Wind Generator Testing with MCEMAX

The EMAX tester provides the user with a variety of tests for quantitative and qualitative analysis while the motor/generator is running:  Power Test, High Resolution Current Analysis, In-Rush/Start-Up, and Process Analysis. For generators our primary interest is the condition of the ground wall insulation and the health of the stator windings and rotating rotor field. 

The Power test provides the following:

• Time Domain of 3-phase Voltage and Current using simultaneous sample and hold data acquisition, therefore delivering extremely accurate power factor information (Figure 1)

• Harmonics evaluation to the 50^th harmonic (Figure 2)

• Detailed Results Page including information on voltage, current, impedance, power, distortion, load and sequence data (Figure 3)

The High Res Current Analysis test provides the following:

• High Re

  

  solution Current Time Domain for load and process analysis (Figure 4)

• High Resolution Armature Current Spectrum for peak detection of repeated modulations throughout the time domain of current

The In-Rush/Start-Up test provides the following:

• Graph of enveloped current showing the instantaneous magnetization current and stresses felt at startup (Figure 5)

• Process analysis of the paralleling operation of the generator with the distribution system for condition assessment of the electronic controls (Figure 5)

The MCE tester provides the user with a variety of tests for analysis while the generator is de-energized: MCE Standard test, Polarization Index, Dielectric Absorption Ratio, Step Voltage, and Rotor Influence Check. The MCE test results provide a comprehensive picture of the electrical condition of motors and generators. Some of the MCE tests provide enough information to call a motor good or bad, based on results from one test. Other MCE tests provide data, which is best suited for trending and comparison. Trending means comparing sequential test results for the same motor over time. Comparison means comparing individual test results on one motor with test results from an identical motor operating in a similar environment.

The MCE Standard test (Figure 6) provides the following for the rotor and stator of the generator:

·        Measured and temperature corrected resistance-to-ground in megohms

·        Capacitance-to-ground in picofarads

·        4-wire bridge resistance in ohms

·        Inductance in mili henries

The resistance-to-ground (RTG) measurement indicates the cleanliness and health of the insulation system. Temperature correction is necessary since the resistance of insulation decreases significantly as its temperature increases. To compare readings obtained today with readings obtained six months from now, it is important to compare like results. The way to do that is to calculate the resistance to a given temperature, MCE uses 40^o Celsius.

The capacitance-to-ground (CTG) measurement is indicative of the cleanliness of the windings and cables. An increasing trend indicates that the motor needs to be cleaned.

4-wire bridge resistance is the measured DC resistance between phases on the stator or rotor. These resistance values are used for trending and comparison with identical units. A change in these values can indicate high resistance connections, coil-to-coil or turn-to-turn current leakage paths, open windings, etc.

The Polarization Index (PI) and Dielectric Absorption (DA) tests (Figure 7) indicate the condition of the insulation system under test. The PI is calculated from the 10-minute megohm reading divided by the 1-minute value. IEEE 43-2000 recommends a minimum PI value of 1.5 for Class A insulation and 2.0 for B, F, and H insulation. The DA ratio is calculated from the 1-minute megohm reading divided by the 30-second value, a value of greater than 1.5 is recommended.

The Step Voltage (Figure 8) or Leakage Current vs. Voltage testing of insulation systems is a process of applying a DC test voltage for a specific amount of time, usually 60 seconds, and recording the leakage current at scheduled times, usually 60 seconds, for a series of voltage steps up to a predetermined level of voltage. The level and steps of voltage applied and the amount of allowable leakage current are set prior to beginning the test. Maximum voltage applied during the test is normally well above the AC peak voltage. Moisture and dirt in the insulation are usually revealed at voltages far below those expected in service. The effects of aging or mechanical damage in fairly clean and dry insulation may not be revealed at such low voltage levels. When the voltage is increased in steps to produce electrical stresses, which approach or exceed those in service, local weak spots in the insulation will be observed in the insulation resistance. Advanced technology has basically eliminated the term “infinity” with respect to leakage-to-ground resistance. Advanced testers reach the tera ohm range, 10¹² or 10 million million ohms. The newer test capabilities and computerized data collection have given the technician the ability to evaluate new motor insulation systems. These advancements in data collection provide legitimate, repeatable data that can be recorded and used for trend evaluation.