How To Select The Reactance Rate (K Value) Of A High-voltage Series Reactor Based On The Background Harmonics Of The Power Grid?

In rail transit power supply systems, the selection of high voltage series resistivity ratio based on grid harmonic background should follow the following principles in order to effectively suppress harmonic pollution and avoid resonance risks:
I. Core Principle: Matching Harmonic Background and Adjusting System Impedance
Harmonic Impedance is inductive
The core function of the resistor is to make the overall harmonic impedance inductance of capacitor connection point and avoid parallel resonance the grid inductive reactance.
Suppression of Specific Harmonics
The reactance ratio should be selected according to the main harmonic series in the grid (e.g. 3, 5, 7) to ensure that the capacitor branch induces the target harmonic and prevents harmonic current from flowing into the capacitor.
ii. Reactance Ratio (K Value) Selection Guidelines
1.When the third harmonic dominates
Recommended K-value: 12%~13%
Function: Set the resonance point to 139Hz to 144Hz (three harmonics below 150Hz) to avoid three harmonic amplifications.
Case 1: Three harmonic aberrations in a subway substation are too large. The third harmonic voltage amplification rate decreased from 1.21 to 0.50, which meets the standard requirements.
2.When the fifth harmonic dominates:
Recommended K value: 4.5%~6%
Functions: adjust the resonant point to 204Hz~235Hz (5 harmonics above 250Hz) and suppress 5 harmonics.
Example 2: a 110kV substation with 6% reactance avoids resonance by reducing voltage amplification from 1.0 to 0.69 for 5 harmonics.
3.When the third and fifth harmonics exist simultaneously:
Recommended solution: Hybrid Reactor: Some capacitor banks are configured with a 12% reactance rate (to suppress the third harmonic) and some banks are configured with a 4.5%~6% reactance rate (to suppress the fifth harmonic).
3. Single Reactor: If the third harmonic content is low, choose 4.5% 4.5% ~ 6% a reactance rate, can suppress the fifth harmonic, the impact on the third harmonic is less.
Case study: In one substation, the third and fifth harmonics were exceeded, using a 12% and 6% hybrid reactor, reducing the total harmonic distortion rate from 4.33% to 3.9%.
4. When all that is required is to limit the flow
Recommended K value: 0.1%~1%
Function: To protect switching equipment by limiting the instantaneous inrush current when capacitor is connected by inductive reactance.
Note: This reactance rate does not affect harmonic suppression and may even amplify three harmonics.
III. Key Considerations
Harmonic Measurement and analysis: Before selecting the reactance rate, the harmonic background of the power grid (e.g., the content of 3 or 5 harmonics) must be clearly determined by field measurements to avoid resonances arising from deviations between theoretical and practical calculations.
Avoiding Resonance Risk: Calculate the matching of capacitor capacity to reactance rate to ensure that the resonance point is away from the main harmonic frequency of the power grid.
For example, if the capacitance capacity is close to 5.1% of the bus short-circuit capacity, a third-order harmonic resonance may be triggered when the the reactance is 6%.
Dynamic Adjustment and monitoring: The orbital traffic load fluctuates significantly, requiring regular monitoring of harmonic levels and dynamic adjustment of reactor parameters or compensation strategies.
Dynamic compensation is to solve the challenge of complex harmonic spectrum by combining active filter (APF).
Standard and Specification Compliance: Ensure that the selection of reactance complies with industry requirements, taking into account standards such as the Parallel Capacitor Banks Design Specification (GB50227-2017).