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High voltage DC operation

High voltage DC (HVDC) system has been part of our mostly-AC power delivery system for decades. The technology saw major advances recently when voltage-sourced converters (VSC) became practical at very high power levels. As this report is written, the converter technology that was once called “HVDC-light” by one of the large manufacturers has reached a power level of about 1000 MW.

Technology of High Voltage DC (HVDC) Systems, Their Applications and Problems
Technology of High Voltage DC (HVDC) Systems, Their Applications and Problems (on photo: NEMO Link is an HVDC electrical line of 1 GW connecting the grids of Belgium to UK; credit: André Bouffioux via Twitter)

It can no longer be called “light”. The possibility of being able to operate such a large converter in any quadrant of a power/reactive plane makes it very interesting.

These VSC converters are new enough that there is much still to learn about their properties as part of the power system.

Computer models are still being refined for planning purposes, and for load-flow and stability studies, where capabilities must be added to the existing positive-sequence programs that meet these needs.

But the converters are making inroads into the power delivery system.

Questions must be addressed about their impact on markets and regulations, and about their need for permitting. But there is a fundamental technical question that must be answered.

Simplified version of the WECC transmission system
Simplified version of the WECC (Western Electricity Coordinating Council) transmission system

The question is posed by the necessity to control, in real time, in each converter, two separate parameters.

With the “old” current-source converters in a point-to-point configuration, one could control only a total of two parameters. More, if tap-change transformers were used, as the taps could be used to adjust the relationship between voltage and power.

However, tap changers move relatively slowly compared to converter control systems, so they offer a different kind of control.

Typically, the two chosen were the power being transmitted and the voltage on the DC line. Strictly, the extinction angle at the inverter is managed, not the voltage. But thinking of that control as being equivalent to controlling the DC-side voltage is a good approximation and a useful simplification.

With VSC converters, the number of controlled parameters is two at each converter. The parameters are functionally equivalent to the turn-on angle of the rectifiers and the turn-off angle.

These parameters affect the voltage and current, and the phase-relation between them, and are usually embedded in control loops that manage, for example, the power being transmitted.

Title:Technology of High Voltage DC (HVDC) Networks, Their Applications and Problems – H. Kirkham, M. Elizondo and J. Dagle (U.S. Department of Energy)
Size:2.40 MB
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Technology of High Voltage DC (HVDC) Networks, Their Applications and Problems
Technology of High Voltage DC (HVDC) Networks, Their Applications and Problems

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