Lecture Notes on the DC System of 220kV Substations

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Lecture 1 on the DC system of 220kV substations: Basic requirements for the DC busbars in 220kV substations: 1. The company-wide initiative to convert substations into unattended types is set to begin in 2006. Understanding the plans for modifying the DC busbars will help ensure better supervision and inspection during maintenance work, thereby preventing any issues that do not meet the required standards from arising. It is necessary to follow the technical plans provided by the provincial company. 2. Battery banks, chargers, and DC busbars 2.1 Two sets of batteries shall be installed, with the capacity of each set being determined based on the requirement that a single battery can supply power to the entire substation’s DC system. 2.2 Two operational rectifiers and one standby rectifier are provided for charging and floating charge purposes; the standby rectifier can take over and replace any of the operational rectifiers in case it fails and stops working. 2.3 The DC panel is equipped with two sections of DC busbars, and there is a sectioning switch between these two sections. Under normal conditions, the two DC buses operate separately, with two sets of batteries and two rectifier units connected to each DC bus. 2.4 In substations equipped with circuit breakers featuring an electromagnetic closing mechanism, two sets of closing busbars should also be provided on the DC panel. 2.5 The 220kV system is equipped with two DC distribution panels. Switchboard I is equipped with 1 set of control busbars (KMⅠ) and 1 set of protection busbars (BMⅠ) ； The power distribution panel II is equipped with 1 set of control busbars (KMⅡ) and 1 set of protection busbars (BMⅡ). 2.6 The 110kV system is equipped with one DC distribution panel, which contains one set of control busbars (KM) and one set of protection busbars (BM). 2.7 When the relay protection panels for 10kV/35kV systems are installed centrally in the control room or protection room, a DC power distribution panel shall be provided in the control room or protection room. 2.8 The power supply for the signaling system is derived independently from the DC feed panel. 2.9 In the central signaling system, the accident signaling system and the advance signaling system have separate DC power supplies. 2.10 The DC power supply for each set of signaling systems is supplied through two independent feeders, and it can be powered by either of the two DC busbars in the two DC systems. 2.11 The circuit-breaker control circuit open-circuit signal, accident signal, and system power-loss signal are fed into the pre-warning signal system ； The pre-warning signal of system power failure is connected to the relevant monitoring circuit of the control system. 2.12 The starting power for each branch of the accident audio busbar shall be supplied by the power source of the accident signal system ； The starting power for each branch of the pre-warning signal busbar should be derived from the power supply of the pre-warning signal system. 2.13 The DC power supply for public measurement and control systems, network cabinets, remote control cabinets, protection fault information management cabinets, the dispatching data network, and UPS is supplied directly from the DC feed panel. II. Current status of DC systems: In 2005, among a total of 6,217 DC charging units owned by State Power Corporation, 3,476 were of the high-frequency switching power supply type, accounting for 55.91%. There are 2,631 phase-controlled units, accounting for 2.32%, and 110 magnetostrictive units, accounting for 1.77%. Of the total 6,897 battery sets, valve-regulated types accounted for 80.27% with 5,536 sets, acid-resistant types made up 10.37% with 710 sets, and nickel-separated types accounted for 9% with 621 sets. According to statistics, from 2002 to 2004, there were a total of 14 failures in the DC systems used at 110-500Kv stations, all of which were faults within the DC power supply systems themselves. There were 12 such failures at 110Kv stations, accounting for 85.7%, while there were 2 failures at 220Kv stations, representing 14.3%. Out of the 14 failures, 5 were battery-related, accounting for 35.7%; one of these occurred due to damage to the monitoring module of the charging equipment, which led to overcharging over an extended period and resulted in all batteries in that set swelling and being damaged ； The battery exploded and caught fire, resulting in a loss of direct current voltage at both the 220 and 110 kV substations, once in each case ； The battery terminals were burned out once ； An internal short circuit in the battery caused a drop in DC voltage across the entire station once. There were 9 failures of the charging equipment, accounting for 64.3%; among these, a silicon rectifier failed, resulting in a drop in direct current voltage across the entire station once ； A short circuit in the DC circuit caused the main fuse to blow, resulting in a loss of DC voltage across the entire station ； When DC circuit fuses and air circuit breakers are used together without proper grading, in the event of a short circuit in the DC circuit, the branch air circuit breakers do not operate; instead, the main fuse for the batteries blows as a result ； Charging device control plug-in, etc., 6 times. The main causes of defects in DC systems are: first, the high defect rate of high-frequency switching modules, which affects the operation of the equipment. Second, the batteries have serious defects, including open circuits, capacity degradation, low terminal voltage, and leakage. Third, monitoring devices frequently suffer from defects, including control failures and poor control accuracy, which severely affect the quality and lifespan of batteries. Fourth, charging devices have numerous inherent defects, including DC voltage drop caused by damaged silicon rectifiers, insufficient stability and current regulation accuracy in charge controllers used with valve-regulated batteries, device crashes, poor lightning protection and interference resistance, as well as damage to voltage reduction units or other components. Fifth is the poor operational reliability of the insulation inspection device, including crashes, incorrect wire selection, failed alarms, and device damage. III: DC busbar wiring methods: 1. General regulations for the operation of DC systems: (1) 220Kv substations generally adopt a single-bus segmented wiring method, while 110Kv substations generally use a single-bus wiring method. In a DC loop circuit, only one of the two power supply switches may be closed; this is because when the busbar switch is open, if both switches are closed, they will function as the busbar switch. Such a setup has a low switching capacity and a small cross-sectional area, and it does not comply with the requirements for sectional operation. The physical location of the sectional switches in the DC loop circuit must be clear; when forming a loop, the busbar switch should be closed first, followed by closing the other feed switch on the DC panel. (2) Each DC feedline busbar must be powered by a battery. (3) Chargers cannot operate in parallel. (4) Under normal conditions, the busbar switch should be in the off position. (5) The insulation inspection device and the voltage inspection device are always in operation. (6) When connecting the charger, start from AC to DC first. The order is reversed during a power outage. (7) When paralleling the busbars, first disconnect one charger, engage the busbar switch, and then disconnect the maintenance battery. (8) When switching the bus from parallel mode to sectionalized mode, first close the maintenance battery, disconnect the bus tie switch, and then activate the charger. 2. Example of DC bus wiring method 1 (Wang section): (1) For this wiring method, there are two sets each of insulation inspection devices, voltage inspection devices, and flash devices. (2) Normal operating mode: Charger No. 1 supplies power to the DC bus via QK1 to operate in place of the battery; the DC bus then supplies power to Section 1 of the DC feed bus through QK2 and the voltage-reduction silicon stack (the same applies to Charger No. 2). QK5 and QK6 are in the off position. This wiring method belongs to negative voltage regulation. (3) Short-term maintenance of 1# charger: Disconnect QK1, and battery 1 is in discharge mode. Close QK1 when powering on. (4) Long-term maintenance of Charger No. 1: Disconnect QK1, and close QK5 or QK6 ； During power supply, disconnect QK5 and QK6, and close QK1. (5) Maintenance of Battery 1: Disconnect QK1, close QK5 or QK6, and disconnect the fuse for Battery 1. Close the fuse for battery 1, disconnect QK5 and QK6, and close QK1. (6) Conduct charge-and-discharge tests on battery 1 alone: Disconnect QK1, close QK6, and disconnect QK2; the battery discharges at the specified current, and once discharge is complete, the charger automatically charges it to the floating charge state. After the experiment is completed, return to the normal operating mode: disconnect QK1 (which is in charging mode at this time), close QK2, disconnect QK6, and then close QK1. 3. Example 2 of DC bus wiring scheme (Nangong): Note: The four silicon stacks from left to right in the diagram are DN5, DN7, DN8, and DN9 respectively. (1) Under normal conditions, Q7, Q8, Q11, Q12, and Q13 are in the off position. (2) Short-term maintenance of Charger #1: Disconnect Q3 and Q1; Q7 should be in the disconnected position, and Battery Pack #1 should be in discharge mode. Close Q1 and Q3 during recovery. (3) Long-term maintenance of Charger No. 1: Disconnect Q3 and Q1; Q7 should be in the disconnected position, and connect Q12 and Q13 (or Q11). During recovery, disconnect Q12 and Q13 (or Q11), and close Q1 and Q3. (4) Maintenance of battery 1: Disconnect Q3 and Q1; Q7 should be in the disconnected position. Connect Q12 and Q13 (or Q11), and disconnect Q5. During recovery, close Q5, disconnect Q12 and Q13 (or Q11), and close Q1 and Q3. (5) Conduct charge and discharge tests on battery 1 alone: disconnect Q3 and Q1; Q7 should be in the disconnected position. Close Q12 and Q13 (or Q11), disconnect Q5, and close Q1 and Q7. During recovery, close Q5, disconnect Q12 and Q13 (or Q11), and close Q1 and Q3. Disconnect Q7. (6) Under normal conditions, DN5, DN6, Q9 together with DN8, and Q10 together with DN7 ensure that the positive bus of the DC feed line does not lose power. (7) Under normal conditions, Q12 and Q13 serve as the main busbars, while Q11 functions as a backup main busbar; Q12 can also act as a backup for Q9 and Q10. 4. Example of DC bus wiring scheme 3 (Jindian): (1) Under normal operation, ZK7 and ZK8 are in the disconnected position, while ZK6 is in the connected position (due to the isolating effect of diodes, the bus is theoretically considered to be segmented). (2) ZK1 and ZK2 are in the bus position, ZK5 is in the disconnected position, and ZK3 is in the connected position. (3) Maintenance of 1# charger: Disconnect ZK1. (4) Maintenance of Battery No. 1: Disconnect ZK1 and ZK3. (5) Conduct charge and discharge tests on battery 1 alone: Place ZK1 at the battery position and disconnect ZK3. 5. Example of DC busbar wiring scheme 3 (Longyao): (1) ZK3 and ZK4 are two bus coupler switches that serve as backups. (2) Machine 3# can charge and discharge Battery 1# individually, while Machine 2# cannot. IV. Operating Status of the Battery and Precautions (1) When the battery pack is operating normally in the float charge mode, it should be fully charged if its voltage is low, or at intervals specified by the manufacturer. (2) The floating charge voltage of the battery is 2.23 volts at 25°C, and the constant-current charge voltage is 2.35 volts. (3) The charger operates in automatic mode, but it is necessary to monitor the individual voltage of the batteries as well as their appearance (such as corrosion or swelling), and to control the temperature in the battery room at 25°C. (4) After the battery fuse blows and sends a signal, the busbar switch should be closed immediately (at this point the chargers are connected in parallel for a short time), then the charger that is charging that specific battery bank should be disconnected, followed by an inspection of the battery. (5) Although there is a battery inspection device, the voltage of each cell still needs to be measured daily. (6) A short circuit in the battery will cause a drop in voltage, and in some cases the voltage may even drop to zero. (7) When the battery is open-circuited, its terminal voltage is higher than the normal value, with the voltage level varying depending on the degree of the open circuit. (8) Overcharging batteries can cause serious damage, leading to swelling and explosion. (9) Poor contact at the pole terminals can easily cause burning out. V. Charger operating status and precautions (1) The main circuit switch of the DC system should be in the normal position; this varies from station to station. (2) All monitors should be turned on. (3) The main and auxiliary silicon chains should normally be in automatic mode; when they cannot meet the requirements of the DC bus, they can be set to an appropriate manual mode. For dual silicon chains, only one of them needs to be adjusted. (4) The microcomputer monitor is equipped with an insulation monitoring device, but it cannot be used for line selection; each bus section has its own insulation monitoring device that can be used for line selection. In most stations, each bus section has an insulation monitoring relay, but it cannot be used for line selection as well. (5) When the insulation inspection switch at Longyao Station is in position 0, the insulation inspection device built into the microcomputer monitor becomes active, as does the insulation inspection device for each bus section. 1 unit of the 1# insulation inspection device is in operation. The insulation inspection device in the 2-digit microcomputer monitor is not functioning. The 3-position 2# insulation inspection device is in operation. The insulation check relay is always active. (6) The charger is affected by temperature, a problem that is particularly evident in the 518 model. (7) Monitors sometimes give false alarms; their power can be turned off for a short period of time, but this should not be done frequently, as it may damage the display screen. (8) Each DC feedline bus shall be equipped with one spare module. VI. Detection of DC system grounding 1. General principles for detecting DC system grounding (1) After the “DC grounding” signal is triggered, it is possible to identify the number of the grounded branch and its grounding status using the DC panel monitor and the insulation monitoring device. The numbers of the branches are usually arranged in order, either from top to bottom or from left to right, according to the feed switch locations on the DC feed panel. The insulation monitoring device can also display the grounding resistance; an alarm is issued when the grounding resistance is less than 15–20 kOhms. To determine the extent of grounding, the voltage across the positive terminal with respect to ground, as well as the voltage across the negative terminal with respect to ground, can be measured using the insulation monitoring switch. Sometimes, the insulation inspection device is unable to determine which branch is at fault and only reports a \"DC bus ground\" condition; in such cases, it could be either the DC bus that is grounded or a branch. (2) Once the DC ground signal is issued, work on the secondary circuit must be stopped. The duty officer should inquire in detail about the situation and promptly correct any improper behavior by the maintenance personnel. (3) Use a multimeter to measure the voltage with respect to ground for both the positive and negative terminals, in order to verify the accuracy of the insulation testing device. The multimeter must have high internal resistance, 2000 ohms per volt; otherwise, it will cause another ground point to be created. (4) Test the emergency lighting circuit at the substation. (5) Test the DC power supply circuit in the maintenance room. (6) Test the 380-volt DC power supply circuit. (7) Test the communication and remote control power supply circuit. (8) Disconnect the battery. (9) Deactivate the charger. (10) Transfer the load from Bus 1 to Bus 2, and determine whether Bus 1 is grounded. (11) Use a ground finder to check each of the control, protection, and signal circuits one by one. 2. Examples of finding the ground connection 1 (jewelry store): (1) The emergency lighting in this location is powered by alternating current under normal conditions, with G1, G2, and G3 in the closed position. (2) After the loss of alternating current supply, the 2C and 3C relays are activated, providing direct current power to the emergency lighting circuit. (3) Disconnect the 110 protection compartment. (4) Disconnect the 220V protection compartment. (5) Disconnect G3. (6) Disconnect G2. (7) Disconnect the emergency lighting switch on the DC feed panel. (8) When locating a DC ground fault, it is necessary to clarify the circuit and eliminate possibilities step by step, starting from powering down the lowest-level components. 3. Examples of grounding searches 2 (Jindian 110 DC energy storage circuit) (1) ZK1 and ZK2 are in the closed position. (2) The loop-breaking switch for the entire circuit is in the off position. (3) Disconnect the DC energy storage switches one by one to determine whether the circuit from the terminal box to the mechanism box is functioning properly. (4) After confirming there are no issues as mentioned above, sequentially turn off the main circuit switches starting from the sectionalizer to determine whether the main circuit is functioning properly. VII. Precautions for the acceptance of DC equipment: (1) If the duty officer does not conduct a thorough inspection, it will lead to difficulties in handling any subsequent issues. (2) Request the as-built drawings. (3) Request the device manual. (4) Request specialized tools and spare parts. (5) Check whether the battery serial number is correct and whether the screws are tightened, to prevent loosening from causing the battery not to discharge and leading to more serious accidents. (6) Check whether the on-site photon card and monitor’s audio-visual alarms are functioning properly. Various fault signals should be simulated and sent to the backend machine. (7) The DC ground signals must be checked one by one to prevent the reported branch number from not matching the actual one. VIII. Term Explanation: DC busbar: The positive and negative main busbars within the DC power supply panel. Closing busbar: The DC busbar within the DC power supply panel that supplies power to components such as the circuit breaker’s electromagnetic closing mechanism and other power-consuming devices. DC power distribution panel: A DC power supply panel installed at various DC load centers to enable classified power supply. DC busbars: Those located outside the DC panel, such as various DC busbars on top of control or protection panels, as well as those within DC distribution panels. DC feed panel: A transitional panel that supplies power from the DC bus to the DC sub-buses and DC distribution panels. DC feed line: The DC power cables that run from the DC feed line panel to the DC busbars and the DC distribution panels. Verification discharge: For batteries in normal operation, in order to determine their actual capacity, the battery pack is taken out of service and subjected to a constant-current discharge at a specified discharge current; the discharge should be stopped as soon as one of the individual cells reaches the specified termination voltage. Float charging: The charger supplies power to compensate for the self-discharge of the battery at a constant voltage, while also meeting the normal DC power demands of the substation; it has a poor ability to discharge power instantaneously. Equalization charging: The charger performs regulated voltage charging on the battery for the period specified by the battery manufacturer (3 months) and at the specified voltage (2.35V), thereby activating the battery that was in a dormant state; once equalization charging is complete, it switches to floating charge mode. Constant charging: Traditional phase-controlled chargers and magnetically saturated chargers do not have the ability to automatically detect the condition of the battery; instead, manual intervention is required to charge the battery with an appropriate current (with a constant current and varying voltage, at 0.01Q/12A). Constant charging stops once the voltage reaches the specified value.