Greg Carttar
Member
- Location
- Kirbyville, MO, USA
Greetings to all.
I am doing a study and experimentation for a problem that has emerged since the wide use of switch mode power supplies for large audio amplifiers (10kw+) has become commonplace in my field. These power supplies are very non-linear, and under some circumstances can cause very large neutral currents in Y120/208 stage power distributions.
During conservative operation on 120v (wide crest factor) the neutral currents are manageable, but still excessive. When these amplifiers are driven very hard and limiting kicks in, the crest factor narrows and the neutral currents can rise very quickly to large values. A quick rough calculation that does not take into account the efficiency of an amplifier suggests (and testing confirms) that a 10KW amplifier can ask for 80 amps or so peak on 120v.
Clearly this is an untenable situation with 20 or 30 10kw amplifiers wailing away. So I am looking into operating the amplifiers on a delta connection. Most of these amplifiers are voltage sensing and auto range when they see the voltage to which they are connected. Some have "world" power supplies that can operate on single-ended (120 North American or 240 European with neutral), split phase (120/240), 3-phase wye (208 North American) or 3-phase delta with a variety of world power formats.
Some do not. My area of experimentation is with those that will only operate on 120 with neutral and ground, 240 with neutral and ground, 240 with two hot legs and ground, and in a pinch, on "single phase" 208 (two legs of North American Y120/208 with ground.
In these power distributions, there is also the need for 120v circuits for "trivial" loads (no device exceeds more than 10 amps or so) for mixing consoles, processing equipment, stage power for guitar amps, etc.
I have determined that a particular amplifer I am working with that is designed for single ended 120v or 240v and split phase 240v, will also run satisfactorily on 208v "single phase" (two legs of 208 and ground).
It is common practice in stagecraft to mix Y and delta connected loads on Y120/208 (3 legs, neutral, ground) sources. For example, the electronics run on 120v (single leg, neutral, ground) and other equipment such as 3-phase hoists to suspend loudspeakers (3 legs and ground) run on a delta connection to a Y secondary source.
Nothing suffers from this mixed connection.
A quick note about how these amplifiers are used. In a perfect world, with normal music, there is a wide differential between the average power output level and the peak power output level, as much as 10db, i.e. 1000w vs. 10,000 watts. The amplifiers are generally capable of short term bursts of 200ms or so at the high power level and they spend most of their time at the lower power value. The joule storage in the amplifiers generally take care of these peaks. When operating in this profile, the average current draw is actually very low. 10-20A or so per amplifier.
However, some types of modern music contain passages with sustained low-frequency notes that become long-term RMS in character rather than peaking. That's when these amplifiers draw BUCKETS of current as the joule storage cannot be replenished and the switch-mode power supplies have to ramp up to supply the output stages.
Some of these systems are VERY large. 200-400kw or more systems are not unusual, and there has recently been a major tour go out with a system that approaches, and can exceed, a megawatt of peak power. In these very large systems, we are generally talking about amplifiers that operate from a 208v delta connection.
My question is about medium to large systems that have amplifiers that do not operate from 208 delta, and operate from Y120/208 with neutral. There can be astonishing neutral currents in the power distributions during peak power output and especially long-term maximum power output. Worse, the character of the current draw is a pulse width modulated power supply in each amplifier that is running at a 180khz to 250khz switching frequency. There is all manner of garbage on the system neutrals. The power supplies are free-running and do not utilize a master clock, so their current draw is not synchronized and the spectral content of the garbage on the neutral is very complex.
What I want to do to largely eliminate or control neutral problems, given that mixed wye and delta connections are commonplace on a wye facility source transformer or generator (generator being a whole additional issue), by running amplifiers that are not designed to run on 3-phase, but which will operate satisfactorily on 2 legs of 208 on individual connections to 2 legs of a delta connection to a wye transformer in rotation (i.e. amp #1 on X-Y, amp #2 on Y-Z, amp #3 on Z-X) in arrays of 3 or 6 or 9 amplifiers while still using a the wye connection for trivial 120v loads.
It is my perception that since the neutral of the wye secondary of the facility source transformer or a wye connected generator is bonded to ground at that source, that the essential function of the grounding of the delta connection is satisfied, as if there were a zig-zag or delta-wye grounding transformer for ground fault protection.
The amplifiers all have substantial chassis ground connections, and the power supplies float within the chassis, since they can accept either hot/neutral/ground or hot/hot/ground by design.
Is my perception that ground fault protection for both Wye and Delta connected equipment exists by virtue of the neutral of the wye source secondary being bonded to earth correct?
Whew!
Thanks for considering my question. I read and watch your forum and facebook page religiously.
Greg Carttar
I am doing a study and experimentation for a problem that has emerged since the wide use of switch mode power supplies for large audio amplifiers (10kw+) has become commonplace in my field. These power supplies are very non-linear, and under some circumstances can cause very large neutral currents in Y120/208 stage power distributions.
During conservative operation on 120v (wide crest factor) the neutral currents are manageable, but still excessive. When these amplifiers are driven very hard and limiting kicks in, the crest factor narrows and the neutral currents can rise very quickly to large values. A quick rough calculation that does not take into account the efficiency of an amplifier suggests (and testing confirms) that a 10KW amplifier can ask for 80 amps or so peak on 120v.
Clearly this is an untenable situation with 20 or 30 10kw amplifiers wailing away. So I am looking into operating the amplifiers on a delta connection. Most of these amplifiers are voltage sensing and auto range when they see the voltage to which they are connected. Some have "world" power supplies that can operate on single-ended (120 North American or 240 European with neutral), split phase (120/240), 3-phase wye (208 North American) or 3-phase delta with a variety of world power formats.
Some do not. My area of experimentation is with those that will only operate on 120 with neutral and ground, 240 with neutral and ground, 240 with two hot legs and ground, and in a pinch, on "single phase" 208 (two legs of North American Y120/208 with ground.
In these power distributions, there is also the need for 120v circuits for "trivial" loads (no device exceeds more than 10 amps or so) for mixing consoles, processing equipment, stage power for guitar amps, etc.
I have determined that a particular amplifer I am working with that is designed for single ended 120v or 240v and split phase 240v, will also run satisfactorily on 208v "single phase" (two legs of 208 and ground).
It is common practice in stagecraft to mix Y and delta connected loads on Y120/208 (3 legs, neutral, ground) sources. For example, the electronics run on 120v (single leg, neutral, ground) and other equipment such as 3-phase hoists to suspend loudspeakers (3 legs and ground) run on a delta connection to a Y secondary source.
Nothing suffers from this mixed connection.
A quick note about how these amplifiers are used. In a perfect world, with normal music, there is a wide differential between the average power output level and the peak power output level, as much as 10db, i.e. 1000w vs. 10,000 watts. The amplifiers are generally capable of short term bursts of 200ms or so at the high power level and they spend most of their time at the lower power value. The joule storage in the amplifiers generally take care of these peaks. When operating in this profile, the average current draw is actually very low. 10-20A or so per amplifier.
However, some types of modern music contain passages with sustained low-frequency notes that become long-term RMS in character rather than peaking. That's when these amplifiers draw BUCKETS of current as the joule storage cannot be replenished and the switch-mode power supplies have to ramp up to supply the output stages.
Some of these systems are VERY large. 200-400kw or more systems are not unusual, and there has recently been a major tour go out with a system that approaches, and can exceed, a megawatt of peak power. In these very large systems, we are generally talking about amplifiers that operate from a 208v delta connection.
My question is about medium to large systems that have amplifiers that do not operate from 208 delta, and operate from Y120/208 with neutral. There can be astonishing neutral currents in the power distributions during peak power output and especially long-term maximum power output. Worse, the character of the current draw is a pulse width modulated power supply in each amplifier that is running at a 180khz to 250khz switching frequency. There is all manner of garbage on the system neutrals. The power supplies are free-running and do not utilize a master clock, so their current draw is not synchronized and the spectral content of the garbage on the neutral is very complex.
What I want to do to largely eliminate or control neutral problems, given that mixed wye and delta connections are commonplace on a wye facility source transformer or generator (generator being a whole additional issue), by running amplifiers that are not designed to run on 3-phase, but which will operate satisfactorily on 2 legs of 208 on individual connections to 2 legs of a delta connection to a wye transformer in rotation (i.e. amp #1 on X-Y, amp #2 on Y-Z, amp #3 on Z-X) in arrays of 3 or 6 or 9 amplifiers while still using a the wye connection for trivial 120v loads.
It is my perception that since the neutral of the wye secondary of the facility source transformer or a wye connected generator is bonded to ground at that source, that the essential function of the grounding of the delta connection is satisfied, as if there were a zig-zag or delta-wye grounding transformer for ground fault protection.
The amplifiers all have substantial chassis ground connections, and the power supplies float within the chassis, since they can accept either hot/neutral/ground or hot/hot/ground by design.
Is my perception that ground fault protection for both Wye and Delta connected equipment exists by virtue of the neutral of the wye source secondary being bonded to earth correct?
Whew!
Thanks for considering my question. I read and watch your forum and facebook page religiously.
Greg Carttar
