794 AC Off-Highway Truck Electronic Control System Caterpillar

Note: If you are not familiar with the components and the operation of the Drive Train Electric Drive System, refer to Systems Operation, UENR2267, "Electric Drive System" before performing this procedure. A knowledge of the components in the system and how the components are connected in the system can be an aid when trying to determine the location of a fault to ground.

Illustration 1	g06375618
Ground fault detection system schematic

Illustration 2	g03866092
Ground fault detection components (1) Ground fault detection sensor - location: crowbar compartment (2) Shunt resistor - location: crowbar compartment

Illustration 3	g03866090
Ground fault detection components (3) Discharge resistor assembly - location: upper rear left-hand cabinet compartment

Insulation of the system electrical conductors provide the isolation between the conductors and frame ground. A ground fault in a power system occurs when the insulation deteriorates to where a detectable amount of electrical current is “leaked” to frame ground. The greater the degree of breakdown in the insulation or the isolation, the greater the amount of current leakage to ground.

A small amount of leakage usually does not cause a problem in the system. A larger amount of leaked current to ground can damage the system components.

An EID 988 DC Ground Fault Event is activated when the detection system has detected a ground fault in the DC section of the system. The EID 988 can be activated by either the "Motor Control 1" ECM or the "Motor Control 2" ECM. The code can be activated as a "Level 1" or a "Level 3" Ground Fault Event.

The discharge resistor assembly connected between the DC positive bus and the DC negative bus is used in the ground fault detection system. For ground fault detection, the ground fault detection sensor is connected between the resistor common connected center point of the voltage divider circuit and the "Frame Ground 1" connection located in the following:

• Next to the Timing Capacitor in the Rear of the Inverter Cabinet.

This configuration effectively connects the voltage sensor to the frame ground reference center point or "neutral point" of the DC Power Bus.

Note: The "Frame Ground 1" connection is the only ground connection (through resistors) between the DC Power Bus and frame ground in the high-voltage section of the "Inverter Cabinet". This ground connection must be disconnected before any insulation testing of the DC system is performed.

When a ground fault occurs, the leakage of DC current to frame ground causes a shift of the ground reference center point of the DC bus. This shift is detected by the voltage sensor and the ECM modules as a deviation from the normal frame ground reference point. The amount of deviation from the ground reference point will be displayed in the "Advisor" or in Cat^® ET as a ground fault percentage. The amount of ground fault leakage that is detected determines if the event is activated as a "Level 1" event or a "Level 3" event.

The percentage range that can be displayed for a DC ground fault is 0.0 percent to 100.0 percent.

The following DC ground fault percentage values and the indications are:

• 0.0 to 49.9 percent - The amount of detected DC ground fault leakage is in the acceptable range. No Events will be activated. Normal machine operation can continue.

• 50.0 percent to 99.9 percent - The amount of DC ground fault leakage is elevated. The ECM will activate a "Level 1" (active only) DC Ground Fault Event. Machine operation can continue with limited propel and will go into an "Auto Grid Dry" mode of operation. If the "Level 1 DC Ground Fault" Event remains active more than 7 minutes, a "Level 3" Event will then be activated.

• 100.0 percent or greater - A high level of DC ground fault leakage has been detected. A "Level 3 DC Ground Fault" Event is activated and an immediate safe shutdown of machine operation is required. At the detected level of ground fault leakage, system components can be damaged if machine operation continues.

The components in the electric drive train system that could be involved when an E988 DC Ground Fault Event is active are:

• Any of the components that are connected to the DC Power Bus in the "Inverter Cabinet".

• Cables or wires that are used to connect components in the rear of the "Inverter Cabinet" to the DC Power Bus.

• The grid system resistors or the cables that are used to connect the grid resistors to the DC Power Bus.

• The "Retarding Grid Blower Inverter" or the cables that are used to supply DC power to the RGBI and the RGBI capacitor.

Points to consider before performing these procedures:

• Moisture in the "Retarding Grid Assembly" can be a common cause of faults to frame ground. If the machine has recently been washed or parked in a wet environment, use the "Advisor" to run the "Grid Dry Mode".

• Moisture at a connection point or inside a component can cause faults to frame ground. Check the condition of the connectors and the seals. Ensure that moisture has not entered the connectors or the "Inverter Cabinet". If moisture is detected in the cabinet, thoroughly clean and dry the wet components before operating the truck.

• Excessive dirt or contamination around connectors or components can cause a fault to frame ground. If a maintenance procedure has recently been performed that involved electrical components or conductors, check for dirt or contamination at the connections. Connectors and connector enclosures must be free of dirt, dust, and other forms of contamination. If any of the high-voltage connections in the "Inverter Cabinet", or in the drive train system requires cleaning, do not use solvents or detergents. Use clean air to remove dry contaminants. Use isopropyl alcohol or a solution that contains at least 90 percent isopropyl alcohol to clean the contacts.

• In some instances, a "Level 3" ground fault will be severe enough to leave visual evidence in the area of the fault. When possible, a visual check of the areas where the fault could occur should be the first action that is taken. Look for damaged insulation, damage at the connection points or discoloration of the frame.

• A resistance of 2.0 mega ohms is the low threshold considered to be an acceptable level of insulation resistance for wire insulation, bus bar isolation, motor windings, or generator windings. When these components are in good condition, the insulation resistance to ground will be much greater than 2.0 mega ohms. A resistance above 2.0 mega ohms by a small margin indicates that the insulation has deteriorated to a level that is adequate, but is not considered to be in good condition. Consideration should be given to replacement of a conductor or a component that tests above 2.0 mega ohms of insulation resistance by a small margin.

• The DCPT2 ground fault voltage sensor provides a signal to each "Motor Control" ECM. Both of the ECMs should activate the same ground fault event. If only one ECM has activated the ground fault event, refer to the Troubleshooting, "MID 169/170 - CID 3047 - FMI 03" procedure in this manual for the ECM that has not activated the ground fault event.

Note: ERM251-Up were manufactured with a 360-2920 Converter As. The converter assembly is also called a "Retarder Grid Blower Inverter". A resistance of 500,000 ohms to ground is the low threshold for the "Retarder Grid Blower Inverter". If any tests are getting down close to 500,000 ohms resistance to ground, there is a problem. The test should show 500,000 ohms or greater resistance to ground.

Note: ERM1-250 were manufactured with the 313-4089 Inverter. The inverter is also called a "Dynamic Retarder Inverter". If the "Dynamic Retarder Inverter" has been replaced, check the unit. The service replacement for the 313-4089 Inverter is the 360-2920 Converter As RGBI. Earlier machines that contain the newer 360-2920 Converter As must use the test specifications for the newer machines. A resistance of 500,000 ohms to ground is the low threshold for any ERM machine that contains the 360-2920 Converter As "Retarder Grid Blower Inverter".

Illustration 4	g01975173
243-3141 5KV Insulation Tester (Megohmmeter or "megger")

Note: When troubleshooting a suspected fault to ground in the system, a 243-3141 5KV Insulation Tester, also called a Megohmmeter or "megger", will be used to check the integrity of the insulation for the conductors and components in the system. This insulation tester can test the components at a maximum of 5000 V. Do not test the insulation resistance of any of the conductors or components at greater than 2500 V. Some components can only be tested at a lower voltage. Follow the instructions for the test voltage to use when testing the insulation. Damage to the components can result if the insulation is tested at too high of a voltage. The user must read the operating instructions for the 243-3141 5KV Insulation Tester. Become familiar with the operation of the "megger" before attempting to use the tester. The 243-3141 5KV Insulation Tester will be referred to as the "megger" in the following procedures.

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The Power Train Electric Drive System will contain hazardous voltage levels during machine operation and for a short period of time after engine shutdown. Do not remove any covers that will expose energized high voltage electrical components while the engine is operating. Any type of maintenance on the following components can only be performed after the Power Train Electrical System Service Shutdown procedure has been followed: High voltage compartments in the inverter cabinet The rear axle housing that contains the electric drive traction motors The generator The retarding resistor grid, the grid blower motor and the grid system cabling The excitation field regulator The high voltage cables and connection enclosures Failure to follow these instructions could result in personal injury or death.

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In order to avoid the buildup of hazardous live voltages on the exposed surfaces, all grounding wires and grounding straps must be properly connected at all times during engine operation. Any disconnected grounding wires, including the grounding wires for all high voltage components and the grounding strap for the inverter cabinet must be properly reconnected before the engine is started. Failure to follow these instructions could result in personal injury or death.

Note: Use the "794 AC Electric Drive Train High-Voltage Component Connections" systems schematics page in the back of this manual for a reference to identify the high-voltage connections for the AC components in the system.

Test Step 1. LEVEL 1 DC GROUND FAULT PROCEDURE

If the "Level 1", E988 DC Ground Fault Event, is active or if moisture has entered the "Inverter Cabinet", follow these steps:

The key start switch and the disconnect must be in the OFF position to begin these procedures.

If required to open a high-voltage compartment or enclosure for this procedure, use the high-voltage meter to verify that the voltage between the DC bus connections is less than 50.0 VDC before any other action is taken.

1. Stop machine operation and run the "Grid Dry Mode". Check the status of the ground fault event.

2. Determine if any type of moisture or contamination may have entered the "Inverter Cabinet" by washing, operating in the rain or snow, or by other means. If contamination or moisture in the "Inverter Cabinet" is suspected, remove the compartment covers to find the problem. Check all high-voltage connections for moisture, dirt, or other forms of contamination. Clean any contaminated areas or dry any wet areas. All connections must be clean, dry, and tight. Install the covers and check the status of the ground fault event.

3. If the Engine has not recently been in operation, operate the engine at high idle for 5 minutes. Check the status of the ground fault event.

Expected Result:

The Level 1, E988 DC Ground Fault Event is no longer active.

Results:

OK -The Level 1, E988 DC Ground Fault Event is no longer active. The ground fault condition is no longer present. STOP

NOT OK-The Level 1, E988 DC Ground Fault Event is still active. The condition causing the ground fault is still present.

Proceed to Test Step 2.

Test Step 2. PERFORM THE POWER TRAIN ELECTRIC DRIVE SYSTEM SERVICE SHUTDOWN PROCEDURE

This procedure will require entries into compartments that contain high-voltage components.

When an area that contains high-voltage components must be entered, the "Power Train Electrical System Service Shutdown" procedure must be performed. Perform the procedure before the cover for the compartment or housing is removed.

Reference: Refer to the Troubleshooting, "Electrical Shutdown and Voltage Discharge" section of this manual. Perform the "Power Train Electrical System Service Shutdown" procedure to verify that the voltage in the power train system has been discharged. Once the voltage of the DC Power Bus in the "Inverter Dynamic Retarding Contactor" compartment is verified as discharged and a jumper wire is in place, return to this point in this procedure and proceed.

Results:

OK - The "Power Train Electrical System Service Shutdown" procedure has been performed, the voltage is verified as discharged, and the jumper wire is in place. If a "Level 1" ground fault is active, go to Test Step 1. If a "Level 3" ground fault is active, go to Test Step 3.

Test Step 3. LEVEL 3 DC GROUND FAULT PROCEDURE

Illustration 5	g03866106
Inverter Cabinet compartments that contain components that could cause a DC ground fault (4) All phase module compartments (5) Retarding contactor compartment (6) Retarding grid system cable connection compartment (7) High-voltage cable entry compartment (8) Generator and traction motor cable connection compartment

Illustration 6	g03866099
"Inverter Cabinet" compartments that contain components that could cause a DC ground fault (9) Power rectifier assembly compartment (10) Resister assembly compartment (11) Retarding grid blower inverter (RGBI) compartment

The key start switch and the battery disconnect switch remain the OFF position.

The inverter cabinet compartment cover for the retarding contactor compartment is removed.

1. At the "Inverter Cabinet", remove the cover for the crowbar assembly compartment. Remove the covers from the high-voltage cable connection compartment. Use the high-voltage meter to measure for any voltage between the DC positive and negative bus connections or the AC connections in these compartments.
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   Illustration 7	g03854147
   Preliminary disconnection of cables and ground prior to testing the insulation in the "Inverter Cabinet". (12) Generator and traction motor cables disconnected and insulated from contact with the bus bars.

2. Disconnect the "Ground 1" wire from the lug in the back by the timing capacitor. Position the wire where no contact with the frame or with any other surface can occur.

3. A ground fault that is severe can sometimes be detected with a multimeter. At the front of the contactor compartment, use a multimeter to check the resistance between the “DCP” positive bus bar and cabinet ground. Check the resistance between the “DCN” negative bus bar and cabinet ground. If both resistance measurements are high - greater than 2.0 mega ohms or “infinite”, proceed to the next step. If either of the multimeter resistance measurements is low - less than 2.0 mega ohms or “infinite”, there is no need to conduct another test. There is a severe short to ground somewhere in the DC system. Proceed to Step 4 (“ISOLATING A DC GROUND FAULT”.

4. In the inverter cable connection compartment, disconnect the high-voltage three-phase cable connections for the "Generator", "Traction Motor 1", and "Traction Motor 2" (12). Disconnect the "Traction Motor 1" and "Traction Motor 2" cable connections. Place a non-conductive material between the cables ends and the bus bars to prevent contact with the bus bars.
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   Illustration 8	g03854178
   Jumper wire connected and the "megger" connected in the retarding contactor compartment. (13) Jumper wire connected between the DCP and the DCN bus bars. (14) "Megger" black negative clip connected to the ground lug on the side of the compartment. (15) "Megger" red positive clip connected to the DCN bus bar - the red positive clip can be connected to either the DCN bus or the DCP bus.

5. At the front of the contactor compartment, connect a jumper wire between the "DCP" (positive) bus bar and the "DCN" (negative) bus bar (13). The jumper allows the complete system to be at the same potential. This jumper wire must remain connected between the DC positive bus bar and the DC negative bus bar continuously during the testing.

6. Connect the "megger". Connect the red (positive) clip to either the "DCN" bus bar or the "DCP" bus bar (15). Connect the black (negative) clip to the ground lug (14) on the side of the compartment.

7. Ensure that all personnel are not in contact with any of the DC conductors. Ensure that all disconnected cables are not in contact with any of the DC bus conductors.
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   Illustration 9	g01976073
   Testing the isolation of the DC system at 2500 V - This test detected a resistance of 109.7 mega ohms to frame ground

8. Set the dial on the "megger" to 2500 V. Press the “START/STOP” button on the "megger" to start the test. Test the DC system at 2500 V. Continue the test until the test voltage has stabilized for 10 seconds. Press the “START/STOP” button to stop the test. The "megger" will discharge the system. Wait until the voltage readout on the "megger" display is 0.0 V. If the resistance measures greater than 2.0 mega ohms, proceed to the next step. If the resistance is less than 2.0 mega ohms, proceed to the "Expected Result:" step.

   Note: The resistance measured greater than 2.0 mega ohms. Since the retarding contactors are OPEN, the "megger" test did not include the retarding contactor grid system which also supplies power to the "Retarding Grid Blower Inverter". In the next step, this part of the DC system will be checked.

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   Illustration 10	g03854202
   Grid system cable connection enclosure (16) “IN” cable connection (17) “IP” cable connection

9. Remove the cover from the retarding grid system cable connection enclosure. Disconnect the "Retarding Grid Blower Inverter" power supply cables "IP" and "IN" from the standoff. Isolate the cable ends to avoid contact with each other or with any other surface.
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   Illustration 11	g03866049
   "Megger" connected to test between the "Retarding Contactor Grid Resister" circuit and ground.

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   Illustration 12	g03866060
   "Megger" connected to test between the "Retarding Grid Blower Inverter" supply circuits and ground at 1000 V.

   Note: In this step, the section of the "Retarding Contactor Grid" resister circuits will be isolated and tested. When using the "megger" to test the circuits that supply power to the "Retarding Grid Blower Inverter", do not test at greater than 1000 V. Damage to the "Retarding Grid Blower Inverter" can occur if tested at greater than 1000 V.

10. Use the "megger" to test the following circuits. Use only the listed test voltage:

    1. Perform two tests: test between the "IP" cable going out to the grid assembly and frame ground at 2500 V. Test between the “IN” cable going out to the grid assembly and frame ground at 2500 V.

    2. Test between the "IP" cable going back into the cabinet to the "Retarding Grid Blower Inverter" and frame ground at 1000 V. A reading above 200K but below 2M for the RGBI is considered Good.

    3. Test between the “IN” cable going back into the cabinet to the "Retarding Grid Blower Inverter" and frame ground at 1000 V. A reading above 200K but below 2M for the "Retarding Grid Blower Inverter" is considered Good.

11. In the previous step, if each of the grid insulation tests indicated a measured resistance of greater than 2.0 mega ohms, proceed to the "Expected Result:" step. In the previous step, if any of the insulation tests fail, there is a problem in the section of the "Retarding Contactor Grid" circuits that measured the low resistance. Disconnect the cables in the section to isolate the "Retarding Grid Blower Inverter" or the resistors from the cables. Check each cable or component visually and using the "megger" to isolate the problem. To check a cable individually, disconnect both end connections, "megger" between the cable and frame ground with the cable in place. Replace any cables or components that are determined to have faulty insulation.

    STOP.

Note: 360-2920 Converter As, the converter assembly is also called a "Retarding Grid Blower Inverter". A resistance of 500,000 ohms to ground is the low threshold for the RGBI. If any tests are getting down close to 500,000 ohms of resistance to ground, there is a problem. The test should show 500,000 ohms or greater resistance to ground.

Expected Result:

The DC resistance to ground test at 2500 V is less than 500,000 ohms.

Results:

YES - The "megger" is registering a bus to ground resistance less than the expected result. This measurement indicates that there is a breakdown of the system isolation somewhere in DC system. The breakdown is in the "Inverter Cabinet" or in the retarding contactor grid circuits. For instruction on isolating the section of the system that contains the ground fault, proceed to Test Step 4.

NO - The insulation tests of the DC system in the "Inverter Cabinet", the "Grid 1" circuit, and the "Grid 2" circuit are not indicating a fault to ground. Each of the tests measured a resistance greater than the expected result. There is no indication of a ground fault in the DC system. Proceed to Test Step 5.

Test Step 4. ISOLATING A DC GROUND FAULT

The DC positive bus and the negative bus remain connected by a jumper wire in the contactor compartment.

The mega ohm meter remains connected to the DC bus and to the ground lug in the contactor compartment.

The retarding contactor grid circuits have been tested for insulation resistance and have passed or the retarding contactor grid circuits have been ruled out as a cause of the ground fault condition.

When a ground fault is present the DC system and the location of the ground fault is not obvious, individual components must be disconnected from the system to isolate the cause. An internal short circuit in a DC connected component could cause a fault to ground.

1. Rule out the chopper grid circuits as the location of the ground fault. Disconnect the "CHP" and the "CHN" cables from the standoff in the cabinet retarding grid system cable connection compartment.

2. Use the "megger" to check the resistance between the chopper grid circuits and ground.

   1. Connect the positive clip to the disconnected cable "CHP" or the "CHN" cable that is going out to the grid.

   2. Connect the negative clip to a frame ground point, preferably on the grid housing.

   3. Test the resistance at 2500 V.

   4. The resistance should be greater than 2.0 mega ohms. If the resistance is lower than 2.0 mega ohms, follow the cables out to the grid connections. Disconnect and isolate the cables. Check the cables individually and check the grid resisters individually. Replace any components that are shorted to ground.

   5. The cables "CHP" and "CHN" that go into the cabinet should be disconnected at the front of the crowbar assembly tray. Test the cables individually at 2500 V to rule out a cable as the cause of the problem. Replace the cable if a short circuit to ground is found.

3. With both grid circuits eliminated as the location of the ground fault, the indication is that there is a short to ground within the "Inverter Cabinet".
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   Illustration 13	g03854205
   "Inverter Cabinet" rear view (18) Resister Assembly Compartments (19) "Retarding Grid Blower Inverter" (RGBI) compartment (20) Rectifier Assembly Compartment

   1. Remove the covers on the rear of the "Inverter Cabinet".

   2. Many of the system components in the rear cabinet compartments are connected by wire conductors to the DC bus. Visually check the wiring, the connections, the bus bars, and the components for signs of a short to frame. Look for areas where a wire could be in contact with the cabinet frame. Look for signs of moisture or contamination in the compartments.

   3. If signs of a short circuit are not observed, refer to the high-voltage connection diagram in the back of this manual. Start disconnecting components that are connected to the DC Power Bus one at a time. Each time that a component is disconnected, use the "megger" to test the DC system from the contactor compartment at 2500 V. Determine if the disconnection of a specific component enabled an acceptable higher resistance between the DC bus and ground. Replace any components that are determined to be causing the problem.

4. If a cause is not found, methodically check the components in the front of the cabinet. Individually check all the components that are connected to the DC Power Bus. The components mainly consist of phase modules. Visually check the connections, the bus bars, and the components for signs of a short to frame. Look for areas where a wire could be in contact with the cabinet frame. Look for signs of moisture or contamination in the compartments. Replace any components that are determined to be causing the problem.

Expected Result:

OK - The cause of the ground fault condition has been resolved. Proceed to Test Step 5.

NOT OK- There are no obvious signs of a short to ground. All sections of the DC system have been checked and a cause of the ground fault condition has not been identified. All the DC connected components have been checked and eliminated as a cause of the condition.

Repair: Ensure that all the DC connected components connected to the DC Power Bus have been checked and are OK. Ensure that the circuits in the grid system have been checked and are OK.

Check the bus bars that are in the "Inverter Cabinet". When the components are disconnected and the insulation test is still indicating a short to ground, a possible short circuit between the positive and the negative bus bars should be investigated.

Start disconnecting section of the bus bars and recheck the status of the resistance to ground. The "megger" connections can be moved to check different sections of the bus bars.

Replace any sections of the bus that are found to be faulty.

If all these steps have been followed and the insulation test is still registering a low resistance to ground, proceed to Test Step 5.

Test Step 5. CHECK THE STATUS OF THE GROUND FAULT EVENT

1. Reconnect all the bus bars, connections, and components that were disconnected during the steps of this procedure. Tighten all connections to the proper torque.

2. Verify that the three-phase cables for the generator and the traction motors are properly connected and torqued.

3. Verify that the "Ground 1" ground connection in the "Crowbar Assembly" compartment is connected and properly tightened. Do not start the Engine with any ground wires or ground straps disconnected.

4. Verify that any jumper wires that were used for the testing have been removed.

5. Install all compartment covers, protective covers, or enclosure covers that are removed. Check all the "Inverter Cabinet" compartment cover seals. The seals should be intact and in good condition. Verify that all the "Inverter Cabinet" compartment covers are installed and the bolts are properly tightened.

6. Turn the battery disconnect switch to the ON position. Start the Engine and run the engine at low idle.

7. Determine if the E988 "DC Ground Fault Event" is active.

Expected Result:

The E988 "DC Ground Fault Event" is not active.

Results:

OK - The E988 "DC Ground Fault Event" is not active. The problem does not exist currently.

Repair: Verify that all the steps that were outlined in this procedure were followed. Verify that all the sections of the system that could be checked for a ground fault were checked.

If all these steps have been followed and the insulation test is still registering a low resistance to ground, call the Dealer Solutions Network (DSN). Indicate the machine conditions that were present when the event was activated. Indicate the steps that have been taken to try to find the cause of this problem and the findings that have been discovered during the testing procedures. The DSN personnel will provide instruction on the actions that must be taken to resolve this problem.

STOP.

NO -The E988 "DC Ground Fault Event" is still active. The problem has not been corrected.

Repair: Verify that all the steps that were outlined in this procedure were followed. Verify that all the sections of the system that could be checked for a ground fault were checked. If all these steps have been followed, and the insulation test is still registering a low resistance to ground, contact your dealer technical communicator (TC) for possible consultation with Caterpillar. This consultation may greatly reduce the required repair time and expense. Indicate the machine conditions that were present when the event was activated. Indicate the steps that have been taken to try to find the cause of this problem and the findings that have been discovered during the testing procedures.

STOP