3 minutes Ride Through for VSD


Thread Starter

Derek Chan

I have a project which requires to keep a number of VSD Driven exhaust fans running for 3 minutes in the event of power failure. 3 minutes is the maximum time that the Generator need to cut in. The VSD Drives we are using are :- Allen Bradley 13361 PLUS II Adjustable Frequency AC Drives or equal Input is 480 VAC, and Fan is rated at 175 HP. We expect to have 4 x 175HP taking DC power from a common battery bank(3 run + 1 standby). The Battery bank VDC will be fed to the VSD DC BUS probably through a diode/isolation switch, transistors. In designing the circuit, some considerations we have thought of are :- - Calculation of DC Bank Battery Power rating - Calculation of Discharge Voltage to keep within the DC BUS +/- Tolerance during the 3 mins - Protection against regeneration effect from the DC bus feeding back to the Battery Bank - Consideration of Isolating the Batteries when it is being charged as charging voltage will probably be higher than the DC BUS - Speed of Detection of Power Failures, or DC BUS LOSS etc - Cutting out the battery bank if power returns before the 3 mins is up - Prevent further discharge until the battery is charged up again. - Design for disabling the battery circuit, if the VSD is in the OFF position - Interlock to prevent the Battery Bank from starting up the VSD on black start etc... Anybody out there with experience in putting together a Battery Backup RIDE THROUGH for VSD ? or can recommend a company which provide such products or services ? Any importants, or comments will be welcome. Thanks Derek Chan

Gee, Jeffrey

Instead of constructing your own backup system, have you thought about buying an uninterruptable power supply? (UPS) Try www.mgeups.com They manufacture very large systems (up to 750 kVA), however they will take up a lot of space and are quite expensive. However, it should take out a lot of the risk involved with designing your own. Maybe you have already thought of these and eliminated it from consideration...just an idea...good luck! Jeff Gee

Darold Woodward

You might find equipment that is better suited for the application at Robicon. They make both UPSs and VFDs and probably have some experience with this type of application. You should also look closely at the requirement that is driving the specification and verify that you have properly evaluated the requirements. This seems pretty stringent and will be expensive. Darold Woodward PE SEL Inc. [email protected]
Consider operating all the drives from a common dc bus. You can support that dc bus with an diode isolated battery source which has a value less than the normal ac line support dc bus value(650VDC). When input power is lost, power will automatically flow from the lower voltage(<680VDC) to the common bus for all drives. The output voltage may be reduced slightly due to a lower dc bus voltage but unless it causes a bit more motor current no action should be required. If too much additional motor current is drawn, a slight reduction in motor speeds will compensate for additional motor current. When ac line recovers, normal dc bus will backbias diodes and stop current flow from batteries. This method can be used with or without common dc bus but will require separate diodes for each drive to individual battery groups. When a single dc bus supply is used only one ac source feeds that supply. Contact vendor for specifics on how to use common dc bus with existing equipment. Sizing of battery source would be responsibility of user since only user knows how much energy is required for the 3 minutes. Important to design and size system to avoid problems associated with precharge circuits during power recovery.
Using UPS as you said will requires a VERY large systems. As it is now, DC battery Bank System is already very expensive considering just the batteries alone. The example I gave for for one such battery bank. We really need 8 battery banks supporting a total of 22 drives ranging from 175HP to 30 HP. It is not a practical solution to use UPS as we will then be converting the UPS DC Voltage to AC, which the drives will convert back to DC anyway ! Best solution will be to look at Ride Through Modules from suppliers that makes this kind of hardware, and use it with DC battery bank. The CRITICAL component we are looking for is a FAST switch that cuts in the Battery Bank when it detects a power failures or a DC Ride Through Module which monitors dip in the DC Drives to cut in the battery bank. We are not really going to built from scratch. We want to "Integrate" a system using off the shelf components. That is what we are looking for. Safety Interlocks circuits, operation & maintenance considerations can be built by us around the critical components.

Amr Elaguizy

Derek, I just came from a project that require a UPS system to keep Motor control centers and other equipment (a ton of load of servers running) till the utility is restored or the generators are on line. I realize your application requirement is lower in KVA but you could use a similar system. The company is called PowerWare. They are located in or close to North Carolina. If you can not locate the company E-mail me and I will get you their address. Batteries were stacked each had 700AmpHour(?) if my memory served me right.
We have actually reviewed the method using only diode blocks. Then problems are the following a)Problem No. 1 We have the keep the Isolated Battery Bank below the nominal voltage, assume nominal-5%. This means, in the event of power failure, while discharging the battery, we have only a very small range before the voltage drops below the minimum, which is nominal-15%. We only have 10%. Not enough span for voltage to drops, and also means lots more battery even if we go this way. b) Problem No. 2 The DC BUS is really not like your normal equipment we accepts DC voltage with a plus/minus tolerance. The DC BUS is part of the drive energy reserve, to keep drive speed constant. For example when the drives accelerate, it draws more current, DC bus voltage will drops. When the drives decelerate, back emf pump energy back into the dc bus, and voltage goes up. With the battery bank connected, the DC battery will be charging the bus when the drives accelerate ! c) Problem No. 3 In a black start, the VSD has a pre-charge circuit which limits the current to charge up the capacitors across the bus. This means we need to isolate the battery bank on a black start. In summary, it is not a good idea to just use a diode block. We will need a additional transistors switch to turn on the battery bank in the event of power failure. Why transistor switch ?...when the power fails, the DC BUS drops very quickly, and we need to turn on QUICKLY before the drives speed is affected. I am still trying to get a resonable feel of this time. For starters, the drives can survive a power dip(NOT TOTAL LOSS OF POWER) for 500 mSec. I assume with total power loss, we may be looking at 100 mSec maybe ?. This switch will have to turn off, when power returns. So we are really loking for a company/individual which can provide a custom design for this switch. Or Alternatively, we are looking for DC RIDE THROUGH module with batteries which can last 3 mins. Any leads ? Derek Chan


FAST. CHEAP. GOOD. Pick two, Maybe it is just me, but this thread is starting to take on a tone of " I want to have a backup system to power my entire production facility, that will respond in the microsecond time frame, and I want to pick it up down at Wal-Mart (tm)." I think the bottom line is that you are not going to be able to string together a couple of DieHards from down at Sears and make this work. Without knowing what you have for a process, I would also look at: 1. Can your generators be brought on-line in less than 3 minutes? That is really a long time if this is a critical as you make it sound. 2. Investigate what your VSD's are capable of. At a previous employer, we had several pumps that ran off of drives, that we set up to restart automatically if it encountered a power loss while running. This made it easy to ride through momentary power loss ( a few seconds) as the motors would coast, and then pick back up. I do not know how this would affect your operation, but consider it, as it might not be as bad as you think to get a slowdown. This would give time for contactors to close, etc. to bring your bank of DieHards online. If your process could survive a .5 second power loss, this would be a viable option. 3. Finally, and perhaps someone can help with the specifics, but I recall that Control, Control Engineering, or Plant Engineering had an article about 6 to 9 months ago about flywheel style backup power generators. These consisted of large flywheels that were kept spinning by motor power, and if a blackout occurred, the motor would regen power back into the system. There was a controller that would do the switching to minimize the bump that was seen by the equipment. This was an engineered solution, however. --Joe Jansen
Thanks all for the discussion. We did managed to find an off-the-shelf solution. Have a look at http://www.bonitron.com They are into making ride through for VSD, and they have a BATTERY version which can substain for 4 mins. Anybody knows of similar competing products/Companies ? Derek Chan
There are two possible solutions in your case: 1. An AC UPS system, which is the connection of a standard UPS to feed your VSD’s 2. A DC UPS system, which is the connection of a battery charger in parallel to the rectifier(s) of the drives and a battery bank in parallel to the DC link. The first solution seems more standart and less problematic, but it is not. You have to consider the harmonic content of the VSD input current, which is about 38% in 6 pulse and about 12% in the 12 pulse system. If your load was not a fan, then you have to think about the starting current demand of the motors. Most AC UPS systems can deliver 150% overcurrent for a limited time, whereas the starting current demand of, for example, a transfer belt motor can be 250% of the nominal current. Altough this is not a problem in your case, because of the harmonic content you have to oversize the AC UPS. For example, for 200 kVA of VSD size I would suggest at least 300 kVA of AC UPS (even more with a mixer load). Another disadvantage of this solution is dublication of components. A standard UPS is basically consisted of a controlled rectifier, a DC link with battery and an inverter (I use the term inverter as a block which converts DC to AC). But you already have the inverter in VSD. Therefore this solution is more costly in investment, maintenance and less efficient, because of the losses in the UPS inverter. The second solution is cheaper (at least 2-3 times), more reliable due to fewer components, needs less maintenance and more efficient. Let’s discuss and design the necessary components for the second case then. I wil suggest to use one rectifier with enough power to feed the DC link and connect the 4 inverters to this DC link. This solution is cheaper than using standard VSD’S (having seperate rectifier/inverter blocks). All major companies (A-B, Siemens, ABB) have this common DC link solution. However, you can use also prefer standard VSD’s, but in this case you must have 2% input commutating reactor to maintain the current sharing. If we come to the DC UPS part, we first find out what should be the voltage level just before the interruption and what should it be after 3 mins. The battery bank will be connected parallel to the DC link, thus it should be at the same nominal voltage of the DC link. For 480 V AC input, the nominal DC link voltage will be 678 V. Since the commercially available lead-acid battery (which is the cheapest) blocks are either 12 V or 24 V (6 V is also available but more expensive), we have to connect here 57 blocks of 12 V batteries in series which makes a nominal voltage of 684 V. A 12 V battery block is a series connection of 6 cells, because the nominal voltage for a lead battery is 2 V/cell. The lead battery should not be discharged below a certain level (about 1.7 V/cell), which is called “deep discharging” otherwise Pb particles will be to high in the acid solution to cause short circuits in the battery cell. However, there is also a lower limit which most inverters can accept (say 10% to be in the safe side), thus we simply say that the end of discharge voltage is 684 x 0,9 = 616 V or 1,8 V/cell. In order to maintain lead batteries are charging, we should have a voltage higher than 2,2 V/cell. We can safely choose a “floating voltage” of 2,3 V/cell, because 15% of DC overvoltage is acceptable in all VSD’s. If you want “fast charging” (like in NiCd case), you must disconnect the battery bank from the DC link, which I do not recommend. Now we have some figures: Starting voltage: 2,3 V/cell End of discharge voltage: 1,8 V/cell 57 blocks of 12 V batteries in series connection for 600 kVA load for 3 mins. Now you can calculate Ah capacity and type of batteries. You have to choose a battery especially designed for UPS use. You know, UPS draws peak currents from the battery. Battery should deliver this. Just look at the preferred manufacturer and use their batteries. You can buy the battery charger from the same manufacturer. The battery charger is a current limiting device and thus should be chosen from the Ah capacity of the batteries. Lets come to your problems: >Calculation of DC Bank Battery Power rating. I did it above, leaving you the manufacturer. >Calculation of Discharge Voltage to keep within the DC BUS +/- Tolerance during the 3 mins. I did it above > Protection against regeneration effect from the DC bus feeding back to the Battery Bank. Very simple: You don’t need to protect the bank. It will accept up to 2,65 V/cell, which is higher than your inverter will fail. When your inverter fails, there will be no charging current to the bank. >Consideration of Isolating the Batteries when it is being charged as charging voltage will probably be higher than the DC BUS. In this solution the charging voltage is in the limits. >Speed of Detection of Power Failures, or DC BUS LOSS etc This solution is uninterruptible. >Cutting out the battery bank if power returns before the 3 mins is up No problem because the battery charger detects when the power is back. You do not and should not cut the battery bank, it should be charged for the next interruption. >Prevent further discharge until the battery is charged up again. In this solution, you cannot discharge the battery further if the mains is on. >Design for disabling the battery circuit, if the VSD is in the OFF position. You can disconnect the battery charger anytime you want. All of them accept digital signals. >Interlock to prevent the Battery Bank from starting up the VSD on black start etc.. You can make a black start from the battery bank. The battery charger will prevent it from deep discharge. There is a little addition to the whole circuit I explained above, which is my know-how gained after some experience, but any power electronics engineer can reach to that result with trial-and-error. I hope I could help you. Best regards, Hakan Ozevin