注入信号法发电机定子接地保护受定子绕组对地电容的影响，灵敏度较低，难以满足大型机组高灵敏度继电保护的要求。而采用外加小电抗器、选择合适的注入信号频率，以补偿定子绕组对地容抗、测量故障接地电阻或测量信号电压、信号电流相位差进行接地保护则可改善大型机组定子接地保护的灵敏度。计算、分析与仿真表明：该方法具有较高的灵敏度。

A principle for resonance measurement based on injectingresonant frequency signal is presented in this letter. The groundingcapacitance of lines and equipment and the grounding fault resistancecan be measured on-line in ineffectively grounded systems.Petersen-coil automatic tuning is implemented, and 100% statorgrounding fault protection for generator stator windings is also proposed.The prototype for Petersen-coil tuning has been tested on networksfor many years. In addition the new method for generator statorgrounding fault protection is verified by simulation.

Fault location using traveling wave has been applied inextra-high voltage power grids successfully. Due to itscomplication and high cost, it is not easy for this technique to beaccepted for use in distribution system. A new traveling wavefault location system is developed simply in a cost-effective wayfor power networks (especially for distribution system) in thispaper. Two traveling wave sensors are developed to capture thecurrent traveling wave flowing from the capacitive equipment toearth and the voltage traveling waves in all three phases. Theoutputs of the sensors are then applied to the trigger and timetagging by using Global Position System (GPS) receiver. Thefault position is calculated by the traveling wave arrival times inevery power station where only one fault locator is installed. Thefault location system is tested in the power system. Testing resultsshow that the fault locator has high precision and robustness.

受干扰、卫星失锁等因素的影响，全球定位系统（GPS）时钟难以满足高可靠性同步控制领域的要求。文中根据高精度晶振与GPS时钟精度互补的特点，采用高精度晶振对GPS时钟进行监测，建立了最小二乘估计模型，估计出GPS时钟的方差及晶振的累计误差，给出了GPS时钟误差的在线修正方法，研制出了具有较高性能价格比的高精度时钟发生装置，并成功地应用于电力系统继电保护与控制。

介绍了能同步记录并分析电力系统各种扰动变化过程的GPS同步暂态录波仪及其软件系统。介绍了基于暂态录波仪的输电网故障行波定位系统。该仪器性能可靠、操作方便并通过了模拟试验。

As a result of deregulation, distributed generation(DG) systems with distributed generators installed in the middleor low voltage networks become popular in distribution systems.Traditional protections developed in radial systems are difficult incoordination and in protecting against high impedance fault(HIF), sometimes it may cause nuisance tripping in the DGsystems. A decentralized multi-agent based protection withcapability of HIF detection, fault location and load shedding forthe DG systems is proposed in the paper. Digital relay is designedas a relay agent which is capable of searching for informationfrom other relay agents, interacting with other relay agents andperforming tasks of protection with autonomy and cooperating.The agent based protection scheme is also presented. EMTPsimulation results show that the proposed agent based protectionscheme can remove the influence of load switching operations,protect against HIF, electric shock. The prototype of relay agentis developed, the research is now in progress on testing theprototype in dynamic power system.

Many mining power systems utilize ineffectivelygrounded sources to restrict the residual current of single-phaseearth fault in order to reduce outage and shock hazard. Inpractice, with the system expansion, topology changing, andinsulation aging, the potential residual current and zero-sequencevoltage for earth fault vary dynamically and some arcing earthfaults can easily cause over-voltage and induce multiple faults. Inorder to improve the system safety, on-site condition monitoring,safety evaluation, and preventive maintenance for earth fault areproposed in this paper. Some techniques for online parametersmeasurement based on resonance-measurement principle arepresented. Power system operation states are classified intonormal secure state, alert state, incipient fault state, and faultstate. Some security enhancement methods (preventive action andremedial action) for each state are implemented. The prototypefor safety evaluation has been developed and installed in industrypower systems for several years.

Many industrial power systems have been operatedwith floating neutral or high resistance grounding. Withtraditional fault detection methods, based on zero sequencecurrent, it is difficult to satisfy the requirement of distributionautomation (DA). In order to improve this, some noveltechniques for ground fault feeder detection are presented in thispaper, and they are implemented by using the informationobtained from the feeder terminal unit (FTU). The faulty feederis identified with fuzzy analysis. These protection schemes havebeen verified by EMTP simulation. A prototype has beendeveloped and tested in a dynamic power system. Results showthat these proposed protection schemes have high sensitivity androbustness to detect all kinds of ground faults.

Power quality is influenced by neutral grounding methods in distribution systems. Optimization for neutral grounding methods is discussed in the paper. Petersen-coil in parallel with high resistance earthed technique has a better performance in safety and power quality than other neutral point grounding techniques. To improve power quality, a novel method for measurement of zero sequence impedance in distribution systems is presented, Petersen-coil inductance is adjusted to compensate the total capacitance, and the resistor which is in parallel with the Petersen-coil is adjusted to control the over-voltage; some other novel techniques for grounding fault feeder protection based on testing the fault resistance or the energy dissipated in the fault point are also presented. The method of Petersen-coil tuning has been developed. Experimental results show that the optimization for neutral grounding methods and the protections proposed can improve the power quality by reducing the over-voltage, the residual current and the grounding fault duration

Many High impedance ground faults (HIGF) that happen in low voltage (LV) system often cause customer supply lost, fire and human safety hazard. Traditional ground fault protection is provided by Residual Current Circuit Breaker 0RCCB). The RCCB often causes nuisance tripping and is difficult to detect HIGF. Wavelet analysis based HIGF protection is developed in the paper. Wavelet transform is applied to filter out some frequency bands of harmonics from residual current and line current. The root mean square (RMS) value of harmonics are calculated using their wavelet coefficients directly. HIGF is identified from disturbance by the RMS difference between the residual current and the line current. The digital protection scheme is designed. EMTP simulation results show that the new protection is able to detect HIGF and prevent electric shock with high sensitivity and robustness.