Difference Between Closed Delta and Open Delta Arrangement of Elements

The terms "open delta" and "closed delta" are normally used to describe the nautre of a power source. I cannot recall a time where they were used to describe the conenctioins of a load. Could be... but do not remember such (oh no... the years are catching up I guess).

I'm working on heating element arrangement at thyristor load end. Thyristor load can be configured in open delta, Closed delta, star and star with neutral. I want to know difference between these arrangements and principle behind these arrangements.

1. Star - All elements need to be equal to phase voltage/1.73 as there will be two elements across each phase. The star point forms a floating neutral. You would use 2 SCR control units for this configuration, but because the Thyristors fire individually, balancing is erratic. Using phase voltage rated elements in a star configuration will result in roughly 58% of the rated element wattage being obtained

2. Star with neutral - Similar to #2. With the inclusion of a neutral conductor, 3 SCR units can be employed, which will enable better control. Be aware though that the neutral will carry any out of balance current, ie. if one element fails then that current would normally be equal to the vector sum of the other 2 phase currents, however with phase angle control, due to independent operation of the 3 SCR units, the neutral current can be around twice that of individual phase currents.

3. Closed Delta - All element operating voltages need to be equal to the phase voltage as there will be two phases across each element. You will require only 2 SCR control units for the 3 phases. Heat control is not as accurate as for open Delta.

4. Open Delta. Note that this term is not the same as is used for open delta connected transformers. This system is essentially a normal closed delta configuration with the exception that the thyristor controls are inserted in the phase to element wiring. All element operating voltages need to be equal to the phase voltage as there will be two phases across each element. Enables more accurate heat control due to the ability to switch the 3 phases independently, but is more complex and costly to implement with 6 wire control vs 3 wire of closed delta. You will require 3 SCR control units for the 3 phases.

For both Delta types, element currents and voltages will depend on the duty cycle in effect at any specific time, but you must allow for full phase voltage on the elements and the SCRs.

Circuit current will be a function of the element wattage and phase voltage, and again you should allow for maximum duty cycle for cable calculations. For example:-

Volts = 415, Watts across phase = 6000, therefore phase current will be I=P/E, = 6000/415, = 14.45 amps. Therefore line current will be 14.45 x 1.73, = 25 amps.

Hope my sums are correct.

Thanks ... really useful ...

Do you mean "open transition" vs "Closed transition" ? North is quite right, i haven't heard of what you mention.

I have never used an Open Delta transformer (only two windings) connection for feeding an SCR bridge.

For a closed Delta (the normal three winding connection) for a full DIODE bridge:

Erms/Edo = 0.74 ratio of ac voltage to DC voltage

fr/fs = 6 ratio of ripple frequencies

Iav/Id = 0.333 (arm av current to output Av current)

Irms/Id = 0.577 (arm current rms to output dc average)(note fuses dimensioned for rms)

Transformer data:

Irms/Id = 0.816 secondary rms to average output current

Pp/P = 1.05 Primary rated power to ideal output power

Ps/p = 1.05 Secondary rated power to ideal output power

Pf = .955 (fully turned on, at half voltage Pf is approx .5

Conduction period 120deg (continuous conduction region)

Now all the above would get messed up with an open Delta. I have never seen any literature on that configuration.

From experience I do know a Scott T transformer messes things up also. I will never be talked into using a Scott T for a phase controlled rectifier again - it just is not worth the pain.