paralleling an inverter with the AC mains


Thread Starter

Andre Roop

i was wondering if it would be possible to parallel an inverter with the AC mains to provide power for a common load,,,,such that whatever power the inverter supplies would go to the load,,and if the load requires more power it will take it from the AC mains. .....if the inverter cannot supply power,,the load will take all from the AC mains,,,, ,,can this be accomplished,,if so, how?

Hakan Ozevin

Even the inverter is in the mains frequency NO because: 1. The sinusoidal currents will not be in the same phase angle (phi) and the current will flow from the inverter to the mains or the other way round(a kind of short circuit). Consider V=Vmax(sin wt + phi). 2. Provided that you have the same phase angle, you could not have the "load/current sharing", since impedance of the inverter will be much more higher than the by-pass circuit and thus nearly all the current will be drawn from the by-pass circuit. 3. If the voltage waveform of the inverter is: a) PWM, then voltage differences to the by-pass circuit will occur, b) "Pure" (which is not possible) sinusoidal (with LC circuits at the output of the inverter), when your mains frequency and voltage change slightly, high currents will occur between the inverter output and the mains. Maybe other reasons can be add by the list. Thus such a connection will mostly result in the destruction of the inverter. Even when using two parallel inverters, you have to have special controls (e.g. high speed syncronisation, impedance matching). Hakan Ozevin
Hey man thanks for the advice....what you said is just what i thought..however i am trying to explain this to my lecturer, but i can't elaborate on the theory to convince him enough that it cannot work. i would greatly appreciate it if you can just help me elaborate on the points you mentioned, to disprove this concept...... thanks andre,,,,,

Hakan Ozevin

I have to note that in theory, it is possible. You match the output frequency and voltage of the "pure sinusoidal" inverter to the mains and make load sharing circuit (e.g. placing equal impedance on the by-pass line as the inverter). But in practice these are almost impossible, at least with ordinary VSD's. If we are talking about practical conditions, then you can convince your lecturer by making such a circuit and showing the destruction of the inverter! Hakan Ozevin
Don't ever try this! You will damage the inverter etc. This is why in inverter bypass apps, the contactor must totally disconnect the inverter before connecting the mains. I believe a special type of motor-drive technology called doubly-feed uses this general concept but it is exotic and mostly a researcher curiosity.

Randy DeMars

This will not work! The inverter controls the speed of the motor by varying the frequency of the voltage it presents to the motor, as well as varying the voltage. (Motor speed is directly proportional to the frequency.) The power line, on the other hand, will be 60Hz (or 50 Hz, depending on where you are located), and will be at a constant voltage. Wiring these two in parallel will result in black smoke.

Alastair Fordyce

Parallelling standby supplies with the mains is a pretty common thing. We commonly supply systems which allow standby generators to synchronise and operate with the mains supply for peak load lopping applications and true standby power. These systems will re-synchronise with the mains after a power outage and transfer the load back without interrupting the user (difficult to get around the outage when the mains first fails though). I have installed UPS systems that do a synchronised transfer of load to and from the mains which does require the inverter output to connect direct to the mains. These UPS's did have extensive output filters which improved the waveshape. Alastair Fordyce REA, NZCE,CQA Systems Engineer Bremca Industries Limited PO Box 7169 Christchurch NEW ZEALAND tel +64 (3) 332 6370 fax +64 (3) 332 6377 e-mail [email protected]
Beg to differ.... If this could not be done then we would have no DC power transmission (HVDC converters coupled to AC systems) and no regenerative VSD systems. The basic trick is to synchronize the inverter to the system voltage, keep the inverter output voltage higher than the system voltage and filter the inverter output. If the system voltage rises above the inverter output voltage then rectify it and return power to the inverter power source (another part of the system, a motor or batteries). If you want to know details then check out how such systems operate. Also, static transfer switches are available which are capable of 2 millisecond operation. Therefore, as long as the inverter voltage is higher than the system voltage, is sinusoidal, suitable output filtering is supplied and the inverter can be very quickly disconnected in case of line voltage increase such paralleling should work. PTa.

Anthony Kerstens

I agree with the other postings. Don't do this. But I am wondering what it is precisely you need and how did you came up with this notion? Anthony Kerstens P.Eng.
There is an interesting article sort of related to this issue : "UPS Systems and Engine-Generator Compatiblity" by Mike Mosman & Brett Korn. You can find the article at : This first paragraph follows: quote: "Uninterruptible power supply system manufacturers and users have long known about the challenges between interfacing engine-generators (E-G) and UPS modules, especially concerning current harmonics generated by module rectifiers. Current harmonics have adversely affected a wide range of systems, including E-G voltage regulators and UPS synchronization circuits. In response, UPS system engineers developed input filters and applied them to UPS modules, allowing them to successfully control current harmonics in hundreds of UPS applications, these filters have led to critical issues to UPS/E-G compatibility." unquote: An alternative: ------------------------------------------ You might also investigate ROTORY exciters. They have the same characteristics of a electric generator..........mainly because that's exactly what they are. These units are basically a DC motor driving a AC generator with a large flywheel. The controls would require both Watt and Var setpoint control. A autosyncronizer that would be required to allow the rotory exciter to syncronize itself to the AC mains. I've never seen this done before, but, technically it should work. -------------------------------------------------- T.W.(Wade) Carlson, P.Eng. Senior Instrumentation & Control Engineer Instrumentation & Control Department Engineering Services Division Power Production Business Unit SaskPower Corp. [email protected]
okay man thanks...for the advice...i see that there are some inverters that have utility capabilites like the advanced engineering inc GC-1000,,,,,however the one i have to use does not have these i think i will have to demonstrate the destruction of the inverter to him......if you have some time check out and tell me what you think of the inverter....whether it can be paralled,, ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,andre
Several have answered as I have. My assumption (maybe wrong) is that Andre is referring to a "motor" drive inverter i.e. adjustable frequency drives, versus a power supply (UPS) inverter. I back my reply for motor drive inverter apps. After all, this is an "automation" forum versus a power quality etc. forum.

Anthony Kerstens

Did I miss something? The conversation seems to have gotten into UPS's, but the original question was about paralleling an _inverter_ with AC mains. When I hear the word inverter, I think VFD. Anthony Kerstens P.Eng.
,,okay..i see now that you are talking about "motor" drives,,,,,,well cool man,,,,i'm glad that we cleared that up,,, thanks > Several have answered as I have. My assumption (maybe wrong) is that Andre is referring to a "motor" drive inverter i.e. adjustable frequency drives, versus a power supply (UPS) inverter. I back my reply for motor drive > inverter apps. After all, this is an "automation" forum versus a power quality etc. forum. >
I couldnt understand yor problem. Do you have to make a parallel connection to this GC-1000 or what? I checked the site you told me, but it does not give any details. However, I can tell you once more that ANY VSD which does not match frequency, phase angle and voltage to the mains will be destroyed with a connection you mentioned. I told you IN PRACTICE all of them will be destroyed (at some load of course). GC-1000 or any other thing does not make any difference. If your aim is to see a destruction of a VSD, you can choose the one that you hate most! > okay man thanks...for the advice...i see that there are some inverters that have utility capabilites like the advanced engineering inc GC-1000 < This one will fail as I told you. If it does not destroy, it will not work, which is not functioning and no use for us, engineers at all. We, engineers, do not like to investigate how things will fail or not work. This is a very sad point from my side. Hakan Ozevin

Johan Bengtsson

But you won't have any use for the inverter, it would just be a way to use up more electrical power than you actually use (unless you count the heat as "use") The DC power transmission lines do of course work, but they take the power from one AC net and bring it to another, they don't take power from one AC net and feed it back to itself. If you want to syncronize the output from the inverter with the AC network it would be better to turn off the inverter as soon as that is accomplished and transfer to directly driving the motor only, that process could of course be reversed as soon as the inverter is "needed" again. The inverter is only needed when the motor should have a speed different from what it should be when driving it directly and during the times when the output is sychonized to the AC line it could as well be disconnected from the motor and thereby save energy. If the inverter could not be locked to the AC line (since you want to have a different speed) the output could never be parallelled to the AC line anyway. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: ----------------------------------------
Respecting the gentleman's comments, a UPS is nothing more than a VFD with a fixed output frequency. Both have a break before make arrangement of mains bypass (with the exception of course of true UPS operation, or cheap VFD's.)::

Lee White, PhD EE