Thyristor-Switched Capacitor Bank for Power Factor Correction

Inductive loads — induction motors, transformers, welding sets, fluorescent lighting — consume reactive power that does no useful work but still flows through cables, switchgear and transformers, increasing current, copper losses, voltage drop and the maximum-demand charges utilities bill. Correcting the power factor with shunt capacitors is standard practice, but conventional contactor-switched banks are slow and slam capacitors onto the bus with inrush transients and contact wear. Thyristor-switched capacitor (TSC) banks solve this by switching each capacitor step at voltage zero-crossing: no inrush, no arcing, and response within a cycle or two, which is why TSC/TCR schemes dominate modern dynamic compensation. This project makes that industrial technique demonstrable on a bench: a single-phase prototype where an Arduino samples voltage and current, computes the displacement power factor, and switches three capacitor steps through optically isolated thyristor switches at zero-crossing, holding the power factor near unity as a variable inductive test load changes.

1. Voltage and current transformers (or isolated sense dividers) feed scaled-down voltage and current waveforms to the Arduino's ADC.
2. The firmware detects voltage zero-crossings and measures the time delay to the current zero-crossing, converting it into the phase angle and hence the displacement power factor.
3. When the measured power factor falls below the configured target minus hysteresis, the controller selects the next capacitor step to switch in.
4. Firing pulses are issued to the opto-isolated thyristor drivers exactly at the next voltage zero-crossing, connecting the capacitor step with no transient.
5. The controller re-measures after each switching event; if the power factor overshoots (leading), it steps capacitors back out, again at zero-crossing.
6. The LCD continuously shows voltage, current, power factor, active and reactive power estimates, and which steps are engaged.
7. If the bus voltage exceeds the overvoltage setpoint, all steps are shed immediately and a fault lamp lights until reset.

Tech stack:

• Arduino Uno (ATmega328P)
• Zero-crossing detector (optocoupler-based) for timing reference
• Back-to-back thyristor (SCR pair) or TRIAC switch modules with opto-isolated drivers
• Stepped shunt capacitor bank (3 steps) with discharge resistors
• Voltage and current sensing stage (isolated)
• 16x2 character LCD
• Arduino IDE (C/C++ firmware)
• Variable inductive test load (choke + lamp bank)

• [Zero-crossing thyristor switching] Each capacitor step switches through back-to-back thyristors fired only at voltage zero-crossing, so steps engage with no inrush transient and no contact wear — the core TSC principle.
• [Automatic power-factor measurement] Voltage and current are sampled on the Arduino and the displacement power factor is computed from the phase angle, with a live readout on the LCD.
• [Three-step capacitor bank] Three switched capacitor steps give seven discrete compensation levels, letting the controller track a varying inductive load and hold the power factor near the target.
• [Configurable target power factor] The target power factor and step-switching hysteresis are set in firmware, so the student can demonstrate under-compensation, correct compensation and the hunting behavior of a badly tuned controller.
• [Overvoltage and discharge protection] Each capacitor carries a discharge resistor, and the controller sheds steps if the bus voltage rises above the set limit, demonstrating safe capacitor-bank practice.
• [Manual and automatic modes] A selector switch allows manual step switching for teaching, while automatic mode runs the closed-loop controller for the main demonstration.
• [Inductive test load rig] A variable inductive load (choke plus lamp bank) is included so the examiner can watch the power factor dip and the controller bring it back.

• Working TSC power-factor correction prototype (Arduino, zero-cross detector, thyristor switch modules, capacitor bank, sensors, LCD, test load)
• Complete firmware source code (PF measurement, step logic, zero-cross firing, LCD)
• Circuit and wiring documentation with capacitor sizing notes
• Component list with voltage/current ratings
• Calibration and tuning procedure (buyer-run: verify switching levels and target PF)
• Project report PDF (reactive power theory, TSC principle, methodology, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (power factor, displacement vs true PF, TSC vs contactor banks, zero-crossing)
• Setup and demonstration guide