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Tech Note: Surge Capacitors

Motors or Generators equipped with Surge Capacitors that have internal discharge resistors will have a lower insulation resistance reading during I/R testing. This is because of the internal discharge resistors being connected in parallel to the capacitor frame ground. The resistors provide a low resistance path to ground which causes the total motor or generator system resistance to be lowered to typically between 3 – 4 megohms. The discharge resistors are molded inside the capacitor assembly and therefore cannot be removed. Insulating the Surge Capacitor assembly itself from ground will correct the insulation readings but it is not recommended because the Surge Capacitors need to be grounded to operate properly. To eliminate the low insulation resistance readings a Surge Capacitor assembly must be used that does not have internal discharge resistors. These type of Surge Capacitors are available from manufacturers such as GE and ABB for all operating voltages, but need to be special ordered without the discharge resistors. Note: These type surge capacitors need to be manually discharged with a ground strap during the normal lock and tag out procedure before working on the equipment.

The Advantages of Permanently Installed Automatic (PIA) IR Testing equipment:

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.

Problems with Manual IR testing:

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:

  1. Manual IR testing allows for human error to become a factor during the testing which will cause the IR test readings to be inaccurate. Improper testing procedures and mistakes made during testing can result in damage to the equipment being tested and injury to personnel performing the testing.
  2. Manual IR testing exposes personnel to dangerous Arc Flash and Electrical injury type accidents. Personnel accessing electrical control cabinets to perform IR testing is one of the leading causes of electrical Arc Flash injuries and fatalities.
  3. Manual IR testing allows for the improper type of testing device to be used, such as an AC HI-Pot tester, which requires a skilled and experienced technician to operate. When HI-Pot testing is done incorrectly it can cause damage the electrical insulation itself and can harm personnel coming in contact with the equipment being tested.
  4. Manual IR testing only provides a snapshot of the equipment’s insulation condition for that moment in time.  The IR readings can change at any time after the test is completed and making the equipment unsafe to operate. This type of testing leaves operators without any way of knowing if the IR reading has changed until another IR test is performed.
  5. Manual IR testing requires consistency in the testing equipment itself, the frequency of the testing, the person performing the test, and making a temperature and dew point correction to obtain the actual IR readings. Any of these elements can change at any given time after the initial test which causes inaccurate and false testing data.

Understanding the Results of IR Testing

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.

Importance of using the correct DC Test Voltages for IR testing:

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 Important is Insulation Resistance Testing?

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.