Reactive Power Characteristics Research Articles

Optimal Configuration Model for Large Capacity Synchronous Condenser Considering Transient Voltage Stability in Multiple UHV DC Receiving End Grids

In a multi-fed DC environment, the UHV DC recipient grid faces significant challenges related to DC phase shift failure and voltage instability due to the high AC/DC coupling strength and low system inertia level. While the new large-capacity synchronous condensers (SCs) can provide effective transient reactive power support, the associated investment and operation costs are high. Therefore, it is valuable to investigate the optimization of SC configuration at key nodes in the recipient grid in a scientific and rational manner. This study begins by qualitatively and quantitatively analyzing the dynamic characteristics of DC reactive power and induction motors under AC faults. The sub-transient and transient reactive power output model is established to describe the SC output characteristics, elucidating the coupling relationship between the SC’s reactive power output and the DC reactive power demand at different time scales. Subsequently, a critical stabilized voltage index for dynamic loads is defined, and the SC’s reactive power compensation target is quantitatively calculated across different time scales, revealing the impact of transient changes in DC reactive power on the transient voltage stability of the multi-fed DC environment with dynamic load integration. Finally, an optimal configuration model for the large-capacity SC is proposed under the critical stability constraint of dynamic loads to maximize the SC’s reactive power support capability at the lowest economic cost. The proposed model is validated in a multi-fed DC area, demonstrating that the optimal configuration scheme effectively addresses issues related to DC phase shift failures and voltage instability resulting from AC bus voltage drops.

Modeling method of photovoltaic power generation grid connection based on particle swarm optimization neural network

Aiming at the complex structure, numerous equipment, intricate control and protection logic, as well as the existence of numerous unmodeled dynamics and black-box device models in photovoltaic (PV) grid-connected systems, a modeling method based on Particle Swarm Optimization Neural Network (PSO-NN) is proposed to address the inability of pure mechanism models to accurately simulate their operational dynamics. Utilizing the differences in active power response waveforms under Voltage-Frequency (Vf) control, Power-Reactive Power (PQ) control, and Droop control as criteria for control strategy identification, a PSO-NN model is constructed for PV grid-connected systems, with inputs comprising temperature, humidity, light intensity, voltage, and frequency disturbances, and outputs being active and reactive power. To validate the model's effectiveness, a PV grid-connected system model is built in a self-developed simulation software and connected to an IEEE 14-bus distribution network for simulation verification. The results demonstrate that the proposed PV grid-connected model can effectively identify the types of Vf control, PQ control, and Droop control strategies, and accurately reflect the dynamic response characteristics of active and reactive power under various voltage and frequency disturbances.

Distributed optimal Volt/Var control in power electronics dominated AC/DC hybrid distribution network

AbstractThe integration of large‐scale distributed power sources increases the voltage fluctuation in AC/DC hybrid distribution network (AD‐HDN). Power electronics devices such as photovoltaic (PV) inverters, soft open point (SOP), and voltage source converters (VSCs) can be utilized for voltage/var control (VVC) to alleviate the risk of voltage fluctuation and violation. This paper proposes a distributed optimal VVC method in power electronics dominated AD‐HDN. Firstly, the reactive power and voltage characteristics of PV inverters, SOP, and VSCs are analysed, and an optimal VVC optimization model for AD‐HDN to minimize node voltage deviation, PV curtailment, and network loss is proposed. Then, the second‐order cone (SOC) relaxation technique is used to re‐formulate the model into a convex optimization model. A distributed optimal VVC framework based on the alternating direction method of multipliers (ADMM) is constructed. Based on the residual balance principle and relaxation technique, an accelerated ADMM method is further proposed to solve the proposed model. Finally, case studies are conducted on the IEEE 33‐node and 85‐node systems to verify the superiority and effectiveness of the proposed method.

Reactive Power Characteristics and FRT Strategy for Grid-Forming Converters with Virtual Impedance-Based Current Limiting

Reactive Power Characteristics and FRT Strategy for Grid-Forming Converters with Virtual Impedance-Based Current Limiting

Coordinated Optimization for Multi-Infeed LCC-HVDCs Transient Control Considering Short-Term Voltage Stability of Receiving-End Grid

With local synchronous generators in receiving-end grids being replaced by multi-infeed line-commutated converter-based (LCC) high-voltage direct currents (HVDCs), the dynamic voltage supportability of the power grid decreases. Moreover, the reactive power consumption of LCC-HVDCs during the recovery process brings risks to voltage stability. Hence, the transient control of LCC-HVDCs should be optimized coordinately to support short-term voltage stability under credible contingencies. In this paper, the dynamic reactive power characteristics between AC/DC systems are derived mathematically. Based on the coupling mechanism among the voltage-dependent current order limiter (VDCOL), AC dynamic voltage, and HVDC power recovery, an “autonomous-synergic” optimization strategy incorporating short-term voltage stability constraints is proposed for multi-infeed LCC-HVDCs. To handle the problem of large-scale differential-algebraic equations (DAEs), the optimization is iteratively solved based on time-domain simulation (TDS) and trajectory sensitivities. The stability is quantified by an indicator and the trajectory sensitivity analysis is utilized to relax stability constraints. The effectiveness and feasibility of the proposed methods are verified with a benchmark system and a real system in China. The short-term voltage stability under all credible contingencies can be guaranteed, while the total power recovery of LCC-HVDCs is improved.

Experimental validation of onboard electric vehicle chargers to improve the efficiency of smart charging operation

Electric vehicles (EVs) are at the center of the power and transport sector coupling; however, smart charging is required to not compromise the integrity of the grid. In this work, we propose, test, and validate a method for investigating EV onboard chargers via the OBDII port. We present the charging efficiency and reactive power characteristics of 38 different EV models from the last 11 years. Data show that, due to added losses, smart charging through current modulation can increase global charging energy demand from 1%–10%. In addition, EVs consume a relatively large amount of reactive power at lower currents, and some models violate the power factor limits for the low-voltage grid. Our projections show an efficiency of 88%–95% by 2030 and a saturation between 90%–96% by 2035. Therefore, the newly presented AC-to-DC conversion efficiency values help achieve better results when calculating life cycle assessment, grid integration and energy simulation that consider EVs. Curtailed smart charging can further integrate charging needs by implementing phase balancing and matching with behind-the-meter local generation. Finally, our results urge regulators and automakers to further improve charging technology and legislation based on other technological experiences, e.g. solar inverters.

Clustering Combined Multi-Objective Optimal Operation of Transmission Systems Considering Voltage Stability

In recent years, power systems have undergone major changes called energy transitions during which synchronous generators have been replaced with power electronics-based generation. Therefore, the voltage stability of power systems has become a major concern owing to the absence of synchronous generators. This study proposes multi-objective optimization using the non-dominated sorting genetic algorithm III to achieve optimal reactive power reserve procurement and improve the voltage stability of the overall system. These systematic approaches require high computational power and are unsuitable for the operational frameworks currently used for large-scale power systems. Previous works have rarely considered the local characteristics of reactive power or generation de-commitment with sufficient re-dispatch owing to greater renewable energy integration. We propose a framework for achieving systematic optimization by considering various objective functions while utilizing the regional aspect of reactive power via spectral clustering-based voltage control area (VCA) identification. The proposed method comprises systematic and regional approaches to optimizing systems for voltage stability improvement based on VCAs. The results demonstrate that the proposed method shows satisfactory performance. These results will be helpful for decision making for power system operations in harsher environments with more renewable energy.

Reactive Power compensation Optimization of Distribution Network with Distributed Power Supply

In recent years, distributed generation has been fully developed, and its permeability is rising, which also brings great challenges to the reactive power regulation of the power distribution network. By analyzing the output characteristics of active power and reactive power of distributed power supply, in this article, a multi-objective optimization mathematical model of reactive power compensation is established, which takes the economy and security of the power distribution network into consideration. The results indicate that the reactive power compensation optimization model and the solution algorithm in this paper have good effectiveness and superiority. This thesis will provide the theoretical foundation and technology support for the design and realization of the DG reactive power compensation scheme.

Experimental determination of static characteristics of loads in nodes of a 6 kv power supply system

Subject of research: static characteristics of nodes of 6 kV power supply systems with asynchronous load.
 Purpose of research: experimental determination of static characteristics by active and reactive power, calculation of the reserve coefficient by static stability of a node with asynchronous load.
 Object of research: power supply systems with a complex load, which includes asynchronous motors.
 Main results of research: the results of an active experiment to determine the static characteristics of the load node are presented. The measurements were carried out using devices that record the quality of electricity. Voltage regulation was carried out by the RPN device, the current transformers on the input switch were selected as the connection point of the device. Static characteristics of active and reactive power were constructed using interpolation and extrapolation tools. The critical voltage of the load node is determined. The coefficient of static stability margin is determined.

Impact of Distributed Generators Penetration Level on the Power Loss and Voltage Profile of Radial Distribution Networks

The Distributed Generator types have different combinations of real and reactive power characteristics, which can affect the total power loss and the voltage support/control of the radial distribution networks (RDNs) in different ways. This paper investigates the impact of DG’s penetration level (PL) on the power loss and voltage profile of RDNs based on different DG types. The DG types are modeled depending on the real and reactive power they inject. The voltage profiles obtained under various circumstances were fairly compared using the voltage profile index (VPI), which assigns a single value to describe how well the voltages match the ideal voltage. Two novel effective power voltage stability indices were developed to select the most sensitive candidate buses for DG penetration. To assess the influence of the DG PL on the power loss and voltage profile, the sizes of the DG types were gradually raised on these candidate buses by 1% of the total load demand of the RDN. The method was applied to the IEEE 33-bus and 69-bus RDNs. A PL of 45–76% is achieved on the IEEE 33-bus and 48–55% penetration on the IEEE 69-bus without an increase in power loss. The VPI was improved with increasing PL of DG compared to the base case scenario.

Coordinated voltage control for improved power system voltage stability by incorporating the reactive power reserve from wind farms

The absorption and output characteristics of reactive power of the doubly-fed induction generator (DFIG) greatly influence the voltage stability of PCC (Point of Common Coupling) where the wind farms are integrated into the bulk power grid. This study proposes a reactive power compensation strategy for coordinated voltage control (CVC) of PCC with large-scale wind farms to achieve the expected voltage quality of the power grid through a minimum amount of control actions in emergencies. To this end, the mechanism of reactive power and voltage control inside DFIG is first analyzed. Then, the concept of reactive power reserve (RPR) sensitivity concerning control actions is introduced and an index of voltage stability margin is proposed to evaluate and analyze the distance between the current operating point and the voltage collapse point by analyzing the relationship between reactive power reserve and voltage stability margin. In the event of an emergency, critical reactive power reserves are obtained to reduce the dimension and complexity of the control problem. The sensitivity of reactive power reserve and the control are formulated into a convex quadratic programming problem to optimize the control strategies for voltage stability. The proposed technology has been validated on the IEEE 39-bus system.

A Double-Ended Protection Principle for an LCC-VSC-MTDC System with Strong Anti-Interference Ability

The DC grid technology of multi-power supply and multi-drop-point power reception is an effective solution for large-scale renewable energy integration into the power grid. Line-commutated converter-Voltage source converter (LCC-VSC) power grids are one of the more important developmental directions of the future power grid that have occured in recent years. But the multi-terminal high voltage direct current system has the problems of inconsistent boundary characteristics, inconsistent control, and fault response characteristics, which puts higher requirements on the protection scheme. Thus, a completely new protection principle is proposed in this paper. Firstly, the fault characteristics of distributed capacitance current are analyzed. The reactive power calculated by the distribution parameters of different frequencies is different. Subsequently, the fault characteristics of DC reactive power are analyzed, and a DC reactive power extraction algorithm is proposed. The polarity of the multi-band DC reactive power is used to construct the protection scheme. Finally, the LCC-VSC power grid model verifies the correctness and superiority of the proposed protection scheme. The proposed scheme uses DC reactive power instead of fault current to solve the long delay problem caused by distributed capacitance. Compared with the prior art, the proposed solution is not affected by distributed capacitance and has a stronger anti-interference ability (600 + 10 dB + 1 ms).

Research on Optimal Allocation of Reactive Power Compensation in Offshore Wind Power Transmission System via AC Cable Based on Controllable Reactor

The capacitance of submarine cables with the same length and the same voltage level is more than 20 times that of overhead lines, which makes the reactive power for AC transmission system of offshore wind power greatly increase and the power flow is complicated, and it is easier to produce a series of problems such as reactive power return, low power factor, increased network loss, and terminal voltage rise, which puts forward higher requirements for reactive power compensation configuration scheme. Combined with practical engineering cases, this paper introduces the reactive power and voltage characteristics of the sea breeze system, as well as the problems and shortcomings of conventional reactive power compensation schemes. Finally, the reactive power allocation optimization scheme based on the new magnetically controlled controllable reactor MCR is proposed, which has the technical and economic advantages of low running loss, saving investment and land occupation, etc. It can be used as a beneficial supplement to the reactive power allocation scheme for the transmission system of offshore wind power with AC cable and has the value of engineering popularization.

Improvement of Dynamic Reactive Power Characteristics of New-type Condenser by Excitation Parameters

The new-type condenser has the capability of dynamic reactive power compensation under subtransient, transient and steady-state, which could effectively solve the problem of insufficient dynamic reactive power reserve under the background of large-scale development of UHVDC transmission. In this paper, the excitation system parameters of the condenser are taken as the research object. First, the basic principle of new-type condenser and the excitation system model are introduced. Then, an effective gain of reactive current is defined as the quantitative index of the dynamic reactive power characteristics of the new-type condenser, and the effect of excitation system parameters on the dynamic reactive power characteristics of new-type condenser is studied by frequency domain trajectory sensitivity analysis. Finally, the simulation model of the condenser grid connected system is built based on PSCAD/EMTDC, the steady-state reactive power output level and transient response characteristics of new-type condenser with different excitation parameters are studied, and the optimization direction of the condenser excitation system parameters is determined. The simulation results show that reducing k A, k p, k D, T m and increasing k I T D, T Z are beneficial to improve the steady-state reactive power output of the condenser. Decreasing k A, k p, k D and increasing k I, T D, T m, T Z are beneficial to improve the transient response characteristics of the condenser.

Dynamic reactive power characteristic analysis of inverters under the commutation failure faults in HVDC systems

In the traditional high voltage direct current (HVDC) systems, commutation failure (CF) fault is one major challenge, and the transient var characteristic of the inverter station is an essential factor in the counter measures against CF fault. Based on the detailed switching process of valves during CF, the var demand characteristic is analysed. From the key factors during the recovery of CF, the dynamic var demand characteristic is investigated. It is shown that, the dc side of converter is short-circuited and the ac side can be regarded as open-circuit. Moreover, the var demand of inverter station is mainly influenced by the changing ratio of dc current and commutating voltage. At last, the case studies verify the validity of the reactive power characteristics of the inverter station proposed in this paper.

Active and reactive power coordinated optimal dispatch in active distribution network based on model predictive control

With the proliferation of the distributed energy resources (DERs), the scheduling and control of the distribution network have become more complicated. To cope with the uncertainty nature of distributed generation, a multi-timescale optimal dispatch method in active distribution network (ADN) based on the model predictive control (MPC) is proposed in this paper. First, based on MPC, a hierarchical scheduling framework for ADN is established, including long-timescale stage, and short-timescale stage. Then, via coordinated control of various resources in the ADN, i.e., distributed generators, energy storage, capacitor banks and OLTC transformer, the impact of intermittent renewable energy and load forecast errors can be reduced. Finally, considering the coupling characteristics of active and reactive power in the ADN, a joint active and reactive power optimization model is proposed to further reduce the network loss. Numerical simulation on a modified IEEE-33 distribution network system verifies the correctness and superiority of the proposed scheduling approach.

Prediction method of successive commutation failure for multi-infeed high voltage direct current systems under fault of weak receiving-end grid

The complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) expand the effect of commutation failure (CF). The rapid prediction of CF is crucial in ensuring the stable operation of such systems. The simultaneous CF caused by grid fault has received widespread attention in recent years. However, the coupling between the LCC-HVDC with CF and the adjacent normal LCC-HVDC is not considered. When the state of the main circuit suddenly changes and the control system subsequently starts, the LCC-HVDC with CF inevitably produces an effect on the adjacent LCC-HVDC through the coupling of grid voltage. This study describes a new phenomenon, where the former produces a successive CF in the latter under the fault of weak receiving-end grid. The dynamic reactive power characteristics of inverter station and the induced mechanism of successive CF are analyzed. And the dynamic reactive power model of LCC-HVDC is established. A fast prediction method for successive CF is then proposed by calculating the extinction angle of LCC-HVDC with consideration of the effect of adjacent LCC-HVDC with CF. The proposed method can improve the accuracy of CF prediction in multi-infeed LCC-HVDC systems through theoretical derivation and simulation verification.

Reactive power characteristics and vibration properties under SISC in synchronous condensers

A stator interturn short-circuit (SISC) fault in synchronous condensers can bring about serious damage if the fault is not detected in time. At present, researches on SISC fault characteristics and diagnosis methods are still scarce because only in recent years are synchronous condensers adopted in power grids. Different from previous studies, this paper considers the practical operation condition of synchronous condensers that need to change exciting current frequently and work in either under-excitation or over-excitation according to the actual demand of power grids. Detailed reactive power and unbalanced magnetic pull (UMP) expressions under SISC are deduced firstly based on the analysis of the magnetic flux density variation. It can be seen that the reactive power increases under under-excitation running and decreases under over-excitation running after SISC. The second, fourth and sixth harmonics of stator UMP/vibration are enlarged under the SISC. Specifically, the sixth harmonic augments with the increase of exciting current in the over-excitation running, while the variation of the sixth harmonic with exciting current depends on parameters and fault degree of synchronous condensers under the under-excitation operation. Moreover, the two-dimensional finite-element analysis (FEA) and experimental tests are performed on a customized synchronous machine. In addition, the FEA of a TTS-300-2 synchronous condenser is accomplished in order to validate the proposed theoretical analysis further. The research conclusions of fault characteristics in this paper provide potential solutions for the monitoring and diagnosis of SISC in synchronous condensers.

A Data Driven RUL Estimation Framework of Electric Motor Using Deep Electrical Feature Learning from Current Harmonics and Apparent Power

An effective remaining useful life (RUL) estimation method is of great concern in industrial machinery to ensure system reliability and reduce the risk of unexpected failures. Anticipation of an electric motor’s future state can improve the yield of a system and warrant the reuse of the industrial asset. In this paper, we present an effective RUL estimation framework of brushless DC (BLDC) motor using third harmonic analysis and output apparent power monitoring. In this work, the mechanical output of the BLDC motor is monitored through a coupled generator. To emphasize the total power generation, we have analyzed the trend of apparent power, which preserves the characteristics of real power and reactive power in an AC power system. A normalized modal current (NMC) is used to extract the current features from the BLDC motor. Fault characteristics of motor current and generator power are fused using a Kalman filter to estimate the RUL. Degradation patterns for the BLDC motor have been monitored for three different scenarios and for future predictions, an attention layer optimized bidirectional long short-term memory (ABLSTM) neural network model is trained. ABLSTM model’s performance is evaluated based on several metrics and compared with other state-of-the-art deep learning models.

Modeling and Analysis of Transient Reactive Power Characteristics of DFIG Considering Crowbar Circuit under Ultra HVDC Commutation Failure

Ultra high voltage direct current (UHVDC) transmission is an effective means of long-distance transmission of renewable power generation, which has obtained a lot of research and practical applications. The commutation failure is a common DC transmission fault, which will cause the voltage amplitude of the sending ac grid in UHVDC system to first decrease then increase. The existing transient mathematical models of the wind power generation system (WPGS) are difficult to apply to scenarios where the grid voltage changes continuously. A mathematical model suitable for commutation failure is established to analyze the transient reactive power characteristics of the doubly fed induction generator (DFIG)-based WPGS with the consideration of the crowbar circuit trigger. The correctness of the mathematical model is validated by an experiment based on the control hardware-in-loop (CHIL) platform. Based on the proposed mathematical model, the influence of the crowbar parameters on the reactive power output of the DFIG is analyzed, and the selection of crowbar parameters to suppress the overvoltage of the sending ac grid is investigated. A simulation model is built based on Matlab/Simulink to verify the overvoltage suppression effect of the proposed selection scheme.