Challenges and Improvements of Wind Power Dispatch in Northwest China

Besides common characteristics of wind power,there are some special characteristics in China power system,including large-scale,long distance transmission and lack of flexible regulating power sources.These special characteristics make power dispatch more challenging in China.Many studies have been carried out and some improvements are presented including wind power monitoring and control as well as evaluation of wind power integration capabilities.As a demonstration project,the technologies are integrated into the energy management system and are implemented in the Northwest China power system.They provide effective measures for wind power dispatch in the grid.

Influences of ±800 kV Yunnan-Guangdong HVDC System on Security and Stability of China Southern Power Grid

The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recovering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only higher power transfer-limit of the parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.

Application of Risk-based Flexibility Assessment Methods to Evaluate System Expansion Plans

This paper proposed a flexibility assessment approach based on and risk assessment methodology. System planners prioritize each planning scheme with consideration of three aspects: reliability, economics, and flexibility. In the past, there is lack of quantitative indices to measure flexibility of a power system. This paper proposes applying probabilistic risk assessment method to quantify system flexibility. The proposed approach is demonstrated to compare two transmission planning schemes during Guangdong expansion planning process.

An optimization strategy of controlled electric vehicle charging considering demand side response and regional wind and photovoltaic

Renewable energy,such as wind and photovoltaic(PV),produces intermittent and variable power output.When superimposed on the load curve,it transforms the load curve into a‘load belt’,i.e.a range.Furthermore,the large scale development of electric vehicle(EV)will also have a significant impact on power grid in general and load characteristics in particular.This paper aims to develop a controlled EV charging strategy to optimize the peak-valley difference of the grid when considering the regional wind and PV power outputs.The probabilistic model of wind and PV power outputs is developed.Based on the probabilistic model,the method of assessing the peak-valley difference of the stochastic load curve is put forward,and a two-stage peak-valley price model is built for controlled EV charging.On this basis,an optimization model is built,in which genetic algorithms are used to determine the start and end time of the valley price,as well as the peak-valley price.Finally,the effectiveness and rationality of the method are proved by the calculation result of the example.

Frequency Control at the Power Sending Side for HVDC Asynchronous Interconnections Between Yunnan Power Grid and the Rest of CSG

For a large-scale high voltage direct current (HVDC)asynchronous interconnected power grid, the frequency issue atthe power sending side under DC faults is a crucial problem.To solve this problem, based on rotor motion equations, theeffect of unbalanced power on the system frequency under DCfaults is analyzed. The characteristics and dynamic developmentprocess of frequencies after the injection of disturbances areanalyzed. In addition, the actions and coordinated strategies ofvarious frequency control measures are also investigated. Basedon the testing projects of an asynchronous interconnection in theChina Southern Power Grid (CSG), the frequency features arestudied according to the measured PMU data. The outcome showsthat the frequency problem of the Yunnan Power Grid after anasynchronous interconnection can be solved and controlled. Italso shows that the frequency limit control (FLC) is importantfor the frequency regulation of large scale HVDC asynchronousinterconnected DC power grids. As demonstrated, DC FLC caneffectively suppress the deviation of the transient frequency.However, reasonable frequency regulation parameters shouldbe set and other area frequency control measures should becoordinated to maintain the frequency stability of the system.

The Single-active-part Structure of the UHVDC Converter Transformer with the UHVAC Power Grid

The studied sending-terminal’s UHVDC converter transformer with the UHVAC grid is currently one of the largest-capacity converter transformers,which is characterized by strong support for the large-scale grid power delivering out,strengthening grid’s fault tolerance and backup capability,improving grid operation’s flexibility and security,lowering the short-circuit currents.The proposed single-active-part converter transformer structure is well-designed,including the single-phase five-leg iron core,the special winding arrangement,the special structure of the spacers and angle rings,and so on,which is a smaller footprint,easier to install and test on site,but is more challenging in the structural design due to the critical railway transportation limitations on aspects of both size and weight.The feasibility of the proposed structure schema is verified by a series of tests,concerning the wave process,the short circuit force,the temperature field,the main insulation,and so on.Among them,the main insulation’s electric fields at the import positions are verified to be reasonable,including the AC impressed voltage test,the DC impressed voltage test,and the polarity reversal test.It shows that the proposed single-active-part design scheme is practicable and conductive to manufacture and application for the UHV large-capacity converter transformers.

Key Technologies of Ultra-high Voltage Hybrid LCC-VSC MTDC Systems

The key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems are investigated,focusing on the design of system configurations,converter topologies and the control and protection system.A double converter per pole of VSC connection is proposed along with the design of a 5000 MW VSC valve to develop a±800 kV/5000 MW large-capacity power transmission.The hybrid MMC topology capable of clearing the DC faults and the control strategy are developed to effectively improve the reliability in case of overhead line faults.The control and protection system of the LCC-VSC MTDC system is introduced to offer flexible operations under both normal and abnormal conditions,which includes voltage/current margin-based coordination,converter switch-in and switch-out,re-connection and drop-off of a third station.Simulations of an LCC-VSC MTDC system based on the LCC-VSC MTDC project are performed.

Design Aspects of Hybrid HVDC System

Hybrid high voltage direct current transmission(Hybrid HVDC)is a new type of HVDC technology developed in recent years.It combines the characteristics of large-capacity and low cost of the line commutated converters(LCC)and non-commutation failure and dynamic reactive power support of the voltage sourced converters(VSCs)in one HVDC system.It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs,giving it broad application prospects.This paper defines the basic form of hybrid HVDC first and discusses the characteristics and challenges of different hybrid HVDCs.Then,it analyzes the matching characteristics between LCC and VSC in the station-station hybrid HVDC,studies the key technologies,such as VSC topologies and control strategies for clearing the DC overhead line faults,and introduces the key technologies for analyzing and suppressing the high-frequency resonances between VSC and the power grid.In this paper,using the Wudongde UHV multi-terminal HVDC as an example,the proposed key technologies are analyzed and verified.

Effect of different coupling agents on the interfacial properties of tube-internally insulated foam materials interface in a composite cross-arm under water environment

In recent years,electrical breakdown accidents for composite cross-arms in rain-impacted environments have occurred which might be ascribed to the degradation of the tube-internally insulated foam materials interface.This research has focused on the different coupling agent treatments(KH-550,KH-560,KH-792,and IPTS)of the tube-internally insulated foam materials interface.The effect of different coupling agents on the inter-facial electrical and physical properties in a water environment have been studied.The leakage current and compressive shear strength of the interface were investigated after water diffusion pre-stress.Three-dimensional computed tomography(μCT)was used to study the interface cross-section tomogram,interface three-dimensional volume rendering,and relative interface porosity.The interfacial hydrolysis resistance ability and the interfacial strength modified by various coupling agents were effectively reflected by the compressive shear strength and relatively defect ratio.The results revealed the ranking of the comprehensive coupling effect was KH-560>KH-550>KH-792>IPTS,which provides significant guidance for modifying internal insulation interface performance for composite cross-arms.

Synthesis of high-purity mesoporous nanosilica microspheres from retired composite insulators based on orthogonal experiment

Silicone rubber is an indispensable insulation material for power equipment and is difficult to degrade naturally,posing a serious threat to the environment.It remains challenging to dispose this alarming amount of waste.In our previous research,a green pyrolysis recycling method was proposed for waste composite insulators.In this article,the experimental device was further improved,and the recycling process was optimised by orthogonal experiments to obtain nanosilica microspheres(NSMs)with higher purity and controllable particle size for the state-of-the-art engineering applications.Firstly,the effect of three factors(calcination temperature,constant-temperature duration,and heating rate)on the particle size of NSMs were investigated by single-factor analysis.ANOVA and response analyses were then used to examine the goodness of fit of the multi-parameter fitting equation and the interaction impact of multiple factors on the particle size.Finally,homogeneous mesoporous NSMs with higher purity(increased from 97%to 99%)and smaller particle size(decreased from 160 to 103 nm were prepared under the guidance of model.The SiO_(2)-recovery rate of this method was 74.7%,revealing superior economic value.

Day-ahead Network-constrained Unit Commitment Considering Distributional Robustness and Intraday Discreteness:A Sparse Solution Approach

Quick-start generation units are critical devices and flexible resources to ensure a high penetration level of renewable energy in power systems.By considering the wind uncertainty and both binary and continuous decisions of quickstart generation units within the intraday dispatch,we develop a Wasserstein-metric-based distributionally robust optimization model for the day-ahead network-constrained unit commitment(NCUC)problem with mixed-integer recourse.We propose two feasible frameworks for solving the optimization problem.One approximates the continuous support of random wind power with a finite number of events,and the other leverages the extremal distributions instead.Both solution frameworks rely on the classic nested column-and-constraint generation(C&CG)method.It is shown that due to the sparsity of L_(1)-norm Wasserstein metric,the continuous support of wind power generation could be represented by a discrete one with a small number of events,and the rendered extremal distributions are sparse as well.With this reduction,the distributionally robust NCUC model with complicated mixed-integer recourse problems can be efficiently handled by both solution frameworks.Numerical studies are carried out,demonstrating that the model considering quick-start generation units ensures unit commitment(UC)schedules to be more robust and cost-effective,and the distributionally robust optimization method captures the wind uncertainty well in terms of out-of-sample tests.