Advanced Partial Discharge Testing of 540V Aeronautic Motors Fed by SiC Inverter under Altitude Conditions
Summary (7 min read)
- Optimization of energy sources aboard aircraft and permanent improvements in more electric technologies are pushing the aeronautic industry to aim for the more electrical aircraft (MEA).
- Power generation, distribution and conversion are easier because achieved than with traditional hydraulic systems needing large, heavy and maintenance intensive distribution system.
- But, this shift towards MEA is not without consequences on the electrical stress the insulation system has to withstand.
- Thus, with primary voltage increasing, power distribution architecture evolution and high power density power electronics, partial discharge (PD) is now a serious cause of concern for aircraft integrator, system designer and component manufacturer.
- Once PDs are occurring regularly, degradation of the insulation system until premature failure of the aircraft system is irremediable.
Scope and Goal of the Work
- This topic of interest has already been discussed by several key actors of the aerospace industry, research center such as US Air force laboratory or international academic actors.
- Among key issues of aeronautic companies that has to be investigated electric motors in an electrically non-intrusive way and under representative aeronautic environment when fed by a silicon carbide (SiC) inverter drive will be addressed in this paper and will be the main contribution.
- The aim of this paper is to demonstrate the feasibility of partial discharge test of conditions, both electrically and environmentally.
- The study helps to limits of the insulation system under various pressure and electrical conditions.
- But, as a whole, a key link in the testing chain was still missing.
- In a second step, key features of more electrical aircraft will be recalled and how they relate to the innovative experimental set-up used here.
- Then, its main characteristics will be presented and an analysis is made regarding the relation between time, dV/dt and overvoltage.
- The impact of a SiC inverter drive on the non-intrusive detection and wavelet numerical processing will be presented as a mean to double-check partial discharge inception (PDIV) values.
- Then, details of the 3 stator will be presented and will be followed by tests results and analysis.
- Stator n°1 illustrates the potential consequences of undetected partial discharge on the insulation system and its consecutive failure.
An electrical discharge that only partially bridges the insulation between conductors. A transient gaseous ionization occurs in an insulation system when the electric stress exceeds a critical value,
- And this ionization produces partial discharges.
Consequences of Partial Discharge
- Partial discharge events have been known and studied for more than a century, starting with Paschen, Peek and Townsend among others.
- Partial discharges are feared because they contribute to the degradation of the insulation system and lead to its premature failure  as will be shown in this paper.
Partial Discharge in More Electrical Aircraft
- Since this variation is non-linear with pressure, it is assumed that no simple scale factor could be applied to predict partial discharge inception voltage from results at atmospheric pressure.
- It is thus mandatory to carry out test across the whole range of pressure.
Partial Discharge Inception Voltage
- The lowest voltage at which continuous partial discharges (PDs) permissible background noise) occur as the applied voltage is including the rate at which the voltage is increased as well as the previous history of the voltage applied to the winding or component thereof PDIV could be expressed either at RMS or peak voltage depending on the applied voltage during test.
- During partial discharge test of equipment, PDIV is usually a representation of the insulation system quality.
- In other words, it partial discharge occurs.
- It is thus of a critical importance to that tests are robust, easy to reproduce and relevant for the foreseen application.
- Have dramatic consequence on its reliability, leading to short-circuits creating electrical arcs.
More Electrical Aircraft Paradigm
- The MEA approach underlines the use of electrical system for non-propulsive application such as environmental control systems.
- Other applications such as electrical taxiing or anti-icing are also aircraft has no on-engine hydraulic power generation and bleed air.
Key Role of Highly Integrated Power Electronics
- A key enabler of such development is the breakthrough in high power density, high voltage and reliable silicon-based power semiconductor switching device such as insulated gate bipolar transistor (IGBT).
- It is expected that state-of-the-art switching device such as Silicon Carbide (SiC) or Gallium Nitride (GaN) will dramatically improve inverter drive, converters, motor controller and other semiconductor.
- The evolution of power electronics towards fast switching in the primary voltage is putting even more stress on motor insulation system [9, 10, 11, 12].
- The combination of harness length, fast rise time is creation overvoltage at motor terminals and an uneven voltage distribution is found on coils increasing electrical stress on the turn-to-turn insulation system.
- The following experimental set-up aims to recreate such electrical stresses.
High Speed High Voltage Power Amplifier
- 10kV peak and a maximum of 40mA RMS current with a bandwidth tested such as AC at various frequency, square bipolar with adaptable duty cycle, frequency and other point-per-point signals.
- An inverter drive has been designed at IRT Saint Exupéry to recreate voltage, two 15kW 500V ElektroAutomatic DC power supplies are in series with the mid-point grounded, the total DC bus voltage could remotely be varied from ± 10V to ± 500V.
- The inverter drive consists in three 1700V SiC Cree power modules delivering fast rise times on each phase with driver boards accepting Matlab-Simulink model could be used to control the inverter drive thanks to the use of an OPAL-RT real time calculator.
- Switching frequency, carrier frequency and modulation factor could easily be per phase (enable and command) allowing the use of the inverter as a High Voltage Measurements Voltage level are monitored using a Testec TT-SI 9010A (1/1000.
Analog to Digital Conversion
- Data are displayed and recorded using a Keysight DSOS204A oscilloscope with a sampling rate of 20GSa/s and numerical bandwidth numerically, the high resolution mode (12 bits) is used instead of the peak detect mode at the higher sampling frequency available.
Analog High Pass-Filtering
- When performing measurement using non-intrusive sensor, it is usual remove noise coming from inverter drive switches or power have been used in the following experiments.
Partial Discharge Sensors
- Technological research at IRT Saint-Exupéry are focused on non-intrusive sensors.
- These sensors and associated method have both electric motor in automotive  and aeronautic test benches .
- The non-intrusive sensor used to detect partial discharge is taking sensor behavior in the next part.
- This sensor has already been used in numerous study, particularly  and .
- All the tests have been carried out within a grounded vacuum chamber, acting as a faraday chamber, connected to a vacuum pump.
Rise Time and Overvoltage Measurement
- Time dependent rise time and overvoltage analysis has been carried out while feeding a three phase electric.
- Measurements were probe between the DC- and the output of the power module.
- Table 1 details the used operating point.
- Typical SiC inverter behavior could be observed on both rise time and overvoltage which is both are current dependent.
- As a result, the electrical stress created by the combination of dV/dt and overvoltage is time dependant.
Capacitive Sensor Study
- Impulses are transduced into voltage variations through the capacitance between the copper wire and oscilloscope impedance.
- Due to its small geometry, achieving good contact and adequate holding on the 3D printed pliers to ensure good contact regardless of the cable gauge and vibrations Sensor sensitivity can therefore be increased simply by improving the coupling capacitance value.
- This is achieved by increasing the contact surface by putting copper tape on the surface of the power cable to control the interacting area and thus the value of the induced capacity ( ).
- It is important to note that this sensor is sensitive to the dynamic of the partial discharge compared to the "standards" sensors, which are only sensitive to the amplitude of the discharge.
Switching Noise Magnitude Evolution: Relation with Common Mode Current
- When performing non-intrusive PD detection, periodic pulses numeric, it is important to track how maximum switching noise is evolving with DC bus voltage.
- Each of these switching is intrusive sensor.
- This source of noise collected by the sensor is dependent of the common mode capacitance of the whole set-up value.
- Whereas the common mode capacitance is decreased when stator frame is will be low when the stator frame is grounded, as it is usually the case in aeronautic applications.
- The sensitivity of the measurement of PD pulse high frequency current will thus be function of the sample common mode capacitance, detection capacitance and the spread of frequency content between switching common mode current and partial discharge current.
Analogical Filters Performance
- Since the sensor is sensitive to quick changes in current, any phenomenon that induces this type of variation can cause a sensor response.
- When seeking to detect PDs in equipment supplied by inverter using PWM., a key challenge is to distinguish the raw signal with respect to electromagnetic noise induced by fast switching inverters.
- The magnitude of the signal associated with partial discharges generally has an amplitude of several tens of mV, while the magnitude of the noise signal is on the order of several hundred mV and depends on the voltage magnitude as has been demonstrated previously.
- A study was carried increased capacitance.
- It can be seen in that the noise increases with the amplitude of This is explained by the fact that the dV / dt is not only dependent on the amplitude of the voltage but also the switched current.
Noise Suppression Improvement
- In aeronautical environments, some equipment operates under pressure and temperature conditions that can vary greatly.
- Signal reconstructed by CWT (red) at atmospheric pressure Moreover this method sets all the wavelet transform 21].
- This method has real no longer required to remove noise in signals.
- It should be noted that although PDIV could be determined with this method, no calibration or PD pulse shape analysis could be carried out at the moment with the proposed algorithm.
Electric Motors Description
- In order to validate their detection method and to make the best use of their test bench based on the SiC technology, three low voltage stators chosen of designed.
- The table below summarizes some of their properties.
- These are stators of three-phases Permanent Magnet Synchronous Motor (PMSM), they are designed to be powered with Pulse Width Modulation (PWM) inverters.
Stator n°1: Laboratory Dummy Sample
- The design of this stator has been extensively detailed in  are tables regarding insulation and electric design are recalled below (Tables 5, 6, 7).
- The two terminals of each of his three phases are accessible.
- This allows to test each phase separately.
- It is very important to highlight that the stator is not impregnated and that it has been rewound before starting the test campaign to ensure a PD free previous history.
Stator n°2: High Mechanical Integration Constraint Stator
- Pressure (VPI process), this allows the varnish to penetrate as much as possible within the windings.
- Its end-windings are mechanically compressed to optimize the axial length of the machine ( ).
- The neutral point is not accessible so the evaluation of the PDIV of each phase alone (turn to turn PDIV) is not possible.
- Therefore, only the star-connected case will be tested.
- Due to non-disclosure agreement with their industrial partners, insulation and electrical design details could not be provided for stator n°2 except that this is a three phases motor with four parallel coils per phase.
Stator n°3: Low Cost Stator
- Unlike the other two stators, this stator is cooled by air.
- The insertion of its coil is automated, this allows a more reproducible winding but end-windings are not compressed and are insulated by relatively thick tapes.
- It is also varnished using dipped impregnation instead of vacuum pressure impregnation as shown in Each of its three phases (U, V and W) is composed of four coils (U1, U2, U3, U4, V1, V2, V3, V4 and W1, W2, W3, W4).
- This stator is designed for electric traction application under atmospheric conditions.
- Both 100mbars and atmospheric pressure except for stator n°3.
- The purpose of these tests is to investigate the PDIV under SiC inverter drive electrical stress compared to typical AC tests usually carried out.
- On one hand, AC tests are used to determine the PDIV between phases (if neutral point is disconnected), and between phases and stator frame.
- On the other hand, SiC inverter tests will be used to determine PDIV in a more realistic approach with fast rime time impulse compared to aeronautic standard (>5V/µs).
PDIV of Experimental Set-Up
- Before starting any investigation, PDIV of the experimental set-up Table 8).
- Stator n°1 has been designed at IRT Saint-Exupery to represent a typical aeronautic electric motor insulation system.
Phase to ground
- Grounded to test the slot insulation only.
- Then each phase is tested separately in respect to the stator frame.
- Tests are performed with 50 Table 9) Results are similar with their previous technical report , showing that even a complete rewound of the electric motor lead to stable between phases could be noted suggesting that slot insulation is balanced in all slots.
Phase to Phase
- The phase to phase insulation is made with the same material as the slot insulation (insulating paper).
- Results (Table 15) are consistent with this observation and should lead to better phase-to-phase insulation performance, although PDIV value are also more widely distributed.
- Another possible cause for this better performance is the concentric windings.
- Stator n°2 is also an aeronautic electric motor with the neutral point star connected meaning only few tests are possible both in AC and possible.
Phase to Ground Tests
- A typical phase to ground insulation test is made on all phases at the Phase to ground insulation performance is better than for stator n°1 which presents similar insulation characteristics except for test for the VPI varnishing process.
- It is believed that the VPI process helps most of air gaps.
Stator Supplied by Three-Phase SiC Inverter at Atmospheric Pressure
- The electric stator is tested at low pressure (100mbars).
- It can be seen on that the discharge appears at 641 Vpeak at 100 mbar for the maximum voltage.
- Any insulation could trigger a This PD location is not characteristics of any insulation on the contrary to the previous case.
- Other parasitic signals of lower amplitude at 100 mbar could be observed and are labelled as residual noise.
- Detection with high-pass test and numerical signal processing.
- Stator n°3 is only tested at atmospheric pressure.
Phase to Ground
- In table 14, PDIV values for each phase are more widely distributed regarding stator n°3 than for both previous stators which is phase-to-ground insulation performance.
- Lowest phase to ground PDIV for stator n°3 is very similar to stator n°1 average phase to ground PDIV value.
- Since 12 coils are available, each one is tested relatively to each other within each phase.
- This is not a conventional test since these coils are supposed to be connected in parallel within each phase and should coils of the same phase are in close vicinity to each other and the magnet wire and varnish is the only insulation and it could lead to a fairly low PDIV if the start of one coil is close to the end of the other.
- Or coils are wide away meaning that PDIV should be very high compared to other insulation.
- Phase U presents the lowest coil to coil insulation of all phase at This weakness obviously does not appears with AC test because all coils are at the same voltage.
- This may be explain by the fact that this motor has already be tested numerous time regarding partial discharge level with an inverter.
Coil Configurations and Electric Stator Characteristics
- Take advantage of having access to its 12 coils and connect them to • ) • ) • ).
- In terms of functional performance, the number of turns per phase ), therefore the base b b represents the speed from which the consequently the current consumed by the motor.
- In the nominal case when all phases are star-connected, the PDIV could not be determined which is consistent with the fact that the most stressing case is when only one phase is tested.
- When comparing results with coils results for phases V and W one could observe that it is likely that phase V presents a turn-to turn PD whereas phase W presents a coil-to coil PD.
- This is due to the fact that the current supplied is lower and the tendency of SiC switch to produce faster dV/dt under low current, the applied electrical stress regarding turn insulation is thus higher.
Preventing PD in Turn-to-Turn Insulation
- The critical area of insulation for phase V is the turn-to-turn insulation of coil 2 (named blue/yellow).
- It thus seems interesting to change the coil connection to try to reduce the electrical stress applied to it.
- The weakest coil V2 was connected in series to coil V4 which does not present turn-to-turn PD.
- It can be seen in that the electrical stress applied to the turn-to-turn critical area has been decreased because PD are no longer observed at coil V2 PDIV.
Preventing PD in Coil to Coil Insulation
- Coil insulation between W1 ( yellow) and W4 .
- The authors can see in that the electrical stress decreased because there is not any PD at the voltage previously observed.
- As before, voltage is now divided between coils in series which, in turn, reduce the coil to coil stress.
- Bus voltage has been increased up to 970VDC without observing any PD signals.
Discussion about Winding Configuration Influence
- Using results from coil-to-coil and impulse tests, the advantage of the same insulation system.
- What could have been expected from a simple voltage distribution analysis on coils have been tested experimentally on a typical electric stator.
- This holds true for this segregating coils and coils of few turns which does not prevent Although these conclusions could not be directly extended on any enhanced PDIV performance on other electric motor even without knowing a priori of all insulation in the motor.
- Obviously, this better PDIV performance come at a cost regarding electromechanical torque and speed.
Summary and Conclusions
- This paper illustrates with stator n°2 the robustness of non-intrusive detection method at low pressure under SiC impulse voltage.
- Both PDIV with a better signal to noise ratio for the latter.
- The experimental set-up consisting in a three phases SiC inverter drive and a vacuum chamber is able to reproduce any PWM waveform on a wide range of electric stator both at atmospheric pressure and at low altitude.
- The failure of stator n°1 illustrates the destructive potential of non-detected partial discharge.
- This study showed how, without any knowledge of PDIV of each similar insulation system and identical electromechanical performance to evaluate insulation performance.
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