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On 20xx xx 22xx, an incident occurred at x Power Plant where operational personnel incorrectly used a multimeter while engaging the protection panel, resulting in the tripping of units 1 and 2 due to human error. The brief situation is reported as follows. All units are requested to organize relevant personnel to study carefully, learn from the accident, and check against the measures proposed in the report to prevent similar incidents from occurring again.1. Brief Overview of the Incident On 20xx xx xx afternoon, x Power Plant had 4 units running. At 14:23, the central control ordered the plant to modify the protection settings of the stability device, executing the “220kV x Power Plant Operational Setting Document“, and required completion within 4 hours. The duty supervisor instructed a control operator to assist the maintenance department’s protection personnel in modifying the stability device protection settings. At 15:30, the duty control supervisor arranged for operators Qin and Zhang to assist the protection personnel. Qin processed the “Stability Device Protection Inspection Card” electrical operation ticket, with Zhang as the operator and Qin as the guardian. The two arrived at the phase I protection room to assist the protection personnel, first withdrawing the stability device protection panel as requested by the protection personnel. At 15:45, the protection personnel completed the modification of the protection settings and notified the operational personnel to restore the stability device operation. The duty operator Qin reported to the supervisor Chen, who agreed, and began to restore the withdrawn stability device protection panel. Qin followed the operational sequence to restore each functional panel first, then the output panel. When restoring the output panel, it was necessary to measure the voltage between the two ends of the panel to confirm that the protection output had not acted. However, Qin did not check the position of the multimeter probes and mistakenly inserted the probe meant for voltage measurement into the current measurement socket. This caused the unit to trip as the negative of the output circuit was grounded through the multimeter, sending a trip signal. At 15:49:02, the main switch of unit 2 2202 tripped, shedding a load of 183MW; subsequently, at 15:49:13, the main switch of unit 1 2201 tripped, shedding a load of 180MW. Qin and Zhang did not notice any abnormalities at this time and continued operations. When they reached the “6LP2 Other Functions Cut” panel for unit 2, maintenance personnel informed them that the units had tripped, prompting them to stop operations.
2. Analysis of the Cause of the Incident(1) Direct Cause of the Incident The direct cause of this incident was the incorrect use of the multimeter by the operational personnel. The duty operator Qin, while measuring the voltage across the protection panel, only checked whether the multimeter was set to the voltage range and did not check the probe connection position. He failed to notice that the probe was incorrectly inserted into the current measurement socket, which caused the unit to trip when measuring the voltage at the output panel’s upper end, as the negative of the output circuit was grounded through the multimeter, triggering the trip signal.(2) Main Cause of the Incident The main cause of this incident was the failure of the operational personnel to strictly adhere to the operational supervision system, leading to violations of operational protocols. On the operation ticket, Qin was designated as the guardian and Zhang as the operator, but during the actual operation, both operated together. Qin measured the voltage while Zhang assisted in engaging the panel, completely losing the supervisory role, which led to the failure to recheck the measuring instrument before use and the lack of analysis of abnormal measurement results, resulting in blind operations that caused the incident.(3) Important Cause of the Incident The low skill level of the operational personnel was an important cause of this incident. The operational personnel, in cooperation with the maintenance personnel to modify the stability device protection settings, withdrew more than 20 output panels. Before re-engaging the output panels, it was necessary to measure the voltage at each panel’s upper and lower terminals one by one to confirm that the voltage was normal before engaging the panels. In the case where the protection output did not act, the voltage at each panel’s upper and lower terminals was -110V and 0V, while the multimeter, with the probes incorrectly inserted, displayed 0V for both terminals. However, neither operator suspected this abnormal situation and continued operations until the sixth output panel was engaged, at which point nearby maintenance personnel reminded them that the units had tripped, prompting them to stop the erroneous operation. If the operational personnel had promptly detected the abnormal voltage measurement at the upper end of the panel and terminated operations, the incident could have been avoided.
3. Issues Exposed by the Incident(1) Lack of Responsibility Among Operational Personnel The operational personnel did not carefully verify the multimeter settings and probe connections before use, and failed to analyze the abnormal voltage measurement at the upper end of the panel, exposing a lack of diligence and responsibility among the involved personnel.(2) Strict Enforcement of the “Two Tickets” System The operational supervision system is an important measure to prevent human error. The group company’s “Technical Specifications for Power Plant Operation Tickets” specifies the supervisory requirements during the execution of operation tickets, explicitly prohibiting “guardians from replacing operators”. However, in this incident, the responsibilities of the operational personnel were unclear, and the division of labor was ambiguous, severely violating the group company’s requirements, revealing significant loopholes in the “Two Tickets” management.(3) Insufficient Technical Capability of Operational Personnel The occurrence of this incident also reflects the insufficient technical capability of the involved operational personnel. The power plant’s operational department conducted training on relay protection technology this year and held examinations, but lacked relevant on-site skills training, and the follow-up on training effectiveness was inadequate, exposing deficiencies in the training of employees’ skills and professional knowledge.(4) Inadequate Management of Tools and Equipment The multimeter used in this incident, model FLUKE17B, did not have an alarm sound when the probe connections and settings were inconsistent, while another multimeter, model FLUKE189, provided an alarm function for incorrect probe connections. The power plant provided two similar-looking multimeters with different functions to the operational personnel, which could lead to incorrect judgments during actual use, exposing flaws in the management of tools and equipment at the power plant.(5) Vulnerabilities in Standard Operating Tickets The standard operating ticket used in this incident, “Stability Device Protection Inspection Card“, states in point 26 that “the voltage at the main cabinet output panel should be 0V under normal conditions and 220V when the output acts. Unless specified, all panels should be engaged, and should not be withdrawn unless there is a protection fault or maintenance, and the engagement of panels should be based on the central control order.” This is inconsistent with the operational regulations (Power Plant 1 Auxiliary Machine Operation Regulations) section 11.2.1.4, which states that “the voltage across the relay protection device panels must be measured with a high-resistance voltmeter before engagement, and direct measurement of the voltage difference across the panels with a multimeter is prohibited,” potentially misleading operational personnel.(6) Need for Improved Site Equipment Identification The identification of spare panels, trip panels, and protection function panels on the power plant’s stability device protection cabinet is the same color, and the failure protection cabinet has the same issue, which is insufficient to alert on-site operational personnel.
4. Measures to Prevent Recurrence of the Incident
(1) Strengthen Employee Responsibility Education and Implement Safety Responsibilities It is particularly important to emphasize the safety responsibilities of operational personnel and cultivate a serious and meticulous work style among operational personnel.
(2) Strengthen “Two Tickets” Management and Strictly Implement the Operational Supervision System Strictly implement the group company’s “Technical Specifications for Power Plant Operation Tickets” and “Technical Specifications for Power Plant Work Tickets”. Electrical operations must strictly adhere to the operational supervision system, and strictly follow the requirements for ticket singing, repetition, simulation operations, etc., to resolutely prevent guardians from replacing operators.
(3) Enhance Employee Skills Training Strengthen on-site skills training and practical training for operational personnel, and enhance the inspection of training effectiveness to effectively improve employee skill levels.
(4) Improve Standard Operating Tickets Conduct a comprehensive review and improvement of standard operating tickets to ensure they are correct and complete.
(5) Equip Dedicated Voltage Meters for Engaging and Withdrawing Protection Panels Each plant should provide dedicated voltage meters for engaging and withdrawing relay protection panels, specifically for measuring protection panel voltages, with no current range and high resistance, to eliminate the possibility of short circuits due to incorrect probe connections. Until the dedicated voltage meters are purchased, the current multimeter’s current measurement socket should be blocked.
(6) Improve Protection Panel Identification Use different colors to identify protection panels, which will aid in their recognition and help prevent erroneous engagement or withdrawal of protection panels, thus preventing operational errors. Typically, functional panels are marked in yellow, protection output (trip) panels in red, and spare panels in white or natural color.
(The case is sourced from the internet and is for learning and communication purposes for power plant personnel!)