Products Description
Off-Circuit Three-Phase Linear Tap Changer (10kV–35kV)
An off-circuit three-phase linear tap changer is a de-energized (off-load) regulating switch that adjusts the transformer turns ratio to hold the output voltage steady across the full tapping range. Because the tap is changed only while the transformer is isolated from the supply, this linear-type tap changer - also called an off-load tap changer (OLTC-free, de-energized tap changer, DETC) - delivers a simple, highly reliable, maintenance-light solution for oil-immersed distribution and power transformers rated up to 35kV (maximum equipment voltage 40.5kV).
Designed for three-phase transformers, our linear tap changer is mounted directly on the transformer tank cover or tank wall and operated without excitation. With 3 to 9 tapping positions and a uniform 2.5% regulation per position, it covers the standard ±5% and extended tapping ranges used in distribution networks, industrial transformers, and renewable-energy step-up transformers.
Rated voltage: 10kV / 20kV / 35kV (≤40.5kV max equipment voltage) Rated current: 63A / 125A / 250A / 300A (wider range available on request up to 500A) Tapping positions: 3, 4, 5, 6, 7, 9 Regulation: 2.5% per position Insulating medium: mineral oil or silicone oil
parameters
Key Features of the Off-Circuit Linear Tap Changer
De-energized (off-circuit) operation - tap change is performed only with the transformer isolated, eliminating arc-quenching contacts and dramatically extending service life.
Three-phase, bar-type (linear) construction - compact, single-shaft linear layout that fixes directly to the transformer tank cover or wall.
Full 10kV–35kV coverage - one product family for the most common distribution and sub-transmission voltage classes (max equipment voltage 40.5kV).
3 to 9 tapping positions - supports symmetrical and asymmetrical tap schemes; 2.5% regulation per step.
63A / 125A / 250A / 300A current ratings - matched to distribution transformer ratings from small pole-mounted units to large oil-immersed power transformers.
Oil-immersed contact system - designed for mineral oil or silicone oil, ensuring stable contact resistance and low temperature rise.
Direct tank-cover or wall mounting - simple M8×30 bolted installation with standard φ43 cover hole; no separate driving mechanism required.
Maintenance-light architecture - few moving parts, no arcing contacts, ideal for long unattended service life.
Technical Specifications
2.1 General Specification (WST family)
| Parameter | Specification |
|---|---|
| Product type | Off-circuit / off-load, three-phase, linear (bar-type) tap changer |
| Rated voltage | 10 kV, 20 kV, 35 kV (max equipment voltage 40.5 kV) |
| Rated current | 63 A, 125 A, 250 A, 300 A |
| Tapping positions | 3 / 4 / 5 / 6 / 7 / 9 |
| Regulation per position | 2.5 % |
| Rated frequency | 50 Hz / 60 Hz |
| Insulating medium | Mineral oil or silicone oil |
| Mounting | Transformer tank cover or tank wall (fixed, off-axis) |
| Operating condition | De-energized only (off-circuit) |
| Thread interface (Md) | M6 (63A) / M8 (125A) / M10 (250A) / M12 (300A) |
| Cover hole diameter | φ43 |
Series I - Terminal-Point Regulating (WST I)
Regulating position: terminal point (end of bar) | Tapping positions: 3 / 5 / 7 | Regulation: 2.5% per position
| Rated current | Rated voltage | Model (representative) | Positions |
|---|---|---|---|
| 63 A | 10 kV | WST I 63/10-3×3 | 3 |
| 125 A | 10 kV | WST I 125/10-3×3 | 3 |
| 250 A | 10 kV | WST I 250/10-3×3 | 3 |
| 63 A | 35 kV | WST I 63/35-3×3 | 3 |
| 125 A | 35 kV | WST I 125/35-3×3 | 3 |
| 250 A | 35 kV | WST I 250/35-3×3 | 3 |
| 63 A | 10 kV | WST I 63/10-5×5 | 5 |
| 125 A | 10 kV | WST I 125/10-5×5 | 5 |
| 250 A | 10 kV | WST I 250/10-5×5Z | 5 |
| 63 A | 35 kV | WST I 63/35-5×5 | 5 |
| 125 A | 35 kV | WST I 125/35-5×5 | 5 |
| 250 A | 35 kV | WST I 250/35-5×5Z | 5 |
| 63 A | 10 kV | WST I 63/10-7×7 | 7 |
Series II - Central Regulating (WST II)
Regulating position: central (middle of bar) | Tapping positions: 3–9 | Regulation: 2.5% per position
| Rated current | Rated voltage | Model (representative) | Positions |
|---|---|---|---|
| 63 A | 10 kV | WST II 63/10-4×3A | 3–4 |
| 125 A | 10 kV | WST II 125/10-4×3A | 3–4 |
| 250 A | 10 kV | WST II 250/10-4×3A | 3–4 |
| 63 A | 10 kV | WST II 63/10-6×5A | 5–6 |
| 125 A | 10 kV | WST II 125/10-6×5A | 5–6 |
| 250 A | 10 kV | WST II 250/10-6×5ZA | 5–6 |
| 63 A | 20 kV | WST II 63/20-4×3A | 3–4 |
| 125 A | 20 kV | WST II 125/20-4×3A | 3–4 |
| 250 A | 20 kV | WST II 250/20-4×3A | 3–4 |
| 63 A | 20 kV | WST II 63/20-6×5A | 5–6 |
| 125 A | 20 kV | WST II 125/20-6×5A | 5–6 |
| 250 A | 20 kV | WST II 250/20-6×5ZA | 5–6 |
| 63 A | 35 kV | WST II 63/35-4×3A | 3–4 |
| 125 A | 35 kV | WST II 125/35-4×3A | 3–4 |
| 250 A | 35 kV | WST II 250/35-4×3A | 3–4 |
| 63 A | 35 kV | WST II 63/35-6×5A | 5–6 |
| 125 A | 35 kV | WST II 125/35-6×5A | 5–6 |
| 250 A | 35 kV | WST II 250/35-6×5ZA | 5–6 |
Product display
Bar type off-load tap changer with G11 fiberglass epoxy board for oil immersed power transformer
Dark phenolic bar type tap changer off-load voltage tap switch for distribution transformer
Factory finished off-load tap changer with position indicator for 10kV power transformer
Mass production off-circuit tap changer transformer voltage tap switch bulk supply from manufacturer
Round type off-circuit tap changer high voltage transformer voltage regulating switch assembly
Transformer tap changer installed on winding core for oil immersed transformer voltage adjustment
How to Select the Right Off-Circuit Tap Changer
Voltage class - match the tap changer to the transformer's highest system voltage: 10kV, 20kV, or 35kV (confirm max equipment voltage ≤40.5kV).
Current rating - size the tap changer to the transformer's rated current (63A / 125A / 250A / 300A). Contact thread Md follows the current rating (M6 / M8 / M10 / M12).
Regulating position - choose terminal-point (WST I) for compact end-of-bar tap schemes, or central (WST II) where the tapping zone should sit at the middle of the winding.
Tapping positions & range - select 3 to 9 positions with 2.5% per step to match the required voltage-regulation band (±5%, ±7.5%, etc.).
Direction of operation - specify positive or reverse operating direction (WST II is available in both).
Insulating liquid - confirm mineral oil or silicone oil compatibility.
Mounting interface - verify cover-hole φ43 and bolt pattern (M8×30 bolt, φ8 flat washer, φ8 spring washer, M8 nut - normally supplied by the transformer factory).
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Application
Applications
Off-circuit three-phase linear tap changers are widely used where reliable, low-maintenance voltage regulation is required during planned de-energized tap changes:
Oil-immersed distribution transformers (pole-mounted and pad-mounted) in 10kV and 20kV networks
Oil-immersed power transformers up to 35kV for industrial and utility substations
Renewable-energy step-up transformers (solar and wind farms) requiring seasonal tap adjustment
Industrial transformers in steel, cement, chemical, and mining plants
Pre-commissioning and maintenance tap setting on new and retrofitted transformers
Installation & Operation Notes
Mount directly on the transformer tank cover or tank wall in the selected regulating position (terminal for Series I, central for Series II).
Off-circuit only - the tap must be changed only when the transformer is de-energized and isolated. Never operate under load.
Cover hole φ43; fasten with M8×30 bolt, φ8 flat gasket, φ8 spring washer, and M8 nut (one set each, normally provided by the transformer factory).
Verify contact alignment at each tapping position before energizing, and confirm the regulating position matches the nameplate tapping table.
Oil compatibility - ensure the contact system is immersed in the specified mineral oil or silicone oil.
FAQ
What is the difference between an off-circuit tap changer and an on-load tap changer (OLTC)? An off-circuit (off-load) tap changer changes the tapping only when the transformer is de-energized; it has no arcing contacts. An OLTC changes taps while energized. Off-circuit tap changers are simpler, more compact, and lower-maintenance.
Which current and voltage ratings are available? Standard three-phase ratings are 63A, 125A, 250A, and 300A at 10kV, 20kV, and 35kV (max equipment voltage 40.5kV). A wider range up to 500A is available for certain configurations.
How many tapping positions can I get? From 3 up to 9 positions, with 2.5% regulation per position. Series I covers 3–7 positions (terminal regulating); Series II covers 3–9 positions (central regulating).
Is it compatible with my oil-immersed transformer? Yes - the tap changer is designed for oil-immersed transformers using mineral oil or silicone oil, and mounts directly to the tank cover or wall with a standard φ43 cover hole.
certificate
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