1、 Ultra high insulation strength and electric field control

Strict insulation level requirements

Ultra high voltage transformers need to withstand extremely high power frequency, lightning, and operating overvoltage. Taking a 1000kV AC transformer as an example:

Power frequency withstand voltage: 1100 kV (5 minutes)

Lightning impulse withstand voltage: 2250 kV (full wave)/2400 kV (truncated wave)

Operating impulse withstand voltage: 1800 kV

In contrast, the lightning impulse withstand voltage of 500kV transformers is only 1550kV, the power frequency withstand voltage is only 680kV, and the insulation strength of ultra-high voltage is increased by about 45%.

Electric field optimization design

Permissible field strength selection: Based on the electric field strength with a partial discharge probability of ≤ 1 ‰ as the design criterion, to avoid excessive insulation margin (increasing volume) or insufficient insulation margin (causing breakdown).

Large oil gap segmentation technology: The arc-shaped conformal partition is used to divide the large oil gap between the body and the oil tank into multiple layers of small oil gaps, preventing oil gap breakdown and field saturation, and improving insulation reliability.

2、 Challenges in Ultra Large Capacity and Structural Design

Large capacity and leakage control

The single unit capacity reaches 1500MVA (three column structure, single column 500MVA), and the magnetic flux density distribution is complex. Multiple shielding measures are required to reduce magnetic leakage:

Magnetic shielding: A magnetic shunt is installed at the end of the device to guide the leakage magnetic flux to form a circuit and reduce clamping losses.

Electrical shielding: Copper shielding is installed on the inner wall of the fuel tank, which uses eddy currents to counteract magnetic leakage and prevent local overheating (fuel tank temperature rise ≤ 20.9K).

The iron core adopts a single-phase five column structure (such as a 1000kV transformer), which disperses the magnetic flux density and avoids single column overheating.

Solution to transportation restrictions

Disassembly design: The transformer is divided into a main body (weighing 397 tons) and a voltage regulating and compensating transformer (weighing 79 tons), which are transported separately and assembled on site, breaking through the weight limit of single piece transportation.

Compact layout: Optimize the fuel tank structure (cylindrical+plate reinforced iron) and control the external dimensions.

3、 Heat dissipation and loss control

Efficient cooling system

The main body adopts forced oil circulation air cooling (OFAF), with S-shaped oil channels designed inside the wire cake to evenly distribute oil flow and control hot spot temperature rise (winding ≤ 65K, hot spot ≤ 78K).

The pressure compensation transformer adopts natural oil cooling (ONAN) to reduce auxiliary energy consumption.

Low loss materials and processes

The iron core adopts high magnetic conductivity silicon steel sheets, and the coil adopts self-adhesive transposition wires to reduce eddy current losses. The no-load loss is ≤ 200kW, the load loss is ≤ 1490kW, and the efficiency reaches 99.83%.

The fuel tank uses non-magnetic steel plates to avoid leakage of magnetic heat in high current areas.