Power Flow in hybrid AC/DC line.

[Mahmoud Salem]

When a HVDC line and AC line are connected in parallel for power transmission, what determines the ratio of power that both lines would carry? Is it the resistance of the lines?

 

[cranky108]

The setting on your AC to DC converter.

 

[davidbeach]

That, in combination with the setting at the DC to AC converter. ;-)

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

The phase angle of the DC to AC inverter is the main determination of the power flow within limits.
The current limit on the AC to DC inverter may set one of those limits.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cuky2000]

In AC parallel transmission lines, the power flow in the steady states is mainly a function of the line reactances. For instance, a line 1 with reactance X and line 2 with reactance 2X the power flow in line 1 is proportional to X1/(X1+X2) =2/3 while line 2 is only 1/3. A transient power flow is a little bit more challenging particularly if a series compensation is implemented.

Conventional control methods used in AC networks may not give satisfactory solutions for HVAC/HVDC parallel links. It is recognized that AC/DC operation is primarily related to the coordination between AC and DC power flows and how each system reacts to any important disturbance.

Simulation and control method-based in mathematical models of AC and DC systems based on artificial intelligence techniques including fuzzy logic is an alternative commonly used for controlling the power flow in multi-area interconnected HVAC and HVDC link as indicated in a simplified diagram illustrated below. The transfer function parameter such as step response, error signal, and its time derivative is obtained by experiment.

 

[waross]

Think of the DC to AC conversion as an independent generator. The power that the Dc line transfers is not a fixed share. It is what the operator wants it to be.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cuky2000]

Hi Waross,

Of course that the power flow is what the operator wants or allows it to be by the regional system operator (ISO). However, due to the high cost of the HVDC link, efficiency, and operation flexibility (including reversing power flow), the HVDC link has usually a larger transfer capability than the AC line.

An interesting transmission design issue will require considering a fault scenario on the HVDC link where all the power momentarily flows on the AC circuit creating potential transient stability that needs properly addressed. One possible solution is to use a series capacitor compensation or deploy other FATS devices on the AC circuit to mitigate the probability of overload or blackouts.

 

[waross]

Thanks for the additional clarification, Cuky.
My point was that the sharing of power is not related to transmission line impedances as it would be with parallel AC lines, but is under operator control.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!