0 Overview Improving the automation level of urban power supply systems is an important aspect of power grid transformation, and the technical level of substation protection and control is a key link in this process. Currently, in the vast majority of urban power grid substations in China, the widely used relay protection devices are still composed of traditional mechanical contact relays. Often, completing a basic protection or control task requires multiple relays working together; for example, overcurrent protection and automatic reclosing control of a 10 kV feeder requires dozens of various relays. Because relay contacts frequently open and close, they are prone to damage, reducing the reliability of power supply and increasing the workload of equipment maintenance. At the same time, the numerous connecting wires between relays not only make commissioning and maintenance extremely difficult but also make it almost impossible to connect the various parts of the substation into a complete automation system. Therefore, traditional mechanical contact relays clearly cannot meet the requirements of substation automation for relay protection devices. Programmable Logic Controllers (PLCs) are a new type of microcomputer-based power distribution controller. Their main feature is that they replace traditional mechanical contact relays with various internally defined auxiliary relays (each PLC can have up to thousands of internal relays), and use software programming to replace the actual hardware connection lines with internal logical relationships. Because of this characteristic, introducing PLC into relay protection devices can overcome the various drawbacks of using traditional relays. Furthermore, it is compatible with design concepts and technical solutions based on traditional relays. Especially for complex contact signal processing and operation output control, PLC programming simplifies the design process. This article illustrates this through application examples. [b]1 PLC Programming for Low-Frequency Load Shedding and Automatic Backup Power Supply[/b] The newly built Mawangdui 110 kV/10 kV substation in Changsha is located in the suburbs and is designed according to unmanned operation standards. The selected secondary protection device is the SEPAM digital multi-function relay from MERLIN GERIN, France, whose functional block diagram is shown in Figure 1. As can be seen from the figure, this is basically the same as a typical microcomputer protection diagram. The slight difference is that this device decomposes the usual computer relay logic circuit into two parts: a protection function relay group and a PLC. Depending on the protected object (differential protection for main transformers, busbar protection, capacitor protection, line protection, etc.), different groups of relays with different protection functions are combined to create several standard models of the device. All individual functional elements follow positive logic rules, and their action nodes are defined in the PLC. For example, when an overvoltage element operates, a corresponding normally open node closes (0→1) in the PLC, and the same applies to an undervoltage element. The PLC programming uses a ladder diagram method most similar to traditional secondary circuit diagrams, and the EEPROM storing the program is externally pluggable, facilitating easy modification of the design. [img=262,137]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9908/image8/t9-1.gif[/img] Figure 1 Functional Block Diagram of SEPAM Device 1.1 Design of Low-Frequency Load Shedding Function According to regulations, substations should be equipped with a sufficient number of automatic low-frequency load shedding devices. When the power system experiences a power deficit due to an accident, the automatic low-frequency load shedding device must disconnect a portion of the secondary loads to prevent excessive frequency reduction and quickly restore it to a certain value, ensuring the stable operation of the system and the normal operation of important loads. In order to ensure the accuracy of the operation, the low-frequency load shedding device must have at least the following functions: (1) In order to prevent the malfunction caused by the sudden increase in short-circuit power and the sudden drop in frequency during the short-circuit process, the low-frequency operation output must have the frequency drop rate (df/dt) blocking function; (2) In order to prevent the malfunction caused by the undervoltage interval during the automatic reclosing or automatic backup power supply operation, the output delay of the low-frequency operation must be independently adjustable; (3) It can determine which loads are connected to the basic section (fast operation section) and what frequency level they should belong to according to the important procedures, and which loads are connected to the backup section (long time limit operation section) and how long the output delay should be set, and these connection and setting work can be carried out without power interruption. In this example, the 10 kV feeder protection device is the SEPAM S07 model. In addition to the overcurrent, instantaneous overcurrent, and zero-sequence overcurrent components that are essential for line protection, the protection function relay group of this model also includes two low-frequency components named F561 and F562. With the help of these two components, a PLC low-frequency load shedding program that meets the above requirements can be programmed (see Figure 2). From the program and annotations in Figure 2, we can conclude that: (1) The difference between the setting value of F561 and the setting value of F562 is Δf, and the setting value of T1 is Δt. When the system frequency drops from above the F561 setting value to below the F562 setting value within Δt, it indicates that the frequency drop rate Δf/Δt is too high. The cause of the low frequency in the system may be a sudden increase in short-circuit power or a sudden power failure in the system, rather than an overload. At this time, the low-frequency action output is blocked. Conversely, the relay K7 generates an output pulse command to trip the circuit breaker. (2) A special backup delay output is specially designed in the program. Its function is that, regardless of the value of Δf/Δt, as long as the system frequency is lower than the action frequency setting value (F562) and cannot be restored within a certain time (T3 setting value), the output action will disconnect the load. This special function can be selected by using an internal switch KP1 of the PLC according to the actual situation. It should be noted that the operating values ​​of the protection relays F561 and F562, as well as the time relays T1, T2, and T3 and the internal switch KP1 in the PLC, can all be set during system operation using a handheld programmer. The states of the switch signal input nodes I2 and I3 depend solely on the connection between the device's signal input interface and the power busbar. Operators can determine whether the load is connected to the low-frequency basic section or the low-frequency backup section simply by closing the corresponding connection. Using the PLC's internal switch instead of the external connection has the same effect; the external connection is used merely for clarity. When programming the low-frequency load shedding program, the traditional relay's operating time and return coefficient concepts are idealized in the ladder diagram. This is possible primarily due to the microcomputer nature of the PLC. The program in the PLC executes cyclically. The time interval between two executions (cycle period) is determined by the timing coordination of all stages of the device, from sampling to filtering to data processing (13.3 ms for the SEPAM device). However, the execution time of the program itself is measured in microseconds and is negligible. Obviously, if the relay node is placed after the corresponding coil in the ladder diagram, the node's operating time can be ideally considered to be zero. This is also an important factor that makes the complex low-frequency load shedding function simple to implement. [img=242,180]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9908/image8/t9-2a.gif[/img][img=237,133]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9908/image8/t9-2b.gif[/img] Figure 2 Low-frequency load shedding section of the line protection PLC program [img=270,139]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9908/image8/t9-3.gif[/img] Figure 3 Wiring structure of the 110 kV side of Mawangdui Substation 1.2 Automatic switching program design of backup power Mawangdui Substation is a terminal station of the 110 kV system of Changsha power grid, with double incoming power supply. The wiring structure of its 110 kV side is shown in Figure 3 (isolation switch is omitted). The automatic backup power supply switching scheme is designed according to the following four operating modes: (1) Mode 1. Line 1 supplies two transformers, and line 2 is on standby. At this time, circuit breakers 502 and 500 are in the "closed" position, and 504 is in the "open" position. If line 1 loses voltage, 502 is disconnected and 504 is closed. (2) Mode 2. Line 2 supplies two transformers, and line 1 is on standby. At this time, circuit breakers 504 and 500 are in the "closed" position, and 502 is in the "open" position. If line 2 loses voltage, 504 is disconnected and 502 is closed. (3) Mode 3. Lines 1 and 2 each supply one transformer. At this time, circuit breakers 502 and 504 are in the "closed" position, and 500 is in the "open" position. If line 1 loses voltage, 502 is disconnected and 500 is closed. (4) Mode 4. Each of the No. 1 and No. 2 incoming lines supplies one transformer. At this time, circuit breakers 502 and 504 are in the "closed" position and 500 is in the "open" position. If the No. 2 incoming line loses voltage, 504 is disconnected and 500 is closed. The control program for automatic transfer of backup power must also meet the following specific requirements: (1) The automatic transfer function of backup power in the four operating modes can be put into use or deactivated respectively; (2) The output only operates once; (3) Backup power is put into use only after the working power is disconnected; (4) When an overcurrent fault occurs in the substation and causes the incoming line to lose voltage, the automatic transfer function of backup power should be locked and automatically reset after a delay after the fault current is eliminated. It should be noted here that whether the incoming line circuit breaker is tripped due to an overcurrent fault is determined by the fault protection program. Considering the above operating modes and specific requirements, 34 SEPAM B04 type devices are selected, each corresponding to one operating mode. The low-voltage elements (UAB, UBC, UCA) and overvoltage elements (UAB, with a setting value lower than the rated voltage) in the protection relay group of this type of device are used to determine whether the voltage of incoming lines 1 and 2 and the bus voltage on both sides of bridge circuit breaker 500 are undervoltage or overvoltage, respectively. The overcurrent elements in two of the devices are used to determine whether fault current flows through incoming circuit breakers 502 and 504. The PLC program logic block diagrams corresponding to operating modes 1 and 3 are shown in Figure 4 (operating modes 2 and 4 are omitted in the figure due to the symmetrical relationship between the two sides). Each operating mode is activated or deactivated using an internal switch KP. As can be seen from the figure, only a small number of external wires are needed between the devices through I/O ports to connect the programs and form a complete automatic backup power supply system. Furthermore, this system has a high degree of modularity and is easy to operate and manage. 2 Conclusion The design, installation, commissioning, and subsequent operation of the PLC-based relay protection device in the substation over the past year have demonstrated that: the high flexibility of the PLC provides optimal solutions for promptly resolving problems encountered during commissioning; the high operational stability and accuracy of the PLC make the relay protection more reliable; PLC programming technology is easy to master, and the program itself closely matches traditional relay protection design diagrams, allowing designers to fully utilize their expertise; relay protection equipment using PLCs is more standardized, and selection and use are convenient. [img=275,276]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9908/image8/t9-4.gif[/img] Figure 4 Flowchart of automatic backup power supply switching procedure for operating modes 1 and 3 [b]3 References[/b] 1. (Japan) Hiroji Ota. Protection and Control of Power Systems. Beijing: Electric Power Industry Press, 1975 2. Yong Zhang. Logic Design of Relay Circuits. Tianjin: Science and Technology Press, 1981 3. SCHNEIDER DOCUMENTS. Logipam. France, 1997