At the Changji converter station in the remote northwest of China, an engineering marvel sits in the desert landscape. The enormous transformer, along with 27 others like it, makes up the world‘s largest HVDC transmission system sprawling 3,284 km across the country.
This system represents a true revolution in power engineering. Let‘s explore the story behind the planet‘s biggest transformer and the HVDC technology that enables its feats.
A Brief History of HVDC Transmission
The beginnings of high-voltage direct current technology trace back almost a century. An early commercial HVDC link went into service in 1954 connecting the Swedish mainland with the island Gotland. Voltage levels in the tens of kilovolts allowed efficient submarine power transfer.
Over the following decades, HVDC slowly gained traction for niche long-distance overhead transmission too. But the voltages remained limited compared to AC technology of the time.
In the 1970s, the availability of thyristor valves provided a breakthrough. These electronic switches allowed development of Voltage Source Converters (VSCs) to efficiently change between AC and DC.
VSC-HVDC systems operating at hundreds of kilovolts were soon constructed in Canada, Brazil, and other countries. Efficiencies improved dramatically.
But the voltages used in these early schemes seem minute compared to today‘s huge HVDC infrastructure.
The World‘s Largest Transformer
Now we arrive at the Changji-Guiguan HVDC link commissioned in China in 2018. This system dwarfs previous installations in every respect.
Some vital statistics illustrate why this is the new benchmark for HVDC transmission globally:
- Voltage: 1,100 kV – the first project operating above 1,000 kV
- Capacity: 12,000 MW – able to deliver 12 gigawatts
- Distance: 3,324 km across China – longer than the distance between Chicago and Phoenix!
- Efficiency: Above 98% even over huge distances
The centerpiece is the enormous transformer units built by Siemens. Each single-phase transformer has a capacity of 587.1 MVA and stands 14.4 meters tall.
Laying these titans on their sides, they measure an incredible 37.5 meters long. With a weight of 909 metric tons, the logistics of installation boggle the mind.
How HVDC Transmission Works
But what exactly is HVDC, and why does this system represent such a leap forward?
In simple terms, HVDC transmits power as direct current rather than alternating current (AC). This provides major advantages for efficient long-distance transmission.
Here is how modern VSC-HVDC systems operate:[Table comparing HVDC and HVAC transmission]
- AC power is converted to DC using electronic switches called Insulated-Gate Bipolar Transistors (IGBTs).
- The DC flows through overhead cables or underground/submarine cables.
- At the receiving end, the DC is inverted back to AC.
- The AC power is synchronized with the grid and distributed.
By using DC for transmission, the system achieves far greater efficiency over enormous distances. HVDC avoids the capacitive charging current issues suffered by AC cables.
Dramatic Efficiency Improvements
Early HVDC links operating at just 20-40 kV provided little benefit compared to AC transmission. But as voltage levels increased, HVDC‘s advantages became clear.
The China Central – East HVDC system demonstrated this by achieving 97.9% efficiency over 2,068 km. Yet the Changji-Guiguan link takes this further – exceeding 98% efficiency over a distance of over 3,000 km!
Another metric that quantifies the advantage of HVDC is the break-even distance. This is the distance where HVDC becomes cheaper than an equivalent AC transmission system.
For the Changji-Guiguan link operating at 1.1 million volts, studies suggest the breakeven point is under 350 km . This truly changes the economics of very long-distance transmission.
Driving the Energy Transition
With its unique benefits, HVDC is set to play a major role in the global energy transition:
- Connecting renewable energy – HVDC links allow efficient transfer from distant solar/wind sites to population centers.
- Interconnecting grids – HVDC enables improved ties between networks, balancing supply and demand.
- Smart grid control – Precise power flow control allows optimal management of complex smart grid systems.
- Offshore wind – Submarine HVDC cables avoid AC cable capacitive issues and are ideal for offshore wind farm connections.
- Ultra-high voltages – Pushing DC voltage above 1 million volts further improves efficiency as in China‘s system.
The Future is HVDC
What started as a niche application has now become the future of electricity networks. HVDC transmission has proven its immense capabilities, and installations are proliferating globally.
With advanced multi-terminal HVDC grids now envisioned, the technology promises to catalyze the transition to renewable energy and forge tomorrow‘s smart, robust infrastructure.
The scale of the engineering achieved at Changji stands as a monument to this HVDC revolution. The world‘s largest transformer is just the beginning.