Detailed Explanation of 10kV High Voltage Switchgear: Differences and Functions Between Infeed Cabinets and Outfeed Cabinets
Answer in one sentence: The incoming cabinet is the "main switch", and the outgoing cabinet is the "branch switch"; One tube is for the whole dish, and the other tube is for the bowl end.
By understanding this point, all subsequent selection, cost, operation and maintenance, and upgrades will not fall into pitfalls.
——01——
Current direction: "Total" first, then "Separate", the order cannot be reversed
Imagine a home distribution box: the external power grid is first connected to the main switch, and then distributed from the main switch to the kitchen, living room, and air conditioning. Similarly, for a 10 kV system, the busbar is the 'busbar', with the incoming cabinet at the busbar head and the outgoing cabinet at the busbar tail.
Once the sequence is switched, the protection settings will be completely disordered, and in the event of a fault, either refuse to operate or jump beyond the level, resulting in a "one pot end" of the entire busbar.
——02——
Parameter difference of one level, price difference of 100000
The rated current of the incoming cabinet generally starts at 3150 A, and the starting price for short-circuit breaking is 31.5 kA; The common types of outgoing cabinets are 630 A and 1250 A, and they can pass the test by disconnecting 25 kA.
For every one level increase in current, the cross-sectional area of the copper bar, the thickness of the silver plating layer on the contacts, and the size of the arc extinguishing chamber of the Circuit Breaker increase exponentially.
Same brand and series, a single incoming cabinet is often 30% to 50% more expensive than an outgoing cabinet. If the incoming cabinet is reported as a "high current outgoing cabinet" during bidding, the budget will instantly explode.
——03——
Protection configuration: The incoming cabinet should "take into account the overall situation", and the outgoing cabinet should only "self sweep the snow in front of the door"
The protection setting of the incoming cabinet must be in line with the main transformer differential, bus differential, and backup automatic switching. The action time of one line difference of 0.1 seconds may exceed the level.
The outgoing cabinet only focuses on its own circuit, and the "three piece set" of overcurrent and grounding can be used for work; Occasionally adding zero sequence protection is also a simple and straightforward way of saying 'whoever's child will take them away'.
90% of the offside trips on the operation and maintenance site are caused by artificially amplifying the fixed value of the incoming cabinet, fearing "misoperation", resulting in "no movement" when a real fault occurs.
——04——
After 2024, cabinets must have 'long eyes'
The 2023 version of State Grid's "Technical Specification for Intelligent Switchgear" includes "online monitoring" as a mandatory clause:
-Incoming cabinet - busbar temperature rise, mechanical characteristics of circuit breakers, SF density, all three online monitoring items must be included;
-Outgoing cabinets - classified according to importance, only mandatory for A+areas (hospitals, subways, data centers), and can be installed slowly in ordinary factories.
One sentence: In the future, cabinets without sensors will not even qualify for bidding.
——05——
Environmental red line: SF probation for three years
The greenhouse effect of SF is 23500 times that of CO ₂, and it will be completely banned by the European Union in 2026. The pace in China is slightly slower, but from 2025 onwards, the proportion of environmentally friendly new projects will not be less than 30%.
There are two alternative options:
1. Dry air insulation - the volume ratio is 15% larger than SF ₆, and the replacement of old distribution rooms requires measuring the height;
2. Vacuum arc extinguishing+solid insulation -20% more expensive, but maintenance free, suitable for high salt spray and high altitude.
We still have inventory of SF containers in our hands. Let's sell them early and get back the blood while someone is taking over the order now.
——06——
Digital Twin: Moving Cabinets into the 'Metaverse'
The Shenzhen pilot project of Southern Power Grid has implemented a 1:1 three-dimensional mapping of the entire station's 10 kV switchgear, with real-time synchronization of temperature, current, and opening and closing stroke to the cloud.
The benefits are visible to the naked eye:
-Simulate in the "metaverse" before power outage maintenance, reducing the probability of misoperation by 70%;
-The lifespan of contacts and the wear trend of circuit breakers are calculated using algorithms, and a "courier" reminder is received two weeks before the actual fault occurs;
-The maintenance cycle has been extended from 5 years to 8 years, and the power outage window saved is like real gold and silver.
Digital twin is not just a 'flower job', but will be included in the 'hard terms' of the power supply plan for A+users in the Greater Bay Area by 2025.
——07——
Graphene contact: lifespan+40%, but don't rush to place an order
The laboratory data in 2023 is indeed impressive, with an increase in electrical lifespan from 20000 to 28000 cycles and a temperature drop of 8 K.
At present, only two companies have received type test reports, and their production capacity has just increased. The unit price is three times that of ordinary contacts.
Suggestion:
Key nodes (main transformer incoming line, high load outgoing line) should be piloted first, instead of replacing the entire station at once. Batch updates can be carried out when the scale reaches 2025, which can reduce costs by half.
——08——
Selection and Avoidance Guide: Take One Table with You
| Scenario | Key points for selecting incoming cabinets | Key points for selecting outgoing cabinets |
|---|---|---|
| Capacity expansion of the old station | It is necessary to verify the short-circuit capacity of the busbar. If 3150 A is not enough, then 4000 A should be added. Don't worry about money | The original cable should not be replaced, and the current should not exceed the original carrying capacity. No matter how large the cabinet is, it is useless |
| New site acquisition | Directly choose environmentally friendly+online monitoring, once in place, saving the need for secondary renovation and power outages | Select based on a load rate of 70%, leaving a 30% margin, to provide "advance quantity" for future photovoltaic and charging piles |
| High altitude | SF ₆ low pressure, using dry air or solid insulation, with a capacity reduction factor of 0.9 above 3000 meters | vacuum arc extinguishing priority, small outlet cabinet volume, easy transportation |
| Salt spray corrosion | Copper bar silver plating layer thickened to 8 μ m, shell made of 304 stainless steel | Contact silver plating+epoxy sealing, salt spray test starting from 1000 hours |
——09——
Operation and maintenance "three piece set": temperature measurement, partial discharge, mechanical characteristics
- Infrared temperature measurement is conducted twice a year, once before the peak load in summer and once before the peak heating period in winter;
- Partial discharge testing with dual channels of ultrasonic and ground wave. If the data is greater than 20 dB, locate it. Don't wait for "firing";
- Mechanical characteristic testing should not only measure the "opening and closing time", as the travel curve contains the "secret language" of spring fatigue and lubrication drying up.
Make these three items into QR codes, and you can scan your phone to see the historical curve. You don't have to "beat your head" during the next maintenance.
——10——
Conclusion: Treat the cabinet like a person
The incoming cabinet is the 'big brother', carrying the maximum current and providing the strictest protection; The outgoing cabinets are 'little brothers', with a large quantity and light responsibility, but even if something really happens, they cannot escape.
After 2024, the distribution room should have both the "big brother" strong and robust physique, as well as the "little brother" sharp ears and eyes.
Next time standing in front of the switchgear, don't just look at the nameplate parameters, ask yourself three questions first:
- What will happen if the current takes one wrong step?
- Which is more terrifying, protecting against misoperation or refusing to move?
- Will the money for environmental protection and intelligence double tomorrow if we don't spend it today?
By understanding these three questions, the incoming and outgoing cabinets are no longer "iron boxes", but rather "gatekeepers of the power grid" who can speak.