Fault Risk Assessment of Transmission Equipment Based on Condition Monitoring

This paper evaluates the risk of transmission equipment fault combined with the safety of transmission equipment based on condition monitoring. First of all, the monitoring system monitors important parameters related to the environment, electricity, and machinery in the operation of the power transmission equipment to determine the operating status of the power transmission equipment. Secondly, the impact of transmission equipment on power system risks is reflected by risk indicator. The proposed risk indicator combines the probability of transmission equipment failure and the importance of the equipment. Finally, a reliable and safe maintenance method is determined while taking into account the operating status (reliability) of the transmission equipment and the impact (importance) of the equipment on the system. This method can more reasonably allocate maintenance resources and improve the reliability and safety of power system operation. Introduction Power transmission equipment is the key to the operation of the power system, because there are a large number of power transmission equipment in a system. And their characteristics are different (age, failure probability, impact of failure), so their maintenance methods are very complicated. When formulate a transmission equipment maintenance strategy, not only the operation status of the transmission equipment itself, but also the impact of the transmission equipment outage on the system should be considered. The operation status of the transmission equipment should be evaluated and analyzed according to the selected condition monitoring parameters. This method uses state monitoring parameters to evaluate the overall state of the power transmission equipment and estimate the probability of the fault status of the power transmission equipment, and propose a comprehensive maintenance decision combining the operating status (reliability) of transmission equipment and the impact of equipment on the system (safety). The purpose of this method is to reduce the risk of power system operation. According to the maintenance decision index, the power transmission equipment is divided into normal status, timely maintenance, emergency maintenance and replacement maintenance. The rest of the paper is organized as follows. Section II discusses the methods for risk assessment. Section III introduces the state recognition technology of transmission equipment. The section IV analyzes the failure probability of transmission equipment, and proposes a risk assessment model of transmission equipment combined with importance. Section V introduces the maintenance strategies for transmission equipment. An example analysis is performed in Section VI. The last part gives the conclusion. Risk Assessment Method for Transmission Equipment In the past few decades, power system risk assessment and management has been widely used in almost every level of power systems, namely power generation, transmission and distribution systems [1] . As shown in figure 1, power system risk assessment usually involves general steps. (1)The first step is to monitor the condition of important components of the power transmission equipment and collect monitoring data. Because the state monitoring data of power transmission equipment come from various sources, in order to make data reflect different states directly, status data requires signal processing. (2)In the second step, the processed monitoring parameters are compared with the allowable values of the parameters to evaluate the status of the transmission equipment, and the working status (fault) of the transmission equipment is determined. (3)The third step uses the associated components to determine the failure probability of the transmission equipment, and at the same time to assess the consequences of each failure event. (4)The fourth step, the risk index associated with each failure event can be calculated by the product of the failure probability (reliability) and the failure consequence (safety). (5) Finally, the corresponding maintenance strategy is developed according to the risk index. The risk index calculation formula is shown in Eq.1 R (Ei) =P (Ei) ·C (Ei) (1) where C (Ei) represents the consequence of the failure event, P (Ei) represents the probability of the failure event, R (Ei) represents the failure event risk index. Transmission equipment Detection signal Compared Allowed parameters Signal acquisition Signal processing Status recognition Maintenance strategy Failure probability Failure consequence Risk indicator Figure 1. Risk assessment of transmission equipment based on condition monitoring. Power Transmission Equipment Status Recognition The process of monitoring the status of power transmission equipment is to identify whether the status of the power transmission equipment is normal, timely discover and determine the nature and location of the fault, determine the development trend of the fault. During the operation of power transmission equipment, signals such as temperature, pressure, current, voltage, vibration, and energy will inevitably be generated during the operation of power transmission equipment. According to different status monitoring requirements, different signals can be selected to indicate the status of power transmission equipment. The system monitors several key parameters of the transmission equipment operation, finds potential failures, and judges the operation status of the transmission equipment. These parameters may be related to the environment in which the transmission equipment is located, mechanical wear, gas density, and electrical quantities during operation. In the power transmission equipment monitoring technology, some sensors are used, such as pressure sensors, fiber optic sensors, etc., in combination with microprocessors to perform monitoring tasks. The signal processing and comparison process of power transmission equipment mainly extracts the monitoring parameters related to the fault from the monitoring signals, and compares the monitoring parameter signals with the allowed parameter values to determine whether the parameters are within the allowed range. By analyzing the monitoring parameter signals, the parameter signals of the equipment are extracted and are used to identify equipment failures based on signals and discrimination criteria. Once the equipment deviates from the normal operating state, it is necessary to further analyze the cause of the failure and provide a reasonable maintenance strategy based on the actual operating conditions. Risk Assessment of Transmission Equipment Based on Importance The first method to study the failure rate of transmission equipment is based on the bathtub curve and the Weibull distribution, and calculate the failure rate of the equipment [2, 3] . The second method is to study the relationship between equipment health and equipment failure rate after defining the health of the equipment [4] . Such as the EA formula: P(X) =K×e-C×X (2) where X is the state health of the equipment, K is the proportionality factor, C is the curvature coefficient, and P is the probability of failure. Each fault event in the transmission system can be analyzed from the perspective of the impact of the fault, so that the impact of the fault is quantified, and the results may include cost, reliability, safety, stability, and importance. When formulate a maintenance strategy for transmission equipment operating in the power system, not only the operating conditions of the transmission equipment, but also the safety of the transmission equipment in the power system should be considered. The safety of power transmission equipment can be translated into the importance of the equipment. The importance of transmission equipment is how much the equipment will affect the system after a fault. The higher the importance of the equipment, the equipment may be in an unsafe operating state even if minor faults occurs. Equipment is less important, and equipment is in a more secure state. Different standard maintenance strategies are developed based on the importance of the equipment. Transmission equipment with high importance in the system are developed high maintenance standards, and transmission equipment with low importance in the system are developed appropriate maintenance standards. Power system equipment should be maintained and operated under certain importance constraints, and any violation of these constraints will have an adverse impact. Table 1. Equipment importance degree division diagram. Equipment types Equipment condition Key equipment (1) In the normal operation mode, equipment failure may cause one or more electric safety accidents. (2) Power equipment worth 10 million or more. (3) A device failure will directly cause a power interruption to a critically important user. Important equipment (1) In normal operation mode, equipment failure may cause a level 1 power safety event. (2) Power equipment worth 8-10 million (3) Equipment failure will directly trigger the interruption of power supply to the first-level important users. Concerned equipment (1) In normal operation mode, equipment failure may cause two or three power safety times. (2) Power equipment worth 5-8 million. (3) Equipment failure will directly trigger the interruption of power supply to the second-level important users. General equipment Equipment other than the above equipment A maintenance plan is developed by combining the probability of equipment failure with the importance of the equipment. Risk index for power transmission equipment in shown in Eq.3. R (Ei) =P (Ei) ·I (Ei) (3) where P (Ei) is the probability of an abnormality in the power transmission equipment. I (Ei) is the importance index of the power transmission equipment in the system. R (Ei) is the power transmission equipment risk index, which can indicate the degree of influence of the power transmission equipment on the system risk in th