- SURGE CAPACITORS WITHOUT INTERNAL DISCHARGE RESISTORS
IR testing devices have made significant technology developments over the years from the first portable hand held type models. The latest designed systems now have the ability to be permanently installed in the motor control cabinets or generator switchgear and can perform automatic continuous inline IR testing on the equipment whenever it is DE-energized. This allows maintenance personnel to better obtain the true IR leakage current in a more accurate and safer manner. Some of the major benefits to this new technology are as follows: 1. (PIA) IR testing systems are permanently installed and have the ability to perform a continuous inline IR test while the equipment is offline and isolated from the power source. This provides the means to capture both the real time IR values as well as the overall IR values. The (PIA) IR testers can detect any changes in the IR readings that may occur quickly, as well as those that occur over a longer period of time. This data can then be used to trend and track the electrical condition of the equipment’s insulation thru a DCS or SCADA type of system. 2. (PIA) IR testing systems provide a significant increase in personnel safety because of their automatic “No Hands On “testing and display capabilities. The systems are permanently installed inside the MCC or Switchgear cabinets and they no longer require maintenance personnel to access the inside of the cabinets to perform manual IR testing. The (PIA) IR testers help maximize a company’s NFPA 70E compliance for personnel safety. 3. The ability of (PIA) IR testers to perform insulation testing over long periods of time makes the test readings temperature independent and so they do not require a temperature correction. This type of overall IR test data is far more informative than the data gathered during a spot type manual IR test. An overall long term IR test is the only way to capture the true leakage current occurring in the equipment’s insulation. 4. (PIA) IR testers provide an electronic method of tracking and trending the equipment’s electrical insulation condition, which is important to be able to accurately predict when a failure may occur. This (PIA) IR testing method provides far more consistent test results which helps eliminate false data during testing of the insulation. 5. (PIA) IR testers provide the ability to perform a pass/fail test on the equipment before each and every start up, which is the time when most electrical failures do occur. A manual IR test would not be feasible before every start up for obvious reasons. Performing an IR test on the equipment before every start up will help eliminate catastrophic electrical failures that can lead to expensive rewind repairs and unscheduled plant downtime costs. Equipment Reliability and operating life can now be maximized with the use of these type of IR testers. 6. (PIA) IR testers are the only devices capable of detecting and recording both the hot IR readings along with the cold IR readings. A manual IR test can never be performed in time to capture the hot IR reading due to access time restraints. The reason that the hot and cold readings are important is because of the fact that good insulators have a negative temperature coefficient which means that their IR readings are then inversely effected by temperature. So the higher the temperature of the insulation the lower the IR reading. If the insulation is in good condition the IR reading will then rebound very quickly as soon as the temperature returns to the ambient temperature. This data is very useful in both determining the age of the insulation materials, as well as predicting the amount of operating life that remains in the insulation.
Because manual IR testers are normally portable, they are used in the field for problem troubleshooting or to perform a final “Spot type” check to confirm the reliability of the electrical equipment’s insulation. This is to ensure that there are no leakage currents present from unintended faults in the insulation caused by age, moisture, contamination, or thermal breakdown. For example, a grounded stator winding caused by surface moisture present on the coils would be obvious from the IR testing results. However, there are several downsides to this type of manual IR testing. They are as follows:
The IR test measurement obtained is intended to indicate the integrity of the electrical insulation, whereas the higher the IR level the better the condition of the insulation. Ideally, an IR test reading should be at an infinite level. However since no insulators are perfect and leakage currents will flow through the dielectric material, between the conductive parts, it ensures that a finite resistance value can be measured. When performing IR testing there are generally three types of current flow detected. It is important to understand all three types of current and how they affect the IR readings: Capacitive charging current is the current that flows upon application of the DC voltage to charge the capacitance between the insulation system under test and earth. This current level will be high in the first instance before dropping off quickly to zero as the capacitor is charged (i.e. within 1 second. Dielectric Absorption current is the polarizing current that is drawn by the insulation system to align the dipoles within the dielectric material, with the applied electric field. This current level is high initially but then gradually drops off as the dipoles in the insulation become increasingly polarized (i.e. on the order of 10 minutes to hours). Leakage current is the resistive current that continuously flows through the insulation to ground via any leakage paths that may exist in the dielectric materials. Obviously, a low leakage current level implies that an insulation system is in good condition. The leakage current level should also stay more or less constant over time in good insulation materials. While it can be useful to monitor all three types of current during IR testing, both the Leakage current and Dielectric Absorption current are the two measurements that are most commonly used in evaluating the insulation’s overall integrity. However, the Capacitive charging current measurement is also a useful indication of the age and performance of the insulation’s dielectric materials. It is very difficult to detect that type of current unless you are using a permanently installed automatic (PIA) IR testing system to test the equipment. That’s because the capacitive charging current causes the IR reading to initially be very low and then it will quickly rise to a higher reading within seconds. This occurrence indicates that the insulation is in good condition and the operating life expectancy is also good. If the IR reading were to drop immediately to a very low level and then not quickly raise up higher, it would indicate that the insulation is aging and the operating life left has been diminished due to defects in the insulation materials. The Dielectric Absorption or Polarization current measurement is time dependent, because the current decreases slowly over time while the DC test voltage is being applied. This type of current measurement is commonly referred to as a Dielectric Absorption Ratio (DAR) or Polarization Index (PI) ratio test. When performing these tests the IR reading is used to create a ratio between a 30 second and a 60 second IR reading for the DAR test, or a one minute and a ten minute IR reading for the P.I. test. A minimum ratio of (1.6) for the DAR test or a minimum ratio of (3) for the PI test indicates that the insulation is in acceptable condition. Any ratio less than these minimum values indicates that there are parallel leakage paths through the insulating materials which indicates that there is a problem. The most common cause of insulation degradation is surface moisture on the dielectric insulation materials. The moisture creates parallel leakage paths to ground through cracks or defects in the insulation materials. These tests are very useful in determining the present “Real Time” electrical condition of the insulation materials. The Leakage current measurement is the most commonly used value to indicate the overall dielectric condition of the insulation, and is the current being measured during an IR test. Leakage current is time and frequency dependent, which means the number of tests performed and the duration of time that the testing is performed affects the testing results. Ideally, the IR test reading should increase slowly over time and then maintain a stable consistent level. This type of test, when done manually, is typically performed as a “spot type” test and is affected by the ambient temperature of the insulation at the time of the test. The IR reading then requires a temperature conversion to obtain the true leakage current at that time. However, with the new style permanently installed automatic (PIA) type of IR testers, the leakage current testing is done continuously over long periods of time. The numerous test results then become averaged over time, resulting in leakage current readings that are therefore temperature independent, meaning these IR readings do not require a temperature correction and the resulting value is the “True” leakage current of the insulation. A low IR leakage current reading that is maintained over a long period of time and does not fluctuate is, in theory, in acceptable condition, even if the IR reading is below the recommended minimum safe level as outlined in IEEE Standard 43-2000. An IR test reading that begins at a high level and then has a significant decrease over time is then considered to be unacceptable and an indication of defects in the insulation. An IR test reading that begins at a low level and then slowly rises to a level greater than four times the initial level, indicates that the insulation is in new or excellent operating condition. The new method of permanently installed automatic (PIA) IR testing is far more accurate and has become a more reliable method of determining the “true” IR leakage current in the insulation.
A critical component of the IR test itself is the DC test voltage level used during the process. The amount of leakage current that can be measured in an insulation’s dielectric material is directly dependent on the test voltage level being applied. IEEE, NETA, and ABS standards all confirm that when performing an IR test, the higher the test voltage level used the greater the ability will be to detect any defects that may be present in the insulation materials. Those defects, such as dirt or moisture, are what breakdown the insulation materials causing the insulation resistance to drop to an unacceptable level and eventually making the equipment unsafe to operate. Typically a 500 VDC or 1000 VDC test voltage is used for low voltage equipment and either a 2500 VDC or 5000 VDC test voltage is used for medium and high voltage equipment. IEEE Std.43-2000 and NETA MTS-2011 both contain industry standard guidelines for choosing the correct minimum test voltage to be used when performing IR testing on equipment operating at various voltage levels. These minimum IR testing voltages must always be adhered to in order to accurately measure the Insulation Resistance in all electrical equipment. Any test done at a lower test voltage level is considered to be inaccurate and misleading at best.
How significant is Insulation Resistance Testing?
Since over 80% of electrical maintenance testing involves evaluating insulation integrity, the answer is Yes, it’s a very important test. This is because electrical insulation begins to age as soon as it’s manufactured and aging causes deterioration in the performance of the insulation. Harsh operating environments will also cause further deterioration, especially where the electrical insulation is exposed to extreme operating temperatures, moisture, and chemical contamination. As a result, personnel safety and operating reliability can both be compromised. It’s extremely critical to always know the electrical condition (IR) of the insulation in your equipment at all times.
What is an Insulation Resistance Test?
The Insulation Resistance (IR) test, commonly known as a “Megger” test, is normally used as a “Spot type” test to measure the insulation’s dielectric condition at a given moment in time. The test is performed by applying a current limited DC test voltage between the conductors (Windings) and the chassis of the equipment (Ground), and then measuring any current leakage across the insulation’s dielectric materials. The current may be measured in Milli-amps or Micro-amps and then calculated into Meg-ohms of resistance. The lower the current level measured, the greater the insulation resistance.
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This week’s Tech Note talks about what needs to be done after an Automatic Resistance Tester indicates a ‘low insulation alarm’.
Automatic I/R Testing and Monitoring systems are designed to detect early signs of insulation breakdown in Motors and Generators in order to prevent catastrophic failures. Once a low level alarm occurs there are several steps that can be taken to try and correct the problem before removing the motor or generator and sending it to a repair shop for servicing. Removing the motor or generator should be the last possible solution. The first steps are as follows:
Determine that the Automatic I/R Testing and Monitoring system is operating and calibrated correctly by pressing and holding the system “TEST/CAL” button. The meter needle should align with the “TEST” mark on the meter dial and after a short delay the alarm contacts will all change state, signaling that the unit is operating and calibrated correctly. If not, follow the operating instructions included with the unit for the proper calibration procedures. Note: If the Automatic I/R system does not have the “TEST/CAL” button feature then consult the factory for proper testing procedures.
Separate the stator windings from the power circuit to determine if the problem is in the motor or generator, or in the power cables and connections. Using a hand held Megohm meter, with the same test output voltage as the Automatic I/R unit, test the stator windings and the power cable circuit separately. If the low reading is in the power cable circuit then further isolation of the cables and testing is required to determine where the cause of the low reading is coming from. If the low reading is in the motor or generator stator windings, then proceed to step 3.
Check to see if the internal space heater circuit is installed and operating correctly. Make sure the space heater circuit is energized and doesn’t have a blown fuse or tripped circuit breaker. Make sure that the space heaters are sized correctly for that size motor or generator. Consult the factory for proper sizing of the space heaters. Note: The general formula for sizing space heaters is 1 to 1.5 watts per H.P. or KW. The heaters should be placed inside the stator housing, near the air inlet, at the 5 o’clock and 7 o’clock position. They should never be installed outside or on the feet of the motor or generator. The space heaters are designed to control the ambient temperature inside the motor or generator in order to prevent large temperature swings that can cause condensation to form on the coils. They are not intended to dry out the stator windings when they become wet, however they can help in the drying process as described below in step 4. If they are not operating then check the power source, run contact, and circuit protection to be sure they are working correctly. If the heaters are still not operating then test each heater for an open circuit with a volt ohmmeter. If the heaters test open then replace them with new space heaters.
The most common cause of low insulation readings in motors and generators is surface moisture and contamination on the windings. This moisture accumulates each time the motor or generator cools down too quickly after being shut off. Condensation forms on the surface of the coils and is held there by dirt and contamination. This surface moisture can normally be removed in place by applying some heat to the motor or generator, either internally or externally. There are several approved methods of drying electrical equipment in place, which are outlined below:
INTERNAL – USING ELECTRIC SPACE HEATERS Energize the internal space heaters located inside the motor or generator and cover the unit with a heavy tarp to contain the heat and restrict air flow. This process typically takes a minimum of 24 hrs to see a positive change in the megohm readings. It is very common during this process for the megohm readings to drop off significantly and then raise up again to a higher level, indicating that the moisture is being removed. In some cases it is necessary to add additional space heaters in the motor or generator to dry off the moisture. Typically these would be a flexible mat type heater that can be installed between the stator core and the housing wrapper if the inside of the motor or generator can be easily accessed. Once the insulation readings return to a safe level, the heaters should be turned off and the tarp left on until the motor or generator returns to the ambient temperature.
INTERNAL- USING DC CURRENT IN THE WINDINGS Another method is to use the stator windings as the source of heat to dry out the motor or generator. This method requires a well trained maintenance person to perform this procedure. There are commercial systems available for drying out electrical windings using this method that inject a controlled DC current into the stator windings to cause the heating. If one of these systems is not available, a DC welder can be used. A problem with using a welder is when it is connected across two of the three phases, the third phase is left with no heating. So it requires moving the welder connections during the dry out process to evenly heat the entire winding. The portable commercial drying units have the ability to place controlled current into all three phases to evenly heat the windings at the same time. A heavy tarp should also be used to aid in this process. Note: You must take extreme caution with this method not to let the current flow exceed 60% of the nameplate operating current of the motor or generator. Also, the stator winding temperature needs to be monitored and should not exceed 180 degrees at any point in the process. The goal is to build just enough heating in the windings to gradually dry them out without causing any steam pockets that could damage the windings.
EXTERNAL HEAT If the inside of the motor or generator cannot be easily accessed to heat the windings or if a DC current source is not available, then an external method of heating can be applied. It will require making a “tent” like structure around the motor or generator with a heavy tarp or other suitable material and inducing heat from an external source. Typically a portable type of fuel heater ( Salamander ) or an electric space heater source is used to create the heat needed to dry the equipment. This process takes a little longer than the internal heating process but it can be effective in removing surface moisture. Note: Caution needs to be taken not to get the heat source too close to the tent or the motor or generator to avoid a fire or damage to the equipment. The object is to raise the internal temperature in the tent high enough to dry out the moisture on the windings. Another method for generators is to dry out the stator windings by disconnecting the excitation circuit and running the prime mover. This creates air flow through the windings and induces some heat from the engine or turbine back into the generator housing which can help remove the surface moisture. Note: Cleaning the stator windings on site to correct insulation problems with air or solvents is not a suggested method for removing surface moisture or contamination. The cleaning process just forces the dirt, dust, and chemicals back up into the windings which restricts the airflow cooling slots and causes heating. This type of procedure needs to be done at a service facility where the motor or generator can be completely dis-assembled during the process.
If the proceeding steps do not correct the low Insulation condition, then at this time it is necessary to remove the motor or generator and send it to a repair facility for what is known as “ RECONDITIONING”. By preventing the potential failure before it caused any damage to the stator core and/or windings, the motor or generator will most likely only need to be reconditioned instead of rewound to correct any problems. The reconditioning process consists of the following: Dis-assemble the motor or generator, steam clean all the windings, bake the windings to remove all moisture present, Dielectric test the windings to ensure they meet the minimum insulation requirements, Hi-Pot or Surge test the windings for any defects, re-insulate the windings with new varnish or epoxy insulation material, ( Epoxy VPI process is preferred), bake the windings to cure the insulation material, re-test the windings again to ensure they are in proper condition, clean, sandblast, and repaint all housings and covers, replace bearings and any seals, replace any other defective or worn parts, re-assemble the motor or generator, run test, paint and ship.
Reconditioning vs Rewinding is about a 50 to 60% savings over a rewind cost and about half the time to do the repair work. This results in a significant cost and time savings and more importantly allows the customer to maintain the original operating life of the equipment. Anytime a motor or generator is rewound, the operating temperature rises due to core damage caused by the burn out process or slot failure and the operating life of the equipment is shortened. The motor efficiency is often reduced as well . If extreme care is not taken during the rewind “burn out” process or if there is too much lamination damage caused during the failure, the motor or generator operating life is reduced significantly and the motor or generator will need to be replaced very soon.
CONCLUSION: The use of Automatic Permanently Installed I/R Testing and Monitoring systems can be very effective in preventing catastrophic failures in electrical equipment. The systems provide an early warning signal to maintenance personnel, before the equipment is energized, in order to prevent a failure from occurring on start up. By detecting electrical insulation problems ahead of time, such as those caused by moisture, chemical or dirt breakdown, heat damage, vibration, etc., repairs can usually be made on site. If not a local repair shop can correct these problems at their facility by doing what is called a Recondition and thus eliminating the need for a total rewind. These systems also allow the maintenance personnel to do their scheduled I/R testing without accessing the control cabinets and being exposed to dangerous Arc Flash conditions and/or electrical accidents. This not only increases plant safety but also ensures maximum equipment reliability and increased operating life.