoming new technologies, it is necessary to know the impact of power systems disturbances in IM’s in view of future substitutions. This paper presents a detailed analysis of the impact of different percentages of under and over voltage unbalances on the temperature and performance of electric motors classes IE2, IE3 and IE4. Results show that the IE4 hybrid motor presents non-linear characteristics, being observed this motor shows less dependence on voltage variation, mainly undervoltage unbalance. An analysis of how this phenomenon impacts on the current harmonic distortions in the induction motors operation is also developed through Spearman’s correlation matrices. Finally, models that represent the temperature increase of each motor with different voltage unbalance conditions are presented.

Fault Ride-Through Enhancement of Grid Supporting Inverter-Based Microgrid Using Delayed Signal Cancellation Algorithm Secondary Control

The growing level of grid-connected renewable energy sources in the form of microgrids has made it highly imperative for grid-connected microgrids to contribute to the overall system stability. Consequently, secondary services which include the fault ride-through (FRT) capability are expected to be possessed characteristics by inverter-based microgrids. This enhances the stable operation of the main grid and sustained microgrid grid interconnection during grid faults in conformity with the emerging national grid codes. This paper proposes an effective FRT secondary control strategy to coordinate power injection during balanced and unbalanced fault conditions. This complements the primary control to form a two-layer hierarchical control structure in the microgrids. The primary level is comprised of voltage/power and current inner loops fed by a droop control. The droop control coordinates grid power-sharing amongst the voltage source inverters. When a fault occurs, the participating inverters operate to support the grid voltage, by injecting supplementary reactive power based on their droop gains. Similarly, under unbalanced voltage condition due to asymmetrical faults in the grid, the proposed secondary control ensures the positive sequence component compensation and negative and zero sequence components clearance using a delayed signal cancellation (DSC) algorithm and power electronic switched series impedance placed in-between the point of common coupling (PCC) and the main grid. While ensuring that FRT ancillary service is rendered to the main utility, the strategy proposed ensures relatively interrupted quality power is supplied to the microgrid load. Consequently, this strategy ensures the microgrid ride-through the voltage sag and supports the grid utility voltage during the period of the main utility grid fault. Results of the study are presented and discussed.

A Novel Segmented Component Injection Scheme to Minimize the Oscillation of DC-Link Voltage Under Balanced and Unbalanced Conditions for Vienna Rectifier

This paper investigates a Vienna rectifier as a charger for series-connected battery packs. Focusing on carrier-based pulsewidth modulation (CBPWM), the ripple current flowing through the neutral point (NP) results in the voltage oscillation if the loads are resistive. To reduce the ripple of average NP current with mitigated distortion under balanced and unbalanced dc-link voltages conditions, a novel CBPWM with segmented component injection scheme (SCIS) is proposed in this paper. After dc component injection, continuous intervals for optimized component injection and clamping intervals for compensation component injection are identified. Optimized components are calculated originally based on unbalanced factor to make the average NP current zero-size in one switching period. Moreover, unique compensation components generate suitable NP current to shape the sinusoidal input currents according to the circuit analysis. In consequence, the SCIS not only keeps the input current with low-harmonic distortion, but also minimizes the oscillation of dc-link voltage under balanced and unbalanced conditions. In addition, the value of the NP current during the clamping intervals is analyzed under various operating conditions. The effectiveness and the performance of the proposed SCIS are verified by simulation and experiments.

Modified Venturini Modulation Method for Matrix Converter Under Unbalanced Input Voltage Conditions

Based on Venturini method, it is in favor of the modulation technique for controlling the matrix converter due to only use of the comparison between the duty cycles in time domain and the triangular carrier wave for generating the gating signals and the achievable voltage ratio between fundamental output magnitude and fundamental input magnitude to 0.866. However, even with simple modulation method and achieving maximum fundamental output magnitude, the possible input voltage unbalance conditions accordingly influence on the output performances (more reduction and distortion). Thus, a control strategy based on Venturini method is presented in this paper, in order to solve the impacts of unbalanced input voltage conditions on the matrix converter performances. Conceptually, this strategy is done by modifying the mathematical model for controlling the modulating waves to satisfy the desirable feature, as generated in the event of normal situation. Up to this approach, it can support either single-phase condition or two-phase condition. Performance of the proposed control strategy was verified by the simplified simulation model in the MATLAB/Simulink software. It is clearly shown that the matrix converter can be controlled for regulating the balanced output voltages with showing good steady-state and dynamic operations without the energy storage devices.

Extended Range of Ultra Sparse Matrix Converter Using Integrated Switched Capacitor Network

Ultra sparse matrix converter (USMC) is known for ac-ac conversion with sinusoidal input currents, output voltages, and variable frequency operation. However, its operation is restricted for voltage gain up to 0.866 and load power factor angle up to 30° along with requirement of protection circuit for pulsating loads. This paper proposes switched capacitor USMC to overcome the restrictions of USMC with improved performance. Experimental results are provided to evaluate the converter capability and behavior under balanced and unbalanced input voltage conditions. Furthermore, the proposed converter is compared with the other counterparts.

Ways to Reduce Power Losses in Mining Power Supply Lines

Electric energy is the most common and universal form of energy. It can be produced in large quantities near energy sources, transmitted to large distances, easily distributed between consumers and converted into mechanical, thermal, and light energy. Ensuring reliable and economical quality electric energy supply to mining enterprises with the optimal use of energy resources is one of the most important tasks facing the country's energy sector. In the Russian Federation, high degree of concentration of generating capacities at power plants has been achieved. The main capacities are concentrated in power plants with installed capacity of more than 1 million kW. High importance of the electric power industry is determined by the infrastructural nature of the industry and the direct relationship between the country's economic growth and the level of its development effectiveness. One of the most pressing problems of modern electricity supply is the problem of ensuring the quality of electric energy. The main reason for the deterioration in the electricity quality is widespread non-linear loads that create non-sinusoidal currents and voltage unbalance during their operation. The voltage unbalance is most often caused by the presence of an unbalanced load. Unbalanced load currents flowing through the elements of the power supply system cause unbalanced voltage losses in them. This results in appearance of unbalanced voltages on the terminals of electric apparatus. The voltage deviations at electric apparatus overdriven phase may exceed the normally acceptable values, while the voltage deviations at electric apparatus other phases may remain within the normal limits. In addition to the deterioration of the voltage mode at electric apparatus under the unbalanced voltage conditions, the operating conditions of both the electric apparatus and all the network elements are significantly worsened, and the reliability of the electrical equipment and the power supply system as a whole decreased. The paper considers the issues of reducing the voltage losses in power supply networks by improving quality of electric power.

Coordinated‐control strategy of scalable superconducting magnetic energy storage under an unbalanced voltage condition

Modular multilevel converters (MMCs) have the advantages of high-power density and small-harmonic distortion because of their modularity and flexibility, thus providing a new avenue for research into scalable superconducting magnetic energy storage (SMES) in renewable energy generation. This study presents coordinated control for a three-phase four-wire (3P4W) MMC-based SMES system under unbalanced voltage applications. First, the positive- and negative-sequence mathematical models and the port-controlled Hamiltonian with dissipation model of the 3P4W MMC-SMES system are established by introducing an additional path for zero-sequence current. Second, a multi-objective passivity-based control strategy that can effectively improve the power quality and system robustness and eliminate both double-frequency active and reactive power fluctuations or double-frequency active power fluctuation and negative-sequence current is proposed. The simulation results based on MATLAB/Simulink demonstrate the effectiveness of the proposed topology of the SMES and its control strategy.

Inter-Turn Fault Detection of Dry-Type Transformers Using Core-Leakage Fluxes

In this paper, a new technique for low-severity inter-turn fault detection under online operating conditions of dry-type transformers is proposed. The proposed methodology requires just two inexpensive flux sensors and is based on the fundamental component of the core-leakage fluxes at the transformer's outer limbs. Theoretical analysis using the proposed analytical models and experimental validation on a 10-kVA experimental prototype transformer are carried out to evaluate the performance of the proposed methodology under various fault severities. The real-time fault-detection capability of the proposed technique is also evaluated using a fabricated prototype health monitoring device, which reveals that as low as 2-turns bolted fault out of 150 turns can be detected within 3 fundamental cycles. The methodology is immune to loading and supply voltage unbalance conditions and on par with the classical differential negative sequence current based technique with an additional advantage of 78% reduction in the total number of sensors required for a three-phase three-winding transformer.

Thermal Analysis of a Directly Grid-Fed Induction Machine With Floating Neutral Point, Operating Under Unbalanced Voltage Conditions

Significant changes may occur in the thermal behavior of a directly grid-fed induction machine when subjected to unbalances in the voltage supply. This article studies and analyzes the thermal behavior of a low power, three-phase, squirrel-cage induction motor, connected in star configuration with floating neutral point, when subjected to different levels of unbalanced voltage. The dependence of the thermal motor behavior on the severity level of the unbalance is studied and analyzed. In addition to amplitude unbalances, this article focuses on the effects of phase unbalances, as well, which were not addressed in detail in previous published studies. Moreover, situations of mixed unbalance, where amplitude and phase unbalances occur simultaneously, are also studied. The finite element method was used to simulate the thermal behavior of the machine. The experimental setup consists of a three-phase programmable AC power supply, suitable to precisely emulate unbalanced conditions that may occur in real-scale power systems, supplying a 2.2 kW induction motor. Experimental data were acquired resorting to resistance temperature detectors PT100, placed in the machine phase whose supply current value changed the most. Finally, the simulation results are verified and critically discussed through experimentally obtained results.

Open-Circuit Fault Diagnosis and Tolerance Strategy Applied to Four-Wire T-Type Converter Systems

Due to the zero-sequence currents in the four-wire systems, the three-phase currents become more complex on both amplitudes and phases in the unbalanced load conditions, so that the existing open-circuit (OC) fault diagnosis methods of the switching devices cannot be directly used. In this paper, an OC fault diagnosis strategy applied to four-wire T-type converter systems is proposed, which is implemented by decomposing the circuit model into a positive- and negative-sequence model and a zero-sequence model, and locating the OC fault based on the law of positive- and negative-sequence voltage error offsets and zero-sequence voltage error offsets. The proposed diagnosis method is suitable for both unbalanced load and unbalanced input voltage conditions. Meanwhile, the differences of fault-tolerant control between four-wire and three-wire systems are analyzed and compared. An effective fault tolerance strategy for inner OC faults applied to four-wire T-type converter systems is presented, which owns the ability to suppress dc bus imbalance. Simulation and experimental results are presented to verify the correctness of the proposed strategy.

Optimal Shaping of the MMC Circulating Currents for Preventing AC-Side Power Oscillations From Propagating Into HVdc Grids

A constrained optimization problem based on the Lagrange multipliers method is formulated to derive the circulating current references of modular multilevel converters (MMCs) directly in abc coordinates. The resulting analytic expressions for calculating the circulating current reference signals are designed to eliminate oscillations in the dc-side power flow, independently of the ac-side operation of the MMC. As a result of the constrained optimization, the circulating currents are shaped to optimally utilize the degrees of freedom provided by the internal energy buffering capacity of the MMC, to effectively decouple the ac-grid conditions from the dc bus. This property of the proposed control method makes it especially suitable for preventing oscillations due to unbalanced ac-grid voltage conditions from propagating into multiterminal high-voltage dc systems. It is shown that the power flow at the dc-side of the MMC will be most effectively decoupled from ac-side transients if the desired steady-state power flow is imposed by acting directly on the circulating current references instead of by acting on the ac-side current references. The operation of an MMC controlled by the proposed approach is demonstrated by simulation studies, verifying the ability to keep the dc power flow free of second harmonic oscillations, independently of the power control objectives applied for calculating the ac-side current references of the converter.

Voltage balancing control based on gate signal delay in series connection of SiC‐MOSFET

AbstractRecently, research and development of SiC power devices have been done, and SiC power devices have become commercially available. The SiC power devices are suitable for realizing to medium voltage applications. Since, the voltage rating of commercial power devices is limited to less than 1.2 kV, they should be connected in series to maintain a higher voltage rating. However parasitic parameters of these devices are not the same, and therefore the voltage sharing during turn‐off operations cannot be controlled. This article proposes a digital control for the voltage balancing of series connected SiC‐MOSFETs during turn‐off operations. In order to compensate the voltage unbalance conditions, this article presents a time‐adjustment gate‐drive circuit using a digital delay line; experimental results for the feedback control using a buck chopper circuit are also presented.

An Online Coordinated Charging/Discharging Strategy of Plug-in Electric Vehicles in Unbalanced Active Distribution Networks with Ancillary Reactive Service in the Energy Market

The global acceptance and off-grid charging of plug-in electric vehicles (PEVs) are expected to grow tremendously in the next few years. Uncoordinated PEV charging can cause serious grid issues such as overloading of transformers and unacceptable voltage drops. Single-phase residential charging can also initiate or contribute to voltage unbalance conditions in the distribution networks. A potential solution and key challenge for PEV integration is shifting of the charging activities to off-peak periods. This paper proposes a new PEV coordination approach based on genetic algorithm (GA) optimization to perform online centralized charging and discharging considering transformer loading and node voltage magnitude and unbalance profiles. It allows PEV as source of active and reactive power to participate in energy market based on different prices during a day, without any degradation. Finally, the impacts of uncoordinated and the proposed GA coordinated PEV charging/discharging strategy are simulated for a real unbalanced Western Australian distribution network in the Perth solar city over 24 h.

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A sliding mode approach to enhance the power quality of wind turbines under unbalanced voltage conditions

An integral terminal sliding mode-based control design is proposed in this paper to enhance the power quality of wind turbines under unbalanced voltage conditions. The design combines the robustness, fast response, and high quality transient characteristics of the integral terminal sliding mode control with the estimation properties of disturbance observers. The controller gains were auto-tuned using a fuzzy logic approach. The effectiveness of the proposed design was assessed under deep voltage sag conditions and parameter variations. Its dynamic response was also compared to that of a standard SMC approach. The performance analysis and simulation results confirmed the ability of the proposed approach to maintain the active power, currents, DC-link voltage and electromagnetic torque within their acceptable ranges even under the most severe unbalanced voltage conditions. It was also shown to be robust to uncertainties and parameter variations, while effectively mitigating chattering in comparison with the standard SMC.

Modified Venturini Modulation Method for Matrix Converter Under Unbalanced Input Voltage Conditions

A Venturini method is one of the popular modulation techniques for controlling the matrix converter due to its simplicity of gating signal generation and a maximum voltage ratio of 0.866 between fundamental output magnitude and fundamental input magnitude. However, even with simple modulation method and achieving maximum fundamental output magnitude, the possible unbalanced conditions of the three-phase input voltages affect the reduction and distortion of the output performances. Thus, a control strategy based on Venturini method is presented in this paper, in order to solve the impacts of unbalanced input voltage conditions on the matrix converter performance. Conceptually, this strategy is done by modifying the mathematical model for controlling the modulating waves as generated in the event of normal situation. Up to this approach, it can support either singlephase condition or two-phase condition. Performance of the proposed control strategy was verified by a simplified simulation model in the MATLAB/Simulink software. It is shown that the matrix converter can be controlled by the proposed algorithm without the energy storage devices for regulating the output voltages, which results in a good steady-state and dynamic operation.

Optimal PI controller of DVR to enhance the performance of hybrid power system feeding a remote area in Egypt

This article proposes optimal PI controllers of Dynamic Voltage Restorer (DVR) to enhance the performance of stand-alone hybrid renewable energy system that is feeding a new community located in Egypt. The hybrid system includes three renewable energy sources, namely, solar PV cells, wind turbines based permanent magnet synchronous generator and fuel cells. These three sources are connected to a common DC link by three boost converters, one for each source. The common DC link is integrated to the AC loads via DC/AC inverter. The optimal size of the three proposed renewable sources is obtained using the HOMER software package. The main contribution of this study is designing optimal PI controller for DVR for enhancing the performance of a hybrid PV-wind-fuel cell. Two PI controllers are used to regulate the IGBT pulses of the voltage source inverter (VSI) driving DVR by adjusting the D-Q axes voltage signals. The D-Q axis components of the load voltage represent the inputs to the two PI controllers respectively. While the D-Q axis voltage signals driving the IGBT pulses of the DVR-VSI IGBT are the outputs of the two PI controllers respectively. A Cuckoo Search (CS), as an optimization technique is used for determining the optimal PI control parameters used to drive the DVR at different operating conditions. To validate the effectiveness of the DVR based PI-CS control, four test scenarios are investigated includes (three phase fault, sag, swell and unbalance voltage conditions). The enhancement of the system performance is achieved through improving the voltage, the current and the power waveforms for each source, improving the dynamic performance of the wind turbine generator and keeping the continuous operation of the three sources during faulty conditions. The proposed system is modeled using MATLAB/Simulink.

Mitigation of harmonics and unbalanced source voltage condition in standalone microgrid: positive sequence component and dynamic phasor based compensator with real-time approach

Penetration of Distributed Energy Resources (DER) is in high demand to supply power to the load where the grid is not available. Many of these sources are a single phase source used to form standalone Microgrid (MG). Single phase connectivity of these sources results in an unbalanced source voltage condition (UbSVC). Interfacing power electronic devices also inject the harmonics into Point of Common Coupling (PCC) voltage. The effect of this unbalance and harmonics on the operation of standalone MG is analysed in this paper in a twofold manner. One at a reduced power transfer from DER to load and the other is an error produced in Phase Locked Loop (PLL) operation. Positive Sequence Component (PSC) based and Dynamic Phasor (DP) based compensation techniques are proposed in this paper to mitigate the effect of UbSVC. Simulation validates that both the proposed methods are capable to provide balanced load voltage condition under UbSVC in terms of Voltage Unbalance Factor (VUbF). It also enhances the power transferred from DER to load during an UbSVC. The performance of the proposed compensator during UbSVC and harmonic presence is validated in real-time simulation using Opal-RT and dSPACE simulators.

Analysis of energy saving potentials in intelligent manufacturing: A case study of bakery plants

To address the global challenge of the climate change, more strict legislations worldwide on carbon emission reductions have put energy intensive industries under immense pressure to improve the energy efficiency. Due to the lack of technical support and financial incentives, a range of technical and economic barriers still exist for small-medium enterprises (SMEs). This paper first introduces a point energy technology, which is developed for SMEs to improve the insight of the energy usage in the manufacturing processes and installed in a local bakery. Statistical analysis of electricity consumption data over a seven-day period is conducted, including the identification of operational modes for individual processing units using an enhanced clustering method and the voltage unbalance conditions associated with these identified modes. Two technical strategies, namely electrical load allotment and voltage unbalance minimisation, are then proposed, which could attain more than 800 kwh energy saving during this period and the current unbalance could be reduced to less than 10%. In addition, the genetic algorithm is deployed to solve the job shop scheduling problem based upon the commercial electrical tariffs, and this reduces the electricity bill by £80 per day in the case study. Implementation of the recommendations based on the above analysis therefore may potentially yield significant financial and environmental benefits.