1. DIGITAL EL-CID TEST ON STATOR CORE:
Digital EL-CID test is an improvisation over the conventional core flux test. Core flux test consists of looping excitation cables around the stator core and energizing it with a 230 V supply and passing current through a certain number of amperes turns that will excite the core to 100 % flux. This magnetic flux around core will heat the core after 30 to 60 min and thermal imagery would reveal presence of hotspots. These locations would correspond to laminar shorting.
For large generators, power supply requirements for 100 % flux excitation would be very large and require looping using large gauge cables, which is not practical at site though can be managed if feeder supply from a power transformer is directly made available. So, EL-CID test requires passing of only 4 % flux. This can be done by using a normal single/two phase variac of 230 V/ 415 V. A chattock coil (flux sensor) is then moved along the length of each slot covering all slots and thus complete periphery. Flux linkage due to core excitation will generate voltage in this coil sensor that is measured as a waveform over distance and calibrated directly as “current measurement”. Any core laminar shorting will produce localized eddy current between these laminations. This will generate its own magnetic field/flux which will either add or subtract from main field. This will produce a peaking or trough in measured waveform and hence detect the defect, as coil passes over this region.
2. INSULATION PROFILING BY CHARGE-DISCHARGE METHOD:
1. IEEE Std 43-2013: IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery
2. "Dielectric Test Methods for Rotating Machine Stator Insulation Inspection" – R. Goffeaux, B.Comte, B. Fruth, IEEE Transaction, Electrical Insulation and Dielectric Phenomena, Oct. 1998. Annual Report. Conference
A suitable DC Voltage is applied on each phases of stator winding individually for a fixed time period while recording the current with other two phases grounded. Then, the stator winding is shorted to ground via PDC circuit, and discharge current is monitored as a function of time. The plot of both the charge and discharge current is plotted against time to facilitate comparisons between the two currents.
The Insulation Resistance (IR) values at 1 min interval is accepted as a parameter of significance. Its value determines the state of dryness or wetness of insulation. Also, where the values of IR are lower than 5 MOhm, another parameter i.e. Polarization Index (Ratio of IR 10 min / IR 1 min) is computed and values above 2 are considered acceptable.
The traditional test was determined for Class B thermoplastic insulation that is hygroscopic in nature and wetness of insulation was a major cause of concern. For present day Class F thermosetting type insulation (e.g. Epoxy resin on mica glass fibrous tape) and processes like VPI, moisture penetration is minimal and very high values of IR are obtained. The values do not deteriorate to below acceptable levels specified in IEEE 43, even in highly contaminated machines.
The insulation is modelled as three polarization mechanisms:
• Movement of slow ions in the impregnating resin
• Relaxation of electric charge being present in the HV insulation
• Interfacial Polarization due to presence of different material (mica-resin) Thus, depolarization current is given by
• Idepol = I0 + a1.e(-t/t1) + a2.e(-t/ t2)+ a3.e(-t/ t3)
The charging curves and discharging curves are plotted over time to get polarization current and depolarization profiling curves.
Two parameters of significance are calculated
• Conduction Index (CI): Defines the global state of insulation and insulation condition linked to slow polarization mechanisms
• Ageing Factor (A): This determines the general ageing/degradation
• Normalized Discharge Resistance (RCdch) – defined by IEEE 43, lower than 2000 values signifies lack of curing, thermal ageing, moisture absorption in bulk wall of insulation.
IR Tester: Megger MIT520 and a 100 Ohm Resistor box + Fluke DMM
3. INSULATION LEAKAGE SPECTRAL ANALYSIS:
Several studies have been conducted on leakage currents on surface of glass as well as polymer insulators in normal and polluted conditions. The FFT spectrum of such leakage currents, showed presence of odd harmonics (3, 5, 7, 9 harmonics). The increase in ratio of 3rd/5th harmonic had a strong correlation with increase in leakage current flow in old and contaminated insulators and which increased steely with arcing.
The usage of non-linear dielectric composite materials in stator insulation (epoxy-mica, conductive tape in slot section, semi-conductive stress grading system at slot-exits) and phenomena like contamination on end-winding surface, space charge polarization due to delamination between tape layers, interfacial polarization between layers of composite dielectrics or between the layers of semi-conductive and main insulation, insulation ageing and degradation etc. add to non-linearities and increase in odd/even harmonic content in insulation. At higher voltages, the partial discharges occurring in insulation voids or on overhang surfaces increase the ratio of 3rd/5th harmonic.
HV Source: DELTA 4110 (Megger-Make) / Power Electronical ILSA Kit: Qualitrol Deltamax-15 or DSO – TBS102B (Tektronix) Shunt: 1 Ohm, Voltage Divider box
4. TAN DELTA AND CAPACITANCE ANALYSIS:
The test will be conducted as per CIGRE 552 & IEEE 286: 2000, only on assembled stator windings. The standard is mainly used for testing of stator coils, but for assembled units, it helps to determine the insulation condition or health and is thus an important test. Test shall be for each phase wise (wherever Star point is external) and also for combined phase with respect to ground. Tan delta test shall be done upto rated phase voltage in 5 or more steps from 20% (or 0.5 kV) to 100% rated phase voltage.
Besides the tip-up information which helps in assessing occurrence of partial discharges and effects due to the same, PE goes a step further. We assess the base values and its variation with voltage for both tan delta and capacitance curves and then combine it with the IR profiling, PD test and other test results to determine the state of resin cure, occurrence of de-lamination, ageing/dielectric loss, presence of moisture/contaminants, quality of contact of stress grade system with stator coils.
HV Source: DELTA 4110 (Megger-Make) or Power Electronical make & Tan Delta Bridge: DELTA 4110 (Megger-Make) or Deltamaxx-15 (Qualitrol make)
5. PHASE RESOLVED PARTIAL DISCHARGE ANALYSIS:
The test will be conducted as per the new IEEE 1434: 2000 and IEC 60270, IEC 60034-27.1 & 27.2: 2006, only on stator windings. Test shall be done for each phase (wherever star point is external) and also for combined phase with respect to ground. The PD inception voltage and extinction voltage shall be measured. Online PD will be conducted only if the coupling capacitive sensors are permanently installed at motor terminals and BNC output is providd externally for measurements.
Besides, the PRPD patterns at rated test voltage, PE will provide a set of plots at different test voltages and at different center frequencies that are chosen based on site conditions and patterns obtained, varying from low frequency to high frequency. This gives better insights into actual PD signals and noise elimination, increasing accuracy of the analysis of insulation condition. The report will indicate severity of PD and its location overview with likely cause, which helps in focused maintenance actions.
HV Source: DELTA 4110 (Megger-Make) / Power Electronical PD Kit: Omicron MPD600 or Qualitrol Deltamax-15 Capacitor: 1000 pF
6. MOTOR CIRCUIT ANALYSIS:
The test is often recommended for LT motors (but can be applied to HT motors) and follows IEEE 388:1992 and 389:1992 standards. It is also listed in IEEE1415:2006 Guide for Induction Machinery Maintenance Testing and Failure Analysis.
It mainly consists of inductance, capacitance and phase angle checks. Often the LT motors can have interturn fault, it may continue to operate (unlike HT units that will burn immediately causing ground fault). So the test is used mainly as a troubleshooting tool and helps in assessing presence of such faults. The measurements can also used to ascertain cable faults, joints issue, phase unbalance, turn insulation problem.
All Test Pro 5 from ATPOL
7. DC WINDING RESISTANCE:
It is also listed in IEEE1415:2006 Guide for Induction Machinery Maintenance Testing and Failure Analysis.
This is not a diagnostic check but mainly used to ascertain balance in three phases. It is often trended to check for any developing high resistive joints etc. This is checked using a micro-ohmmeter which gives better accuracy than a multi-meter given the low values. Test is often important post a rewinding operation but generally no variation is expected for healthy motors. While it is included in Winding Circuit Analysis, this measurement will be cross checked as a micro-ohmeter gives high accuracy for such low resistances.
Micro-ohmmeter from Prestige or Motwane
8. SURGE COMPARISON TEST:
The test follows IEEE 522 standard.
The main aim of the test is to assess health of inter-turn insulation which is not obtained in a Megger test. A repetitive voltage surge of known voltage level (corresponding to rise time and motor rated voltage is applied at motor terminals and its damped characteristic is measured and compared for different phases. Any mismatch indicates shorted turns. The test also gives a developing interturn indication and is effective than inductance/impedance measurements.