Abstract: This article introduces the renovation of the power distribution equipment of a chemical plant. It starts with the problems existing in the original design, and also includes the renovation ideas, equipment selection, renovation plan and the effect after renovation. A 6kV substation in a chemical plant supplies power to a polypropylene plant with an annual output of 70,000 tons. The power distribution equipment is designed and configured by the contractor along with the production equipment. The two 6kV busbars of the substation adopt a single busbar segmented wiring method, and a standby automatic transfer device is installed on the bus tie switch. The equipment connected to the two busbars are capacitor banks, motors and transformers. The capacitor banks are manually switched in groups. The main wiring diagram is shown in Figure 1. 1. Problems existing in the original design (1) A standby automatic transfer device is installed on the bus tie circuit breaker qf3. The undervoltage detection element of the standby automatic transfer device is installed on the incoming lines tvi and tv2, and the impact of the secondary voltage circuit disconnection on the standby automatic transfer is not considered. Therefore, there are two problems: First, the automatic transfer switch can only be activated when the incoming line loses voltage; if the incoming line circuit breaker qf1 or qf2 trips erroneously, the incoming line still has voltage, but the busbar has lost voltage, the automatic transfer switch cannot be activated, thus failing to achieve the intended function of the automatic transfer switch device; the voltage detection element should be installed on the busbar tv3 or tv4. Second, when a fuse in the secondary voltage circuit blows, it may trigger the automatic transfer switch to start, causing voltage fluctuations in the primary power supply system, which may lead to production fluctuations or even shutdowns, which is not allowed in chemical production. (2) All 6kV motors are equipped with undervoltage protection. On the 6kV section II busbar, except for one motor which does not have an automatic start function, all others have an undervoltage automatic start function, that is, after a delay, the motor automatically starts after the voltage drops instantaneously and then recovers, and the busbar power loss time is allowed to be no more than 3s. The voltage detection element of the 6kV motor with automatic start function is installed on the section II busbar tv4. For motors that do not have self-starting function (one for each bus section), the voltage detection element for undervoltage protection is installed on the secondary circuit of the incoming lines tv1 and tv2 of section i and section ii. When one incoming line is under maintenance and the other incoming line is used to drive the two bus sections, one motor will inevitably fail to work due to the undervoltage protection action. Neither of these motors has a corresponding backup motor. If one of them fails to start, the entire process operation of the polypropylene unit cannot be guaranteed. This will seriously affect the normal operation of high-load continuous chemical production, which is not allowed. Although the incoming line circuit breakers qfi and qf2 were later used as the criteria for undervoltage protection action to meet the production needs, the undervoltage protection circuit of the motor is relatively complicated and not conducive to operation, maintenance and testing. (3) The capacitor bank is not equipped with undervoltage protection. When the capacitor bank in operation suddenly loses voltage, the following two adverse consequences may occur: First, because the qf'3 is equipped with an automatic transfer switch, when a section of the line loses voltage and causes the automatic transfer switch to operate, the bus voltage can be restored quickly. However, if the residual voltage on the capacitor bank does not drop below 0.1 times the rated voltage, the capacitor bank may be damaged due to bearing a voltage higher than 1.1 times the rated voltage. Second, when the substation loses all power and the voltage is restored, if the capacitor bank cannot be manually disconnected in time, the upper transformer may cause a capacitor combination switch to generate a resonance overvoltage that damages the capacitor bank. In addition, in the early stage of voltage restoration in the substation, the load is very small and the bus voltage is high, which may also cause overvoltage damage to the capacitor bank. (4) The voltage range of the voltage detection element is too small, and the lowest operating voltage value can only be set to 70V. It also has two problems: First, when one of the power supplies in the entire plant loses power, the 6kV bus tie automatic transfer switch may act before the upstream automatic transfer switch, increasing the operation of restoring normal power supply and making it impossible to match the operating voltage of the upstream automatic transfer switch; Second, when the voltage is below 70V, the automatic transfer switch is activated, which is too sensitive and not conducive to the stable operation of the power supply system. 2. Modification Ideas In view of the problems existing in the original system of automatic transfer switch and undervoltage protection, the main purpose of the modification is: (1) To reconnect the voltage detection element of the automatic transfer switch to bus tv3 and tv4. The bus voltage is detected instead of the incoming line voltage, and the disconnection blocking function is added, that is, to ensure that the automatic transfer switch does not act when the secondary voltage circuit is disconnected. (2) To simplify the motor undervoltage protection circuit. The voltage of the low voltage protection circuit is taken from bus tv3 and tv4. The criteria related to the incoming circuit breakers qfl and qf2 are removed, which is beneficial for operation, maintenance and testing, and prevents the motor low voltage protection from failing to operate due to abnormal operation of the circuit breaker auxiliary contacts. In addition, the motor low voltage protection also realizes the disconnection lockout, which prevents the motor low voltage protection from malfunctioning due to the disconnection of the secondary circuit of tv3 and tv4. (3) The capacitor bank is equipped with low voltage protection with disconnection lockout. (4) In order to meet the requirements of the upper and lower level coordination, the voltage detection action range is increased and the voltage detection element is replaced. However, due to the limited space in the cabinet, the above purpose cannot be achieved by using traditional relays. Therefore, it is considered to use a comprehensive protection voltage relay. 3. Equipment selection Through analysis and comparison, it is decided to use the MRU3-2 type comprehensive voltage relay. It has the following advantages: (1) Small size and flexible installation method. It can be installed on the panel or in the cabinet, which is especially suitable for occasions with limited installation space. (2) It has 4 programmable low voltage, over voltage or negative sequence relays, each relay also has two pairs of output contacts, and can be set to operate only when the three phase voltages drop simultaneously, thereby realizing the disconnection lockout function. (3) It has a fault recording function. (4) The operating voltage setting range is wide, the low voltage operating setting range is 0-90V, and the overvoltage operating setting range is 70-120V, which fully meets the voltage setting requirements under various conditions. 4. Modification plan and steps (1) Remove the contacts of the relevant relays for detecting power loss in the original automatic transfer circuit. (2) Remove the low voltage protection of the motor without automatic start requirement. (3) The three-phase voltage input circuit of the MRU3-2 integrated voltage relay is taken from the secondary circuit of bus tv3 and tv4. In order to meet the requirements of low voltage protection for motors and capacitor banks, its output relay 1 is set to operate when the three-phase voltage is simultaneously lower than 60V. The first pair of output contacts of relay 1 is used to start an intermediate relay. The contacts of the intermediate relay are used to start the trip circuit of the motor and capacitor on the corresponding section to realize the low voltage protection of motors and capacitors. (4) For the voltage detection of automatic transfer switch undervoltage start, relay 2 is set to operate when the voltage is lower than 25V. In order to realize the disconnection lockout function, the first pair of output contacts of relay 2 is connected in series with the second pair of output contacts of relay 1 to start an intermediate relay. The output contacts of the relay replace the relevant contacts in the original automatic transfer switch circuit for detecting the input line undervoltage, so as to move the power supply undervoltage detection element of the automatic transfer switch circuit to bus tv3 and tv4 and realize the disconnection lockout function. (5) The wiring diagram before and after the modification of the motor low voltage protection circuit is shown in Figure 2. As shown in Figure 2(b), the modified protection wiring is greatly simplified, and a protection pressure plate has been added, facilitating the maintenance and testing of voltage transformers. 5. Results After the modification, a linkage test was conducted on the system. The automatic transfer switch (ATS) circuit operated correctly. The ATS circuit only activated when one busbar truly lost voltage and had no overcurrent, while the voltage of the other busbar was normal. The low-voltage protection for motors and capacitors activated during the ATS start-up, and motors with self-starting capabilities could start automatically after the ATS activated. This modification completely eliminated the hidden dangers and deficiencies of the original design, enabling the protection system to better coordinate with the power supply circuit and achieve safe and reliable power supply to the production equipment.