EMERGENCY GENERATOR SET OVERHAUL

By Larry Bush

Ref: detroit diesel generators, detroit diesel generator, temporary generator, temporary power distribution, temporary power, esg engineering, winding insulation class, cable damage curves, winding insulation

Summary: "Larry relates to issues with their Detroit Diesel generator set for emergency temporary power distribution, in the hope that the information might be helpful to the reader"

There were a number of incidents that occurred while maintaining the Saudi Arabian Oil Company owned Emergency Supply Generators (ESG). I will relate some of them in the hope that the information might be helpful to the reader.

The Company had an Engineering Building with a computer center in Dhahran that utilized a 1460 HP Detroit Diesel generator set for emergency power. There was an emergency battery setup in the building that provided emergency bridge-over power for up to ½ hour.

Nine months prior to our taking over responsibility from the company's existing generator maintenance service providers, the Engineering Building generator faulted out and was removed from service. A temporary replacement emergency generator was rented from the local Detroit Diesel vendor.

Five hundred MCM cables were temporarily installed and hooked up from the generator to the connection points over 200 feet away. A replacement part was ordered, "Rush", from the United States.

The maintenance unit waited and waited and waited. In the meantime, costs for renting the replacement generator continued to mount. Renting a 1400 HP generator in Saudi Arabia was not inexpensive.

I learned that the long awaited part was due to arrive on site just two days after the turnover date of the maintenance responsibility. I thought that was rather fortuitous timing. We would be able to immediately repair a money-consuming monster and look good doing it.

When the part was delivered, I saw that the part did not match the part that was needed. The wrong part had been ordered or sent.

Fortunately, I had a contingency plan. The damaged part was a rectifier that converted AC voltage to full wave DC voltage for the generator field circuit.

I first visited the local Detroit Diesel vendor in the nearby city of Al Khobar. I had visited their offices before and so knew just who to call on. I took the part to them and asked if they had one or could rob the rectifiers from a similar one to repair it.

LOCATING THE DETROIT DIESEL GENERATOR PART

The Parts Manager of the local Detroit Diesel vendor knew right where to look. He searched his files to confirm the location and quickly produced the rectifiers needed to repair the part. He then had his company's shop make the repairs to the part. Problem solved.

IDENTIFYING THE ROOT CAUSE

Before we installed the part, I decided to examine the generator in order to try to ascertain the reason for the failure of the part. I read the manufacturer's manual on the generator and there in black and white was the warning to refrain from carrying out a powered insulation test on the generator circuit containing the field rectifiers or failure of the rectifiers could result.

The generator was directly coupled to the engine shaft. The generator shaft only had one bearing. The drive end of the shaft used the engine bearing.

While checking the generator, I noticed a small amount of oil in the bottom of the inside of the generator housing. We removed inspection plates from the engine end of the generator and found the source of the leak.

The engine drive-bearing seal had partially failed and oil was leaking into the generator. The oil had gotten on and into the generator windings and was affecting the insulation on the windings. The generator insulation resistance to ground would need to be tested.

TESTING THE WINDING INSULATION

Abdullah and I re-read the generator manual to make certain we knew what we were doing before testing the generator. We insured that all delicate circuits were disconnected and then used a 1000-volt, hand megger on the windings.

The windings were partially grounded and that was why they had been meggered by the original unit that had burned out the rectifier circuit. The generator would need to be thoroughly cleaned and then dipped in liquid insulation and baked for a good repair. A new bearing would then be installed on the engine and the seals would also be replaced.

At one time, our Company's heavy-duty repair shop had performed this service. When I checked with them, they said they were only doing this procedure in the Southern Area for oil field equipment.

For the repairs that I needed, they would send the generator out to a contractor and I could expect the repaired generator back in six to nine months. I told them to forget it. My unit could do it faster than that.

ENGINE AND GENERATOR OVERHAUL

I consulted with our crews and we decided to tackle the problems of an in-place short overhaul of the 1460 HP diesel engine and removal/reinstallation of the 4-ton generator.

I checked with our company Cranes and Rigging Unit and they said they would move the generator from the ground to the back of a truck, that's all. No other division or unit would tackle the job.

The generator set was located in the basement of the building. There was a delivery ramp down to the basement floor level. A 5-ton traveling hoist on a horizontal, I-beam track ran the length of the basement to the delivery area loading dock.

All we had to do was uncouple the engine and generator, pick the generator up, set it on a mobile platform, and then move the generator to where the traveling hoist could pick it up. Then do the same procedure in reverse to reinstall it.

I had to design, order the parts and have built the wheeled platform sturdy enough to handle a four-ton load. While the platform was being built, work continued at the generator set site.

Our overhaul/repair crew was ordering parts and laying out the required work on the engine. They would need to replace the bearing with the bad seal and check the other main shaft bearings.

They would also be opening and checking the pistons and heads. They would not be replacing anything unless it was bad. We didn't anticipate having to replace much more than gaskets and other easily replaceable items.

MOVING THE 4 TON GENERATOR

In order to lift the generator after uncoupling, there had to be attachment rings in place above the generator capable of handling more than the full weight of the generator.

Fortunately, there were several rings already in place, probably mounted during initial installation of the generator set. We only had to add one more lifting ring. Three chain hoists each with a four ton capacity would be used to lift and move the generator.

The generator had to be lifted and then moved partially out of the room so the moving platform would have enough room to fit under the generator. We were able to attach another chain hoist to a large welded beam to pull the generator into position.

After depositing the generator on the hard-wheeled platform, it was just a matter of pushing the generator to the traveling hoist. The load was transferred to the hoist on the hoist track and the generator then was moved to the loading dock and onto a truck bed.

The truck delivered the generator to the Detroit Diesel vendor shops for prearranged repairs. The repairs were to be carried out on a rush basis, overtime approved. This was still cheaper than the rental cost.

The generator repairs were to take three weeks, or less. Our repairs on the engine would take about two weeks. We would then reinstall the generator onto the engine and carry out our tests.

TEMPORARY GENERATOR SET PROBLEMS

We turned our attention to the temporary generator and cables lying on the ground while work continued on the engine. I first wanted to test the insulation resistance of the cables to ground.

This was an extremely important safety fact that I knew had been neglected. The cables had been lying on the ground for so many months and they had been dragged over the rough concrete and blacktop to different locations.

The cables sometimes were in the way of vehicular access to and from the loading dock. Some of the vehicles ran over the cables and, apparently, caused damage to the insulation.

Abdullah brought the megger to the temporary generator site. We made certain that the automatic start circuit for the engine was disabled and we began to take insulation readings on the cables. Instantly, we had extremely low readings on two sets of the cables for the A and B phases. The cables were partially shorted together and grounded.

I had noticed during my time with the original crew that they started the temporary generator by hand on a weekly basis, but did not close the circuit breaker to feed power to the emergency circuit over the temporary cables. I was told that the last time the system had functioned was 4 months earlier.

AT some point during the last four months, the cables had sustained enough damage to render them an extreme safety hazard to personnel and the temporary generator set. In addition, there had been no backup power for the building emergency circuits.

We immediately tagged out the generator set, notified our division superintendent and the building maintenance supervisor. Then, we began to check the cables for the likely problem areas.

CABLE FAILURE ROOT CAUSE ANALYSIS

There were two 500 MCM cables for each of the three phases and more for the ground connection. In the loading dock area, the cables had fallen or been moved into a vehicle access area.

We figured this was a good place to start. The cables at this point were lying on the blacktop all twisted around each other with tire marks and scuffmarks.

The loading dock area was also the lowest point in the dock area and housed the sump pit and pump for rainwater drainage and disposal. It didn't rain often in Saudi Arabia, but when it did, we might get or 2 to 3 inches of rain in a one hour downpour.

If there was damage to the insulation, the water had probably exacerbated the damage. Abdullah called over several of our work crews to come to the loading dock to separate and try to repair the cables. The cables were 500MCM and very heavy to handle.

After the cables were separated and tied up off the blacktop, we used insulation splice kits to repair the damaged insulation on the cables. We again meggered the cables after the kits had time to setup.

The repairs were successful and the cables meggered good. Now it was time to start the engine and run a full-scale test.

TEMPORARY GENSET TEST

I contacted the building maintenance supervisor and he requested we wait till most people had departed the building. It was already after my crew's quitting time so I explained that we could not leave the place with no emergency power.

He finally agreed to the test. The engine was started through a simulated power loss feature in the building's plant control room.

The engine immediately came on line and supplied power to the automatic transfer switch. The automatic transfer switch operated and transferred the emergency building power load to the temporary emergency generator.

Success. Our repairs held up. The temporary generator performed as required and all was well. The load transferred back automatically and the generator ran for a preset cool down period and then shut down.

Abdullah and I showed the building maintenance supervisor what had happened to the cables. The cables were only going to be required for approximately 3 to 4 more weeks, but we wanted no more damage to the cables.

CABLE DAMAGE PREVENTION

The supervisor got a crew to put up warning tape and cones to protect the cables. I suggested he get stands and or sawhorses to place in front of the cables with high-voltage, electrical warning signs to keep vehicles and personnel away from the cables.

After two weeks of 24/7 days, the mini-overhaul of the engine was complete. The generator repair facility had called for a time and place to deliver the generator.

The generator arrived at the loading dock and was unloaded by our crew using the traveling hoist and moved to the point closest to the generator room. The generator was then put onto the mobile platform and pushed to the room.

Installation of the generator proceeded smoothly. Coupling the generator to the engine continued until finished. I wanted to be able to test-run the generator set the next morning.

The engine had been started and run for about 4 hours after completion of the repairs and before the generator had arrived. We knew the engine would run, but did not know for certain that the generator/engine coupling was accurate.

MAIN DIESEL GENERATOR SET TEST RUNS

Due to the one-bearing setup of the generator, a slight misalignment of the generator shaft to the engine shaft could lead to a rapid, catastrophic failure before the engine could be shut down. We were all holding our breaths when the button was pressed and the engine roared into life.

The overhaul mechanic crew leader wanted to run the engine and generator set, unloaded, for four hours and then stop for a check of the engine for overheating, bearing problems, and metal shavings in the oil. If all was well, the engine would be started and run for 24 more hours with a mechanic on duty at all times.

The generator set and testing was excellent. The temporary generator and cables were disconnected and the original generator was connected to the emergency system.

Transfer, stability, and power tests were performed on the generator set that same day and the generator set was officially put back into service in less than one month from the time we took over the new unit. That was eight months less than the length of time the generator set had been out of service under the old unit.

ANALYSIS and SUMMARY

The initial problem with the existing generator set failure was due to the failure of the bearing seals on the rear bearing of the engine. This bearing supported the engine and the generator shaft is bolted to the engine shaft with another bearing on the opposite generator end.

The seal failure allowed lubricating oil from the bearing to seep into the generator compartment. The oil dripped onto the generator windings and eventually reduced the insulation resistance of the windings. This caused an alarm to be tripped on the generator control panel.

The maintenance crew decided to use a hand powered, 1000 volt, DC megger to test the insulation resistance windings to ground. The field windings of this particular generator are powered through a full wave rectifier setup to provide DC power to control the output of the generator.

Using a 1000 Volt DC megger on this rectifier circuit would blow right through the rectifiers and ruin them. This is what the maintenance crew did when they checked the windings. There was no spare part on hand. Without this part, the generator set was unusable

The generator set was built and delivered from the United States. The part was ordered from the original dealer through the company's satellite headquarters in Houston, Texas. Houston personnel bought and delivered all the parts for Saudi Arabia from the United States.

I ordered the part through the local Detroit Diesel vendor after getting permission to bypass the normal supply channels. This emergency generator set was crucial to the oil company's operations and maintaining and repairing it was a very high priority.

Root cause analysis was used to identify the actual problems and solve them. The initial cause of the failure was the seal failure and the subsequent seepage of oil into the generator compartment. There were a number of breakdowns in good maintenance practices and I have listed them following:
1) This oil seepage should have been identified by regularly scheduled inspections and the problem corrected long before it damaged the generator winding insulation.
2) The seals and bearing could have been replaced in one 24 hour period with a huge savings in the actual costs.
3) The insulation resistance readings should only have been carried out after reading and following the manufacturer's instructions listed in the generator manual.
4) The spare parts should have been ordered through the local Saudi Arabian vendor with an assurance that the right part would be available.
5) The temporary emergency generator should have been connected to automatically start and assume the emergency load on main power failure.
6) The temporary emergency generator cables should have been placed in a more protected location and tied up off the ground.
7) The temporary cables should have had protective signage and saw horses placed around the cables.
8) The temporary cables should have been tested for grounds and shorts on a regular basis.

Fortunately, there was no long term power outage to the buildings served by the emergency generator during the four months that the cables were grounded and shorted. Had the cables been powered during this period, there could have been an electrical explosion and possible negative consequences to personnel and equipment in the area.

Larry Bush

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About the Author: Larry Bush has been an electrician for 47 years, and in maintenance management for 22 years. Download his new e-Book "Maintenance Policy and Procedures Manual" !!