2025-07-22
Manufacturing a 2500kVA electrical transformer (typically used for distribution or small power applications) is a complex, multi-stage process requiring precision engineering, specialized materials, and rigorous testing.Those in need can contact our company.
Material Selection: Grain-oriented silicon steel (GOES) is chosen for its superior magnetic properties and low core loss. It comes in large rolls.
Cutting: The steel is precisely cut into thin laminations (typically 0.23mm to 0.35mm thick) using specialized cutting machines (stamping, laser, or EDM). Shapes are usually E-I, L-L, or step-lap configurations.
Surface Treatment: Laminations are often coated with an insulating layer (C3, C5, C6 core plate) to minimize eddy currents between them.
Stacking & Assembly: Laminations are carefully stacked, layer by layer, alternating the joints to create a continuous, low-reluctance magnetic circuit. The stacking pattern minimizes air gaps and core losses. Large hydraulic presses clamp the stack tightly.
Clamping: Robust top and bottom frames, clamping bolts, and pressure plates are installed to hold the core under high pressure, preventing vibration and noise.
Material Selection: High-conductivity copper or aluminum is chosen for windings. Copper is more common for efficiency at this size, but aluminum is sometimes used for cost.
Conductor Preparation: Round or rectangular (flat) wire is insulated with paper (Kraft, Nomex), enamel, or DDP (Double Dacron Paper). For larger conductors, transposed cable (CTC - Continuously Transposed Conductor) might be used to minimize eddy current losses.
Winding: Conductors are wound onto temporary mandrels using sophisticated CNC winding machines:
Low Voltage (LV) Winding: Usually wound first, closest to the core. Often a simple helical or layer winding due to higher currents and fewer turns.
High Voltage (HV) Winding: Wound over the LV winding (with insulation barriers), usually with more turns and finer wire. Often a disc or layer winding.
Insulation: Critical layers of insulation (pressboard cylinders, angle rings, end blocks, spacers) are placed between windings, between turns, and at ends to withstand electrical, thermal, and mechanical stresses. Oil ducts are created for cooling.
Drying: Windings are dried in large ovens under vacuum to remove moisture before assembly.
Opening the Core: The top yoke laminations of the core are carefully removed.
Lowering the Coil Assembly: The pre-assembled HV and LV windings (together with their insulation structures) are lowered precisely onto the core's bottom yoke and limbs.
Reclosing the Core: The top yoke laminations are meticulously reinserted and the core is re-clamped.
Lead Connections: HV and LV leads (made of copper bar or flexible cable) are connected to the windings and brought out to temporary connection points.
For Oil-Immersed Transformers - Most Common for 2500kVA
Tank Fabrication: A robust steel tank is fabricated (welded), designed to hold oil and withstand pressure/vacuum. It includes lifting lugs, drain valve, sampling valve, and mounting points.
Core-Coil Placement: The assembled core and coils are lowered into the tank.
Internal Fittings: Radiators/fins, conservator tank (expansion vessel), Buchholz relay (gas/oil flow detector), pressure relief device, and tap changer mechanism (if OLTC or DETC) are installed and connected.
Sealing: The main cover is bolted onto the tank with a heavy-duty gasket.
Vacuum Drying: The entire tank is evacuated to a high vacuum for an extended period (often 24-72 hours) to remove moisture from the core, windings, and insulation.
Oil Filling: While still under vacuum, highly purified, dehydrated, and degassed transformer oil is slowly introduced into the tank. Vacuum is maintained afterward to ensure thorough oil impregnation of the insulation.
Final Sealing: Once filled, all valves are closed, and the conservator is filled to the correct level.
Bushings: HV and LV porcelain or composite bushings are installed on the tank cover, providing the insulated connection points for external cables.
Cooling: Radiators/fins are connected. Cooling fans (if forced air cooling - FA/FOA) and pumps (if forced oil - FOA/ODAF) are mounted and wired.
Control Cabinet: A weatherproof cabinet housing protection relays (Buchholz, temperature, pressure), controls (tap changer control), monitoring devices (oil level, winding temp gauges), and terminal blocks is installed and wired.
Tap Changer: The external drive mechanism and controls for the On-Load Tap Changer (OLTC) or De-Energized Tap Changer (DETC) are installed and connected.
Grounding: Main tank grounding pads and core/coil ground connections are finalized.
Rigorous testing ensures safety, performance, and longevity:
Routine Tests (Mandatory per Standards - IEC 60076 / IEEE C57.12.00):
Winding Resistance Measurement (DC)
Voltage Ratio & Vector Group Verification
Impedance Voltage & Load Loss Measurement
No-Load Loss & Current Measurement
Insulation Resistance & Polarization Index (PI) Test (Megger)
Dielectric Tests: Separate Source AC Voltage Withstand Test (HV & LV), Induced AC Voltage Test (PD measurement often included)
Special Tests (Often Performed or Customer Specified):
Temperature Rise Test (Verifies cooling design)
Lightning Impulse Tests (BIL - Basic Impulse Level)
Short-Circuit Withstand Test (Simulates fault conditions - often type tested on a prototype)
Dissolved Gas Analysis (DGA - Baseline)
Sound Level Measurement
Final Checks: Visual inspection, oil sampling (DGA, dielectric strength, moisture), control wiring check, functional tests of cooling, alarms, and tap changer.
Surface Preparation: Tank and accessories are cleaned (sandblasting often used).
Painting: Multiple coats of high-quality, corrosion-resistant paint are applied.
Final Assembly: Any remaining external parts are installed.
Packaging & Shipping: Secured for transport (often by heavy truck), with lifting instructions and shock recorders sometimes attached.
Size & Weight: Significant (several tons), requiring heavy lifting equipment throughout manufacturing.
Cooling: Usually ONAN (Oil Natural Air Natural) or ONAF (Oil Natural Air Forced) for this size. Cooling design is critical.
Customization: HV/LV voltages, vector group (e.g., Dyn11), impedance, tap range, accessories (OLTC vs DETC), and enclosure type are often customer-specified.
Quality Focus: Precision in core stacking, winding tension, insulation placement, drying, oil processing, and testing is paramount for efficiency, reliability, and long life (often 25-40 years).
