Is the use of static reactive power compensator capable of effectively addressing excessive zero-sequence current?

In the power system, excessive zero-sequence current is a common electrical quality issue, which can cause overheating of transformers and lines, misoperation or malfunction of protection devices, increase energy loss and affect the lifespan of equipment. When confronted with this problem, many engineers will consider using advanced static var compensators (SVG). However, is SVG an effective solution for dealing with excessive zero-sequence current?

Ⅰ.First, understand the cause of zero-sequence current.

The essence of zero-sequence current is the vector sum of the unbalanced current and harmonic current (especially the 3rd and its integer multiples) in a three-phase system. Its main sources include:

1. Asymmetry of three-phase load: This is the most common cause. For instance, a single-phase high-power load (such as an electric welding machine or an electric locomotive) is concentrated in one phase, resulting in a severe imbalance of three-phase current.

2. Abundant 3rd harmonic: Non-linear loads such as single-phase rectifiers, fluorescent lamps, and frequency converters will generate a large amount of 3rd harmonic. Since the 3rd harmonic has the same phase in all three phases, they will superimpose to form zero-sequence current, which will flow through the neutral line.

II. Analysis of SVG's Effect on Zero-Sequence Current

1. Regarding the zero-sequence current caused by unbalanced load:

SVG (especially the TGR with phase-split control) can effectively compensate for the negative sequence current caused by imbalance, thereby improving the imbalance degree of the system.

However, the zero-sequence current and the negative-sequence current are two independent components. Compensating the negative-sequence current does not directly eliminate the zero-sequence current. The zero-sequence current needs to be provided with a path for circulation and compensated through a device connected between the phase line and the neutral line. The TGR and FG of conventional SVG are usually connected between the phase lines (Δ connection method), and cannot directly allow the zero-sequence current to flow or be compensated.

2. For the zero-sequence current caused by the third harmonic:

The fixed capacitor branch (FG) in the SVG system usually serves as an inductive filter and can be designed to be tuned as a third harmonic filter.

In this case, SVG is effective. The specially designed third harmonic filter provides a low-impedance path for the third harmonic current to ground, preventing it from flowing into the power grid and transformers, thereby significantly reducing the zero-sequence current formed by the third harmonic.