One of our clients has or rather had, a BEST FERUPS 18 KVA UPS. It has stopped working and I was given the following report:
Radio guy: The power went out, the generator started, then I heard a bang.
Myself: You heard a bang?
RG: Yeah, a big BANG!
Myself: You heard a big bang.
And so he did:
The primary damage is around two MOV’s mounted on the other side of the board. This is the power sense (voltage sample) input to the board. We have attempted to repair it, but alas, it is not repairable. A replacement board runs over $1,400.00 from Eaton Powerware. Since this unit also needs a new set of batteries, it is likely best to replace the entire unit.
A client of ours, WDST in Woodstock, NY, has this Best FE7KVA UPS. This unit is several years old and has already been through a couple of battery replacements. These are good units, however, Best Power has been bought up by Eaton/Powerware and they are no longer made apparently are still made.
Regardless of that, the UPS is still functional, but like all UPS’s, it needs battery replacement every so often. This unit has three battery cases, each holding four 75 Ah (20 hour rate) 12 volt sealed lead acid batteries. Each battery is slightly larger than an average car battery. There is one important detail here, the batteries must be gas recombinant because the UPS has a four stage battery charging system. Gel or AGM batteries will not make it past the first or second equalizing charge.
A bit about batteries and battery charging. Most battery chargers are three stage units; bulk, absorption, and float. The rate of charge depends on the voltage of the battery. Bulk charging sends the most current to the battery and for 12 volt cells, this anything below 12.5 volts or so. Absorption stage reduces the charging current and supplies a steady current until the battery reaches full charge of 13.8 volts or so. Float or trickle charging draws very little current, just enough to maintain the battery at full charge.
The forth stage of charging is equalizing which is important for multiple battery series/parallel installations. This is when the batteries are deliberately overcharged for a period of time. The reason for equalizing charges is to intentionally boil off some hydrogen gas and knock any sulfur crystals off of the lead plates. With multiple battery banks in a parallel configuration, it is important to maintain the battery resistances as close as possible so that each bank of batteries is drawn on and charged equally.
In flooded lead acid batteries, this works well and the battery either vents off the hydrogen gas or recombines it will oxygen to make water again. In non-gas recombinant cells, the hydrogen will be released into the room which may pose an explosion hazard. Additionally, the electrolyte level will need to be checked after every equalization charge. With a sealed non-recombinant battery, the case my bulge and split, spilling electrolyte and ruining the battery and battery enclosure. Thus the importance of ordering the correct replacement batteries.
After all, the reason for the UPS is to protect the expensive computer equipment connected to it. It simply will not do to install the wrong stuff and do more damage than if the UPS did not exist at all.
Power loss is a critical failure, thus much money is spent to prevent or mitigate commercial power interruptions in broadcast facilities. Backup generators and Uninterruptible Power Supplies (UPS) are the first line of defense against commercial power interruptions. It is prudent to research products and check reliability and interoperability when specifying and installing these systems. However, even the best mechanical and electrical systems will fail, often at the worst possible time. The UPS has a startling tendency to shut down, often at the worst possible moment, due to some internal control circuit or something similar. This can happen when commercial power is being supplied without interruption. The net result is some critical piece of equipment is now dark and the station is off the air.
There is a solution: The Eaton EATS EPDU TPC 2234-A Automatic Transfer Switch.
With this unit, the primary plug is connected to the output of the UPS, the secondary plug is connected to the commercial power source. If the UPS fails, the load is automatically transferred to the commercial power. Typically, the commercial power is also backup up with a generator. The secondary plug can also be connected to a second UPS. In theory, having two UPSs connected in parallel via an Automatic Transfer Switch would increase the Mean Time Between Failure (MTBF) by 50%.
The Eaton products come with a variety of options, including basic network monitoring, advanced network monitoring, switching and management. Those features are available via Ethernet or serial data port.
Multiple layers of redundancy is the best method to avoid those late night, weekend or holiday phone calls.
An issue I had to deal with recently; an unstable generator/UPS relationship. When the generator was running under load, it surged repeatedly causing the UPS to drop out and not recharge. Eventually, the UPS ran out of juice and shut down, killing the power to the Sine Systems remote control and telephone system. Of the two, the remote control was the biggest pain to fix, as it lost it’s timed commands and would not reduce power at sun set for the associated class D AM station.
What went wrong? This is a chart of typical problems with generators operating UPS loads:
Fail to “lock on” to generator power
Improper generator frequency or voltage
Poor generator regulation
Unrealistic performance requirements
Instability of generator
Voltage regulator sensitivity
Control loop compatibility
Governor or AVR problem
Fail to sync bypass
Frequency or voltage out of range
Poor frequency stability
Unrealistic performance requirements
Changes to total load on the system
Generator output voltage distortion
Generator excitation methods can be the culprit in many of these situations. Generators often use one of three types of excitation for their field coils:
Shunt excited SCR (silicon controlled recifier)
Shunt excited PWM (pulse width modulation)
PGM (permanent magnet generator)
Of the three, permanent magnet generator is the most stable since the AVR (automatic voltage regulator) is powered by a separate small generator which is unaffected by the load on the main generator output. SCR and PWM both use the generator output windings, which makes them susceptible to load inducted voltage distortion brought on by non-linear loads. Therefore, in locations where large UPSs are known to be part of the load, PGM excited generators are the best choice.
Sometimes, the generator is already in use before the UPS is installed. In that case, there are some remedial steps that can be taken. The speed which the voltage regulator reacts to changes in the load is often the culprit in many of these situations. It may seem counter intuitive, however, the faster the AVR reacts, the more fluctuations there will be in the voltage and frequency. A UPS can operate under a wide range of voltages and frequency, provided they do not rapidly change.
Depending on other loads, it may be necessary to dampen the gain on the AVR to slow it’s reactions down. This will work if there are not large intermittent starting loads on the generator such as air conditioning compressors.
Another method would be to delay the UPS transfer to generator power until after all the other loads have been satisfied. This will ensure that the generator voltage and current fluctuations are damped by the existing load.
The generator’s size needs to account for the equipment attached to the UPS and the battery charging load. With a larger UPS, the battery charging load can be significant. Generators that are improperly sized will not be made to work under any circumstances, hence the “unrealistic performance requirements” noted in the chart above.
You can read the entire Cummins Power white paper on generators powering UPS loads here.