Paralleling Inverters

[easyser]

For one of our applications, we need to parallel two 440 V, 600 KVA inverters; what are the factors to be considered to see that this works successfully and does not create a problem.

 

Thanks in advance.

 

easyser

[marke]

Hello easyser

 

You need to talk to the manufacturers of the inverters before you attempt to run in parallel. It is very dependant on the inverter design as to if and/how they can be run in parallel.

 

Some systems are designed for parallel operation, others are not. If you attmept to parallel connect inverters that are not desinged for parallel operation, or if you do it incorrectly with inverters that are designed to be run in parallel, you will have difficulty putting all the smoke back in!!

 

Best regards,

[jOmega]

Spot-On, Marke.

 

Consider inverter bridge 1 withoutput A, B, C

and inverter bridge 2 with outputs A', B', C'

connected in parallel by connecting the outputs as follows:

 

A - A', B - B' , C - C'

 

Consder then that a 10-microsecond difference between A & A' or B & B' or C & C' results in a stiff, dc component in the motor.

 

Lots of Current .... Smoke ..... and loud noise .... which is usually followed by the consumption of a large glass of whiskey and a change of clothes. :cool:

[bob]

Hi,

 

Many manufacturers, amongst PDL , Siemens, Asirobicon, are used to this technique and they are working very well. I have got one 1600 kW on site at my place.

 

Regards.

 

Bob

[marke]

Hi Bob,

yes that is correct, but you do ned to ensure that the inverters are designed to be parralled and it is done correctly. It is a case of synchronising the high frequency carrier, not just the low frequency output.

Some manufacturers require a ribbon cable from the master unit to the "slaves" to ensure that the firing signals are all in synch and some manufacturers require output balancing reactors. It is important to work with the supplier to get it right!!

 

Best regards,

[GGOSS]

Good to see you still have your sense of humour jOmega. I really missed that when you dissappeared from eng-tips.

 

Keep it up.

 

Regards,

GGOSS

[marke]

Why do I get the impression that JO has been there done that??

[jOmega]

.... and got the Tee-Shirt, Mark.

 

First experience was in the city of Salamanca, Mexico, at the Petroleos Mexicanos Refinery, and a Cracking Tower Application; Circa 1973. Inverter bridges were 2 x 100 kVA SCR units using McMurray-Bedford commutation scheme. Each bridge housed in stand-alone cabinet approx. 10 feet apart. Bridges fed separate 230v primary wdgs on a specially designed transformer; the 460v secondary of which fed the Byron Jackson motor-pump imbedded in the Cracking Tower.

 

Custom scope probe allowed observation of the A-A' / B-B' / C-C' waveforms; one phase at a time (removed gating to devices of other two phases not under observation at the time).

 

Measured the differential phase - phase' output waveforms to assure that relationship between bridges (phase - phase') was less than 10-microseconds.

 

Once that was confirmed, connected all gating signals to the inverter bridges and ran in three phase mode into the transformer (motor not connected), and ultimately the motor.

 

Transformer was designed by Ken Schrider (an Archetectual Engineer by education) .

 

Exciting times. Great people down there at the refinery. Didn't have much appreciation for my forest green liesure suit and white, mock alligator skin shoes. But I did look good!

 

jΩ

[jOmega]

GGOSS:

 

....awwww shucks, GG

 

I missed you guys too.

 

 

Nice to hear from you again...

 

Kind regards,

 

jΩ

[mariomaggi]

Dera easyser,

I fully agree with Mark, it is absolutely necessary to discuss this problem with the inverter manufacturer.

If the manufacturer is no more active, you must ask a specialized consultancy to avoid risks; in this case you need to install an output balancing reactor and probably you have to resize the total kVA to something like 1000 kVA.

 

Regards

Mario

 

[jOmega]

Mario,

 

I am curious:

 

600 kVA + 600 kVA = 1000 kVA ???

 

So where do you see losses of 200 kVA ?

 

Please explain.

 

Grazie,

 

jΩ

[mariomaggi]

Dear jOmega,

 

So where do you see losses of 200 kVA ?

 

No, there are no losses for 200 kVA. Simply you cannot use 2 units 1xPn at 2xPn

If you read accurately (I'm sorry for my bad English, I'm Italian), I wrote that "if the manufacturer is no more active" (this means that it was not a leader company) and that "probably" it is necessary to reduce the maximum nominal power of inverters. You will have only additional true losses on output reactor. If - for example - stray reactance of cables is different on two units, and carrier frequency is high, you cannot reach an output power of (2xPn) - ReactorPloss.

 

Regards

Mario

[jOmega]

No, there are no losses for 200 kVA. Simply you cannot use 2 units 1xPn at 2xPn

 

Ciao, Mario.

 

If I understand your statement, I cannot connect 2 x 600 kVA inverters in parallel and supply 1200 kVA to a load.

Se capisco la vostra dichiarazione, non posso collegare gli invertitori da 2 x 600 KVA parallelamente ed il rifornimento 1200 KVA ad un carico.

 

Then it must also be true that I could not connect 2 x 600 kVA AC Generators in parallel and supply 1200 kVA to a load.

Allora deve anche essere allineare che non potrei collegare i generatori di CA di 2 x 600 KVA parallelamente ed il rifornimento 1200 KVA ad un carico.

 

So perhaps you will explain why two inverters, connected in parallel, cannot supply their full rating to the load.

Così forse spiegherete perchè due invertitori, collegati parallelamente, non possono fornire la loro valutazione completa al carico.

 

If you are not comfortable explaining in English, then please, explain in Italian and I will try to make the necessary translation.

Se non siete spiegazione comoda in inglese, quindi per favore, spieghi in italiano e proverò a fare la traduzione necessaria

 

I look forward to your reply.

Osservo in avanti alla vostra risposta.

 

 

Grazie.

 

jΩ

 

[marke]

One of the issues must be the accuracy of the sharing and this is dependant on the topology used.

 

Best regards,

[mariomaggi]

Dear jOmega,

thank you for your automatic translations, but automatic translations are not enough good. Please continue only in English. You can make corrections on my bad English, I will appreciate them.

-------------

I agree with marke, topology is an important factor.

 

If I understand your statement, I cannot connect 2 x 600 kVA inverters in parallel and supply 1200 kVA to a load.

 

I'm sure that an actual leader company - inverter manufacturer - could install in parallel 2 units 600 kVA to obtain 1.2 MVA output power.

I'm also sure that if you add an output reactor for balancing, you will have a small derating (i.e.: 1180 kVA) measured at motor cables output.

 

I would recommend - with existent units manufactured by companies no more active (with difficulties to get technical info and/or spare parts) - to avoid to use 2 units in parallel at nominal power. I simply recommend to make some derating, depending from topology.

 

Regards

Mario

 

 

 

[jOmega]

One of the issues must be the accuracy of the sharing and this is dependant on the topology used.

 

Mark E.

 

Absolutely.

 

Additionally, there are other very important issues to consider and to understand just why.... taking two separate inverters (even of the same manufacturer and rating) and paralleling them, should not be undertaken by those who are not well schooled in Inverter / Power Electronics design engineering.

 

More soon ..

 

jΩ