Unexpected 7.5kVa Line Load

Dad

Your post is very confusing to me. I sound like you have a 240 volt generator back feeding a 240 7.5 kva transformer to get a primary voltage of 7.6 kv(or something near this). This primary voltage then feeds a 7.5 kva transformer over 10000 ft of cable. You say that the output of this transformer should be 480 volts but read 560 volts. For some reason you say you added a 600 volt primary transformer to achieve the desired 240 volts. This does not make sense. Your output was to be 480 volts. You say that the main breaker is open but you are reading 63 amps. That does not sound possible. Where are you getting this reading. Is it from the generator to the first transformer? I totally do not understand the need for the 600 volt transformer. Can you clear this up? You may want to add a drawing to your post.

Perhaps I did jump around a bit.

· 11.2kW Genset (Lower Site) was supposed to have had a single-phase 480-volt output. It was actually 240-volt single phase. Had to swap the purchased 480V transformer with the 240V transformer onsite to work with the 240V - ph genset output

· Lower site: Transformer 1 (Cooper MaxiShrub) stepped-up 240V to 7.2kV

· Top Site: Transformer 2 (Cooper MaxiShrub) stepped-down 7.2kV to 480V (but is supplying 560V output on secondaries, taps adjusted)

· Top Site: Transformer 3 (Dry type, 600V/240V) was installed to step-down 560V from Transformer 2 to 240/120V for service voltage at top site

· There is approx. 10,000 feet run of 15kVa-rated single 1/0 conductor from Transformer "1" to Transformer "2."

Problem: The genset at lower site is now pushing 280V to compensate for a load which was not expected (I am not an Electrical engineer). This affects the comm. Gear in the lower site.

· Tested with no load, Main genset breaker open, genset was producing 240V output.

· Readings taken across L1/L2 show a 63-Amp load with the 7.5kV distribution line energized (3 transformers, no load) exceeding the genset capacity of 48-Amps.

· With 7.5kV distribution ISOLATED/Deenergized and local service load for comm. gear, lower site voltage stable at 240V, and 120V service panel balanced at 16 Amps / 20 Amps respectively.

· With Lower Site service isolated, output from genset increased to 305V, feeding only the 7.5kV line distribution, no load at top site

GENSET à>>XFMR 1 (240/7.5kV) à>>à>>XFMR 2 (7.5kV/480V –560V actual)

à>>XFMR 2 -à>>XFMR 3 (600v/240(120V) -à>>Top Site Service Panel via Auto Transfer / Switch panel

Thanks - I will investigate. It would be nice if all I have to do is close the service breaker and let the lights glow. I may have a 2kw heater to test load at the output from the third transformer on a GFCI-protected 240V circuit. (Not yet installed)

With such a load, is it likely the "ferroresonance" factor could be eliminated and the apparent load of a 2kw heater allow the genset at the bottom of the hill to settle down and produce a nominal 240V output with a more normal load of 2 to 10 Amps? I know my crew would be happier not having to break trail to the top of the mountain with a 3oo-pound load in the sled behind 'em.

Great answer. This area of electrical theory is very complex and often overlooked.

Dad

Please let us know what the results are.

Old Man Winter is slowly loosening his grip on the Susitna River and we are planning a site visit to load test the generator. We had to resupply fuel for the top site and used a D8 to get us through the snow and ice on the very steep access trail. (10 days ago)

This morning I received approval to hire a technician w/load tester - they don't rent the tester device. Hope to test the system next week. I also have radio problems to fix, solar panels to reactivate, and cell phone amplifiers to set up for our big quarry operation which got started up again before "breakup" (when the snow and ice turn to mud and slush.) Our seasons in Alaska are "Winter and Construction." There are black bears to bag and tag, but can't bring them out with the work trucks - have to run a river jetboat up there for those critters.

Interesting work mate. Somewhat different from my climatic experiences. The Pilbara region in WA has temps above 40 deg c for around 7 months per year. I used to work there 20 years ago and am now to revisit the place regularly. Heat can kill but your kind of cold is scary.

Could you provide the manufacturer's name and part number for the cable?

For the sake of calculations, I will share the cable MFG. I reviewed many, many manufacturer cable products to select one that can hold up in an extreme Arctic environment, populated by bears, porcupines, and other possible varmints. It is in a remote location, little or no human population, traversing steep slopes, boggy and alder-infested sections, and massive granite above and below the surface. The cost for trenching and burying would have been too great, this cable lays above the surface on the ground, and is buried only at service road crossings. My final choice was Prysmian cable(Drawing Number QXZ902A), 15 kV 1/0 AWG AIRGUARD, UL 1072 Type MV-105. Not cheap, and a fine product - though the price for one of the splice kits could feed a family of four at a nice restaurant.

Wareagle, that is a great answer(#3). This area of electrical theory is complex and often overlooked.

I took a technician with a "suitcase" style load bank tester to Curry last week, in hopes of tracking down a persistent problem there. It had not been adequately tested following installation, and I did not allow it to be cut over to the communications system at the top site to present the normal load. Essentially, all our investment so far is not ready to be used. The load tester NC provided was the CANNON L-69-80 and on final test it blew a small fuse in the fan circuit. The tech did not bring spare fuses, and had never tested a system with a relatively small genset and step-up transformers. His experience seemed to be more in the marine generation and switchgear testing area.

Our genset is actually a 12kw Lister with a 240VAC output, and working current capacity of 52 amps. The system "sees" an initial surge of approximately 63+ amps, and then the Voltage Regulator reduces its output from 30 volts to around 2 volts in the exciter circuit. It then appears the high voltage begins to cause the genset to run over speed and we have to shut it down.

I can only conclude one of two things are occurring:

1. The genset is undersized and cannot overcome the initial line load presented by the (three) transformers and no actual linear load can be added until the power factor is improved.
2. I selected a mismatched pair of Cooper transformers based on the fact the genset is only capable of a 240 Volt output, and not 480 Volts as I first specified when the transformers were ordered. The 240/120 VAC transformer had to be placed at the bottom site to match the primary voltage output of 240 Volts, and the 480/240 volt transformer was installed at the top site.
1. The Voltage on the Secondary windings output from the top site transformer was 580+ volts (stepped down from 7.5 kVA) and I had to add a third transformer
2. Then Hammond dry transformer is rated for 600 VOlts on the Primary and had adequate taps to bring the service output voltage to the desired 240/120 volt range
3. The actual voltage measured on the individual legs of the third transformer was reading approximately 145 volts. Too high for safe operation of the equipment on the other side of the transfer switch.

• Question: Do I change out the top site Cooper transformer to match the 240/120 configuration of the bottom site?
• Is there any possibility of the high power factor being caused by the transformer mismatch?
• Question 2: Do I replace the bottom site generator with a higher capacity genset?
• Will this cure the high power factor on startup?

I have run through the entire allocated budget for this project. I cannot afford to throw money at it, but I can justify making a modification to the system to bring it on line.

Mate,

It sure has turned ugly for you.

At this stage the best suggestion I can make is to test it with a primary resistance starting circuit to limit the current until the magnetic circuits and line capacitance charge up. Seeing the instantaneous overload is 15 amps above design and the current would be better limited to below maximum, say 45 amps I'd suggest 3 X 3.6kw 240 volt heating elements with a good quality 63 amp (or bigger) switch to short the resistance once the voltage stabilises.

Until you can get the Genset to stabilise everything else is a guess.

If it works ok then it would be a matter of automating the process with a hold out timer and a voltage sensing relay (only true output when voltage within range) to detect when the voltage has settled. A second timer to detect fail to settle would also be needed (times out unless stopped by the voltage control. I'd set the first timer at around 10 sec and the fail at something less than a minute.

It sounds primitive I know. The manual test would be the first step.

Here we are one year later and no progress has been made...new winter snow has dropped in and temperatures are plummeting - -15degrees Fahrenheit up on Curry Ridge, and my crew is back at the site to refuel the generator.

All summer I have pleaded my case to Electrical gurus (local engineering con-sulting firms) and even the pros at Cooper Transformers - no joy.

Here is the exact product description for my Prysmian cable, and I have not yet found a reliable source of specification for this exact configuration from Prysmian or any distributorship even travelling (virtually via internet) to AU, NZ, IT....

NEEDED: CAPACITANCE data, other electrical specs

PRYSMIAN 1/C 1/0 AWG 19ST CPRS AL, 220 MIL EPR,

16-#14 AWG CU C/N, SEPARATOR, POLYMERIC

ARMOR, 70 MIL BLACK PVC JACKET-SFM.

15KV/133%. AIRGUARD(TM) CABLE. UL 1072

TYPE MV-105 SUN RES, AEIC CS8, ICEA-649