[align=left] 1. Introduction The disc shear unit in the finishing area of ​​the medium plate mill of Nanjing Iron & Steel Co., Ltd. is a large unit controlled by an ABB ACS 600 multi-drive frequency converter. This multi-drive frequency converter consists of two parts. The first part is the power supply and braking section, composed of an auxiliary control unit (ACU), an input line unit (ICU), and a thyristor power supply unit (TSU). The TSU consists of two anti-parallel thyristor fully controlled bridges. To prevent the thyristor anti-bridge used for braking from failing during the braking process, the thyristor anti-bridge is equipped with an autotransformer to ensure that its AC input voltage is not affected by grid voltage fluctuations, which could cause the AC input voltage to be too low and lead to inverter failure. In this unit, the models of the thyristor positive bridge used for power supply and the thyristor anti-bridge used for braking are... The two types of inverters are: ACN654-2120-500000300907 (U1: 3～500V, i1: 2449A, TA: 40℃, i2: 3000A, module dimensions: B5) and ACN664-1405-500000300900 (U1: 3～500V, i1: 1617A, TA: 40℃, i2: 1980A, module dimensions: B4). The second part is the transmission part, which consists of a common DC bus and multiple inverters connected to the common DC bus. There are a total of 12 inverters in this unit. The selection of each inverter and the motor it connects to is shown in Table 1. Table 1 Equipment Selection for the Transmission Section In October 2006, the circuit breaker Q1 of the incoming line unit (ICU) of the disc shear unit experienced a fault where it closed and then immediately opened, causing the power supply unit of the disc shear unit to lose power and resulting in the entire main circuit of the unit losing power, causing no output from the common DC bus. Although the auxiliary control unit (ACU) of the unit could be powered on after its knife fuse switch F10 was closed, the TSU reported an over-temperature alarm and fault. The seven-segment digital tube on the control board in the thyristor positive bridge chassis alternately displayed A105 (TSU module over-temperature alarm) and F04 (TSU module over-temperature fault). Due to the lack of output from the common DC bus, each inverter reported a PPCC Link fault, and the entire unit could not be put into use. It was necessary to quickly investigate and deal with the fault and restore it to normal operation before putting it into production. 2. Fault Phenomenon and Handling Process 2.1 Fault Phenomenon After closing -F10, all control parts of the unit are energized. During the subsequent main circuit closing operation, -Q1 closes and then immediately opens, causing the disc shear unit's power supply unit to lose power, resulting in the entire unit's main circuit losing power. This causes no output from the common DC bus, and the TSU reports an over-temperature alarm and fault. Specifically, the seven-segment display on the control board inside the thyristor positive bridge chassis alternately displays A105 (TSU module over-temperature alarm) and F04 (TSU module over-temperature fault). Due to the lack of output from the common DC bus, each inverter reports a PPCC Link fault, and the entire unit cannot be put into operation. 2.2 Connections between TSU circuit boards. The attached diagram shows the connections between the various circuit boards of the TSU. Table 2 shows the detailed list of the TSU circuit boards, and the connections between them are shown in the attached diagram. In this system, except for the SDCS-IOB-23 board (digital input signal power supply is 230VAC) which is installed inside the ACU, all other circuit boards are installed inside the TSU thyristor positive bridge chassis. The SDCS-CON-2 board has a seven-segment display (not shown in the diagram) for displaying TSU status codes and a CDP312 control panel interface X34. The CDP312 control panel can display recent fault information, perform fault resets, display three actual values, and display and adjust parameters. The AMC-DC board has three pairs of fiber optic connection interfaces: CH0 is designated for use by the host computer control system, CH2 is unused, and CH3 is designated for PC connection (mainly used for debugging with DriveWindow software). The SDCS-IOE- Board 2 and board SDCCS-UCM-1 are used to detect the intermediate DC circuit voltage UC (i.e., the DC side voltage of each inverter); board SDCCS-PIN-51 is used to detect the three-phase AC power supply voltage of TSU, the DC side voltage of TSU, the two-phase line current of AC side, and zero current detection. At the same time, board SDCCS-PIN-51 uses an NTC type temperature sensor to monitor the temperature of TSU module. In this system, a temperature sensor is installed in the thyristor positive bridge chassis and the thyristor negative bridge chassis. Since the external dimensions of the thyristor positive bridge and thyristor negative bridge modules are B5 and B4 respectively, the pulse branch interface board SDCCS-REB-1 is required. Therefore, in the attached diagram, board SDCCS-PIN-51 with SDCCS-REB-1 (lower right of the attached diagram) should be used instead of board SDCCS-PIN-51 without SDCCS-REB-1 (lower left of the attached diagram). Table 2. TSU Circuit Board Details Table 2.3 Troubleshooting Process After checking the -Q1 control circuit, it was found that the normally open contact of the "main contactor closed" function provided by the SDCS-IOB-23 board in the self-protection circuit of the auxiliary relay -K11 before the closing coil YC, which is also the normally open contact D03 of the output relay of the SDCS-IOB-23 board, was in the open state after the -Q1 closing operation, causing -Q1 to immediately open after the closing operation. The "main contactor closed" branch of the SDCS-IOB-23 board has Di3 as its input, which is the normally open contact 43-44 of the undervoltage coil YC of -Q1, and its output is the normally open contact D03 of the SDCS-IOB-23 board. In this system, -Q1 is controlled by the start switch -S11 and the auxiliary relay -K11. Simultaneously, -Q1 can also be disconnected by the TSU via the normally open contact D03 of the SDCS-IOB-23 board. That is, under normal conditions of -S11, D03 of the SDCS-IOB-23 board, and their circuitry, if the TSU malfunctions or experiences a fault, the normally open contact D03 of the SDCS-IOB-23 board will not close, meaning the SDCS-IOB-23 board is controlled by the SDCS-CON-2 board. After further careful observation, analysis, and troubleshooting, it was found that, except for the inability of the normally open contact D03 of the SDCS-IOB-23 board to close in the self-protection circuit of -K11, the rest of the -Q1 control circuit was normal. Since the unit was not in operation, but the TSU reported over-temperature alarms and faults, further investigation was conducted on the TSU. First, it was planned to check whether the temperature sensor inside the thyristor positive bridge chassis was damaged. The fastening screws of the front panel of the thyristor positive bridge chassis were removed, and the front panel door of the thyristor positive bridge chassis was opened. It was immediately found that the two flat cable plugs connecting to the SDCCS-CON-2 board at X12 and X13 of the SDCCS-PIN-51 board had come loose. After reconnecting the flat cable plugs at X12 and X13, the -Q1 closing test was performed. -Q1 closed successfully and could be maintained. The common DC bus output was normal. After resetting each inverter, the PPCC Link fault disappeared, and the entire unit returned to normal. From the entire troubleshooting and handling process, it can be seen that the two flat cable plugs at X12 and X13 of the SDCS-PIN-51 board came loose. This caused several issues: First, the SDCS-CON-2 board prevented the normally open contact of D03 on the SDCS-IOB-23 board from closing, thus blocking the D03 signal and preventing TSU from closing -Q1, causing -Q1 to immediately open after the closing operation. Second, it resulted in no pulses in the thyristor positive bridge and no AC input power, leading to no output from the common DC bus and causing each inverter to report a PPCC Link fault. Third, it caused the SDCS-CON-2 board to generate false TSU over-temperature alarms and faults, ultimately rendering the entire multi-drive frequency converter inoperable and preventing the disc shear unit from being put into production. 3. Conclusion Through observation, analysis, and troubleshooting of the multi-drive frequency converter, and appropriate and reasonable handling, the fault of the ABB ACS600 multi-drive frequency converter in the disc shear unit was quickly eliminated, restoring normal production in the finishing area and ensuring the normal operation of production in the medium plate plant of Nanjing Iron & Steel Co., Ltd.