0 Introduction In recent years, problems with AC secondary voltage circuits have frequently led to malfunctions in the protection devices of main units, main transformers, and main lines of the Hunan power grid, often accompanied by large-scale power outages. This seriously endangers the safe operation of the power grid and represents a weak link in relay protection work. To improve the safety and stability of the power grid, this paper takes a typical accident caused by abnormalities in the secondary voltage circuit leading to malfunctions of protection devices as an example, analyzes the problems exposed by the AC secondary voltage circuit, and proposes countermeasures, hoping to draw the attention of protection personnel at all levels. [b]1 AC Secondary Voltage Circuit and Protection[/b] The AC secondary voltage circuit refers to the circuit formed by connecting the secondary terminals of the voltage transformer to protection, measurement, metering, and other devices. The protections related to the AC secondary voltage circuit mainly include overvoltage, undervoltage, main transformer overexcitation, distance, power direction, integrated reclosing protection, and underfrequency automatic devices. The AC secondary circuit has few devices and its wiring is not complex, but its problems frequently cause malfunctions in protection devices. For example, on May 25, 1994, a ground fault occurred on phase B of the Xiangquan line, and poor contact in the 3U0 (zero-sequence voltage circuit) on the Xiang side caused the zero-sequence directional protection to fail to operate; on August 18, 1995, when the Leiyang Power Plant operators were switching the No. 2 unit to the busbar, poor contact in the auxiliary contact of the isolating switch caused the secondary voltage circuit to fail to switch, resulting in the No. 2 unit's impedance protection tripping; on October 4, 1998, the 110 kV zero-sequence protection of the 220 kV Quantang substation failed to operate, causing the No. 2 main transformer to trip, resulting in a 110 kV loss of voltage at the Quantang substation. [b]2 Typical Accident Analysis[/b] 2.1 Yuntian No. 2 Main Transformer Distance Protection Maloperation and Main Transformer Tripping Accident 2.1.1 Accident Process On January 13, 1996, a DC ground fault occurred at the Yuntian substation. The operation and relay protection personnel checked the DC ground fault by disconnecting each DC circuit separately. When investigating the 220 kV DC ring network, considering that the power supply for the reactivating relay of the auxiliary contact of the 6X24 II bus disconnector is taken from the 220 kV ring network DC power supply, opening the DC circuit would cause the reactivating relay to lose its magnetization, resulting in a voltage drop on the II bus PT. Therefore, the measure of parallel operation of the secondary voltage circuits of II and IV bus was adopted, i.e., the parallel switch 43S1 was closed. It was found that the DC grounding was caused by a shared AC/DC cable laid during the expansion in 1995. To resolve the DC grounding on site, the grounding core was temporarily disconnected, and then the normal operation mode of the secondary AC voltage circuit was restored. The parallel switch 43S1 was disconnected, the fault horn sounded, the distance protection on the medium voltage side of the No. 2 main transformer was activated, all three switches tripped, and 306 MW of load was cut off. Cause of the accident: The fuse of the reactivating relay at the auxiliary contact of the 6X24 disconnector of the 220 kV II bus was of poor quality. The fuse detached from the solder and poor contact caused the reactivating relay to lose its magnetization, resulting in a voltage drop on the secondary AC voltage of the II bus. The distance protection on the medium voltage side of the No. 2 main transformer tripped because there was no disconnection interlocking device output. 2.1.2 Problems Exposed by the Accident a. The designers did not implement the provisions of Article 11.9 of the Ministry-issued Key Points for Accident Prevention Measures of Power System Relay Protection and Automatic Safety Devices; an alarm should be issued and potentially malfunctioning protections should be blocked in the event of simultaneous loss of voltage in one, two, or three phases of the secondary voltage circuit; a voltage loss blocking device was not installed during the expansion of the main transformer protection. b. The 6X24 position re-activating relay power supply was not connected to a dedicated control power supply, but instead used the 220 kV site ring network closing DC power supply, which does not meet the requirements of the design manual. c. The 6X24 fuse had soldering quality problems. d. The AC and DC voltage circuits shared a cable, violating relevant regulations. e. The cable installation did not meet basic process standards, with excessively long cuts to the cable sheath and no cable ends wrapped. 2.2 500 kV Gangshi Substation No. 1 Main Transformer Overexcitation Protection Malfunction Tripping Accident 2.2.1 Accident Summary On the afternoon of March 15, 1996, Gangshi Substation operators were performing a busbar switching operation to restore the normal dual-busbar operation from the 220 kV II busbar single-line operation mode. During the operation of switching the 612 and 616 line switches to the I busbar, the protection devices issued warning signs for protection faults, protection lockout, and waveform recording activation (which the on-site protection personnel considered abnormal). Then, during the switching of the No. 1 main transformer to the I busbar, the overexcitation protection of the No. 1 medium-voltage side main transformer tripped all three sides of the circuit breaker. After the main transformer tripped, the I busbar panel indicated Uca = 330 kV, Uab = 240 kV, and Ubc = 110 kV. The secondary voltages of the I busbar voltage transformers were Uao = 62 V, Ubo = 62 V, Uco = 95 V, and 3Uo = 0.36 V. The accident investigation confirmed that during the switching operation, the secondary C-phase automatic small switch of the voltage transformer on Bus I had been opened, resulting in a voltage loss on the secondary side of Bus I. When switch 612 switched to Bus I, low voltage recording was initiated, and the protection was locked. When switch 616 switched to Bus I, low voltage recording was initiated again, and the protection of line 616 issued a fault signal. When switch 610 of main transformer No. 1 switched to Bus I, the capacitor across the C-phase automatic small switch was connected in series with the inductive load and was in the resonant region. The measured voltages were Uc = 118 V and U1 = 95 V, causing the load voltage to rise and the three-phase voltage to become unbalanced, resulting in the overexcitation protection of main transformer No. 1 tripping. 2.2.2 Problems exposed by the accident: a. No PT disconnection signal was issued when the secondary automatic small switch of the voltage transformer was opened; b. The 8μF capacitor across the C-phase of the automatic small switch was improperly configured, causing resonance with the secondary inductive load; c. An abnormality occurred during operation. Signals from indicator lights 612 and 616 were issued, triggering a protection lockout. However, this did not attract the attention of the operating personnel, who failed to stop the operation or identify the cause. 2.3 Main Countermeasures After the Accident 2.3.1 Yuntian Substation added a dedicated voltage switching panel and installed a set of line breakage lockout protection devices. 2.3.2 Gangshi Substation changed the parameters of capacitor C from 8μF to 4μF. Tests showed that line breakage in one, two, and three phases of the voltage circuit could activate the line breakage lockout device with sensitivity, and would not generate dangerous overvoltage on the load. [b]3 Causes and Hazards of Unsafe Factors in AC Voltage Secondary Circuit[/b] Through analysis of some accidents, it can be seen that the main problems causing incorrect operation of protection devices due to defects in the AC voltage secondary circuit are: AC secondary voltage circuit undervoltage; problems with the tertiary coil 3UO circuit; and improper selection of the parallel capacitor value of phase C of the automatic small switch in the AC secondary voltage circuit. The main reasons for the above problems are: a. The automatic small switch of the AC secondary voltage circuit often disconnects due to overload and short circuit in the voltage secondary circuit; b. Voltage switching circuit faults are mainly caused by inadequate maintenance of the auxiliary contacts of the isolating switch, poor inspection quality of the switching relay and reactivation relay, and problems with the DC power supply circuits of the switching relay and reactivation relay; c. Poor secondary circuit wiring contact is mainly due to various mechanical contact problems and poor welding; d. Incorrect 3UO wiring is mainly due to incorrect polarity; e. Additional voltage generated by the additional current flowing through two or more grounding points in the N-line circuit. The reason is that in a high-current contact system, when a grounding fault occurs at the substation or line outlet and a large short-circuit current flows into the substation's grounding grid, the potential at each point of the grounding grid is different. If the secondary circuit of the voltage transformer has two grounding points, or if two voltage transformers each have one grounding point and are directly connected through the secondary circuit, the potential difference between the different grounding points will cause abnormal voltage at the relay protection input. If a zero-phase grounding method is used, it will cause abnormal voltages in Uao, Ubo, Uco, and 3Uo, making it unable to correctly reflect the amplitude and phase of the primary voltage, causing the grounding distance protection and zero-sequence direction protection to malfunction. f. The reason for the improper selection of the parallel capacitor value of phase C of the automatic small switch in the AC secondary voltage circuit is that there was no theoretical calculation value during the design. When actually selecting the capacitor, no simulation test was conducted using system voltage and various secondary loads. The capacitor was selected based on experience, so it often cannot meet various operating conditions. Its hazards are: first, it may generate overvoltage, causing overvoltage protection and overexcitation protection to operate, such as the accident in Gangshi Substation; second, when the three-phase automatic small switch is opened, the disconnection blocking device cannot be activated, thus blocking the protection. [b]4 Lessons and Countermeasures[/b] 4.1 Design 4.1.1 Designers should be familiar with the key points of anti-accident measures for power system relay protection and automatic safety devices, and design according to the requirements of the anti-accident measures. 4.1.2 The common switching relays and re-operating relays of the AC secondary voltage circuit should be placed in the control room for convenient operation and monitoring by the operating personnel. Their DC power supply should be taken from the control power supply according to the design manual and should be dedicated. It cannot be like the Yuntian Substation, where it is taken from the nearest site, which makes it inconvenient to find the DC ground and also makes it easy to accidentally disconnect the power supply. 4.1.3 The design of the small switch in the secondary circuit of the voltage transformer should use a single-phase design, not a three-phase linkage design, to reduce the chance of three-phase voltage loss. The parallel capacitor across the automatic small switch should have a theoretical calculation value, not an empirical value. 4.1.4 The design should ensure that simultaneous loss of voltage in one, two, or three phases of the secondary voltage circuit triggers an alarm and locks out potential malfunctioning protections, including measures for the demagnetization of the automatic small switch and recirculating relay. 4.1.5 The secondary voltages for protection and metering should be separately led out from the secondary terminals of the voltage transformer, each with its own independent path. 4.2 Installation and Testing 4.2.1 Construction, installation, and testing personnel should study and be familiar with the countermeasures. Any measures that do not meet the countermeasure requirements must be promptly raised and modified to avoid leaving hidden dangers. 4.2.2 Construction should adhere to proper workmanship, and cables and circuits should be clearly marked. The as-built drawings should conform to the actual site conditions. 4.2.3 The parallel capacitor of the C-phase automatic small switch must be determined through testing. Test data for various loads, including the maximum and minimum loads, should be available to prevent similar accidents from recurring. 4.3 Operational acceptance should be strict and not relaxed. Any failure to meet the countermeasure requirements must be resolutely rectified. If any abnormal situation occurs during operation, operation must be stopped, dispatch must be notified, and a thorough inspection and analysis should be conducted. Operation can only continue after the dispatcher's approval. 4.4 Maintenance 4.4.1 For secondary voltage circuits that do not meet the requirements, modifications should be made according to the key points of the countermeasures issued by the Ministry. For example, the N-line of the secondary and tertiary coils should be separated from the site and reconnected and grounded in the control and protection room. If there is no alarm signal for simultaneous undervoltage of one, two, or three phases in the secondary voltage circuit, an alarm device must be installed, and protections that may malfunction must be locked. 4.4.2 For automatic small switches in the secondary voltage circuit that are three-phase linked, they should be changed to single-phase operation. If the substation is equipped with overvoltage and overexcitation protection, the selection of capacitor C must be tested. If there has been no testing in the past, conditions should be created to conduct testing to prevent similar accidents from recurring. 4.4.3 For voltage switching circuits, the auxiliary contacts of the disconnecting switch, switching relays, and reactivation relays must be inspected annually, with a focus on the contact parts to ensure good contact. Poor contact, especially at the auxiliary contacts of the disconnecting switch, occurs frequently, and clear regulations should be established for primary maintenance personnel. A significant reason for problems in the secondary voltage circuit is the neglect of its regular inspection and requirements; therefore, clear regulations are essential. 4.4.4 Checking the wiring of the voltage circuit under load is a crucial task. Not only should the wiring of the secondary and tertiary coils be checked, but also the wiring of the voltage extraction lines. Practice has shown that checking the unbalanced voltage of the tertiary coil can determine contact problems in the tertiary coil circuit. Generally, the unbalanced voltage of the tertiary coil should be greater than 0.2 V, i.e., 3UO should be greater than 0.2 V. If 3UO is less than 0.2 V, special attention must be paid. Measurements must be performed on all parts of the circuit, from the protection panel and common signal panel to the site voltage terminal box and the switching circuit, including the auxiliary contacts of the disconnecting switch, the reactivation relays, and the switching relays, to ensure good contact in the tertiary coil circuit. Don't assume that a smaller UO value is always better; a value less than 0.2 V may indicate an abnormality. The most fundamental way to check the polarity of zero-sequence power direction is to verify the polarity of the voltage and current transformers. All connections from the transformer terminals to the relay protection panel and the polarity of the zero-sequence directional relays on the panel should be checked under load to make a comprehensive and accurate judgment. [b]5 Conclusion[/b] The preceding sections mainly introduced the problems in AC voltage secondary circuits and the lessons learned and countermeasures. The aim is to plug the loopholes in AC voltage secondary circuits, enhance the reliability of AC voltage secondary circuit operation, reduce the hidden dangers of incorrect relay protection operation, and improve the level of safe and stable operation of the power grid.