UPS/Generator Power Disagreement

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rpmcmahon

Member
Location
Georgia
:confused:
I must start with some background. We have a client who has a 480/277 Service into a 1000A Breaker. Neutral/Ground bond is located here via a busbar. The ground is a ground ring of unknown size and distance. There is also a supplemental ground wire (#6) attached to a ground rod. The service is then routed through an Cutler Hammer ATS. The service is then routed to a distribution panel. The panel has 2 600A 480/277 breakers. It also has various other "Utility" breakers that feed other distribution panels that go to HVAC units and lights/receptacles through another transformer. The 2 600A breakers go as follows. 1 goes to UPS as Mains 1 and the other goes as Mains 2. I am not sure how the UPS manages these two Mains but there is also a MOTS bypass. This is simply a manual transfer switch to bypass the UPS. Now for the problem. The UPS just runs the power through it until it sees a problem. Now, as mentioned before the entire bldg is through an ATS. This ATS is connected to an 800kW Diesel generator with a Marathon DVR2000 Voltage Regulator. When this generator turns on either by power fail or exercise the UPS automatically triggers itself by some interconnect relay to close an internal switch that switches all power to the rectifiers. Now even though the ATS is connected to the utility company the UPS is rectifying power. (maybe in anticipation of transfer, the UPS tech cannot convey why this is) Now when the ATS transfers to ER Power (GenSet) the power is rectified from the Generator. This is raising havoc on our generator. We cannot stabilize the voltage. The current appears to stay the same...around 360 and the Freq is stable at 59.7. However the voltage is floating between 420 and 490. This causes the UPS to switch periodically to batteries. The "powers that be" recommended that we put filtering on the Voltage Regulator to filter and stabilize the voltage. We installed MOVs to clamp the 3 phase sensing wires on the VR and a 3% Line Reactor on the sensing wires. This, as I thought, had no effect. The VR seems to be doing what it is supposed to. We have about 175kW of coming out of the UPS and the GenSet is supplying about 260kW. So this means that about 85kW is being used for the Utility. This means that 67% of the load is PURE Reactive through the UPS rectifiers. We have paralleled a Resistive load bank with the UPS at 250kW and were able to stabilize the voltage. This tells me that we need to add some resistive load to this unit. HOW? Oh yeah...the UPS tech says that his machine must maintain a SEPERATE neutral/ground bond. That makes (2) Two complete neutral ground bonds on the 480/277 buss. How can we stabilize the voltage? IF more info is needed just ask.
 
catchtwentytwo

catchtwentytwo

Senior Member
Wow, this'll take a while to sort out!
  1. First, it sounds like might be talking about a single module Piller UPS System with a MOTS (Manual Overlapping Transfer Switch). Is that the case?
  2. What model (or vintage) is the UPS?
I had a N+1 750 kva Piller Uniblock II UPS system that was being fed by a single 650 KW diesel (while waiting for 3 - 800 kw generator plant to be installed). The actual UPS output load was around 400 kw. When the UPS was on generator, it "went" to the rectifier/inverter path, and that would bog down the engine. More load and more harmonics in this state.

One "temporary fix" we used is to determine how much the battery recharging adds to the total connected load when on generator and utilizing a feature that Piller has to current limit that while on generator. This is done by using a set of contacts on the building ATS that "tell" the UPS it is being fed by generator and to limit the amount of load applied to recharging the batteries.

Before that is done, consult with the battery manufacturer and get their recommendations so you don't void your battery warranty. I seem to remember that the battery recharge time is typically 7 times the length of the discharge but that might be wrong.
 
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dbuckley

Senior Member
Location
Canterbury, New Zealand
Just two quick pionts, without directly helping your problem :)

Most UPSs have seperate mains inputs for the rectifiers, and the static bypass input.

Many gensets have no other load than a UPS, and thus see nothing but the rectifier load, and thus this voltage stability cannot be purely a function of horribleness of load in the general case.

I'd be having a very close look at the voltage regulator, maybe look at the exciter voltages, how do they relate to the genset output voltage...?

One thing does occur: if you look at the input voltage to the UPS on a 'scope, then the rising side of the sine wave normally has a notch out of it where the rectifiers start to conduct and the current starts to flow, so the voltage drops a bit. Maybe the super duper microprocessor based voltage regulator is getting the wrong idea based on too short a time constant...?
 
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rpmcmahon

Member
Location
Georgia
Addition

Addition

WOW! Yes this is a Piller UPS, and I do not have the model number but it is within the last 5 to 7 years. Enough to have just had a firmware upgrade. I am trying to get the Owner to understand that this is not ONLY a GenSet issue. This is a system issue. The Piller folks must have better Lobbyist than we do as we are being looked at to Fix this problem. Anyway, I may need to look at the sign wave with a scope...also I am thinking that maybe a VR that is not so electronic may be a better fit. It usually works that the older stuff had less trouble than the new stuff.....most of the time. Then again the need for an airbag on a model T just wasnt there. Anyway, where is the voltage sway coming from....the GenSets inability to compensate for the DC Switching?
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
I may have missed it in the post, but did you say that the UPS is rated at 750kVA and the load on the UPS was 175kW? That's pretty low load.
If so, put a power quality meter on the mains to the UPS and then on the output of the generators. There may be a leading power factor or really terrible harmonic distortion. See if there is any type of input filter on the UPS that might be able to be disconnected during low load.
Can you stage the load to have the non-UPS load transfer to generator first .... then the UPS? This may help stabilize the generators VR's ability to "watch" the output voltage.
Along the lines of the VR, is it the DVR2000 or 2000E. And is it single phase or three phase monitoring. The Enhanced three phase version seems to do a better job with UPS loads.
 
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rpmcmahon

Member
Location
Georgia
Update!

Update!

Ok...Let me try to wade through and hopefully answer to the best of my ability. The DVR2000 is 3 phase sensing. I don't know the model and ratings of the UPS but will have them posted here by close of business today. I have put an Amprobe power quality meter on this unit before and can see the rapid "stepping" of the voltage. We suggested to them to add a stationary Resistive load bank to the system to come on only when power was out and GenSet was carrying load....however they are shying away from that due to Fuel Consumption. This would take the kW from about 250 to about 600 but would level the voltage.

Secondly, YES this problem started fairly soon after the "firmware" upgrade. However the Piller UPS tech has been there and confirmed his unit is working A OK. It has seemed to fall back to us. I know there is no Magic Wand but I am just trying to understand what is happening.

Test Results from Previous power monitoring
Voltage Harmonics THD=2.17% H1=276% H3=5.01% H5=0 H7=0
Current Harmonics THD=7.8% H1=86.95% H3=0 H5=3.86% H7=2.86%

Also at the instant when the Power switches from Utility to GenSet the recordings show
ON UTILITY
Apparent Power 19500 VA
Real Power 19130 W
Reactive Power (Ind) 0 VAR
Reactive Power (Cap) 3734 VAR

1 MIN LATER ON GENSET
Apparent Power 23960 VA
Real Power 19700 W
Reactive Power (Ind) 13640 VAR
Reactive Power (Cap) 0 VAR
 
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rpmcmahon

Member
Location
Georgia
update

update

Let me correct that......the Power readings are when the UPS switches the static switch to rectifiers. As previously mentioned when the Gen starts the UPS switches the static switch to rectifiers.
 
catchtwentytwo

catchtwentytwo

Senior Member
It wouldn't be unheard of for a firmware upgrade to have problems but I suppose that depends on how long ago it was created. I hope your system wasn't the "alpha site". Did they refuse to do a roll-back? If they did ask why and what issues the "upgrade" was supposed to fix. You should also ask for a list of customers who received it to see if they are having the problems.

I believe you have only one module, is that correct? We once had a situation where each module of a redundant pair was at a different firmware revision.
 
robbietan

robbietan

Senior Member
Location
Antipolo City
just scanned the DVR2000 manual and the manual suggest that for poor voltage regulation:
1) ground the regulator
2) isolate field leads from ground
3) isolate PMG leads from the ground

does the genset works for other loads (with the UPS disconnected?)

however, if the genset voltage is unstable only if the UPS is the load, you got me there. it would seem that the harmonics of the UPS is interfering with the 'solid state' nature of the DVR2000.

mr dbuckley, from the DVR2000 manual:
Over-Voltage
The over-voltage protection feature monitors the regulator-sensed voltage and activates when the sensed voltage exceeds 115% of the adjusted voltage. When activated, the overvoltage LED will light. If the regulator sensed voltage remains above this level for .75 seconds, the regulator will turn off. The generator voltage will go to a residual level and the over-voltage LED will remain lit. The generator must be stopped or the input power
must be removed for a minimum of 10 seconds to reset the circuit and restore normal operation. Upon startup, after the LED sequence scan, the over-voltage LED will turn on for one (1) second before entering the fine voltage adjust mode as an indication that the regulator over-voltage function commanded a shutdown.
 
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