How Can High-voltage Series Reactors Solve The Harmonic Pollution Problem in Rail Transit Power Supply Systems?

In rail transit power supply systems, high voltage series reactor and capacitor banks bank form a series of filters to effectively solve harmonic pollution problem. The core mechanism and implementation elements are as follows:
I. Sources and hazards of harmonic pollution
Harmonics in rail transit power supply systems are mainly derived from rectifier units (such as the 24-pulse rectifier in traction substations). Although multi-pulse technique can reduce low harmonicorder, in practice there will still be 5, 7 and 11 characteristic harmonics, as well as non-characteristic harmonics caused by the asymmetry of the power grid. Harmonics can cause:
Capacitor overheating and damage: harmonic current increases capacitor losses and accelerates insulation aging.
Harmonic amplification and resonance: Capacitors and grid inductive reactance may form resonant circuits that amplify specific harmonics and threaten equipment safety.
Relay protection maloperation: Harmonic interference may lead to differential protection, distance protection, etc., and affect system stability.
ii. Working Principle of High-Voltage Series Reactors
High voltage series reactor suppresses harmonics in the following ways:
Change the impedance characteristic of the system: when in series with a capacitor, the LC filter branch is formed. Its impedance is induced at a specific harmonic frequency, preventing harmonic current from flowing into the capacitor bank.
For example, for 5 harmonics (250Hz), the the capacitor branch is inductive to 5 harmonics when the resistivity is selected at 6% to avoid resonance.
Inrush Current: When the capacitor is connected to a power supply, the inductance of the reactor inhibits the instantaneous inrush current, protecting the switching device and the capacitor itself.
Frequency Division Filtering: by adjusting the reactance rate (e.g., 4.5%, 6%, 12%), key harmonics such as three and five phases can be filtered out. For example:
6% reactance rate: Effective suppression of 5 harmonics while preventing 3 harmonics amplification.
12% reactance rate: Suitable for severe three-phase harmonic contamination, such as substations with large single-phase nonlinear loads.
III. Typical applications of rail transit
Take, for example, a metro 110kV substation:
System Background: Two new 2400kvar capacitor banks have been installed with a series reactors (144kvar) with a 6% reactivity rate.
After debugging, the total voltage distortion rate of the 10kV busbar reached 4.33% (4% above the limit), of which the third harmonic distortion rate was 3.77%% (3.2% above the limit).
Problem analysis: The original reactance rate selection did not take fully into account the system's harmonic background, resulting in three harmonic amplification.
Calculations show that the capacitor's capacity is close to 5.1% of the busbar's short-circuit capacity under 6% reactance, and it is easy to trigger parallel resonance.
Optimization Measures: the reactance rate adjusted to 12% and the harmonic impedance recalculated:
The third harmonic voltage amplification rate is reduced to 0.50, which meets the standard.
This avoids the risk of resonance and limits the excitation inrush.
IV. INTRODUCTION Technical Parameters and Selection Principles
Reactance Ratio Selection:
3rd Harmonic Predominant: Select a 12% reactance ratio to enable the capacitor branch to sense the third harmonic.
5th Harmonic Predominant: Select a 6% reactance ratio to balance filtration effectiveness and cost.
General Scenarios: A 4.5% reactance ratio can handle harmonic suppression of magnitude 5 and above.
Rated Voltage and capacity:
The rated voltage must match the system voltage level (e.g. 10kV, 35kV).
Rated capacity is calculated by multiplying the capacitance capacity by the reactance ratio (e.g. 144 kW = 2,400 kW x 6%).
Structure and materials: Epoxy-reinforced coils and imported silicon steel core ensure low temperature rise and low noise.
Silicone rubber vibration rubber damping pad is suitable for compact space of rail transit.
V. Implementation results and industry practices
Harmonic Mitigation Effectiveness: After adjusting the reactance rate, the third harmonic distortion rate subway substation decreased from 3.77% to 2.8% and the total distortion rate decreased to 3.9%, reaching the GB/T 14549-1993 standard.
Capacitor failure rate reduced by 80%, extending the service life of equipment.
Industry Trends: Dynamic compensation combined with Active power filters (APF) solves the challenge of increasingly complex harmonic spectrum.
Intelligent monitoring system can track harmonic change in real time and optimize reactor parameter adjustment strategies.