# High voltage DC transimission

1. Jul 27, 2013

### DUET

A HVDC connection in a transmission network may reduce the system effective inertia due to the decoupling of mechanical and electrical systems.

Could someone please tell me what does the above sentence suggest?

2. Jul 27, 2013

### jim hardy

Basically that the HVDC link is a 'spongy' connection

The mathematical answer is somewhat beyond my comfort zone

For some scholarly papers try a search on this phrase:

hvdc system stability

3. Aug 2, 2013

### DUET

The VSC-HVDC instantaneous power control is typically based on current vector control. For the vector control using grid voltage orientation, the active power and reactive power are proportional to the d-axis and q-axis currents respectively. Although VSC terminals can have different operation modes, their inner control loops are similar, e.g., using the converter current PI regulator to follow the relevant current references.

What are "d-axis" and "q-axis" currents?

Last edited: Aug 2, 2013
4. Aug 2, 2013

### nsaspook

Any AC synchronous generator on the grid must be locked (with small frequency adjustments to increase or reduce power) to the grids power freqency. Droop speed control

Each generator has stored energy due to it's mechanical rotation that can provide system effective inertia by providing voltage and freqency stability during load changes (a voltage source). If the HVDC connection is seen as a decoupled current source that adjusts it's voltage to keep the current flow into the grid at the current limit on the HVDC controllers set point then changes in load power can't directly use the synchronous generator inertia in the distant generators for grid voltage stability but must rely on the current setpoint control loop to smooth out changes.

5. Aug 2, 2013

### sophiecentaur

It may help if I put this really simply (apologies if it's too simple). Two AC generators, connected to the same grid, will interact. If they are not exactly in phase with each other, the fast one can pour current into the slow one (an embarrassing amount) and the slow one will catch it up and can overtake it. This can produce a really serious oscillations, involving massive rotating equipment, which wastes energy and makes things hot. Changing the two supplies to DC before you connect them together still requires you to match their voltages (so that the right amount of current can flow from one into the other) but it is much easier to just regulate the voltages so that both sources are powering just the load and not chasing each other. But the two system phases (or even the frequencies) need not be the same. So two different grid systems (UK and France, for instance) can do their own thing (a hard enough job in any case), yet still give each other DC power when needed. Isolating the two AC systems in this way is referred to as 'decoupling' because it smooths out the effect of the oscillations (50Hz) at each side of the link.

6. Aug 2, 2013

### jim hardy

Shorthand for "Direct" and "Quadrature", ie in phase or 90 degrees out of phase with the rotating field. One carries the actual power, ie megawatts, the other carries the reactive power ie megavars.

You might search for a tutorial on alternators. TI's Motor Compendium is one I often suggest but it's a 12 meg pdf file,,, google finds it quickly.... there are shorter ones around.