Hi,

Not my field of expertise..but I'm V interested in Q2. As this applies to all sorts of facilities.
My personal preference is a number of stand alone UPS. This avoids everything failing at once, it probably simplifies wiring but increases cost.

There must be some some point of maximum effectiveness.

I s'pose the other alternative is to have your own generator for the whole establishment.

Del

Thanks Del,

We do have a 100 percent back up system through diesel generator sets yet there are some equipment like Nuclear Magnetic Resonance Spectrometer and a hundred others alike, for which uninterrupted power supply is very crucial. Standalone UPS does have own advantages but I have noticed that they not only create disbalance in the three phase system but at the same time pollute the power system network by injecting harmonics. I would appreciate if you can advise after considering these factors.

Regards,

Kamal

Del's commonsense suggestion to#2 is the one we should follow.

< pollute the power system network by injecting harmonics> This may turn out to be a misplaced fear.

Pollutes the mains supply? But mains Transformer's star delta windings absorbs all 3rd harmonic distortions. And you can put in a Low-pass(high stop) filter into UPS.

Thanks Sparky,

I do appreciate your thoughtful comments, as for cable sizing, let us for instance consider a 35 KVA, 3-Phase input and 1-Phase output UPS. Would you please let me know the rule of the thumb for calculating the cable size after accounting for the battery charging current in the event of complete discharge.

Regards,

Kamal

Hello again j_kamal

To answer re cable sizes, one would need to know the length of cable involved, along with voltages used.

As it's almost midnight here, I'll check back in the morning.

Meanwhile another Member may have given you an answer.

Cheers.

Kind Regards from far away....

Thanks Sparky,

The lenth of Input Cable is 15 Meters and the connected to the UPS is 22 KW. System voltage is 400 V Phase to Phase.

Regards,

Kamal

The UPS output is 35kVA. By dividing to UPS efficiency you have the input kVA to UPS (without considering the charging power). consider the efficiency to be 85% the input power will be 35kVA/0.85=41.2kVA

Now you can add the battery bank boost charge current.

If the battery bank is 1*100%, Ni-Cd type, 30 Minute. back-up time and the nominal DC voltage is 110V. Roughly the battery size will be 91*236AH SBH type and the charging current will be about 20% of 236A i.e 47A. So the charging power will be 47A*91*1.5V=6400W

the UPS input power at the worst conditions will be about 41.2+6.4=47.6kVA

for the 400V system the current will be 47.6/(1.73*0.4)=69A.

Now you can follow the cable sizing.

Well Abbas,

That answers my question. Thanks

Regards,

Kamal

Generally the battery bank is mounted close to the UPS and therefore not much of voltage drop will take place & therefore battery cable operating current density of approx. 1.5/2.0 A/sq.mm is sufficient. But when AC supply is distributed then depending on the distance at which load is located, cable sizing has to be done. You can decide how much is the max. permissible voltage drop at the load end. Say V at current I. Cable resistance ( neglecting the reactance ) will then be V/I. From the cable table, for this Resistance value, check suitable gauge size, for the length concerned.

Last but not the least, though costly, high efficiency ups are ALWAYS preferable. Sometimes high efficiency UPS might even be cheaper, if the battery capacity requirement of a high efficiency ups is lower than that of a lower efficiency ups. Alternatively high efficiency ups would give greater backup time.

Thanks and regards

Ashok Toshniwal, Bangalore, India

What exactly means: indiscriminant use of UPS? Having too much back up power? Please clarify.

If single UPS (wow what a battery) safeguards power to the whole campus, single UPS failure (or down time) removes said safeguards for the entire campus...including safeguards like emergency services dispatch.

Yes exactly!

You know in a scientific institute, people are paranoid about availability of uninterrupted power supply. I have come across guys who would buy UPS just for sake of it. Anyways, that is the reason I talked about centrailzed system in the first instance.

Regards,

Kamal

Okay, so you're saying "indiscriminant" means, un-centralized (I think). Seems to me, however, that that's backwards; and that centralized would be (or would tend to be) the most "indiscriminant." (Here I'm ignoring the fact that localized UPSs will probably still be installed, central UPS or no.)

You speak of a scientific institute and of paranoia about uninterrupted power. But you make it sound as if the institute is peopled by a bunch of yahoos without a clue. Would it be unreasonable...paranoia about loss of critical (scientific research and such) information stored in computers and in instrument controllers? Would it be unreasonable to want the (uninterruptible) back-up power resource (online or offline) to be as close as possible to the point(s) of application? For you, could it not be more reasonable to have single point failures in the "system" already partially isolated? Because the UPS's are localized (as well as isolated) near the points of failure symptoms on the grid?

You say you have backup power already in the form of diesel generators, (Possibly you might also procure backup power utility company as is done by businesses in U.S.) So it seems that the concept for centralizing the campus UPS's might be more about "control" and "make work" than about any real need. That said, perhaps a study (involving yourself and end users of the power) might lead (apart from keeping things as they are) to some kind of "hybrid" system (centralize where it makes sense only). Again, bear in mind that total centralizing is quite unlikely to keep localized UPS's from being installed anyway--whether maintenance engineer agrees or not. (Also, there's where you might run into problems--when glitches and transients in the central UPS office begin showing up as (indiscriminant) power failures (UPS switching and alerts) all around the campus. Would you as maintenance engineer want to take on the task of policing all power users to prevent such things?)

So I'm simply asking you to consider carefully whether this is not simply a bright idea looking for an application--as opposed to a real problem looking for a cure.

Hi Mr. Kamal,

You got very good and valuable recommendations. I would like to add only few points.

1. Since your organisation being R&D and under Govt. of India, I hope finance will not be a constrain so long as you can convince the sanctioning authority. So technical considerations are more important. For taking a decision for installing UPS. There are two consideration for calculating the capacity requirement.

(1) What is the frequency of power failure and the time period the power outing is expected. I mean weather your organisation come under regular load shedding from electricity board.

(2) Weather do you really need uninterrupted power supply for all the equipment including lighting and air conditioning or only for few vital machines computers and instruments where uninterrupted power s required for specific process where the power interruption may cause damage to the ongoing research job.

(3) What is the quality of power required for the specific equipment.

From best of my experience I feel you should go for UPS, only for your computers and most frequently used sophisticated equipments.The UPS should be of good quality stand alone UPS (of reputed manufacture) which provide harmonic suppressors and and sine wave output )depending on the capacity of the particular machine with consideration of actual peak load requirement. Here also depending upon the continuity of supply with out break is a point of consideration. If a small break in power supply is not permissible for the process then you should go for online UPS where the equipment is powered from UPS and backup battery and the battery is in floating condition so that power interruption will not have any effect on power outings. Other wise Inverters with change over switch can be used. Why I suggest this is it will reduce maintenance and equipment cost.

For the rest of the load if you anticipate long power failure go for captive diesel generating sets with change over switch. They are cheap and easy to operate and maintain.

As you suggest, the indiscriminate use of UPSs is wasteful on many levels. It takes time to work out what equipment needs to have a maintained supply (eg, CPUs, emergency lights) and what can flicker off and back on when the mains power returns (computer screens, recreational equipment). I recently specified a system that had three levels of backup powering different coloured sockets: white—no backup, direct from mains supply; green—generator backup; red—UPS. There was also a DC line that offered the highest level of power security as the equipment ran from the battery bank all of the time.

The more equipment that can be run off ELV DC power supplies, the better. 12V has advantages in terms of availability of equipment but 24V and higher has advantages in lower losses in the conductors (due to reduced current).

My preference is for centralised systems and good quality deep-cycle cells but distribution distance becomes a problem unless you use a high voltage. If individuals purchase their own stand-alone UPS for their own equipment, it will typically be undersized and overpriced and have too much plugged into it.

It sounds like you have quite a job on your hands! Good luck.

Here's a couple of articles that would be helpful to determine which avenue will be beneficial to your Gov't.

This is the sixth article in a series covering basic engineering and code issues for standby generators and critical systems used in commercial building.

When a power event occurs, the uninterruptible power supply provides the temporary ridethrough until the generator kicks in. In this installment of the series, I summarize some of the rotary and flywheel UPS technology available today. These technologies will be evaluated in conjunction with standby generators for critical environments.

A majority of UPS backup systems that I have designed are provided with a battery arrangements. These systems are typically provided with five to 15 minutes of battery backup at full load. The majority of these systems in a critical environment are backed up with a standby generator. If the generator is also serving life safety loads, as specified in the National Electrical Code Article 700, it must be running and providing alternating current power within 10 seconds of a power outage. Even if the generator is not designed for life safety, it can typically be up and running within 10 to 15 seconds after a power failure. Most of the flywheel UPS systems will supply 15 seconds of full-load power and can actually serve up to several minutes at less than full load. In addition, most of the utility disturbances will last for 5 seconds or less. For these reasons, a rotary or flywheel UPS can be a viable option over conventional battery backup systems.

A DC flywheel system can be utilized by itself or in combination with batteries. The DC flywheel would feed the inverter of a UPS system. This combination approach with a DC flywheel and batteries would represent higher first cost, but would provide for a longer life and potentially simpler maintenance. In addition, a system that only implements a DC flywheel requires a smaller footprint than a battery system. A conventional battery-powered UPS system that is augmented by a DC flywheel could serve the majority of utility outages—typically lasting for five seconds or less—through the flywheel and not the batteries. This would greatly increase the life of the batteries.

Other advantages of a flywheel (kinetic energy) system in lieu of a battery system include the following:

• Flywheel reliability is typically greater than a single battery string.
• A flywheel requires less maintenance than battery systems.
• Flywheel UPS systems do not require the controlled environment that a UPS battery system requires. A battery system must be kept close to 77ºF for good performance and life expectancy. In addition, a battery system can also represent a risk of explosion or acid spill.
• A flywheel will take up less space than a UPS battery system.
• A battery system can require spill control and neutralization and hydrogen sensors. Eye wash stations can also be required with battery installations. In addition, the battery system should be continuously monitored to ensure top performance and life expectancy.

The rotational speed of the flywheel (kinetic energy) has a dramatic effect on the amount of stored energy available for critical loads. Doubling the rotations per minute of the flywheel will quadruple the available stored energy. High-speed flywheel systems require a different design approach than low-speed systems. The former are typically made from carbon or carbon and fiberglass composite materials. These materials can withstand the higher stresses associated with the high-speed systems. In addition, these systems utilize magnetic bearings and vacuum enclosures to reduce rotational friction and system losses.

In a hybrid rotary UPS approach, a motor generator incorporates two fully rated power paths. The first path is through a conventional rectifier and inverter and the second path is around the rectifier and inverter, through a static switch to the motor generator. (See diagram below). In this situation, the rectifier continually provides power to the batteries, but the power is normally regulated and conditioned through the motor generator. The rotating inertia of the motor generator will provide a power ride through for brief utility power interruptions. The motor generator provides for true power isolation. No utility power disturbances pass through to the critical load. When the main power fails, the DC batteries provide power through the inverter and to the load through the motor generator. The inverter is normally off in this situation. The benefits to this design scenario include the following:

• The system provides for two conditioned paths of power.
• The system can provide for two independent power sources. The batteries are only utilized if both paths of power fail.
• The hybrid system can have a greater overload capability.
• The hybrid uninterruptible power supply system approach can offers high fault clearing ability without going into bypass.

This diagram represents a hybrid approach. The motor generator provided continuous conditioned power to the load. The rotating inertia of the motor generator will provide a power ride through for brief utility power interruptions. The inverter will only operate when the power fails for more than a brief interruption and the generator is in the process of starting.

Another UPS system approach is called kinetic power cell technology. The system offers a line-interactive UPS system. Voltage transients are suppressed through the transient voltage surge suppressors. When the system senses a voltage sag, the system takes energy from the flywheel to maintain the output voltage. These systems are matched with the generator to provide for seamless power. These systems claim a UPS system efficiency of 96% to 97%. These systems indicate that there is no restriction on the number of energy discharges. In contrast, a large number of energy discharges on a battery system can reduce life expectancy.

Moreover, flywheel UPS are "generator friendly". By reducing harmonic distortion and by eliminating block loading, the generator systems may not have to be oversized when feeding a flywheel UPS. This is typically not the case with battery UPS. Due to uninterruptible power supply system efficiency losses, the harmonic distortion created by the UPS system as well as the potential simultaneous charging of the battery supply after discharge and serving the critical loads, a generator feeding a battery UPS system may have to be oversized. Voltage distortion can be the result of a heavily loaded generator feeding a large UPS system that creates harmonic distortion.

Some of the flywheel UPS manufacturers indicate that a short ridethrough of 15 seconds can be adequate in lieu of the traditional 15 minutes typically provided in a battery UPS if the system is designed with higher reliability. The intent of the argument is that a highly reliable data center is counting on the generator to start the first time, every time.Strict maintenance and redundant starting circuits can ensure that optimum reliability is attained. In addition, redundant generators are typically designed into a high end data center.

This approach may not be appropriate in a typical Tier I data center or facility installation with a single standby generator. In addition, these installations will typically not have a 24-hour maintenance and operation staff that follow a rigorous fuel and battery system maintenance protocol. Generator starting reliability in these types of facilities is not going to be as high as in a Tier IV data center.

These manufacturers further indicate that the conventional wisdom is that a 15 minute backup system provided with a battery UPS system will allow time for an orderly shutdown and additional cranking of the generator. The argument against this conventional wisdom is that a 15 minute orderly shutdown will still result in an unacceptable loss of load.

Furthermore, if the generator does not start within the first few seconds, the chances are that it will not start within 15 minutes. In addition, flywheel UPS vendor literature indicates that the reliability of VRLA batteries drops off considerable two to three years into the life of the UPS system. This in and of itself can reduce reliability of the battery system over the flywheel option. One study indicates that batteries cause 70% of the failures in a conventional battery operated uninterruptible power supply system.

Generator starting reliability can be improved when a flywheel system provides 24 volts DC to the generator starting system. The power supply from the flywheel system to the generator starting system can be monitored by the UPS system to add even greater starting reliability.

The design engineer should understand the owner's objectives, maintenance and operations procedures and comfort level concerning back up times. With this information at hand, the rotary or flywheel UPS system options do represent a viable solution and should be considered on future projects.

Flywheel vs. Battery

Rudy

A1) This is a matter of discussion with the end user(s).

A2) This is a matter of discussion with the end user(s).

A3) Use the electrical standards prevailing in the country of installation. In the UK, BS7671 applies, for example.