Key Points for Selecting Low‑Voltage Reactive Power Compensation Devices: How to Determine Compensation Capacity, Reactors and Capacitors

Writer： Hengfeng you electric Time：2026-09-10 views：times

Ⅰ Classification of Applicable Products for Low‑Voltage Reactive Power Compensation

Reactive power compensation on the industrial 0.4 kV side mainly includes conventional capacitor‑switching compensation (TBB), thyristor‑switched dynamic compensation (TSC) and Static Var Generator (SVG).

For plants with low harmonic content, contactor‑switched capacitors are acceptable. TSC or SVG is recommended for chemical plants and variable‑frequency workshops with impact loads and heavy harmonics. Reactors connected in series with capacitors suppress harmonics and prevent resonance.

Core components of compensation devices: power capacitors, series reactors, switching switches (contactors / thyristors), controllers, fuses and cabinet enclosures.

Functions: improve power factor, reduce grid reactive power loss, avoid power‑factor‑related surcharges imposed by power utilities, and enhance voltage quality.

Ⅱ Accurate Calculation of Compensation Capacity

Compensation capacity shall not be simply set at 30 % of the transformer rating; calculation shall be based on the actual on‑site power factor.

Basic calculation formula: Qc = P × (tanφ₁ − tanφ₂)

Where: P = active power of the plant; tanφ₁ = tangent of power‑factor angle before compensation; tanφ₂ = target tangent of power‑factor angle after compensation (generally target power factor 0.95).

Selection margin:

1. Ordinary plants: compensation capacity at 20 %‑30 % of transformer rating;
2. Heavy‑duty workshops with numerous motors and air compressors: 30 %‑40 %;
3. Plants with abundant frequency converters and severe harmonics: reserve an extra 10 % margin and adopt series reactors.

Note: Over‑compensation shall be avoided. A power factor higher than 0.98 may cause over‑compensation and excessive voltage rise that damages equipment.

Case 1: Insufficient compensation capacity leads to electricity penalty in a small processing plant A hardware processing plant with a 1250 kVA transformer was fitted with a 200 kvar compensation cabinet. With numerous stamping motors in operation, the actual power factor dropped to 0.82, resulting in substantial monthly power‑factor penalties. Calculations show the reasonable compensation capacity should be 375 kvar. After capacity expansion, the power factor stabilized at 0.95 and penalties were eliminated.

Ⅲ Series Reactor Selection: Differentiated According to Harmonic Conditions

Reactors are frequently overlooked components. They suppress harmonics and prevent resonance between capacitors and the power system.

1. 5 % reactor: mainly suppresses 5th‑order harmonics, widely applied in most workshops with frequency converters;
2. 12 % reactor: for severe 3rd‑order harmonic scenarios, e.g. sites with massive single‑phase rectifiers and chemical rectification workshops;
3. No reactor: only suitable for simple plants with almost no harmonics. Ordinary capacitor cabinets without reactors are strictly prohibited for workshops with many frequency converters.

Case 2: Capacitor burnout caused by missing reactors in a chemical workshop A fine‑chemical workshop with a large number of variable‑frequency stirring devices purchased ordinary compensation cabinets without series reactors. After commissioning, system harmonics were amplified, capacitors bulged and burned out frequently, requiring replacement three times within one year.

Rectification: Compensation devices equipped with 5 % series reactors were installed. Harmonics were restrained and capacitor service life returned to normal.

Ⅳ Key Selection Points for Capacitors and Switches

1. Capacitors: harmonic‑resistant special capacitors shall be adopted under harmonic conditions instead of general‑purpose civilian capacitors.
2. Switching devices: ‑ Contactors for slowly‑varying loads; ‑ Thyristor‑based TSC dynamic switching for heavily fluctuating loads requiring frequent switching; ‑ Directly adopt SVG Static Var Generator where harmonic content is extremely high.

Ⅴ Auxiliary Selection Parameters

1. Compensation controller: with harmonic monitoring, automatic power‑factor‑driven switching, over‑voltage and under‑voltage protection;
2. Cabinet protection rating: IP30 for indoor distribution rooms; IP41 for dusty workshops;
3. Ambient conditions: cooling fans shall be fitted for capacitor cabinets in high‑temperature distribution rooms;
4. Communication: Modbus interface is optional for uploading compensation status and power‑factor data to factory background systems.

Ⅵ Summary of Common Pitfalls in Reactive Power Compensation Selection

1. Capacity configured merely by fixed transformer ratio, resulting in under‑compensation under real‑world load and power‑factor penalties;
2. Omission of series reactors for harmonic‑rich workshops, triggering resonance and repeated capacitor burnout;
3. Ordinary contactors adopted for impact fluctuating loads, causing switching oscillation and accelerated capacitor failure;
4. Excessively high compensation target leading to over‑compensation and abnormal system voltage rise;
5. Lack of heat dissipation in high‑temperature enclosed distribution rooms, accelerating capacitor ageing.

Ⅶ Conclusion

Reactive power compensation device selection shall not copy fixed transformer‑percentage rules. Compensation capacity is determined by actual plant active load and target power factor. Series reactors mitigate harmonics and prevent resonance‑induced capacitor damage. Switching modes shall match load fluctuation characteristics.

For plants with stable ordinary loads, contactors combined with 5 %‑reactor capacitor compensation are preferred. TSC or SVG shall be adopted for chemical and variable‑frequency workshops featuring impact loads and severe harmonics. Selection shall comply with GB 50227 Code for Design of Shunt Capacitor Installations. Comprehensive evaluation of on‑site harmonics and load conditions helps avoid power‑factor penalties and capacitor burnout.

Hengfengyou Electrical supplies complete sets including low‑voltage reactive‑power‑compensation cabinets, TSC dynamic compensation and SVG Static Var Generators. We can calculate required compensation capacity based on plant transformer and load data and deliver complete power‑distribution solutions.

Email: hunter@hengfengyou.com WhatsApp/Mobile: +8618224429139