Commutating Reactance 2

[rail1996]

We often see the term commutating reactance associating with rectifier transformers.
Can someone please tell me if that is just simply the transformer impedance? If not, can we derive the Tx impedance from the commutating reactance?

Thanks in advance!

 

[jbartos]

Suggestion: Reference: IEEE Std 100-2000 "Dictionary"
Commutating Reactance Factor (Rectifier Circuits) The line-to-neutral commutating reactance in ohms multiplied by the direct current commutated and divided by the effective (root-mean-square) value of the line-to-neutral voltage of the transformer direct current winding.

 

[ScottyUK]

Commutating reactance is the combined total of source reactance, cable / line reactance, and any additional reactance added to slow down commutation di/dt. Additional reactance is normally only needed with very high current convertors.

 

[hvcad]

In a rectifier, the hand over of load current between an incomming and outgoing diode does not occur instantanously. This is due to the inductance in the supply lines to the rectifier circuit. When the incoming device starts to conduct, the current rises from zero to the dc current value, at the same time, the current in the outgoing device, falls from the dc current value to zero. There is therefore a period, during which both incomming and outgoing devices are conducting. This is known as the commutation (or overlap) period. The larger the supply inductance L (or the commutation reactance Xc = 2*pi*f) , the longer the commutation period.

For a rectifier transformer the leakage reactance of the transformer will equal its commutation reactance. For a synchronous machine, the commutation reactance is approximately Xc = (Xd" + Xd')/2

A high commutation reactance "rounds off" the steps in the ac side current, reducing harmonic currents. This is at the expense of a poorer power factor, and also gives the rectifier a poorer regulation characteristic.

This is discussed in detail in chap 3 of Kimbark's book on High Voltage DC transmission.


Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.

 

[rail1996]

hvcad,

Thank you, I found your explainations very useful.

rail1996

 

[jbartos]

Suggestion: Somewhat related to the commutation reactance is a commutation failure. Visit

http://www.itee.uq.edu.au/~aupec/aupec02/Final-Papers/SO-Faried107.pdf

for: Commutation failure that is the result of a failure of the incoming valve to take over the direct current before the commutating voltage reverses its polarity, taking into account the need for sufficient extinction time

 

[jbartos]

Suggestion: Reference:
Muhammad D. Rashid "Power Electronics Handbook," Academic Press, 2001, Section 12.2.6 Commutation, page 188, addresses line inductance Ls between the converter and power supply. Ls is part of the commutation reactance.
Normally, the line reactor or filter can be viewed as a portion of the commutation reactance.

 

[hvcad]

Commutation failure does not occur in diode rectifier converters. It will not occur in thyristor converters when operating as rectifiers (the firing angle alpha is in the range 0 to 90 degrees and the real power flow is from the ac to the dc side).

When a thyristor converter operates as an inverter (the firing angle is now between 90 and 180 degrees), commutation failure may occur, as the firing delay angle + the overlap angle approaches 180 degrees).
I assume that the initial posting refers to traction rectifiers (perhaps diode devices), so inverter operation is
not a concern (or possible).

Returning to the original post, the commutation reactance seen by an individual rectifier is the total reactance in the ac supply lines, generally the most significant element of which is the rectifier transformer. On two occasions where there has been a degree of ambiguity in the claimed fault levels of the supply comming into an industrial plant, I have measured the commutation overlap angle in the currents drawn by large dc drives, and from the
other drive parameters have been able to deduce the incomming fault level. This can be very useful!

Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.

 

[jbartos]

Comment on the previous posting: I agree with the commutation failure occurrence.
It is not clear in the original post, if the rectifier transformer is for an AC-DC conversion only or for an AC-DC-AC conversion.

 

[ScottyUK]

HVCad / jB,

Commutating reactance in inverters is required in current-source designs to provide a 'slow' transition between conducting thyristors. This allows the off-going thyristor time to recover its blocking state before being subjected to forward voltage. The commutating reactance causes the characteristic trapezoidal current waveform of the CSI, the reactance determining the slope of the trapezoid i.e. the rate of change of current in the thyristors.

Current source drives aren't as popular now, the ubiquitous PWM voltage source drive having taken their place in all the but heaviest drives.

 

[jbartos]

Suggestion: Reference:
Ned Mohan, Tore M. Undeland, William P. Robbins "Power Electronics, Converters, Applications, and Design," 3rd Edition, John Wiley & Sons, Inc., 2003,
6-3-2 Effect of Ls
The ac-side inductance Ls cannot generally be ignored in practical thyristor (ac-dc) converters. For a given delay alfa, the current commutation takes a finite commutation interval u. In principle, commutation process is similar to that in diode bridge rectifiers. During commutation all four thyristor conduct. ....