Understanding the Basics of d-axis and q-axis Currents in Alternators

In summary: Alternators convert mechanical energy into electric energy. If you have two generators connected to the same AC source, and one of them is a synchronous generator, you need to synchronize it with the grid before connecting it. This means adjusting the frequency so that the generator is running at the same speed as the grid. Synchronization also ensures that the power going to the load is the same at all times.
  • #1
DUET
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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?
 
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  • #2
Basically that the HVDC link is a 'spongy' connection

like trying to pull a load with a "Slinky" instead of a stout rope

The mathematical answer is somewhat beyond my comfort zone

For some scholarly papers try a search on this phrase:

hvdc system stability
 
  • #3
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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?
 
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  • #4
DUET said:
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?

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
DUET said:
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?

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

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.
 

1. What is high voltage DC transmission and how does it work?

High voltage DC (HVDC) transmission is a method of transmitting electricity over long distances using direct current (DC) instead of alternating current (AC). This is achieved by converting AC power to DC power at the sending end, transmitting it through power lines, and converting it back to AC power at the receiving end. This method is more efficient for long distance transmission because it experiences less power loss compared to AC transmission.

2. What are the benefits of using high voltage DC transmission?

There are several benefits to using HVDC transmission. It allows for longer distance transmission with less power loss, making it more efficient for large-scale energy projects. HVDC also has a smaller footprint, as it requires fewer power lines and substations than AC transmission. Additionally, HVDC can connect different power systems with different frequencies, making it easier to integrate renewable energy sources into the grid.

3. How does high voltage DC transmission compare to traditional AC transmission?

AC transmission has been the standard for electricity distribution for many years, but HVDC has several advantages over AC transmission. HVDC is more efficient for long distance transmission, experiences less power loss, and has a smaller footprint. However, HVDC is more expensive to build and maintain, and it is not as suitable for short distance transmission. Both AC and HVDC transmission have their own strengths and are used in different situations.

4. What are the main challenges of implementing high voltage DC transmission?

The main challenges of implementing HVDC transmission include the high cost of building and maintaining HVDC infrastructure, the need for specialized equipment and expertise, and potential environmental impacts. There may also be challenges in integrating HVDC into existing AC grids and in managing the different frequency levels of AC and DC systems.

5. Can high voltage DC transmission be used for renewable energy sources?

Yes, HVDC transmission is often used to connect large-scale renewable energy projects, such as wind farms and solar power plants, to the grid. It allows for more efficient transmission of the electricity generated from these sources to areas with high demand for energy. HVDC can also help to mitigate some of the challenges of integrating renewable energy into the grid, such as fluctuating power output and transmission over long distances.

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