Hao Zhiguo

Optimal storage sizing for composite energy storage and wind in Micro grid

Energy storage system is necessary to maintain steady operation of the micro-grid and decrease its influence on the distribution grid. In this paper, some research has been carried on in optimal storage sizing in composite energy storage system in wind micro grid. The energy storage system is composed of battery and fly-wheel, which combines advantages of both sides. Cost function is used as the objective function and the loss of load and the generation spilled are considered during the calculation. We considered the constraint functions from the power, energy aspects and set rules for composite storage system. The optimization for a one-week example has been studied in this paper. After summary and comparison, some conclusions are obtained.

Simulation and analysis of oil-immersed transformer based on thermal-fluid coupled field

Power transformer plays a pivotal role in the power transmission, whose failures have serious impact on reliability of power supply and operation stability of power system. Partial insulation aging caused by thermal effects in large-scale transformers is one of significant factors which have a negative effect on operation stability. Just because temperature change of in large power transformers is obvious before and after fault, Real time temperature monitoring have contributed to early warning for operation risks and potential faults In actual operation. Some operation experience indicated that temperature-monitoring aimed at main components of transformers is rather sensitive. Therefore, it makes a lot of sense to research and predict the hottest spot temperature and temperature distribution of transformers. Those traditional methods, such as the thermal circuit model, fail to meet the requirements of large transformer design and operation. In order to simulate temperature and flow velocity distribution in an oil-filled transformer under different conditions, mathematical simulation technology is adopted in this paper. Firstly, by analyzing the relation between heat and losses of transformer, a 720MVA/500kV oil-immersed transformer model is built and then the related losses are calculated in the FEA software ANSOFT. In view of thermal-fluid coupling relationship in heat transfer, the viscous fluid-flow model of the above transformer is set up in CFD software FLUENT. Using the above losses as heat sources, temperature and flow velocity distribution of the transformer is solved according to the basic principles of heat transfer and fluid mechanics. Moreover, the factors which affect the temperature field and oil flow field are analyzed and concluded based on the simulations under different conditions. The study in this paper analyzes the coupling relationship between temperature of transformer tanks and oil flow velocity and summarizes the factors which affect the temperature field and oil flow field, including load coefficient and cooling pattern. The simulation results is veritable accurate, which can provide the reference to transformer temperature measuring and lay the foundation for the further study on transformer non-electrical protection.

Research on Monitoring of Winding Deformation of Power Transformer by On-line Parameter Estimation about Leakage Inductance

Transformers are required to demonstrate the ability to withstand short circuit currents. Over currents due to short circuit can cause winding deformation. A novel method for monitoring the state of transformer windings by identifying parameter of leakage inductance of transformer windings is presented in this paper, using least square techniques based on the equivalent circuit equation of transformer. The mathematical model is derived for on-line identification of winding electrical parameter. The identification errors coming from measuring error and model error are analyzed, and correcting model lowers the influence to result owing to model error. Test result indicates that the accuracy of parameter identification is high, which will not be influenced by the fluctuation of load and the changing of power factor. The method can be use to on-line detect the winding deformation effectively.

The research of sub synchronous oscillation in PMSG wind farm

Under some certain conditions, the wind power farm which accesses to the power system may appears the phenomenon of sub synchronous oscillation (SSO). Doubly fed induction generator (DFIG) connected to power grid usually transports power energy by series compensated AC transmission or DC transmission, and the theory of SSO in DFIG wind farm is relatively clear and has been researched mostly. Permanent magnet synchronous generator (PMSG) wind farm connected to weak AC power grid also appears more SSO accidents, which causes multiple PMSG to take off the power grid and the damage of crowbar circuits. Because PMSG wind farm still appears sub synchronous oscillation accidents in the absence of series compensation or DC transmission when transmitted to the outside, SSO in PMSG wind farm is a new type of oscillation waiting to be researched. In order to investigate the mechanism of problem, the equivalent system model of D-PMSG connected with power grid was established in this paper. Through the transfer function, eigenvalue value analysis and electromagnetic transient simulation, the mechanism of sub synchronous oscillation was revealed. The influence on its characteristics from many factors was also analyzed, including delay time and the wind speed. The results show that the voltage delay link of controller feed forward in network side is the direct cause of sub synchronous oscillation. Finally, potential danger and its suppression measures of the SSO problem were discussed according to the characteristics of sub synchronous oscillation of D-PMSG wind farm.

Study on the Transformer Protection Principle Based on The Equivalent Circuit Equation

The mutual flux calculated from the equivalent circuit equation of the primary and second winding is equal during normal operation conditions, magnetizing inrush and external fault. It differs during an internal winding fault, ground fault and phase fault. A new transformer protection algorithm based on the equivalent circuit equation of the primary and secondary winding mutual flux is described in this paper. For a single-phase and a three-phase Y-Y transformer, the mutual flux is calculated. For a transformer with delta winding, the differences of the mutual flux between the two phases are calculated to use the line currents. Because of the accuracy of winding parameter influencing the sensitiveness of protection algorithm, the least square is adopted to identify and correct the parameter value. Compared to a conventional harmonically restrained differential relay, the algorithm is independent of core characteristics and system condition. Test result indicates that the algorithm can successfully discriminate between internal fault and all other operation conditions

Analysis of photovoltaic power generation system low voltage ride through control during asymmetric fault

The electromagnetic transient model of photovoltaic power generation system is established based on PSCAD/EMTDC platform. The mechanism that 100Hz fluctuations appearing of PV output power and PV array operating point during asymmetric fault are simulated and theoretical analyzed. The paper proposes a positive and negative sequence separation control strategy, to filter the negative sequence current, making PV output current do not contain negative sequence component and outputting standard sinusoidal current waveform during asymmetric fault. The paper also proposes a DC voltage fluctuation suppression control strategy, to eliminate the DC voltage fluctuations and make the PV array operating point stable. Finally, simulation verification is made on PSCAD/EMTDC platform.

Study of the interaction between photovoltaic inverter and power quality harmonic and its suppression method

In the process of photovoltaic (PV) generator was connected to the grid, in order not to affect the power quality of grid, the current harmonic component from PV inverter needs to meet the relevant interconnection standards. This topic put-forwards a method based on impedance analysis, which analyses the harmonic interaction between the photovoltaic (PV) grid-connected inverter and power grid containing background harmonic. The basic reason of harmonic resonance problem caused by grid-connected PV system is duo to the existence of impedance intersection at the point of common connection (PoC), which is determined by the equivalent output impedance of the inverter and the equivalent impedance of the grid. At that intersection point, the sum of the impedance reduced to the minimum value and the current amplitude significantly increased, which leads to the harmonic resonance phenomenon. To analyze this problem, this paper established the NORTON equivalent model of the inverter, developed the expression for the equivalent output impedance of the inverter, and analyzed the reasonable range of the equivalent inverter output impedance based on the maximum limit of photovoltaic (PV) grid harmonic voltage, current and harmonic resonance conditions. On this basis, the PI control and the quasi PR control methods were used respectively to adjust the equivalent output impedance of inverter. Based on the simulation experiment, the equivalent output impedance was effectively suppressed, and the harmonic resonance phenomenon was avoided.

Research of VFTO in 110kV minimized GIS

110kV minimized gas insulated substation (GIS) has been widely used in China, due to its great electrical properties. The closing or opening operation of disconnector switches (DS) and circuit breaker (CB), and single-phase ground fault are all probable to cause the very fast transient overvoltage (VFTO), which is a threat to the power system. Compared to the structure of higher voltage GIS, three-phase electrical equipment in 110kV minimized GIS is placed in the same box. So we must establish the three-phase model of GIS and then calculate its VFTO. This paper referred to 110 kV minimized GIS power system. Using the Electro-magnetic Transient Program (EMTP), numerical simulation and analysis of VFTO in different DS, CB operation modes and single-phase grounding fault were carried out. And the data of VETO under different operation modes, including the frequency, amplitude, steepness and waveform were obtained. Then it calculated and analyzed the influence on VFTO of length of branch bus and transformer entrance capacitance, in order to provide reference basis to the structure design of GIS and analysis of VFTO.

The simulation and analysis on transformer gas Protection Maloperation based on multiple physical fields coupling

As a transformer main protection, gas protection is an important and traditional non-electricity protection. For internal faults of transformer, gas protection can directly response to non-electricity characteristic such as oil flow velocity, and then has higher sensitivity and reliability compared with electric protection in principle. However, for external faults, great electromagnetic force and losses of windings, which caused by the heavy through currents, may result in maloperation of gas relay. Thus the reliability of the gas protection is low. From statistical data analysis of several gas protection maloperation accidents happened in recent years, it is clear that the root reasons of low reliability of gas protection should be outdated protective principle and inaccurate threshold value. In order to analyze the mechanism of gas protection mal-operation and provide a proper method to set the threshold value of gas protection, the transient process and the related physics essence of gas protection mal-operation resulted from external fault is analyzed. In the process of external short circuit, electromagnetic force of windings caused by short-circuit impulse current is so huge that the whole windings will generate intense vibration. Meanwhile, insulating oil in transformer will rush into the oil tube by squeezing and shaking, which lead to gas protection mal-operation eventually. From the above analysis, it is illustrated that the process of gas protection mal-operation caused by external short circuit is a multi-physical field coupled process. To simulate the complicated process veritably and accurately, an oil transformer model based on multi-physics field is established in ANSYS Workbench. Firstly, the short circuit electromagnetic force of windings is calculated via the magnetic field analysis, which will be loaded into the dynamic mechanical analysis of windings. Thus stress distribution and strain characteristics, which can describe vibration of windings, is calculated and loaded into the flow field analysis over a defined interface. Eventually, the oil flow velocity through gas relay in the oil tube is achieved by a hydrodynamic model. Simulations with different external fault current are conducted to study the relationship between the oil flow velocity and external fault current in the windings. The results show that factors which affect the oil flow velocity is different between internal fault and external fault, and there exists the obvious distinction of the oil flow velocity between internal fault and external fault. Furthermore, the oil flow velocity of transformer under fault conditions can be obtained in the above simulation more veritably and accurate, which can be used as references for the threshold value of gas relay. The purpose of this study is to overcome the difficulties in setting threshold value, avoid the gas protection misoperation during external short circuit and improve the reliability of gas protection.