Secondary grounding specifications for voltage transformers and current transformers
Voltage transformers and current transformers, as key components in power systems, undertake important tasks of measurement, protection and monitoring. These two types of transformers each play an indispensable role in the power system, and together provide strong guarantees for the stable operation of the power system. Voltage transformers, as the name suggests, are mainly used to measure voltage. Its working principle is based on electromagnetic induction, converting high voltage into low voltage so that it can be safely used in measuring instruments and protection devices. The current transformer focuses on the measurement of current. It can convert large currents into small currents, which is convenient for the measurement of current and the action of protection devices. Below, the editor will share with you the secondary grounding specification requirements for voltage transformers and current transformers.
▌01 Secondary grounding specifications for voltage transformers and current transformers
(1) Voltage transformer:
The neutral line of the secondary circuit that is independent and has no electrical connection with other voltage transformer secondary circuits should be grounded at one point in the switch field, including the secondary circuit of the voltage transformer used for reclosing and synchronization checking devices.
The secondary circuit of the public voltage transformer is only allowed to be grounded at one point in the control room.
The secondary grounding point of the generator neutral point voltage transformer used for generator stator grounding protection should be located in the grounding protection cabinet.
The secondary circuit of the voltage transformer for line voltage extraction and the secondary circuit of the discharge voltage transformer of the high-voltage capacitor bank should be grounded at one point in the switch field.
The neutral points of all PTs are connected to a protective cabinet in the central control room and grounded at one point throughout the station.
(2) Current transformer:
The secondary circuit of the secondary winding of the public current transformer is only allowed to be and must be grounded at one point inside the relevant protection screen. The grounding point is set at the outer terminal of the directly connected protection screen terminal block.
[Explanation] The secondary winding of the common current transformer includes: differential protection, protection of various double circuit breaker main connections, and currents and circuits that are directly physically connected in parallel.
The secondary circuit that is independent and has no electrical connection with other current transformer secondary circuits should be grounded at one point in the switch field. [Explanation] It is more appropriate to ground the secondary winding of the current transformer in the switch field. When the primary winding breaks down, the grounding wire is the shortest and it is most effective to limit the high voltage from entering the secondary circuit.
No fuse is installed in the open triangle, and no fuse is required for the voltage transformer used for excitation.
(3) Grounding requirements and specifications:
Connect the N phase of the voltage transformer to the grounding busbar with a 4mm2 two-color wire, and hang signs on both sides of the grounding wire that read “TV N600 is the only grounding point for the entire station and must not be removed.”
A bare copper busbar with a cross-section of not less than 100 mm2 should be installed in the terminal box of the switch yard, and connected to the equipotential grounding network in the cable trench using a copper cable with a cross-section of not less than 100 mm2.
Under the floor of the panel cabinet where the static protection and control devices are installed, a grounding copper bar with a cross-section of not less than 100mm2 should be directly connected to form an equipotential grounding busbar. The grounding busbar should be reliably connected to form a ring network, and directly connected to the grounding network of the plant and station with a cross-section of not less than 50mm2 and no less than 4 copper bars.
A grounding copper bar with a cross-section of not less than 100mm2 should be installed at the bottom of the panel cabinet of the static protection and control device. The grounding terminal of the device on the panel cabinet should be connected to the grounding copper bar with a multi-strand copper wire with a cross-section of not less than 4mm2. The grounding copper bar should be connected to the equipotential grounding busbar under the ground with a copper bar with a cross-section of not less than 50mm2.
The shielding layer of all secondary cables should be reliably connected to the copper bus of the equipotential grounding grid using multi-strand copper soft wires with a cross-section of not less than 4mm2.
The differential protection current circuit should use 4MM2 cable.
The upper end of the metal pipe of the booster station should be well welded to the equipment base, and the lower end should be well welded to the main grounding grid nearby.
The secondary circuit of the secondary winding of the public voltage transformer is only allowed to be grounded at one point, and the grounding point should be set in the control room. Independent secondary circuits that are not electrically connected to other voltage transformers should be grounded at one point in the distribution equipment area.
The secondary circuit of the secondary winding of the public current transformer is only allowed to be grounded at one point in the relevant protection screen or cabinet. Independent secondary circuits that are not electrically connected to other current transformers should be grounded at one point in the power distribution equipment area.
The shielding layer of the shielded cable should be grounded at both ends of the switch field and the control room. In the control room, the shielding layer should be connected to the grounding copper bar in the panel cabinet on the protective screen; in the switch field, the shielding layer should be grounded in the terminal box at a certain distance from the high-voltage equipment. The secondary circuit of each phase of the transformer is led from the high-voltage box to the terminal box through a two-core shielded cable, and the shielding layer of the cable is grounded at both ends of the high-voltage box and the terminal box.
The (communication) signal transmission cable used between the protection room and the communication room should be a twisted pair double-shielded cable, and the shielding layer should be grounded at both ends.
Shielded twisted pair cables should be used to transmit audio signals, and the shielding layer should be grounded at both ends.
When the distance between the protection and communication equipment for transmitting digital signals is greater than 50m, optical cable should be used.
For cables with low-frequency and low-level analog signals, such as cables for thermocouples, the shielding layer shall not be grounded at two or more points, and centralized one-point grounding is preferred. The shielding layer must be grounded at the most unbalanced end or the grounding point of the circuit itself.
For double-shielded cables, the inner shield should be grounded at one end and the outer shield should be grounded at both ends.
The cable shielding layer should preferably be copper tape shielding.
The analog and pulse weak current signal input loop cable should be a twisted shielded cable, and the core wire section should not be less than 0.75mm2. The core wire section of the AC sampling signal cable should not be less than 2.5mm2;
(4) The requirements of the Southern Power Grid are as follows:
The cable grounding wire should be arranged with uniform diameter and protected with heat shrink tubing. The cable shielding layer of the protection and related secondary circuits and high-frequency transceivers should be reliably connected to the copper busbar of the equipotential grounding network using multi-strand copper soft wires with a cross-section of not less than 4 mm2.
The N lines of the secondary and tertiary windings of the voltage transformer are separated in the terminal box and led out to the control room with different cables. The four incoming wires ABCN from the secondary of the voltage transformer in the switch field and the two incoming wires LN of the open delta winding of the voltage transformer should use their own independent cables.
There can be only one grounding point for each voltage level TVN600 in the entire station. The grounding point should be connected to the equipotential grounding busbar, and signs reading “The only grounding point for TV N600 in the entire station, must not be removed” should be hung on both sides of the grounding wire.
The secondary coil of the voltage transformer that has been grounded at one point in the control room should have its neutral point grounded through a discharge gap or zinc oxide valve plate in the switch field. Its breakdown voltage peak should be greater than 30·Imax volts (Imax is the maximum possible effective value of the grounding current passing through the substation when the power grid is grounded, in kA). The discharge gap or zinc oxide valve plate should be checked regularly to prevent the phenomenon of multiple grounding points in the voltage secondary circuit. — Add a breakdown fuse JBO-220 in the terminal box of the booster station.
The differential protection current circuit should use 4MM2 cable.
An equipotential grounding grid closely connected to the main grounding grid should be laid using bare copper busbars (cables) with a cross-section of not less than 100 mm2 in the main control room, protection room, trenches for laying secondary cables, local terminal boxes in switch yards, and protective combined filters.
In the cable room below the cabinets and panels in the main control room and protection room, lay a 100 mm2 dedicated copper busbar (cable) in the direction of the cabinet and panel layout, and connect the ends of the dedicated copper busbar (cable) to form an equipotential grounding network in the protection room. The equipotential grounding network in the protection room must be reliably connected to the main grounding network of the plant and station at the cable shaft with at least 4 copper busbars (cables) with a cross-section of not less than 50 mm2.
Lay bare copper busbars (cables) with a cross-section of not less than 100 mm2 along the trench of secondary cables to construct an outdoor equipotential grounding network.
Between the dispersedly arranged local protection stations, communication rooms and central control rooms, copper busbars (cables) with a cross-section of not less than 100 mm2 and tightly connected to the main grounding grids of the plant and station should be used to reliably connect the local protection stations and the equipotential grounding grids of the central control room.
A bare copper busbar with a cross-section of not less than 100 mm2 should be installed in the local terminal box of the switch yard, and connected to the equipotential grounding network in the cable trench using a copper cable with a cross-section of not less than 100 mm2.
The lower part of the panel cabinet of the static protection and control device should be equipped with a grounding copper bar with a cross-section of not less than 100 mm2. The grounding terminal installed on the panel cabinet should be connected to the grounding copper bar with a multi-strand copper wire with a cross-section of not less than 4 mm2. The grounding copper bar should be connected to the equipotential grounding grid in the protection room with a copper cable with a cross-section of not less than 50 mm2. —Equipotential copper bars with a cross-section of not less than 100 mm2 should be installed at the bottom of each panel cabinet and box. The ends of this special grounding copper bar are connected at the same time, and then one point of the grounding grid is connected to the ground grid of the control room.
The secondary cables of transformers, circuit breakers, isolating switches, combined filters, current and voltage transformers and other equipment in the switch field should be led from the junction box (box) of the primary equipment to the local terminal box through metal pipes, and the upper end of the metal pipe should be well welded to the base and metal shell of the above equipment, and the lower end should be well welded to the main grounding grid nearby. At the local terminal box, the shielding layer of these secondary cables should be reliably connected to the copper busbar of the equipotential grounding grid using a multi-strand copper soft wire with a cross-section of not less than 4 mm2.
The signal transmission cable used between the protection room and the communication room should be a twisted pair double-shielded cable, and the shielding layer should be grounded at both ends.
14.The secondary circuit of relay protection should be grounded according to the following requirements:
The secondary circuit of the secondary winding of the public voltage transformer is allowed to be grounded at only one point, and the grounding point should be set in the control room. Independent secondary circuits that are not electrically connected to other voltage transformers should be grounded at one point in the power distribution equipment area. To ensure reliable grounding, the neutral line of each voltage transformer must not be connected to a switch or fuse that may be disconnected. The secondary winding and open delta winding of the voltage transformer should be led to the control room or protection room using their own independent cables.
Independent current transformers that have no electrical connection with other current transformer secondary circuits should be grounded at one point in the power distribution equipment area. The secondary circuit of the secondary winding of a common current transformer is only allowed to be grounded at one point in the relevant protection screen or cabinet.
The N line of the AC power supply (lighting, printer and modem) in the panel cabinet of the microcomputer-type relay protection device is not allowed to be connected to the dedicated copper busbar with equipotential.
Follow the principle that the 24V input power supply of the protection device does not leave the protection room to avoid introducing interference.
It is advisable to cancel the grounding method of the secondary B phase of the voltage transformer, or change it to synchronous parallel connection through an isolation transformer.
The neutral point of the transformer should have two grounding leads connected to different trunks from the main ground grid, and each grounding lead should meet the requirements of thermal stability verification. Important equipment and equipment structures should have two grounding leads connected to different trunks from the main ground grid, and each grounding lead should meet the requirements of thermal stability verification. The connecting leads should be easy to check and test regularly.
Shielded cables should be used for current, voltage and signal contact lead-in lines for integrated circuit and microcomputer protection, and the shielding layer should be grounded at the same time in the switch field and the control room.
▌02 Requirements for grounding of the secondary circuit of voltage transformer
The “Eighteen Major Grid Accident Prevention Measures (Trial) of State Grid Corporation of China” stipulates:
Pay attention to the grounding of the secondary circuit of relay protection, and regularly check the reliability and effectiveness of these grounding points. The grounding of the secondary circuit of relay protection should meet the following requirements:
The secondary circuit of the public voltage transformer is only allowed to be grounded at one point in the control room. To ensure reliable grounding, the neutral line of each voltage transformer shall not be connected to a switch or fuse that may be disconnected. The secondary coil of the voltage transformer that has been grounded at one point in the control room should be grounded at the neutral point of the secondary coil through a discharge gap or zinc oxide valve plate in the switch field. The peak breakdown voltage should be greater than 30·Imax volts (Imax is the maximum possible effective value of the grounding current passing through the substation when the power grid fails, in kA). The discharge gap or zinc oxide valve plate should be checked regularly to prevent the phenomenon of multiple grounding points in the voltage secondary circuit.
The secondary circuit of the secondary winding of the common current transformer is only allowed to be and must be grounded at one point in the relevant protection cabinet. The independent secondary circuit that has no electrical connection with the secondary circuits of other voltage transformers and current transformers should be grounded at one point in the switch field.
The neutral line (zero line) of the AC power supply (lighting, printer and modem) in the cabinet of the microcomputer-type relay protection device should not be connected to the equipotential grounding network.
Attachment: Ministry of Electric Power Industry Dian An Sheng [1994] No. 191 “Key Points of Anti-Accident Measures for Relay Protection and Safety Automatic Devices in Power Systems”, if the secondary side of the voltage transformer is star-connected, the neutral point should be grounded, and the B-phase grounding method should be cancelled. “Key Points of Anti-Accident Measures for Relay Protection and Safety Automatic Devices in Power Systems”
Instrument transformer and its secondary circuit
8.1 The secondary circuits of the current transformer and voltage transformer must be grounded at one point only.
8.2 The grounding point of the current circuit composed of several sets of current transformer secondary combinations, such as differential protection and various double circuit breaker main line protection current circuits, should be selected in the control room.
8.3 For several groups of voltage transformer secondary circuits connected via the zero-phase busbar (N600) in the control room, only one point N600 should be grounded in the control room, and the secondary neutral point of each voltage transformer should be disconnected at the grounding point in the switch field; to ensure reliable grounding, the neutral line of each voltage transformer must not be connected to a switch or contactor that may be disconnected.
8.4 For the secondary coil of the voltage transformer which has been grounded at one point in the control room, if it is deemed necessary, the neutral point of the secondary coil can be grounded through a discharge gap or zinc oxide valve plate in the switch field. The peak breakdown voltage should be greater than 30Imax volts. Imax is the maximum possible effective value of the grounding current passing through the substation when the power grid grounds fault, and the unit is kiloamperes.
8.5 It is advisable to cancel the grounding method of the secondary B phase of the voltage transformer, or change it to synchronous parallel connection through an isolation transformer.
8.6 Independent current or voltage transformer secondary circuits that have no electrical connection with other transformer secondary circuits can be grounded at one point in the control room or in the switch field.
