I. Product Overview

With the construction and development of converter stations and DC power grids, the phenomenon of DC magnetic bias in the neutral points of AC system transformers is becoming increasingly common. When a DC transmission system operates in single-pole-to-earth return mode, the earth effectively acts as one of the conductors in the DC transmission line, carrying a current equal to the operating current of the DC transmission system. This leads to significant changes in the surface potential near the converter station, inducing a DC component in the neutral points of nearby transformers and thereby causing DC magnetic bias in the transformers. Such bias can result in transformer magnetic saturation, seriously affecting the stable operation of both the transformers and the AC system.

By installing a DC bias suppression device at the neutral point of the transformer, which is designed to block and suppress DC bias currents, the problem of DC bias in transformers can be effectively addressed. This prevents the adverse effects caused by biased operation of the transformer, ensuring its stable performance. At the same time, this device has no impact whatsoever on the operation of the AC system, thereby resolving the issue of interference with the AC system when the DC system operates via a single-grounded electrode in hybrid AC-DC power supply systems.

II. Scope of Product Application

When a DC current flows into the transformer windings via the neutral point of the transformer, it generates severe harmonics in the AC system and triggers phenomena such as increased transformer vibration and noise—known as DC bias. These effects seriously compromise the stable operation of the power grid. In cases where the DC bias in the transformer is relatively small, a solution involving the series connection of a current-limiting resistor at the transformer’s neutral point can be adopted to isolate the DC current flowing through the neutral point. Additionally, by paralleling a graphite gap with the current-limiting resistor, overvoltage at the transformer’s neutral point caused by three-phase unbalance faults can be effectively suppressed.

III. Product Features

1. The current-limiting resistor used in the product is made of either a stainless steel nickel-chromium alloy (Cr20Ni80) or a non-metallic material (carbon valve片, zinc oxide valve片). These materials feature high electrical conductivity, a small temperature coefficient, a high melting point, strong corrosion resistance, excellent toughness that resists deformation, and good resistance value stability.

2. The enclosure is typically made from stainless steel sheet (or cold-rolled steel sheet), featuring a clean and elegant appearance, high protection rating, excellent corrosion resistance, and a reliable structure.

3. The product has strong instantaneous resistance to high-current surges and low temperature rise.

4. The enclosure features a compact design, small size, excellent heat dissipation performance, and high insulation level.

5. The resistance value of the product’s current-limiting resistor can be adjusted in steps according to on-site conditions, offering flexibility and convenience.

6. The product can be customized according to customer requirements and has wide applicability.