[align=left] 1 Introduction With the rapid development of China's power industry, the power grid is becoming increasingly large, and the operation, dispatching, and management of the power system are becoming increasingly complex and busy. Unmanned remote-controlled substations with complete automation functions can greatly reduce the workload of dispatching personnel and are of great significance to ensuring the safe operation of the power grid. In order to promote unmanned substations in Guangxi, the Guangxi Power Bureau initiated a project in 1994, deciding to use the Guilin Zhishan Substation as a pilot project for transforming an old 110 kV substation into an unmanned substation. To ensure the success of this pilot project, the Guilin Power Supply Bureau established a design team, and with the strong support of the Guangxi Power Bureau Central Dispatch Center, the transformation was successful. Currently, the system is operating stably and reliably. [b]2 Equipment and System Introduction[/b] The Zhishan Substation was originally a conventional 110 kV manned substation, using traditional high-voltage control methods. Figure 1 shows the main wiring diagram of the substation. The substation has two on-load tap-changing transformers equipped with electromagnetic protection manufactured by Xuchang Relay Factory. The 110 kV side is a single busbar with sectionalized bypass, with four incoming lines equipped with SW6 circuit breakers, protected by PxD-32 transistor protection systems manufactured by Nanjing Automation Equipment Factory. The 10 kV side is also a single busbar with sectionalized overcurrent protection, with 32 outgoing lines equipped with JSGC-4 transistor instantaneous overcurrent reclosing circuit breakers manufactured by Nanjing Automation Equipment Factory. Two sets of capacitors are equipped with electromagnetic protection and handcart-type switchgear. All switch operations are manual electrical control, and disconnector operations are also manual. All secondary equipment within the substation, including control, measurement, signaling, and relay protection systems, are conventional. The remote control system for this renovation project consists of two main parts: a main control computer (referred to as the host computer) at the dispatching end and a programmable logic controller (PLC) at the substation end (referred to as the slave computer). The two are connected via a remote control channel, as shown in Figure 2. The core PLC of the remote control system drives the execution relays on the remote control execution panel to perform switch opening and closing, synchronization verification, main transformer voltage regulation, and signal reset. In addition, the PLC has the following comprehensive in-station automation functions: 2.1 Automatic capacitor switching and main transformer voltage regulation: Based on the voltage and reactive power (or power factor) within the acceptable range issued by the dispatching terminal, the PLC automatically operates the main transformer voltage regulation and capacitor switching according to the nine-domain diagram rules. 2.2 Automatic detection and tripping of small grounding current: The PLC collects the zero-sequence current 3I0 of each 10 kV line and the zero-sequence voltage 3U0 of each busbar open delta. When a grounding occurs in the system, the PLC selects to trip the 10 kV line based on the rate of change of 3I0 before and after 3U0 exceeds the limit, or only selects and does not trip, or trips and then recloses. This completely eliminates the various drawbacks caused by using the absolute value of 3I0 as the setting value. [img=515,349]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/47.gif[/img] Figure 1. Primary main wiring diagram of Zhishan substation [img=514,299]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/47-.gif[/img] Figure 2. Schematic diagram of remote control system 2.3 Low-frequency automatic load reduction function When the average value of the PLC's rapid n frequency measurements (n is adjustable) is lower than the set value and meets the locking conditions such as slip, the output trip switch automatically reduces the load, and at the same time, the fault recording device is started by the remote control execution panel-execution relay. This function allows for easy configuration of different circuit breakers tripping in different cycles without modifying secondary wiring. 2.4 Automatic Backup Power Transfer (BZT) Function: When a section of the 10 kV busbar loses power, the bus tie switch is automatically closed to maintain continuous power supply to that section of the busbar. In the event of a 10 kV line fault, if the switch fails to operate and trips the busbar incoming circuit breaker (No. 901 or 902), closing No. 900 would amplify the fault and cause a complete substation voltage loss. To prevent this, before closing the No. 900 bus tie, a trip pulse is applied to the unloaded section incoming circuit breaker to check for faults on the 10 kV line and whether the main transformer backup protection has activated. If so, this function is disabled. After closing the bus tie, the PLC checks for overload on the main transformer and automatically reduces the load to normal according to the set sequence based on the overload condition. The system uses a highly reliable, original American-made MODICON-984 programmable logic controller (RTU) to collect modular and switching signals simultaneously, sharing various information resources. It not only achieves the "four remote" functions (remote control, remote monitoring, remote control, and remote operation) but also realizes many station automation functions within the station's closed-loop control system, truly achieving integrated automation functions such as remote control and automation with a single unit. [b]3 Basic Principles and Ideas of the Retrofit[/b] The basic principle of the retrofit is to add "four remote" functions without significantly altering the original control signals and other functions. Protection and remote control are relatively independent, and the original equipment appearance is largely maintained. After the retrofit, both manned and unmanned operation are possible. The key to this retrofit project is how to modify the existing secondary circuits and equipment to connect them well with the remote control section, forming a complete remote control system. The remote control system operates in parallel with the original station control system, without affecting the use of the original control functions. One-to-one manual operation can still be performed using the KK handle on the original control panel, and all measuring instruments, warning signals, and emergency sounds remain effective. The remote control execution panel serves as the interface between the remote control unit and the secondary unit. Besides performing remote control functions, it also performs corresponding auxiliary functions, such as starting synchronization circuits corresponding to remote closing, acceleration after remote closing, remote tripping and reclosing (discharge), and low-frequency load shearing starting fault recording devices. To ensure the implementation of remote control and auxiliary functions, the PLC-driven relays on the execution panel require an engagement time of approximately 1 second. The remote control execution panel is designed entirely based on actual site conditions, is simple, reliable, convenient, and practical, without unnecessary lead-out contacts or switching switches. Based on the remote control quantities listed in the remote control sequence table, two remote control execution panels and one remote signaling transfer panel are planned. The first remote control execution panel is specifically for tripping and closing control of the 10 kV line, while the second remote control execution panel performs all remote control operations outside the 10 kV line. The remote signaling transfer panel is used to transfer all remote signaling signals to the RTU. Switching between remote control functions is achieved by directly controlling the positive power supply via a pressure plate on the execution panel, creating a clear disconnection point as a safety measure during maintenance. There is no need to set up local or remote switching switches without obvious disconnection points. [b]4 Content and Principle of the Modification[/b] To meet the requirements of unmanned operation in coordination with the remote control unit, it is necessary to modify the control circuit, signal device, measurement circuit, synchronization circuit, reclosing circuit, etc. in the station in many aspects, as follows. 4.1 Modification of Circuit Breaker Control Circuit Replace the manual positioning control switch on the control panel with an automatic reset switch to avoid the trouble caused by the mismatch between the position of the control handle KK and the actual position of the circuit breaker. For example, if KK is in the closed position, the circuit breaker will be in the tripped position after remote tripping, which does not correspond to KK. The newly installed KK is closed by turning it 45° clockwise and open by turning it 45° counterclockwise. After the operation is completed and the handle is released, the KK handle will automatically reset to the vertical position. Add a remote control circuit to the original control circuit. Connect the execution relay contacts on the remote control execution panel to the trip and close circuit, synchronization and closing circuit, reclosing acceleration and interlocking circuit of each circuit breaker as needed, as shown in Figures 3, 4 and 5. 4.2 The modification of the 10 kV line reclosing circuit is shown in Figure 3. Each circuit adds a time relay, activated by the switch tripping position, with a set time of 5–7 seconds to avoid the maximum reclosing cycle. Its delay contact is connected in parallel with the remote tripping relay contact and the KK contact to achieve remote tripping. Manual tripping provides instantaneous discharge to the reclosing circuit. After tripping, the SJ contact discharges continuously to block reclosing. Connecting the DC circuit will restart and restore the circuit. During trial energization, reclosing is guaranteed to not occur. Additionally, the manual closing KK contact is connected in parallel with the remote closing relay contact to accelerate manual and remote closing protection actions. Each 10 kV protection panel adds a signal reset relay, controlled by the remote control panel to reset all eight protection systems on the panel. [img=523,355]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/49.gif[/img] Figure 3 Schematic diagram of 10 kV line control and reclosing modification[/align][img=221,206]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/49-.gif[/img] Figure 4 Schematic diagram of 130, 900, 131 (132), 901 (902) switches and capacitor switches with remote control 4.3 Modification of 110 kV line reclosing circuit is shown in Figure 6. Each circuit adds a time relay and an intermediate relay to replace the original KK Handle. A signal reset relay is used to reset the signal of the entire panel. The improved circuit retains the advantages of the original KK handle circuit and is compatible with the features of advanced computer remote control. Various interlocking functions are automatically completed during remote operation. Manual tripping discharges KK, and remote tripping discharges YTJ. After tripping, TWJ starts SJ for 5-7 seconds, then starts ZJ and self-holds it. The normally closed contact of ZJ disconnects the reclosing positive power supply and locks the reclosing for a long time. Even if DC is switched on, the circuit can restart and return to normal operation. After the switch is closed, HWJ releases ZJ self-holding, and the normally closed contact of ZJ connects the reclosing positive power supply. 4.4 Synchronization Circuit Modification Two low-voltage relays are added to the central signal relay panel to detect the 110 kV busbar de-voltage and line de-voltage. Together with the original synchronizing relay, they constitute the permissible conditions for 110 kV line closing operation. Closing operation is allowed when one of the following conditions is met. [img=528,289]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/50.gif[/img] Figure 5 Schematic diagram of 110 kV line control circuit with remote control [img=517,271]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/50-.gif[/img] Figure 6 Reclosing circuit diagram of 110 kV line ① Busbar without voltage; ② Line without voltage; ③ Busbar and line with voltage and meeting synchronization conditions. See Figure 7. A603Ⅰ and A603Ⅱ are the synchronizing voltages of the 110 kV I and II bus sections, respectively. The voltage A609 of each 110 kV line is led to the remote control execution panel. Through the synchronizing execution relays TQJ of each control unit, voltages A611 and A621 are connected to the no-voltage and synchronizing verification relays of the relay panel. When the closing conditions are met, circuits 725 and 722 are connected, allowing the closing operation. 4.5 Modification of the Warning Signal Circuit: The original signal relays for the main transformer protection, capacitor protection, and underfrequency load shedding panel, which used a tag-drop local reset method, are replaced with current-starting, voltage-holding, and electrically reset signal relays according to the unmanned operation requirements. The normally closed contact of the signal reset execution relay on the remote control execution panel disconnects the signal relay to maintain the voltage reset signal. The illuminated signs on the control panel are replaced with LED illuminated signs with optocoupler output. The flashing circuit is removed (because it affects remote signaling), and the test lamp operation handle is removed; testing lamps is not allowed to prevent DC short circuits. Replace the circuit breaker position indicator lights (red and green lights) on the control panel with LED signal lights with optocoupler output. [img=534,275]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/51.gif[/img] Figure 7 Wiring diagram of the modified synchronization circuit 4.6 Modification of the emergency alarm circuit Before the modification, the emergency alarm was started by the KK handle position not corresponding to the circuit breaker position. After the KK handle was replaced, another new way to start the emergency alarm had to be found. The emergency alarm starting method of this modification must meet two conditions: protection action and switch tripping, as shown in Figure 8. TWJ is the unit switch trip relay contact, and the protection action contact is taken from the spare normally open contacts of all output trip protection signal relays of the unit. If the signal relay contacts are insufficient, intermediate relays are added to increase the number of contacts. 4.7 The modification of the main transformer voltage regulation control circuit is shown in Figure 9. The contacts of the boost relay SYJ and the buck relay JYJ on the remote control execution panel are led to the voltage regulation controller on the main transformer control panel. When boosting operation is performed, the buck operation circuit is locked; when bucking operation is performed, the boost operation circuit is locked. QA3 is the boost button on the operator, and QA4 is the buck button. [img=206,140]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gxdljs/gxdljs99/gxdl9902/image2/51-.gif[/img] Figure 8 Emergency sound start circuit [align=left]4.8 Acquisition of remote signaling quantities The names of all remote signaling signals are mostly the same as the names of the indicator lights and light boards on the control panel. Therefore, most remote signaling signals can be obtained as follows: replace the circuit breaker position indicator lights (traffic lights) and indicator lights on the control panel with LED indicator lights and indicator lights with optocoupler outputs. This retains the original signals while obtaining a large number of remote signaling signals with the same name. It is particularly important to note that switch position remote signals must be obtained from the dual positions on the traffic lights. This monitors both the normality of the switch tripping circuit and the presence of DC operation. The optocoupler outputs are connected in parallel according to signal classification. Additionally, the remote signaling signals for the main transformer voltage regulation positions are obtained using the following steps: ① Create a self-made rectifier, filter, and step-down circuit, installed on the terminal block inside the on-load tap changer outdoor control box, to convert the power supply for the position display from AC to DC to prevent AC zero-crossing from affecting the remote signaling. ② Create a self-made optocoupler position acquisition circuit board, installed on the main transformer control panel, to acquire one-to-one position remote signaling signals from the existing position display circuit. 4.9 Remote Measurement Acquisition: The acquisition of bus voltage, line current, unit equipment current, and active and reactive power remote measurements was achieved by replacing and adding new, high-interference-resistance current-type (4-20 mA) transmitters. Additionally, new transmitters for main transformer oil temperature and 10 kV high-voltage room temperature and humidity were installed, and the 10 kV small grounding current selector on the control panel was upgraded to a small grounding current transmitter. The advantage of using current-type transmitters is that they improve anti-interference capability and allow for monitoring of transmitter malfunctions. When a fault occurs, the output current is outside the 4-20 mA range. [align=left][b]5 Conclusion[/b] This project was a pilot project for the entire region, with significant construction difficulty and workload, and it was not permissible to disrupt normal power supply during construction. Therefore, before construction, technical training was conducted for construction personnel, and a meticulous construction plan and preparation were made before construction began. The construction procedure followed a progression from simple to complex, from no power outage required to power outage construction, and finally, phased power outage construction on the 10 kV outgoing lines. An old substation was transformed into a remotely controlled substation in one operation with minimal disruption to normal power supply. Various tests confirmed the success of the transformation, and it passed the acceptance test by the Guangxi Power Bureau at the end of 1996. The system has been operating for over two years, weathering several rainy seasons without any damage to the PLC, and the "four remote" functions (remote control, remote monitoring, and remote operation) have functioned normally, demonstrating the high reliability of the PLC as an RTU.