Customize power distribution cabinets for small substations
When customizing power distribution cabinets for small substations, it is necessary to combine the voltage levels of the substations (usually 10kV/35kV medium and low voltage systems), load characteristics, and intelligent requirements, and conduct strict designs in terms of technical parameters, equipment selection, safety specifications, etc. The following are the core points to note:
I. Clarify the Application Scenario and Core Requirements
1. **Substation Type and Voltage Level**
- **Type**: Box-type substations (outdoor compact type), indoor substations, prefabricated substations, etc., which determine the cabinet structure (such as fully enclosed, modular) and protection level (IP54/IP65 is required for outdoor use, and IP30 is sufficient for indoor use).
- **Voltage Level**: Medium voltage side (10kV/35kV), low voltage side (0.4kV), which needs to match the transformer capacity (such as 500kVA/1000kVA) and the system grounding method (ungrounded neutral point, grounding through arc suppression coil, etc.).
YouTube：https://youtube.com/shorts/9cFtgGisxJc
2. **Functional Positioning**
- **Medium Voltage Power Distribution Cabinet**: Incoming line, outgoing line, bus tie, PT/CT cabinets, which need to integrate protection and measurement and control devices (such as overcurrent, instantaneous trip, differential protection).
- **Low Voltage Power Distribution Cabinet**: Low voltage incoming line, outgoing line, capacitor compensation, and tie cabinets, which need to support reactive power compensation and harmonic treatment (such as configuring reactors and filters).
- **Intelligent Requirements**: Whether to access the substation integrated automation system (which needs to support IEC 61850/Modbus communication protocol), remote switching on and off, and status monitoring (temperature, partial discharge monitoring).
II. Compliance and Industry Standards
1. **National Standards and Industry Specifications**
- **Medium Voltage Equipment**: It complies with GB 3906 (3.6-40.5kV AC metal-enclosed switchgear) and DL/T 404 (3.6-40.5kV AC high-voltage switchgear), and needs to pass type tests (insulation test, short-circuit withstand test, protection level test).
- **Low Voltage Equipment**: It complies with GB 7251.1 (low-voltage complete switchgear). The 3C certification is mandatory, and grid-connected projects need to pass the grid company's access test (such as the enterprise standard Q/GDW of State Grid).
- **Special Scenarios**: In plateau areas (altitude > 1000m), it needs to meet GB/T 20641 (low-voltage complete equipment for high altitude), and in coastal areas, it needs to prevent salt spray corrosion (select 316 stainless steel as the material or spray weather-resistant coating).
2. **Protection and Grounding System**
- **Relay Protection**: Configure microcomputer protection devices (such as products of NARI, XJ Group, CGC), realize three-stage overcurrent protection and zero-sequence protection, and support remote modification of setting values.
- **Grounding Design**: An independent grounding copper busbar (cross-sectional area ≥ 50mm²) needs to be set in the cabinet. The grounding resistance of the medium and low voltage systems ≤ 4Ω, and the lightning protection grounding needs to be reliably connected to the main grounding grid (resistance ≤ 10Ω).
III. Technical Parameters and Equipment Selection
1. Key Parameters of Medium Voltage Power Distribution Cabinets
| Parameter | Requirement |
| Rated Voltage | 10kV/35kV (needs to match the high voltage side of the transformer) |
| Rated Current | Incoming line cabinet ≥ 1.2 times the rated current of the transformer (for example, for a 1000kVA transformer, the current on the 10kV side is about 57.7A, select 63A/125A) |
| Short-circuit Breaking Capacity | Meet the system short-circuit capacity (such as 25kA/31.5kA, and a calculation report of the local power grid short-circuit current needs to be provided) |
| Protection Level | Outdoor cabinet IP54 (water spray protection), indoor cabinet IP30 (protection against solid foreign objects) |
| Circuit Breaker Type | Vacuum circuit breaker (commonly used for 10kV, such as the VS1 series) or SF6 circuit breaker (commonly used for 35kV, such as ABB SafeRing) |
2. Key Parameters of Low Voltage Power Distribution Cabinets
| Parameter | Requirement |
| Rated Voltage | 400V (AC) |
| Rated Current | Incoming line cabinet ≥ the current on the low voltage side of the transformer (for example, for a 1000kVA transformer, the current on the low voltage side is 1443A, select a 1600A frame circuit breaker) |
| Breaking Capacity | Not lower than the short-circuit current on the low voltage side (usually 50kA-100kA, and a circuit breaker with high breaking capacity needs to be configured) |
| Busbar System | Select the cross-sectional area of the copper busbar according to the current (for example, for 1600A, 3 copper busbars of 100×10mm need to be connected in parallel), and the surface is tin-plated to prevent oxidation |
| Compensation Configuration | The capacity of the capacitor compensation cabinet is configured according to 20%-30% of the transformer capacity (such as 200-300kvar for 1000kVA), and a reactor needs to be configured to suppress harmonics |
3. Selection of Core Components
- **Medium Voltage Circuit Breaker**: Give priority to joint venture brands (such as Schneider RM6, ABB Unigear) or first-tier domestic brands (such as Pinggao, XD Group, Daqo) to ensure reliability and service life (number of operations ≥ 10,000 times).
- **Low Voltage Circuit Breaker**: Frame circuit breakers (such as Schneider MT, ABB Emax), molded case circuit breakers (such as Siemens 3VL, Chint NA8), with electronic trip units to achieve precise protection.
- **Monitoring Instruments**: Configure multi-functional power instruments on the medium voltage side (support three-phase voltage/current, active/reactive power monitoring), and configure smart electricity meters on the low voltage side (support RS485 communication, in line with DL/T 645 protocol).
- **Insulation Materials**: The insulation parts of the medium voltage cabinet need to be cast with epoxy resin (temperature resistance ≥ 130℃), and the insulated wires of the low voltage cabinet are selected as flame-retardant type (such as ZR-BVR).
IV. Cabinet Structure and Process Design
1. **Cabinet Material and Structure**
- **Medium Voltage Cabinet**: Aluminum alloy profile frame (lightweight, good heat dissipation) or aluminized zinc steel plate (thickness ≥ 2.0mm, surface electrostatic powder spraying, anti-ultraviolet coating is required for outdoor use), using a modular design (circuit breaker room, busbar room, cable room are independently separated).
- **Low Voltage Cabinet**: Cold-rolled steel plate (thickness ≥ 1.5mm), drawer-type structure (such as MNS, GCK) is convenient for maintenance, and the breaking capacity of the drawer unit needs to match the circuit (such as a 100A drawer is equipped with a 63A circuit breaker).
2. **Heat Dissipation and Ventilation**
- Outdoor cabinets need to be equipped with temperature-controlled fans (start when the temperature > 40℃) and moisture-proof heaters (start when the humidity > 75%), and the ventilation holes are equipped with dust-proof nets (aperture ≤ 2mm).
- Reserve a heat dissipation channel in the busbar room to avoid local overheating caused by eddy current loss (the spacing between copper busbars ≥ 20mm, and support the installation of infrared temperature measurement modules).
3. **Cable Inlet and Outlet Design**
- The height of the cable room of the medium voltage cabinet ≥ 800mm (convenient for making cable terminal heads), and the incoming line method supports bottom incoming line/side incoming line, and reserve enough bending radius space (15 times the outer diameter of the cable).
- The outgoing line of the low voltage cabinet uses prefabricated cable joints or copper busbar transfer, and the bottom of the cabinet is provided with a multi-hole wire-passing plate (equipped with a rubber sealing ring for waterproof and dustproof).
V. Intelligent and Integrated Design
1. **Integration of Protection and Measurement and Control**
- Configure comprehensive protection devices in the medium voltage cabinet (such as NARI-Relays RCS-9611, XJ Group WGB-871), realize protection logic and fault recording, and support the access of GOOSE/SV signals to the intelligent substation system.
- Install intelligent monitoring modules in the low voltage cabinet (such as Acrel AEM96), real-time monitor the power quality of each circuit (voltage deviation, harmonic distortion rate), and automatically alarm in case of abnormalities.
2. **Status Monitoring Function**
- Connect the position of the medium voltage circuit breaker handcart and the on/off state signal to the monitoring system, and support the switching between local/remote control.
- Install wireless temperature measurement sensors on the cabinet (monitor the temperature of the busbar connection points and cable joints), and give an alarm at the threshold (warning when > 80℃, trip when > 100℃).
3. **Communication and Interface**
- Configure an industrial-grade switch (supporting ring network redundancy), and the medium and low voltage equipment is connected to the substation background through the Modbus TCP/IEC 61850 protocol, and reserve an OPC UA interface to connect to a third-party platform.
- Human-Machine Interface (HMI): Configure a 7-inch touch screen for the medium voltage cabinet (display real-time data, fault information), and configure a 4-inch LED screen for the low voltage cabinet (display main circuit parameters).
VI. Manufacturer Selection and Project Implementation
1. **Qualification and Capacity Review**
- Require the manufacturer to have the **License for Installation (Repair, Test) of Electric Power Facilities** (Grade III and above), the production qualification of medium voltage switchgear (passed CQC certification), and provide performance of similar projects (such as cases of 10kV box-type substation power distribution cabinets).
- Inspect the manufacturer's production equipment: The medium voltage cabinet needs to have CNC bending machines, laser cutting machines, and epoxy resin casting production lines; the low voltage cabinet needs to have fully automatic sheet metal production lines and busbar processing centers.
2. **Technical Agreement and Drawing Confirmation**
- Provide complete design documents: Electrical schematic diagrams (including protection setting value tables), cabinet layout diagrams, cable lists, communication network diagrams, and clarify the brands of components (such as selecting ABB for circuit breakers and CGC Deepblue for protection devices).
- Focus on confirmation: The five-prevention functions of the medium voltage cabinet (preventing misoperation of opening and closing, closing with electricity, etc.), and the short-circuit withstand test report of the low voltage cabinet (an 8/20μs surge test report issued by a third-party testing institution needs to be provided).
3. **Installation and Commissioning**
- Before the cabinet is transported, seismic reinforcement needs to be carried out (in line with the requirements of the seismic motion parameters in GB/T 13540), and after arriving at the site, check the verticality of the cabinet (deviation ≤ 1.5‰).
- Joint commissioning: Conduct a withstand voltage test on the medium voltage cabinet (42kV/1min for a 10kV cabinet), and verify the action value of the relay protection; conduct an insulation resistance test on the low voltage cabinet (≥ 10MΩ), and test the automatic switching on and off of the capacitor compensation.
VII. Adaptability to Special Environments
1. **High Altitude Areas** (> 1000m)
- The medium voltage equipment needs to be used with reduced capacity (for every 100m increase in altitude, the insulation level decreases by 1%), and insulation enhancement components (such as thickened insulation partitions) need to be added inside the cabinet.
2. **High/Low Temperature Environments**
- In extremely high temperatures (> 50℃): The cabinet adopts a sunshade design, and the circuit breaker operating mechanism selects high-temperature resistant lubricant (temperature resistance ≥ 120℃).
- In low temperatures (< -25℃): Install an electric heating belt (maintain the temperature inside the cabinet ≥ -10℃), and select silicone rubber material for the sealing ring (low temperature resistance -40℃).
3. **Explosion-proof Environments** (such as petrochemical plants)
- It is necessary to select explosion-proof power distribution cabinets (in line with the GB 3836 explosion-proof standard). The cabinet adopts a flameproof structure, the cable entry device is an explosion-proof gland, and the internal components are explosion-proof (such as explosion-proof circuit breakers).
VIII. Documents and After-sales Guarantee
1. **Delivered Documents**
- Provide a full set of completion materials: Factory test reports, protection setting value sheets, wiring diagrams, communication configuration instructions, 3C certification and access permits.
- The medium voltage cabinet needs to be attached with a mechanical characteristic test report of the circuit breaker (opening and closing time, synchronism data), and the low voltage cabinet is attached with a busbar temperature rise test report (temperature rise ≤ 60K).
2. **After-sales Service**
- The warranty period ≥ 2 years, provide 7×24-hour remote technical support, and arrive at the site within 4 hours in case of major failures (8 hours in remote areas).
- Agree on the replacement cycle of vulnerable parts: For example, the energy storage motor of the medium voltage cabinet (5 years), the capacitor compensator of the low voltage cabinet (8 years), and the manufacturer provides lifetime spare parts supply (the price is not higher than 10% of the market price).
Summary
The customization of power distribution cabinets for small substations needs to balance safety, reliability, and intelligence, especially paying attention to the short-circuit breaking capacity of the medium voltage system, the compatibility of protection devices, and environmental adaptability. It is recommended to select manufacturers with experience in power engineering projects, communicate with the power grid company in advance about the access requirements, and ensure that the equipment meets the management requirements of modern substations with "unattended operation and minimal maintenance" after being put into operation.
