Open delta transformer - Z calculation



There certainly are advantages for three-phase rectifier circuits as opposed to single-phase full-wave or bridge rectifiers. A three-phase bridge rectifier will give you less repple voltage and higher fundamental ripple frequency if the phases are balanced. It will throw out less harmonic current into the lines.
You could get even less ripple by using zig-zag windings to get even less ripple, but that will not be a big benefit in most cases/
Bill
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wrote:

-------------------------------- Note that I mentioned 3 phase to single phase conversion where the advantages that you indicate, correctly, are not present. This conversion is not good from a motor or an electronic supply situation.
As to converting from single to 3 phase-yes-better filtering at lower cost is available. However- only in the unlikely event that the cost of the filtering for single phase is less than the cost of filtering 3 phase plus conversion costs. In fact, if one goes that route- why not go to 6 or 12 phase as the added cost is relatively insignificant compared to the gains. The choice ultimately comes down to "how can I get what I need in a satisfactory manner at the least cost?"
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Don Kelly snipped-for-privacy@shawcross.ca
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The only way I can picture more filtering cost for three-phase supplies being greater than single phase is if there is little to no engineering cost involved and if large purchases of 120 Hz filter give sufficient price advantage.
I once worked on an electron synchrotron project that used a flywheel for energy storage. Pulses were about one second duration. Ignitrons were used to rectify the alternator output to drive the magnet. When the electron pulse was over, the Ignitrons were used to transfer stored inductive energy to speed up the flywheel. I believe that used 24-phases, but I am not sure.
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>

ran
to
If
Exactly, paying the power co to bring in 3 phase to this location would have been cost prohibitive.
The transmitter was "free" + shipping. And there was no time urgency so the savings over buying a new unit built to spec was considerable. By free I mean it was part of a new acquisition in another market and surplus to their needs.
>What was the transformer rating?
About 20 k VA

It's just a big DC power supply for a RF tube.

It was an interesting project. It involved converting an old 10 kW transmitter which fed an antenna with a gain of less then 1. The idea was to increase signal coverage in certain areas by having more downward radiation in a situation that had rolling terrain.
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Why were breakers tripping? Was it due to ti making connection at the wrong phase? Did you have a gigantic capacitive input filter?
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Salmon Egg wrote:

Perhaps the single phase transformer was so large that the magnitizing in-rush current was large enough to trip the breaker?
daestrom
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The inrush current depends upon the phase at circuit closure. As a rule of thumb, try to close at the supply voltage peak.
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wrote:

----------------
That would work but, in the end, resistors are simpler, cheaper and more practical in many cases.
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It has been decades now, but I do recall a situation using some Wabash regulating transformers. I did not know how they worked but I do know they drew excessive starting current with random switches. I got a fast acting reed relay for triggering a triac at the voltage peak. That helped a lot.
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And after doing this, what did you do with the triac- leave it set to trigger at the peak or cut it out after a few cycles?
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This was a capacitor charging supply. Low level electronics were on between pulses. Each pulse used the relay to energize the (almost worthless) Wabash transformer. The small relay closed at the peak of the supply voltage and was kept closed until the charging cycle was completed. The only trick was to adjust this fast relay to close at the voltage peak.
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wrote:

This was about 5kVA. If I did not say so, the relay kept the thyristors on all the time once the pulse started. The thyristor was used just to keep the transformer starting transient in check.
I also was working on other large (for us) charging system. One charged two banks of capacitor banks to plus and minus 19kV using a vacuum switch to change the polarity of the charging supply. We probably used phase controlled mercury thyratrons so as to keep the efficiency high. These were the days when solid state thyristors and rectifiers were ridiculously expensive. My memory for the details are hazy.
Another technique I used was to have a very leaky (reactance) transformer feeding rectifiers to charge capacitors. This method had a very low power factor, Power factor correcting capacitors were used on the input end so that the power factor started lagging at the beginning of the charge and became leading when the charge was near completion. This arrangement led to very slow charging toward the end of the charging cycle. Extra voltage capability was put into the transformer to get up to the desired charge fairly quickly. The slow charge could, if left going long enough, could charge the capacitors to almost double the desired voltage. Thus, to be on the safe side, two independent methods were used to stop the charge when the desired voltage was reached.
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wrote:

I
hired
Most large transmitter power supplies have some means of limiting starting (inrush) current at start up.
Sometimes called step-start or soft start circuit this is usually only the time it takes a second relay to close thus shorting the resistors (about the same amount of time as it taks to snap your fingers twice). In a smaller 3 kW unit one might expect to find 1/2 ohm 20 W wirewounds. After a storm one might expect to find them melted. Keep a box of spares on hand.
Some newer units use solid state devices like SCRs, triacs to slowly raise voltage, current, power.
Transformer in a typical older transmitter" http://www.goodrichenterprises.com/page_oneweb5-02_012.htm
Some cool pictures of an old Gates http://www.radioblvd.com/Gates%20BC-250L.htm
Some larger stuff: http://www.hawkins.pair.com/wcbs_wfan.html
500kw schematic http://hawkins.pair.com/wlw500schematic.shtml
Many cool pictures for thos that like old radio stations http://hawkins.pair.com/radio.html
I just can not remember how big this particular capacitor was but if I can recall correctly it was about a half gallon in volume. It was likely something similar to this http://www.fairradio.com/catalog.php?mode=viewitem&item 47 only much higher working voltage and less capacitance
I merely had to triple the resistors to 6 ohms each. This isn't a 100% cure as a series of power jolts / fast interruption as during a storm can still pop the breaker.
This just happened to me last Friday on a "new" 3 year old instillation. All 3 phase, 200 amp bolt in type breaker.
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says...

This isn't unique. The large ECL IBM mainframes had "inrush limiters", as well.

The inrush limiter was a bank of power resistors in the exhaust stack cooled much like those used for dynamic braking power dissipation on some diesel-electric trains. The contactor used to switch the resistor bank was controlled by the service processor (another computer inside the mainframe). I got called in when a few $20M systems started smoking in customer's offices. As it turned out, one of the manufacturers of the circuit driving the inrush limiter made parts that couldn't take the inductive kick of the wires connecting the driver to the contactor (yes, the freewheeling diode should have been placed at the driver but it was placed at the contactor for several reasons) and went into secondary breakdown, causing the contactor to stay closed and rooms filled with smoke.

FETs at lower power.

Interesting pages. Michael Terrell pointed me to them a while back.
<snip>
--
Keith

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I really have no experience with more modern power supplies for radio transmitters. At one time I was deeply involved in capacitor charging supplies. Since then, the technology has greatly improved.
The problem with capacitors is that they act as short circuits when discharged. Many modern charging supplies rectify the main ac power and then use solid state switching to match the supplies output voltage to the voltage on the capacitor bank. Using polyphase rectifier reduces ripple. Free wheeling diodes allow circulating current to charge at low voltage so that little power is taken from the line until the capacitor voltage builds up.
There is no reason why large transformers would be needed at transmitters. Rectify the 60Hz directly from the line. Use thyristors or transistors to switch at high frequency, 25kHz and higher, so that small well insulated transformers can be used.
Bill
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wrote:

starting
the
the
3
one
raise
One of my newest transmitters, which is all solid state uses 5 power supply modules, switching of course, which are then combined. They must be matched exactly or trouble light flash and unit will not make full power. The modules are not field serviceable and not field adjustable. Sooner or later this will make a $78,000 + investment so much junk. This same unit used 10 RF amplifiers which combine, also not field serviceable.
Repair of switching supplies is pretty hard compared to linear and the voltages and current involved make it quite deadly to work on.
Now picture you self at the top of a mountain after your snow cat nearly fell off the road a few times. Its midnight and there isn't heat because the transmitter is used to heat the building.and its broke. Now tell me if you would really be in the mood to troubleshoot a switching supply down to component level?
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However open delta has some drawbacks for 3-phase loads, as stated below. ______________
A.C. POWER DISTRIBUTION FOR OPTIMUM TRANSMITTER PERFORMANCE
by J. B. Pickard
For many years, Harris engineers have recommended that the three phase power distribution system should be either a closed delta or WYE configuration to provide better radio and television transmitter performance by helping prevent line unbalance. Operation with substantial voltage unbalance from line to line results in higher than normal signal-to-noise ratio in the transmitter output signal, increased three phase transformer heating, and overly hot three phase motors.
Overheating From Line Unbalance Even a device as simple as a three phase motor should be operated from a power line in which the voltage is balanced within 1%. It takes only a 3.5% line unbalance to produce a 25% increase above normal temperature. A 5% unbalance will cause destructive temperature rises of 50% greater than normal!
Similar characteristics can be expected in the windings of a three phase power transformer down inside the cabinet of your transmitter. Transformers and motors can be designed with extra safety features where thermal rise is limited to acceptable levels; however, in this case, other transmitter parameters cannot be made acceptable at a reasonable cost.
Transmitter Noise Performance The most difficult parameter to meet with power line unbalance is transmitter AM noise performance. Most large transmitters use six-phase or twelve phase high voltage power supplies. The energy storage capacitors are expensive to install and large stored energies make destructive faults inevitable. A good design will have sufficient energy storage capacitors to meet the specified signal-to-noise but not much more. When the equipment is then operated from an unbalanced line, the power supply ripple frequency will be twice the line frequency instead of six to twelve times. It becomes obvious that it would take three times as much energy storage to achieve the original performance goal.
The Causes of Line Unbalance How does a line unbalance occur? It is a rare case in which a large commercial power producer would generate unbalanced voltage, so we must look elsewhere in the system. When you have large single phase power users on a power line this can cause uneven distribution of the line currents in the system. Uneven currents through balanced impedances will result in line-to-line voltage unbalance.
Another likely source of this problem can come from unbalanced impedances in the power distribution system. Unbalanced impedance will always be seen when an "open" delta three phase distribution system is used. Transformer design textbooks clearly show that the voltage regulation of an unbalanced system is poor.
Three Phase Delta Distribution Transformers Figures 1 and 2 show closed and open delta systems. The closed delta impedance looking into each terminal (A, B & C) is exactly the same; but this is not the case in the open delta configuration. Depending on the impedances of the transformers in the open delta circuit, line voltage unbalance sufficient to impair satisfactory operation of the overall transmitter may result.
The only advantage of the open delta is lower cost, and this is partially offset by the fact that two transformers capable of 0.577 the total kVA are required instead of three 0.333 kVA transformers. Harris customers have experienced difficulties with open delta systems -- but when a third transformer was added to close the delta, the problems disappeared.
There is another problem that can occur with an open delta system, and that is caused by lightning and switching transients. When lightning strikes or heavy loads are switched on a power distribution system, high voltage transients are propagated throughout the system. Unbalanced impedances will enhance these transients and can cause transmitter damage, particularly to solid state rectifiers.
Many transmitters are located at the end of a long AC transmission line which is highly susceptible to transient phenomena. Devices such as Metal Oxide Varistors are inexpensive and very effective in reducing overvoltage spikes. These units are limited in the amount of energy that can be dissipated, but will handle, if designed properly, very large currents. You can't take a direct lightning hit and still operate, but not many things will. Several Harris customers, upon installation of a third transformer and transient protection devices, have eliminated power line difficulties.
Three Phase WYE Distribution Transformers The WYE connected system is also considered a symmetrical form of three phase power distribution. All impedances are balanced as seen from each terminal (see Figure 3). It is important when using a WYE connected system that the fourth wire (neutral) is connected to the mid-point of the system as shown in the diagram. When this connection is made it provides a path for the zero sequence currents as well as any harmonic currents that are generated due to rectification of the secondary voltages.
Today, many transformers are supplied with all of the primary terminals available so that either a delta or WYE connection can be made. Table 1 shows the different line-to-line voltages that are available with this configuration.
In summary, both symmetrical power distribution systems are satisfactory because of their balanced impedances. Use either a closed delta or a four wire WYE system for maximum transmitter performance. Never use an open delta system just to cut costs. It could cost you dearly in the long run.
J. B. Pickard was AM Product Development Manager at Harris Broadcast Products Division (retired - 1994).
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