Theory & Background

A 3-phase, 6-pulse thyristor bridge rectifier provides smoother DC than single-phase. Six thyristors (SCR1–6) are arranged in a bridge: top 3 connected to each AC phase (cathode group), bottom 3 with anodes to AC phases (anode group). Firing angle α controls DC output: Vdc = 1.35 × Vline × cos(α). At α=0°: Vdc = 1.35 × Vline (maximum). At α=90°: Vdc = 0. At α > 90°: Vdc is negative (inverter operation, returning power to AC grid).

Components & Requirements

10 components required for this project.

#	Component	Purpose	Qty
1	SCR Thyristors (BT151, 12A/600V)	6-pulse bridge rectifier and inverter	x12
2	Gate Trigger Circuit (synchronizing)	SCR firing angle control	x2
3	Isolation Transformers (230V/110V + 230V/110V)	Simulated sending and receiving end AC systems	x2
4	DC Smoothing Reactor (10mH/5A)	DC current smoothing	x1
5	Smoothing Capacitor (4700µF/100V)	DC voltage smoothing	x2
6	Arduino Due	Firing angle control via phase-locked loop	x1
7	Zero-Crossing Detector Circuit	Synchronizing thyristor firing to AC grid	x2
8	High-Voltage Measurement Probes	Oscilloscope monitoring of DC and AC	x4
9	Variac (1kVA)	Variable AC supply for testing	x2
10	Insulation-rated Cables (600V rated)	Inter-stage connections	x20m

Step-by-Step Implementation

Follow these 3 steps carefully.

1

6-Pulse Bridge Rectifier Theory

A 3-phase, 6-pulse thyristor bridge rectifier provides smoother DC than single-phase. Six thyristors (SCR1–6) are arranged in a bridge: top 3 connected to each AC phase (cathode group), bottom 3 with anodes to AC phases (anode group). Firing angle α controls DC output: Vdc = 1.35 × Vline × cos(α). At α=0°: Vdc = 1.35 × Vline (maximum). At α=90°: Vdc = 0. At α > 90°: Vdc is negative (inverter operation, returning power to AC grid).

2

Firing Circuit Design with Arduino

Detect zero crossings of all three phase voltages. From zero crossing of Phase A, count microseconds: fire SCR1 when angle α is reached. Phase B fires 60° later, then Phase C, then SCR4, SCR6, SCR2 in 60° sequence. Use Arduino timer interrupts for precise firing delay: delay_us = (α/360) × 20000 (for 50Hz). Connect trigger pulses through opto-couplers and gate driver transformers to provide the required 1–2A gate current pulse.

3

DC Link and Transmission Section

In a real HVDC system, the DC link may be hundreds of kilometers of cable or overhead line. In the laboratory model, use a combination of inductor (smoothing reactor) and resistor to simulate the DC line. The smoothing reactor reduces current ripple. Measure DC voltage, current, and power at both ends. The voltage difference represents line losses: ΔV = I × R_line. Power loss = I² × R_line.

Code & Implementation

Core code for hvdc_firing.ino:

hvdc_firing.ino
  C/C++

// HVDC thyristor firing angle control  volatile unsigned long zcTime = 0; volatile bool zcFlag = false; float alpha_deg = 30.0;   void zeroCrossingISR() {   zcTime = micros();   zcFlag = true; }  void fireSCR(int scr_pin) {   digitalWrite(scr_pin, HIGH);   delayMicroseconds(200);    digitalWrite(scr_pin, LOW); }  void loop() {   if(zcFlag) {     zcFlag = false;          unsigned long delay_us = (unsigned long)(alpha_deg / 360.0 * 20000.0);     delayMicroseconds(delay_us);     fireSCR(9);         }      alpha_deg = map(analogRead(A0), 0, 1023, 0, 150);  }

Testing & Troubleshooting

Test High Voltage DC Transmission Model by verifying each subsystem individually before full integration.

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Troubleshooting Tips

Verify power voltages, check ground connections, use serial monitor for debug.

Real-World Applications

*Long-distance power transmission modeling

*Offshore wind farm connection

*Asynchronous grid interconnection

*Back-to-back frequency converter

*HVDC power flow control in meshed networks

*Educational demonstration of power electronics

*Submarine cable power transmission

*Multi-terminal DC grid research

Extensions & Next Steps

Implement VSC (Voltage Source Converter) HVDC using IGBT-based modular multilevel converter
Add cable fault location algorithm
Build a multi-terminal HVDC model with power flow control
Implement dynamic reactive power compensation (STATCOM)
Add power oscillation damping controller for AC system stability

Interactive Playground

Coming Soon

An interactive simulator will be available here — simulate circuits and run code in-browser without hardware.

Frequently Asked Questions

Why is HVDC preferred over HVAC for long-distance power transmission?

HVDC advantages over HVAC at distances > 600–800km (overhead) or > 80km (cable): lower losses (no reactive power losses, no skin effect increase), no stability limit (AC systems face synchronous stability constraints), no reactive power compensation needed at intervals, easier interconnection of asynchronous AC grids (different frequencies or unstable grids), and cables can carry full rated current (AC cables have charging current that occupies capacity). The trade-off is higher converter station cost.

Introduction