Study On Key Technology Of Large Generator's Protection

With the continuous expanding of interconnected power grid, the power system is developing towards "large generating unit, extra-high voltage, large power grid". Many new features appear in some aspects of large generating units such as insulation, structural craftwork, design, operation and so on, which demand higher standards from generator protections. The characteristics of long distance and cross-region power transmission of large power grid result in structural complexity and operational control difficulty. This will cause the security and stability of power grid to become more extreme, and generator-grid coordination to become more urgent. Power grid related protections such as stator and rotor grounding protection, loss of excitation protection and out of step protection are not perfect, their reliability are not enough. Their incorrect, unreasonable operation in large power grid accident aggravated the collapse of the power system. They are the chronic disease of generator protection and potential threats to security and stability of power grid. In this paper, in order to protect the security and stability of large generating units and large power grid, several key techniques of generator protection, such as stator grounding protection, loss of excitation protection, out of step protection and configuration principles are studied.Firstly, this paper analyzes and compares the sensitivity and its matching problem of two traditional generator stator ground fault protections:the fundamental zero sequence voltage protection and the three harmonic voltage protection. The conclusion shows that, the two protection schemes cannot meet the sensitivity requirements when large generator has large stator to ground capacitance, and cannot provide protection when the generator is in static, startup and shutdown state. Thus, the injection based stator ground fault protection need to be introduced.Then this paper studies the principle and scheme design of injection based stator ground fault protection, and the influence factors of its sensitivity. The study found that parameters of the distribution transformer at generator neutral have a great impact on the sensitivity and error of protection, and the parameters need complex debugging and compensation in engineering application, which is difficult to achieve accurate. To solve these problems, new injecting scheme that not be affected by distribution transformer parameters is presented. The new scheme separates injection and measurement circuits, so the protection is not affected by the distribution transformer's parameters, and can achieve very high sensitivity without compensation which simplifies engineering debug. Dynamic model test and field test proved the above conclusions.The traditional static stability limit (SSL) based impedance characteristic of loss of excitation (LOE) protection is studied, its characteristics and shortcomings are analyzed. To prevent the shortcoming of this LOE protection that could be badly affected by load change, a new load adaptive LOE protection scheme is proposed. Simulation proves the validity of the new LOE protection scheme. Operation of the new scheme is more rapid and reliable especially in the light load compared with the traditional SSL based impedance characteristic of LOE.The characteristic of impedance trajectory in generator's LOE process is studied and analyzed. Three new LOE protection criteria are put forward. They take advantage of three electrical quantities in LOE process:the variation range of impedance locus, the central angle of impedance circle of equal active power and the viaration of central angle. A new scheme of LOE protection is proposed combined these three criteria and some auxiliary measures. In the new scheme, LOE could be detected immediately after its occurrence, LOE signal could be sent quickly after a short delay to prevent non-LOE fault. The simulation results show that the new protection scheme can operate accuratelly and reliablly in every LOE and non-LOE conditions.Equal energy principle and equal area criterion based generator out of step (OOS) protections are studied and analyzed. New OOS protection scheme based on real-time mechanical power measurement and equal energy principle is proposed. The new scheme can track real time generator's mechanical power and make the equal energy principle based OOS protection more accurate and reliable under fast valving and other emergency control measures.Transient stability analysis method extended equal area criterion (EEAC) is studied, and a multi-machine OOS protection scheme based on wide area measurement system (WAMS) and EEAC is proposed. The new protection scheme obtains power and angle data of every generator via WAMS, and predicts the stability state of power system by utilizing EEAC. Simulation is carried out to verify the effectiveness of the protection scheme.Finally, the particularity of large generating units and its new requirements for generator protection are summarized. According to the particularity of large generator and the security, the stability requirements of large-scale interconnected modern power grid, configuration principles of relative generator protections are summarized.