UPS supply from Emergency Diesel Generator 4

[zaza123]

Hi All,
i have to give supply to some critical meters which the instrumentation department insists should be given by UPS supply. As there is no source other than DG(Diesel generator) can the supply can be given from DG. In the UPS is there any thing which suggests the incoming suppply should not be from DG.
i am working in Utility , normally one Emergency power supply is given from DG,in case of failure on normal power UPS is run by DG.But in the above case the Supply to meter is continuous.
what u suggest

 

[dpc]

The UPS can run from a local generator source. It is done all the time.

 

[rovineye]

No problem at all.

 

[waross]

Be sure that the UPS will accept a small frequency variation. I have had issues with small UPSs that would interpret any frequency variation as a loss of utility power and go to battery power rather than run on a less than perfect source of power.
Off load to the instrument department the responsibility to select a UPS that will accept slight a frequency variation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Waross has a good point. I've had UPS systems that were somewhat sensitive and ended up on battery power for no apparent reason.

But in general, you should be able to run your UPS from the standby generator - this is really common, for obvious reasons.

 

[rovineye]

Agree with waross. I frequently have to adjust the change-over setpoints on UPSs so that they don't go on battery during normal load fluctuations. Most have that ability, but not all. The procedure is not always in the documentation and can take some hunting to find.

 

[zaza123]

thanks dpc, rovineye & waross for your comments.

 

[ScottyUK]

Check the AVR will behave itself with a highly distorted phase-controlled rectifier load, especially if the UPS is the only load. If you have some AVR-friendly loads like motors or heating then they usually serve to calm things down a bit.


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If we learn from our mistakes I'm getting a great education!

 

[zaza123]

This is going to be a very small UPS meant only for the supply to Instrument meters in site, as there is no other source nearby at site the only way is DG set. there are no other loads to DG.

 

[catserveng]

Depending on the system size, most UPS units will work ok with a stand alone generator.

Some things I have run into,

Some UPS manufacturers have dual setpoints for "on grid" and "island". As pointed out above, utility systems have very good frequency control, and most UPS's go to battery on a very small variation, if you have a generator with a droop governor, and the frequency drops, the UPS will go to battery and not unitl the batteries run out. A UPS with adjustable parameters for voltage and frequency tolerence is usually desirable.

Some UPS units have power factor correction capacitors and when lightly loaded have a large leading power factor, which can cause a generator to lose voltage control and on newer generators cause a shutdown due to an apparent loss of excitation or overvoltage condition.

I have run into a few UPS system with pretty bad harmonic distortion, and have seen both high and low generator voltage as a result depending on how the AVR measures the voltage, other loads, and how robust the excitation system is.

About 80% of the generators going into standby service in our area have a large part of their load as UPS systems.

Some possible recommendations,

Three phase sensing AVR, some of the newer digital regulators do a pretty good job, but the AVR that has never let me down is the Basler SRR (just a tad bit expensive these days)

Permanent magnet or seperate winding excitation power system (like AREP)

Make sure genset manufacturer controls have ability to compensate or disable protections that may cause nuisence trips due to low power factor or high harmonic loads. I recently have been involved with some CAT and Cummins units that the "factory default" protection levels for reverse VAR's or large leading power factor couldn't be disabled and the system have required some other changes to be operated.

Power factor correction capacitors in UPS's should have ability to be bypassed or disabled while on generator power.

EGSA (Electrical Generating Systems Association) has some good guidelines if you need more information.

http://www.egsa.org

 

[itsmoked]

Nice stuff cats!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VTer]

I agree with cats. Another suggestion I would like to point out is to look into High RPM Flywheel (kinetic) UPS system vs. the battery (static) UPS. Flywheel UPS is much more durable, reliable, not temperature sensitive, and has a better LCC due to lower maintenance requirements over years even though the initial cast is higher. One downfall is the fact that it only can provide full load power for a few seconds (typically more than 10 sec) depending how much the UPS is loaded, but if the emergency loads are backed up by a Gen set than all you really need the UPS for is to provide power until the Gen set starts up and “transfer takes place” (not more than 10 sec) in which case flywheel UPS would have the capability to “support” loads until the transfer takes place from normal utility power to emergency generator power. I hope this helps.

Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature. – Nikola Tesla

 

[itsmoked]

Welcome to Eng-Tips VTer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alehman]

I ran in to a case of the loss of excitation problem on a Cummins genset a few years ago. It drove me nuts until I noticed the + sign on the power factor display. If the power factor is such that the generator is outside of its capability curve, there's probably nothing to do except fix the power factor.

 

[zaza123]

thanks catserveng, Vter,alehman for your valuable support.

 

[Handlin]

A couple of important notes on UPS - Standby generator applications.

1) Generator sizing must take UPS harmonics into considerations. Several factors determine final sizing including UPS harmonic filters, generator sub-transient reactance, and non UPS load on the generator.

Rectifier Power
Remember the input power of a UPS is different than the output power, due to its efficiency, losses and battery re-charge. For example a 150KVA UPS has an input power rating of approximately 162KVA.
Some UPS do not have a soft start feature. In this case the generator will have to be sized to accept all the inrush current of the UPS during energization.

Input Harmonics
A standard 6 pulse rectifier generates input current harmonics on the order of 25%-30% these are reflected back to the source and can generate voltage harmonics, which can make it difficult for the generator to regulate voltage properly. These current harmonics can also overheat the alternator windings in the generator.

Sub-transient Reactance “Xd”
It is best for a generator to have as low a sub-transient reactance as possible. This figure directly relates to the voltage distortion that the generator will have to cope with. It is usually given as a percentage. A good Xd is something around 8%-12%

Total Load on Generator
Be sure to take into account other loads that may be on the generator apart from the UPS, such as air conditioning and lighting. Also if there are any pf correction capacitors in the system these can intensify voltage distortion and need to taken into account. The line impedance of the input cabling can also be a factor in the amount of voltage distortion seen by the generator

2) The transfer scheme used between utility and generator needs to be a delayed or closed transition. This will help avoid too fast a transfer and allow the UPS filters to get off line before being connected to the generator. Note Cummins website has a good white paper explaining UPS generator compatability and the generator runaway associated with leading power factor loads