Choosing 10 kV switchgear: KYN28A-12 vs RMU vs gas-insulated

Selecting 10 kV switchgear fixes the layout, protection philosophy and maintenance regime of a substation for its service life. For the 6 kV and 10 kV networks in Uzbekistan the choice normally comes down to three families: withdrawable metal-clad cubicles (KYN28A-12), ring main units (XGN15-12 / HXGN15-12) and fully sealed gas-insulated switchgear (SRM16-12). This article sets out where each is used, what the main switch can and cannot do, and how the 10 kV side pairs with GCS or GGD low-voltage boards.

Three families, one voltage class

All three are metal-enclosed assemblies of the 12 kV class for 50 Hz systems, so the same equipment serves both 6 kV and 10 kV networks. They are designed and built to GB/T 3906 / IEC 62271-200, the standard that also defines internal arc classification; of the three, KYN28A-12 has a type-test report.

Where each type of 10 kV switchgear is used

KYN28A-12: main substations and industrial intake

The withdrawable KYN28A-12 is the standard choice for the 10 kV section of a main step-down substation, a switching station (RP) or a plant intake with many outgoing feeders. Ratings up to 4000 A and 31.5 kA cover incomers and bus sections that a 630 A ring main unit cannot.

• Each feeder has its own vacuum circuit breaker with relay protection, so a cable fault is cleared selectively without fuses.
• Trucks of the same purpose are interchangeable; voltage-transformer, metering and isolating trucks share the same frame as the breaker truck.
• Frequent operation is within the design intent, and a VC7 vacuum contactor truck is available for motor feeders.
• The breaker and cable compartments can be fitted with separate anti-condensation heaters, which matters in unheated switchrooms.

The cost is floor space: a KYN28A-12 line-up needs a switchroom with front access for racking and rear access to the cable compartment.

XGN15-12 and HXGN15-12: cable rings and box-type substations

A ring main unit is what goes into a box-type substation (KTP), a small secondary substation or a switching point on a 10 kV cable ring. The usual configuration is two incoming load-break switches (ring in, ring out) plus one or two transformer feeders with a switch-fuse combination. XGN15-12 uses an SF6 load-break switch inside an air-insulated cabinet with four isolated compartments (busbar, switch, cable, and mechanism with control) and a pressure-relief channel at the rear. HXGN15-12 is the unit-type variant of the same design (HX = box-type ring main cabinet, G = fixed, N = indoor) with the same 630 A / 20 kA ratings and an SF6 main switch. Loading above 630 A or a fault level above 20 kA rules both out, as does a feeder that needs relay-graded protection rather than fuses.

SRM16-12: compact rooms and harsh environments

SRM16-12 places the switching modules (vacuum circuit breaker, load-break switch, switch-fuse combination or isolator) inside a sealed SF6 tank with plug-in cable connections. The G version is fixed, the K version extends the busbar left or right through couplers, and the D version serves as a cable branch box. Because live parts never see ambient air, the datasheet permits -40 to +40 °C, altitude up to 4000 m (state the altitude in the enquiry if above 1000 m) and pollution level III to GB/T 5582: dusty sites, unheated cabins and mountain locations.

Vacuum circuit breaker or load-break switch?

The main switch defines what a panel can do during a fault:

• A vacuum circuit breaker interrupts full short-circuit current (up to 31.5 kA in KYN28A-12) on command from a protection relay. It is suited to frequent operation and needs no gas handling.
• A load-break switch switches rated load current (630 A, far above the roughly 92 A a 1600 kVA transformer draws at 10 kV) and has a rated short-circuit making capacity, so it can close onto a fault, but it cannot interrupt one. Fault clearance is left to the fuses on transformer feeders or to the upstream circuit breaker on ring cables.
• A switch-fuse combination is common practice for transformer feeders of roughly 1250–1600 kVA and below; the feeder current is limited by the fuse rating, not by the 630 A switch. This is a rule of thumb, not a fixed limit: for larger units, or where selective relay grading is required, specify a circuit-breaker feeder; SRM16-12 offers a vacuum circuit breaker module for this case within an RMU footprint.

Protection, interlocks and internal arc classification

Protection relays. KYN28A-12 cubicles are delivered with the relays, instrument transformers and surge arresters defined in the project: typically overcurrent and earth-fault (ANSI 50/51, 50N/51N) on feeders, undervoltage (27) and transformer gas or temperature trips on incomers, plus metering and a SCADA port (for example Modbus, IEC 60870-5-103/104 or IEC 61850). Ring main units usually rely on fuses, whose strikers trip the switch on all three phases, or carry a self-powered overcurrent relay where a circuit-breaker module is used on the transformer feeder.

Interlocks. The "five prevention" mechanical interlocks are standard on KYN28A-12 and on the XGN15-12 family:

1. Prevent mis-opening and mis-closing of the circuit breaker.
2. Prevent racking the breaker truck in or out under load.
3. Prevent closing the circuit breaker while the earthing switch is closed.
4. Prevent closing the earthing switch on a live circuit.
5. Prevent entry into a live compartment.

Internal arc classification. IEC 62271-200 classifies an assembly for internal arc withstand as IAC with accessibility type A (authorised personnel only) or B (unrestricted access) and the tested sides F (front), L (lateral) and R (rear). The KYN28A-12 type-test report is listed on the certificates page; confirm the internal arc classification (for example IAC AFLR) of the configuration actually offered and request its test report with the quotation. Your specification should state the arc current and duration required for the system fault level (for example 20 kA or 31.5 kA for 1 s). Also specify the loss-of-service-continuity category and the partition class (PM, metallic, or PI, insulating). In GB/T 3906 and IEC 62271-200:2011 terms, LSC2B is typical for withdrawable metal-clad switchgear and LSC2A for ring main units (the 2021 edition of IEC 62271-200 merges both into LSC2); the category defines which compartments can be opened while the rest of the switchgear stays live.

Pairing with GCS and GGD low-voltage boards

The 10 kV panel and the 0.4 kV board are sized from the same transformer. For a 10/0.4 kV unit the LV full-load current is S / (√3 × 0.4 kV):

• 630 kVA → 909 A → 1000 A incomer
• 1000 kVA → 1443 A → 1600 A incomer
• 1600 kVA → 2309 A → 2500 A incomer
• 2500 kVA → 3608 A → 4000 A incomer

GCS withdrawable switchgear covers 400 V systems up to 4000 A at IP40, with isolated functional-unit, busbar and cable compartments and drawers exchanged without de-energising the board: the partner for a main LV board or motor control centre behind a KYN28A-12 intake. GGD fixed-type cabinets (380 V, up to 3150 A, IP30) are the economical option for the LV side of a box-type substation or a board with few feeders. Both are built to GB/T 7251 / IEC 61439; Jiangsu Xinhong's GGD cabinets hold a CCC certificate covering main busbars of 400–1600 A.

Two coordination points matter at the interface:

• LV short-circuit level. Assuming a transformer impedance voltage of 6 % and neglecting source and cable impedance (an assumption; confirm on the transformer datasheet, since 10/0.4 kV units of this size are built with 4–6 %), a 2500 kVA unit gives roughly 60 kA at the 0.4 kV terminals, and a lower impedance gives more. The LV busbars must be rated accordingly; check the rated short-time withstand current (Icw) of the offered board against this value (Jiangsu Xinhong's CCC certificates give main-busbar Icw values of 30 kA for GGD and 80 kA for XMCS-K).
• Protection grading. The 10 kV transformer feeder (fuse or relay) must grade with the LV incoming air circuit breaker so that an LV fault is cleared by the ACB, with the 10 kV protection as back-up only.

Checklist before ordering 10 kV switchgear

Before requesting an offer, fix the following:

• Position in the network: main substation or switching station (KYN28A-12), cable ring or box-type substation (XGN15-12 / HXGN15-12), compact or harsh site (SRM16-12).
• Busbar and feeder currents: above 630 A only KYN28A-12 applies.
• System fault level at the busbar: 20 kA (RMU, SRM16-12) or up to 31.5 kA (KYN28A-12).
• Main switch per feeder: circuit breaker with relay, or load-break switch with fuses.
• Ambient temperature, altitude above 1000 m, dust and humidity at the site.
• Internal arc rating and accessibility (IAC AFLR), LSC category and partition class.
• Relay functions, metering, CT/VT ratios and communication protocol.
• LV board type (GCS or GGD), incomer rating and short-circuit withstand.

LALOGO supplies the transformer, the 10 kV switchgear and the 0.4 kV board from one manufacturer, configured to your single-line diagram. Send us the diagram, load list and site conditions and we will return a technical proposal with a quotation: request a quotation.