Overvoltage Problems Research Articles

Coordinated Countermeasures Against Overvoltage Instability in Autonomous Power Systems

Autonomous power systems with a single power station and a high-voltage (HV) network may be susceptible to overvoltage instability, especially during light-load conditions. Moreover, in low inertia systems, considerable frequency excursions may be observed after disturbances. To deal with voltage and frequency stability problems in such isolated power systems, several measures should be taken, such as rules for adequate automatic voltage control of the synchronous generators and load-tap-changers (LTCs), and/or adequate design of actions applied during emergency system conditions such as reactors switching, and redesigning underfrequency load shedding taking into account overvoltage problems. The effectiveness of the proposed countermeasures against overvoltage instability in autonomous power systems is illustrated in this paper. In 2016, the autonomous power system of the Greek island of Rhodes experienced similar voltage/frequency problems leading to a blackout. This event has been accurately reconstructed in this paper. The application of the suggested countermeasures during the simulation of the 2016 blackout event in Rhodes, further highlighted the effectiveness of these measures, providing a guideline for similar cases.

Network Partition-Based Two-Layer Optimal Scheduling for Active Distribution Networks With Multiple Stakeholders

This article proposes a two-layer optimal scheduling strategy to handle the overvoltage problem in high photovoltaic (PV) power-penetrated distribution networks. The voltage regulators can be classified as the power utility and PV owners, which are referred to as stakeholders. The proposed scheduling strategy includes autonomous optimization layer and coordination optimization layer. In the autonomous optimization layer, a min-max robust game model and a mixed-integer second-order cone programming-based model are respectively proposed to minimize the operating costs of PV stakeholders and the power utility stakeholder. A parallel optimization is employed to solve the two models in the autonomous optimization layer. In the coordination optimization layer, a noncooperative game-based model is presented to coordinate scheduling solutions of each stakeholder. Finally, an actual 10 kV, 106-bus feeder in Zhejiang Province, China, and a modified IEEE 123-bus distribution system are employed to verify the feasibility and effectiveness of the proposed approach.

A Distributed Inter-Phase Coordination Algorithm for Voltage Control With Unbalanced PV Integration in LV Systems

Most traditional Var compensation-based voltage regulation methods are developed following the single-phase Volt-Var response rule. These methods typically have competent voltage regulation performance with balanced photovoltaic (PV) integration. However, certain randomness of single-phase rooftop PV installation may lead to significant PV power imbalance across three phases, especially in low voltage (LV) distribution systems. In such unbalanced situations, unintended inter-phase Volt-Var response which is ignored in the single-phase Volt-Var response rule will become significant and greatly challenge the effectiveness of the traditional methods on voltage regulation. This can further cause inverter saturation and consequently makes distribution systems vulnerable to overvoltage problems. In this paper, the mathematical equations of unbalanced three-phase Volt-Var response are first derived and analyzed to identify the strong MVE (mutual Var compensation effect) and the weak MVE. This analysis provides the theoretical foundation for the development of the proposed inter-phase coordinated consensus algorithm, which can successfully overcome PV imbalance-induced voltage regulation challenges (e.g., inverter saturation and network overvoltage), while does not need exact system parameters. The effectiveness of this method has been validated by time-series simulations with a real LV distribution system and recorded data.

분산전원 연계 허용용량 개선을 위한 스마트인버터의 전압-무효전력 기능 적용에 따른 영향 분석

Herein, This paper analyzed the effect of applying the volt-var curve on the smart inverter to solve the over-voltage problem caused by increasing the distributed generation. To analyze the voltage characteristics due to the increase in distributed power connection, the voltage characteristics at the end of the test model were analyzed while increasing the distributed power capacity. The potential hosting capacity was analyzed by calculating the voltage margin up to the upper limit according to volt-var curve types with different reactive power contributions and dead-band. In addition, mounting the volt-var curve to many distributed generation connected to the distribution system faced a practical limitation. Considering this limitation, the voltage characteristics and hosting capacity margins for the penetration level of the volt-var curve were analyzed. In this paper, the effect of the volt-var curve was analyzed using OpenDSS, a quasi-static time-series simulation, considering the characteristics of the distribution system test model in the worst case.

Impact of First Tower Earthing Resistance on Fast Front Back-Flashover in a 66 kV Transmission System

Lightning stroke on a transmission tower structure is one of the major reasons that results in high voltages at the tower arms due to the excessive lightning current flowing through the transmission tower to earth. The Surge voltage seen at the tower cross arm on the first tower close to a substation is the worst case. If this voltage is higher than the withstand level of the insulator string, the insulation of substation equipment will be exposed to transient over-voltage called fast front back-flashover (FFBF). The peak of this transient overvoltage is affected by the value of the system’s earthing resistance. This paper studies the effect of reducing the grounding resistance of both the surge arrestor (SA) and the first transmission line tower adjacent to a 66 kV substation on FFBF. Three case studies using PSCAD/EMTDC software are presented to simulate the variation of the potential sire at the substation equipment with different resistance values for the first tower and SA earthing resistance. The paper also addresses the economic protection system for solving the problem of transient overvoltage. The study proves that the proper design of the first tower grounding system enhances the safety of the system and reduces the cost for the grounding system to the minimum.

High-economic PV power compensation algorithm to mitigate voltage rise with minimal curtailment

Increasing photovoltaic (PV) penetration has caused overvoltage problem in distribution systems. Power curtailment that includes power absorption of PV inverter and charge of battery energy storage is usually used to improve the voltage profile. Most methods prefer to use an excessive local power curtailment to regulate voltage. This inevitably reduces PV generation economy. Hence, a coordinated power compensation algorithm (PCA) is proposed in this paper to regulate bus voltage, aimed at reducing the total power curtailment while serving requisite load demand. Selecting primary control nodes and optimal PV compensation orders based on sensitivity sequence, our PCA achieves an effective voltage regulation with the minimal power curtailment. Final results on a tested PV system model verify the effectiveness.

Impact of Power-to-Gas on distribution systems with large renewable energy penetration

• A Power-to-Gas (PtG) model with electrolyser, H 2 buffer and methanation is set up. • The model is based on measurement carried out on an existing plant. • The PtG model is fully integrated into a distribution system power flow calculation. • Two network topologies and several penetrations of renewable sources are considered. • Annual simulations show positive impacts of PtG also with high share of renewables. The exploitation of the Power-to-Gas (PtG) technology can properly support the distribution system operation in case of large penetration of Renewable Energy Sources (RES). This paper addresses the impact of the PtG operation on the electrical distribution systems. A novel model of the PtG plant has been created to be representative of the entire process chain, as well as to be compatible with network calculations. The structure of the model with the corresponding parameters has been defined and validated on the basis of measurements gathered on a real plant. The PtG impact on the distribution systems has then been simulated on two network models representing a rural and a semi-urban environment, respectively. The testing has been carried out by defining a set of cases that contain critical situations for the distribution network, caused by RES plant placement. The objectives of the introduction of PtG are the reduction of the reverse power flow, as well as the reduction of the overcurrent and overvoltage issues in the distribution system. The results obtained from annual simulations lead to considerable reduction (from 78 to 100%) of the reverse power flow with respect to the base case, and to alleviating (or even solving) the overcurrent and overvoltage problems of the networks. These results indicate PtG as a possible solution for guaranteeing a smooth transition towards decarbonized energy systems. The capacity factors of the PtG plants largely vary depending on the network topology, the RES penetration, the number of the PtG plants and their sizes. From the test cases, the performance in a rural network (where the minimum capacity factor is about 50%) resulted better than in a semi-urban network (where the capacity factor values range between 21% and 60%).

Coordination of OLTC and smart inverters for optimal voltage regulation of unbalanced distribution networks

Photovoltaic (PV) smart inverters can improve the voltage profile of distribution networks. A multi-objective optimization framework for coordination of reactive power injection of smart inverters and tap operations of on-load tap changers (OLTCs) for multi-phase unbalanced distribution systems is proposed. The optimization objective is to minimize voltage deviations and the number of tap operations simultaneously. A novel linearization method is proposed to linearize power flow equations and to convexify the problem, which guarantees convergence of the optimization and less computation costs. The optimization is modeled and solved using mixed-integer linear programming (MILP). The proposed method is validated against conventional rule-based autonomous voltage regulation (AVR) on the highly-unbalanced modified IEEE 37 bus test system and a large California utility feeder. Simulation results show that the proposed method accurately estimates feeder voltage, significantly reduces voltage deviations, mitigates over-voltage problems, and reduces voltage unbalance while eliminating unnecessary tap operations. The robustness of the method is validated against various levels of forecast error. The computational efficiency and scalability of the proposed approach are also demonstrated through the simulations on the large utility feeder.

A New Approach to Model Reverse Recovery Process of a Thyristor for HVdc Circuit Breaker Testing

In the HVdc circuit breaker testing process, a thyristor is used to generate high current for the testing purpose. However, the reverse recovery process (RRP) of a thyristor can induce a significant overvoltage problem, which jeopardizes the reliable operation. It is important to model the RRP of a thyristor. The existing modeling methods usually omit the stray inductances in the circuit, which cannot describe the hard-switching process accurately. Therefore, this article proposes a novel method to model the RRP, considering the stray inductances. There are mainly three original contributions. First, the physical mechanism of the RRP is analyzed, describing the internal charge behavior and dividing the RRP into two stages. Second, this article provides a novel trigonometric exponential (TE) model of the thyristor voltage and current with analytical equations. Third, the extraction method of model parameters is also provided based on external circuit parameters and thyristor characteristics. In order to verify the proposed modeling method, a 1 kV/830 A IGBT-based circuit breaker is implemented with a thyristor to initialize the current. The experimental results show that the negative peak voltage induced by the RRP is as high as 4.26 kV, and the proposed TE model can precisely predict the overvoltage with a relative error of 7.51%.

Reinforcement Learning-Based Distributed BESS Management for Mitigating Overvoltage Issues in Systems With High PV Penetration

High levels of penetration of distributed photovoltaic generators can cause serious overvoltage issues, especially during periods of high power generation and light loads. There have been many solutions proposed to mitigate the voltage problems, some of them using battery energy storage systems (BESS) at the PV generation sites. In addition to their ability to absorb extra power during the light load periods, BESS can also supply additional power under high load conditions. However, their capacity may not be sufficient to allow charging every time when power absorption is desired. Therefore, typical PV/BESS may not fully prevent over-voltage problems in power distribution grids. This work develops a cooperative state of charge control scheme to alleviate the BESS capacity problem through Monte Carlo tree search based reinforcement learning (MCTS-RL). The proposed intelligent method coordinates the distributed batteries from other regions to provide voltage regulation in a distribution network. Furthermore, the energy optimization process during the day hours and the simultaneous state of charge control are achieved using model predictive control (MPC). The proposed approach is demonstrated on two test cases, the IEEE 33 bus system and a practical medium size distribution system in Alberta Canada.

Avoiding overvoltage problems in three‐phase distributed‐generation systems during unbalanced voltage sags

During voltage sags, distributed generation systems must fulfil specific grid-code requirements for reactive current injection. This ancillary service can produce overvoltage problems in networks operating in unbalanced conditions when the amplitude of one of the phase voltages is higher than the others and a balanced reactive current is injected through large grid impedance. This study proposes a control scheme to avoid these overvoltage problems, thus reducing the risk of cascade disconnection that this incident may produce. The derivation of the control scheme starts from the flexible oscillating-power control, introduces the necessary modifications so that this control meets the grid-code requirements for current injection and combines it with a slope voltage control to achieve a good voltage regulation. A theoretical analysis is included to determine the expressions that quantify the voltage support characteristics of the proposal. Finally, selected experimental results are reported to validate the characteristics of the proposed control.

Optimal Parameters of Volt–Var Function in Smart Inverters for Improving System Performance

This paper proposes a method to improve the performance of a distribution system by optimizing volt–var function of a smart inverter to alleviate the voltage deviation problem due to distributed generation connection. In order to minimize voltage deviation and line losses which represent the performance of a distribution system, this paper proposes an algorithm that optimally sets the parameters of the volt–var function. In the process of optimizing the parameters of the volt–var function, the algorithm proposed in this paper considers minimizing the contribution of the reactive power in order not to affect the output of the distributed generation. In order to apply to the field, the distribution system in South Korea considering the configuration and operation regulation was selected as a test model for algorithm verification. As a result, the system performance was successfully improved by optimally setting the volt–var function of the smart inverter which is an effective way to solve the over-voltage problem caused by distributed generation connection. This paper verified the proposed method through OpenDSS, a quasi-static time-series simulation, for the test model considering the characteristics of the distribution system in South Korea.

Optimal reactive power flow procedure to set up an effective local voltage control

These days, due to distributed generation increasing, the operation and control of active distribution grid becomes complicated. Thus, their voltage control is the main topic of many research papers recently. While network reinforcement mitigated voltage rise in passive distribution networks, local voltage control, coordinated voltage control and centralized voltage control have been studied in several research papers. However, many of these approaches need plenty of sensors to gather large number of measurements that could cause complexity. This paper presents a new procedure to set up a local voltage control law devoted to properly manage the voltage profile (e.g. minimizing losses on MV feeders and increasing grid hosting capacity), without relying on advanced telecommunication architectures. The final goal of this paper is in a procedure that could be directly adopted in the today in place electric distribution grids. In particular, the contribution proposed is a method to optimize, offline, the Q(V) characteristic curve of the local voltage control for distributed generation units. The approach proposed has been implemented and tested on a real-life distribution grid of an Italian mountain area, where overvoltage problems could seriously bound renewable exploitation.

Capacitive Power Tapping From Insulated Shield Wire of Overhead High Voltage Transmission Lines With Tuning

Due to a lack of high and stable power supplies for large-scale online monitoring devices, the intelligent development of high voltage transmission lines has always been restricted. This paper presents a method for capacitive power tapping from an insulated shield wire of overhead high voltage transmission lines with tuning. A general equivalent model, which considers the influence of line sag and landform on the equivalent circuit parameters, is established. The calculated equivalent circuit parameters show good agreement with the field measured data obtained by a 220 kV single-circuit transmission line. In order to maximize the tap-off power, a nonlinear tuning method is proposed since linear tuning can cause an overvoltage problem, and a tuned reactor is developed. Based on the measured equivalent circuit parameters, an experiment platform is built in the laboratory to measure the tap-off power under different resistive loads. The experimental results indicate that the maximum power is about 900 W, which represents an improvement of 80% compared with the maximum power without tuning. Additionally, the tuned reactor shows good robustness to the perturbation of equivalent circuit parameters, which validates the effectiveness of the proposed power tapping method.

Optimal Sitting, Sizing, and Operation of Batteries and Passive Filters to Mitigate Over-voltage and Harmonic Problems in Distribution Networks with High Photovoltaic Penetration

Optimal Sitting, Sizing, and Operation of Batteries and Passive Filters to Mitigate Over-voltage and Harmonic Problems in Distribution Networks with High Photovoltaic Penetration

Transient Overvoltage Simulation in 500 kV Transmission Line Plan of Sarawak, Malaysia using PSCAD

Transmission system is a crucial system in electrical power since the system transmit the electricity from power generation to consumer load. According to World Bank, the power losses from transmission lines are rapidly increasing from year to year at the rate of 3.85% in the year of 2013 to 5.792% in 2014. Losses in transmission system are most likely from power quality problems such as transients. Transients are the outcome of high unexpected increment in voltage or current surge magnitudes. The peak values of both voltages and current are usually more than twice of that normal voltage and current amplitudes. The surges due to transients can vitally cause power system failure and breakdown of electrical equipment especially at the substations. There were few known transient overcurrent and overvoltage problems, which are due to faults, lightning and line energizing, respectively. This research work mainly focuses on simulating transients for 500 kV transmission system which employ Sarawak as the case study location. Sarawak currently has main 275 kV transmission line covering the whole Sarawak from Miri to Kuching known as Sarawak backbone, but due to lots of industries and rapid development and urbanization boom in Sarawak, there is a planned of 500 kV transmission line as a backup if the 275 kV transmission line proves inadequate. In Sarawak, the 500 kV is planned to be energized at 275 kV. But, in fact this work is for that transmission line to be operated at 500 kV, hence, monitoring the highest transient may occur. The results revealed that lightning and three-phase faults of 1.0 s fault time duration cause the highest change in amplitude of current on the line up to 9.06 pu and 9.27 pu, respectively. The highest lightning amplitude is observed when lightning was simulated at the receiving end of the line which is near to the Tada substation.

Novel Uninterruptible Phase-Separation Passing and Power Quality Compensation Scheme Based on Modular Multilevel Converter for Double-Track Electrified Railway

Over-voltage and over-current problems of locomotives when passing phase separation and negative sequence current penetration seriously influence the safety of double-track electrified railway and public power systems. In order to solve these problems, this paper proposes a novel uninterruptible power supply phase separation passing and power quality compensation (UPSP-PQC) scheme for double-track electrified railway. Three working modes of UPSP-PQC are put forward, including uninterruptible phase separation passing mode, power quality compensation mode and uninterruptible phase separation passing priority, and power quality compensation optimum mode. A three-leg modular multilevel converter (MMC) topology of UPSP-PQC is proposed and the corresponding control strategy has been studied. PSCAD/EMTDC simulation is performed to verify the correctness and effectiveness of the proposed scheme and its control method.

Use of demand response for voltage regulation in power distribution systems with flexible resources

In low-voltage power distribution systems with high penetration of photovoltaics (PVs) generation and electric vehicles (EVs), the over-voltage problem arises at times because of large PV generation, and under-voltage problem also arises sometimes because of simultaneous charging of massive EVs. Over- and under-voltage problems lead to more difficulties in achieving voltage regulation. Demand response (DR) is expected to be promising and cost-effective in promoting smart grids, and hence, the utilisation of flexible resources (FRs) through DR can be helpful for distribution system voltage regulation. This study introduces a hierarchical control structure of a community energy management system (CEMS) and multiple sub-CEMSs to apply an FR-based two-stage voltage regulation technique. In the first stage, i.e. the day-ahead scheduling stage, each sub-CEMS optimises the FRs' schedules for minimising customers' electricity cost and network voltage violation times. In the second stage, i.e. the real-time operation stage, the voltage sensitivity-based FRs' shifting method is proposed to eliminate network voltage violations caused by errors of estimated day-ahead data. The proposed models and methods are verified based on a realistic distribution system in Japan, where voltage violations, customer electricity cost and a number of on-load tap changer tap operations are proved to be reduced.

Multiple Spatiotemporal Characteristics-Based Zonal Voltage Control for High Penetrated PVs in Active Distribution Networks

The penetration of photovoltaic (PV) outputs brings great challenges to optimal operation of active distribution networks (ADNs), especially leading to more serious overvoltage problems. This study proposes a zonal voltage control scheme based on multiple spatiotemporal characteristics for highly penetrated PVs in ADNs. In the spatial domain, a community detection algorithm using a reactive/ active power quality function was introduced to partition an ADN into sub-networks. In the time domain, short-term zonal scheduling (SZS) with 1 h granularity was drawn up based on a cluster. The objective was to minimize the supported reactive power and the curtailed active power in reactive and active power sub-networks. Additionally, a real-time zonal voltage control scheme (RZVC) with 1 min granularity was proposed to correct the SZS rapidly by choosing and controlling the key PV inverter to regulate the supported reactive power and the curtailed active power of the inverters to prevent the overvoltage in each sub-network. With the time domain cooperation, the proposed method could achieve economic control and avoid overvoltage caused by errors in the forecast data of the PVs. For the spatial domain, zonal scheduling and zonal voltage control were carried out in each cluster, and the short-term scheduling and voltage controlling problem of the ADN could then be decomposed into several sub-problems. This could simplify the optimization and control which can reduce the computing time. Finally, an actual 10kV, 103-node network in Zhejiang Province of China is employed to verify the effectiveness and feasibility of the proposed approach.

Infinitesimal Method Based Calculation of Metro Stray Current in Multiple Power Supply Sections

The calculation of rail potential and stray current in the DC traction power system can guide the protection against the rail overvoltage problem and mitigate the electrolytic corrosion due to stray current. To obtain the accurate stray current distribution, the infinitesimal method based calculation of metro stray current in multiple power supply sections (PPSs) is studied in this paper. The through-type structure characteristics of power supply system and that of return current system which is constituted of rails - stray current collection mats - tunnel reinforcements - ground (RSTG) are considered in the proposed calculation method. In the traction power supply system, the currents injected into the rail by train, and absorbed by traction power substation (TPS) or regeneratively braking train, are solved by the power flow calculation which considers stray current. In the return current system, these currents are used as the boundary conditions for stray current calculation. Then the rail potential and stray current in multiple PPSs are solved by the infinitesimal method. The verification result shows that the proposed calculation method can effectively calculate the rail potential and stray current in multiple metro PPSs. At last, the novel calculation method is used to gain the dynamic stray current along the metro line and evaluate the influence factors of stray current.