Tertiary transformers Dd0y11 2

[ekuegler]

We have a tertiary transformer feeding a DC 690v bus (part of a multi VFD) dist system, DC link section.

Can someone explain why this type of Tx was used for this application, assume cost related...! and also explain how the secondaries that feed the same bus differ in configuration one being delta 0 displacement and 1 star angular displacement 11. Does the difference have any effect on the system...?

 

[sibeen]

From your brief description I cannot be certain but this certainly sounds like a transformer with two secondaries with a 30 degree phase displacement between them. Each secondary will feed a 6 pulse diode or thyristor bridge (Graetz Bridge) and the output of the two bridges will be combined, either in series or or parallel, which gives what is called a twelve pulse system.

This is normally done to lower the upstream harmonic current.

 

[ekuegler]

Hi Sibeen,

Thanks for the reply, yes that's correct there are 2 secondaries one delta, one star configured - primary being delta.

So by combining the secondary outputs to achieve a 12 pulse convertor in essence reduces the harmonic distortion upstream? Is this the single unique advantage of such a system? And does this have an effect on the dc link/inverter behaviour/performance.

Best regards

 

[sibeen]

ekuegler, upstream harmonic reduction is the main reason for using this configuration although it does produce other side benefits such as a slightly higher DC voltage and the DC voltage contains less ripple. For a 50 Hz system, and without any DC filtering, a single bridge produces a ripple with six pulses over 20 milliseconds (one cycle), hence the name 6 pulse bridge. By using two bridges you have 12 pulses in the ripple over the same time period but at a much reduced amplitude.

As a side note, by using phase shifting transformers the number of pulses can be increased further and the upstream harmonic current reduced even further.

The expressions:

Harm = 6k*n +- 1 (1.1) k = any positive integer
n = number of six pulse converters
Magnitude = 1/(6k*n +- 1) (1.2)

provide a simple way to calculate the frequency and magnitude of harmonics that will be present in the ac input line current when multi-pulse topologies are implemented . The frequencies at which the harmonics will appear for an n-pulse converter are computed by multiplying (1.1) by the fundamental frequency of the system.

The magnitude of the harmonics are calculated by multiplying (1.2) by the amplitude of the signal at the fundamental frequency. For example, if a 12-pulse transformer/rectifer system were to be implemented, the first harmonics to contribute to the total harmonic distortion (THDi) of the input line current would be the 11th and 13th. It can be seen that by using a phase-shifting transformer and adding and additional 6-pulse diode rectifer bridge, the harmonic content in the supply current is attenuated up to the 11th harmonic as opposed to the 5th for the traditional 6-pulse bridge rectifer.

So without any additional filtering at the input you get approximately the following THDi for different scenarios:

6 Pulse (single bridge) = 27%

12 Pulse (two bridges) = 13%

24 Pulse (four bridges) = 6%

48 Pulse (eight bridges) = 3%

Basically every time you double the number of bridges you halve the THDi, also the major individual harmonic goes way up in frequency; so for a 6 pulse system the first harmonics are the 5th and 7th, for a twelve pulse system it is the 11th and 13 and for a 48 pulse system it is the 47th and 49th.

Like every bit of engineering there is more than one way to skin the cat so it is rare to see systems higher than 18 pulse, as using filters, either passive or active, on the front end can lower the THDi considerably without the expense of fancy transformers and additional diodes/thyristors.

 

[ekuegler]

Many thanks,

Great information and has helped/ironed out a number of areas.

Thankyou

EK