Design and Testing of Insulation for Adjustable Speed Drives Alfredo Contin DIA University of Trieste (Italy)
[email protected] In cooperation with Andrea Cavallini
[email protected]
Davide Fabiani
[email protected]
University of Bologna (Italy)
Germano Rabach University of Trieste (Italy)
Purpose: to provide information on modern techniques adopted to design and test insulation for adjustable speed drives (ASD). Reasons: ASD dramatically increases electrical stress due to the significant harmonic content of the power supply and can promote premature breakdown. Design and testing criteria are quite different with respect AC applications. Most of the procedures are still under investigation Standards are still under discussion
Problems High slew rate Overshoots High switching frequency Uneven voltage distribution
Design
PD activity Space charge accumulation Localized overheating Increased electrical losses Electromechanical fatigue
New constrains Multi-objective design Accelerated life tests
Evaluation
Space charge measurements
Solutions
Filters (high cost)
PD measurements
New insulating materials (nano-tech)
Summary: 1.
Design of Insulation Systems for AC Applications
2.
Additional Stresses
3.
Degradation Processes in ASD Applications
4.
Tests to Evaluate the Degradation Processes
5.
New Materials and Systems
6.
Design of Applications
7.
New Standards
Insulation
Systems
for
ASD
Design of Insulation Systems for AC Applications Summary:
Insulation technologies
Stresses and Aging
Design of insulation systems
for LV-MV-HV rotating machines Reasons: current design criteria for insulation of ASD are derived from those adopted in AC considering additional stress typologies different impact of typical AC stresses
Random Wound Machines Typical solution for LV, LP rotating machines
Voltage
b
3 1
1) phase-to-phase
a
2
2) phase-to-ground 3) turn-to-turn
Insulation a) phase-to-phase insulation within the slot and on the overhang b) ground insulation c) turn insulation
1 c
Form Wound Coils
d
Random Wound Coils
Wire Insulation Modern magnet wire typically uses 1-4 layers of polymer film insulation. Increasing the temperature range: Polyvinyl
Polyurethane Polyamide Polyester
Polyester-polyimide Polyamide-polyimide (or amide-imide) Polyimide (up to 250°C)
To improve the insulation strength and the longterm reliability, the insulation is often augmented by
wrapping it with fiberglass or mica tapes
using VPI technology
Classification magnet wire is classified by diameter (AWG number or SWG) area (square millimeters) thermal class insulation class. The thermal class indicates the temperature of the wire corresponding to 20,000 hour service life Common temperature classes are 105° C, 130° C, 155° C, 180° C and 220° C (IEC 60085)
Ground-Wall & Phase/Overhang Insulation Wide use Nomex : synthetic aramid paper (with high porosity for VPI applications). Mylar : Polyester film (Polyethylene Terephthalate (PET)). Less adopted Kapton : polyimide film higher performances but expensive Imp Obs: most of LV LP rotating machines are insulated using only organic materials
Materials for MV & HV Motor Coils
Turn Insulation: is designed according to the rated voltage and the thermal class of the machine Volt/turn 10>V/MN
Simulations Transients effects in ASD applications are studied resorting to simulation of the complete system in order to predict the distribution of the voltage stresses to obtain information for insulation design The signal transmission theory is adopted to simulate the connection cables instead the more complex distributed parameter model R0 Ro Z
S1
Ro Z
S S
Where S is the input signal Z characteristic impedance of the cable Ro output impedance of the cable S1 reflected signal So output signal (So=S+S1)
So
Ro Z
S
Coil model The random nature of the coil with unknown position of the turns inside the slot can be approached considering the coupling between two turns:
where Lt, Rt are the turn self inductance and resistance Mij the magnetic coupling between turns Ctt, Ctg the capacitive couplings between adjacent turns and external turns and grounded stator, respectively
After selecting the number of turns, all the possible configurations of turn connections are explored (e.g.,7 turns) Two different simulations are compared with the experimental plot: coil impedance vs frequency Zin-gnd shows two resonant frequencies. These resonances change with the turn arrangement but not in large domain
Winding model A simplified electrical model (derived from the coil model) can be adopted to simulate the whole winding where Lw, Rw are the turn self inductance and resistance Rp, Cp the equivalent parallel resistance and capacitance Rgnd, Cgnd the equivalent phase-toground resistance and capacitance The experimental validation shows a good agreement
I° Solution: Multi-Level Converters
• Reduction of the Jump voltage 0.7(Vdc/(n-1) + Vb) • Reduction of the ph-to-ph Vpp Vdc/(n-1) + 2Vb • Effective but expensive • Valid if adopted for other purposes
2.5
2
2
1.5
1.5 1 1 0.5
0.5 0
2 Levels
0
-0.5
-0.5
-1 -1 -1.5 -1.5
-2 -2.5
-2 0
3 Levels
500
1000
1500
2000
2500
3000
3500
0
2
2
1.5
1.5
1
1
0.5
0.5
0
0
-0.5
-0.5
-1
-1
-1.5
-1.5
500
1000
1500
2000
2500
3000
3500
-2
-2 0
500
1000
1500
2000
2500
0
3000
1.5
500
1000
1500
2000
2500
3000
2
1.5 1
1 0.5
0.5
5 Levels
0
0
-0.5 -0.5
-1 -1
-1.5 -2
-1.5 0
500
1000
1500
2000
2500
3000
3500
Phase-to-Ground V
0
500
1000
1500
2000
2500
3000
3500
Turn-to-Turn V
turn-to-turn stress decreases
Harmonic Filters Two kinds of analog filters can be connected at the inverter output: to reduce the harmonic content of the supply voltage (rise-time increase, overvoltages reduction, delay time increased, voltage stress reduction at bearings) to transform the PWM in sinusoidal wave-shape (avoid the use of shielded cables and EMC problems) inverter output
filter output
The selection of the materials and the design of insulation systems for ASD are here considered But Which quantities associated to the harmonic distortion affect accelerated degradation?
Vpeak
Vrms
Vslope
Freq.
Rep.Rate
(dV/dt)
Do we know the voltage waveforms affecting insulation systems & electrical apparatus (rotating machines)? Do we know the degradation mechanism?
Stresses in ASD Applications Voltage waveforms for ASD are characterized by: High slew rate Overshoots High switching frequency Uneven voltage distribution The effects are: PD activity
Space charge accumulation Localized overheating Increased electrical losses Electromechanical fatigue
Many investigations have been performed to evaluate the impact of typical ASD stresses (Vpp, Vrms, dV/dt, f, rep.rate) on the ageing processes to identify the most important factors that affects the insulation life-time. This was done by conducting accelerated ageing tests (mainly electrical and thermal ageing) using different voltage wave-forms and repetition rate (see e.g.) S.Grzybowski et al. “Accelerated Ageing Tests on Magnet Wires Under High Frequency Pulsating Voltage and High Temperatures”, Proc. of CEIDP, pp.555558, 1999
M.Kaufhold, et al. “failure Mechanisms of the Interturn Insulation of Low Voltage Electric Machines Fed by Pulse-Controlled Inverters”, IEEE El.Ins.Mag., Vol.12, pp.9-16, Sept./Oct. 1996. A.Mbaye et al. “Existence of PD in Low-Voltage Induction Machines Supplied by PWM Drives”, IEEE Trans. on Diel., El. Ins., Vol.3, pp.555-560, August 1996
Effect of Duty Cycle The test was conducted on twisted pairs with polyurethane resin, at Vp0=950 V tr=200 ns tp RR=15 kHz D 100 T= 100°C. T tp: duration of positive pulses T: period the lifetime of the insulation decreases with increasing the duty cycle Duty is proportional to the rms voltage
Effect of the Slew Rate The average ttb under different rise times were recorded using twisted pairs with polyurethane resin, at Vp0=950 V DC=16% RR=15 kHz T= 100°C. fast rising voltage pulses create high capacitive impulsive currents. RT causes spikes, over-voltages and uneven voltage distribution along the winding local dielectric heating space charge formation
Repetition Rate Effect The TTB were evaluate using twisted pairs with polyurethane resin, at Vp0= increased in steps of 50V/s DC=16% RT=200 ns T= 100°, 155° 180°C.
Samples aged at 40 kHz (shorter duration) endured the life test longer than those which were aged at 25 kHz. The dielectric losses are smaller at higher pulsating frequencies due to the prevailing polarization mechanism under pulsating frequencies
Thermal Effects
Because of the short pulse duration and fast RT at high frequencies, voltage waveforms are affected by high order harmonic components high values of the capacitive current (proportional to ) polarization processes insulation conductivity Significant increaseg the insulation temperature (thermal degradation)
Pd CV 2 tg
Pd
n
i C iV i tg i 2
i 1 n
DP
i 1
Pi
P1
Kf= ageing acceleration factor
i
Vi V1
Thermal Effects The TTB=f(T, RR) at Vp0= increased in steps of 50V/s DC=16% RT=200 ns RR= 15, 25 and 40 kHz
the dielectric loss increases with: pulsating frequency rise time of the voltage pulses
+∆ V
+V
Voltage Stresses The impact of the voltage stress must be evaluated considering: Square unipolar (phase-tophase voltage) Square bipolar (phase-toground turn-to-turn voltage) Sinusoidal at different frequencies (dV/dt negligible) With and without overvoltages
0
-V
−∆ V
Sinusoidal Voltages at Different Frequencies At constant voltage, the time-to-breakdown is reduced increasing the frequency of the voltage mainly in the presence of organic materials 2D Graph 1 50 Hz OIL 10 kHz OIL
VEC = 11.7 10000
Voltage (rms value) [V]
Comparison of life curves obtained testing twisted pairs samples immersed in oil to avoid surface discharges. Tests performed at 50 Hz and 10 kHz
VEC = 9.2
1000 0.01
0.1
1 Failure time [h]
10
100
VEC is evaluated for the comparison: VEC=11.7 indicates a longer life with respect to VEC=9.2
1000
With and Without Over-Voltages Over-voltages are due to R/F-T, cable length, mismatch impedance, converter topology Causes an uneven distribution of the stress along the winding In random wound windings, the voltage stress between the different turns depend by their relative position It is highest when the first and the last turn of the first coil are in contact The overvoltage can be consider adiabatic (negligible heating). Rectangular wave shapes heat the insulation in the same way
Unipolar and Bipolar Wave-Shapes Rectangular wave shapes are characterized by: V0p: 0-peak and Vpp: peak-to-peak amplitude The uneven voltage distribution along the winding cause an impulsive stress whose amplitude is related to the time behavior of the turns in contact
2.5 2
V1
1.5
A voltage jump is determined (derivative effect of the winding) whose amplitude is related to V0p (unipolar) and Vpp (bipolar) voltages
1 0.5
V1-V2
V2
0 -0.5 -1 -1.5 -2 -2.5
0
0.5
1
1.5
2
2.5
3 -7
x 10
Partial Discharges Typically, the insulation thickness is designed at rated voltage High values of the voltage jump can incept Partial Discharges (PD) even in low voltage machines
IEC 60270: Partial Discharge (PD) - localized electrical discharge that only partially bridges the insulation between conductors or the adjacent area of a conductor
Life Curves With and Without Partial Discharges Experimental evidence show the huge impact of PD on the life of organic materials (turn insulation) Tests were performed at 50 Hz and 10 kHz, sinusoidal 2D Graph 1 voltage with specimens in air (with PD) and in oil (no PD)
PD
L10 kHz
L50 Hz 200 L50 Hz 14000
VEC = 11.7
Voltage (rms value) [V]
NO PD
L10 kHz
50 Hz OIL 50 Hz AIR 10 kHz OIL 10 kHz AIR
10000 VEC = 8.7
VEC = 4.5 VEC = 6.4
1000 0.01
0.1
1
10
100
1000
Failure time [h]
VEC values clearly indicate the insulation life is strongly shortened in the presence of PD when f=10 kHz
PD is a localized discharge, resulting from transient gaseous ionization where the voltage stress levels exceed a critical value. PD transfer Anode, + – Electrons on the surface anode – Positive ions on the surface catode E0 • These charges induce a local electric field Eq • In opposition with that induced by Catode, the external supply, E0. Anode, + • After discharge, the local field is – Ei=E0-Eq E0 Eq • Charges move inside the solid material and the intensity Eq decrease thus increasing Eo Catode, • Next discharge occurs when E0 > Eif (PD inception field)
The degradation of stator insulation that is exposed to a continuous voltage stress above the PDIV is a physical erosion of the insulation due to the PD attack: in voids Pos. Ions
Electrons
1- charge avalance
4- discharges carbonize the polymer
2- charge cloud hits the void surface
5- both the erosion and carbonization processes enhance the local field
3- the charge impact erodes the void surface
6- tree formation and growth until breakdown
80
On the surface
PD transform a part of the capacitively stored energy in the insulation into heat and radiation as well as mechanical and chemical energies, which can degrade insulation materials The insulation progressively reduces its breakdown voltage, until the breakdown and failure of the whole drive occurs The Paschen Curves Paschen’s Curve defines the relationship breakdown voltage, pressure and airgap
between
PD and Stress Typologies The major factors that affect the PD or corona are voltage dielectric thickness pulsation frequency
humidity
temperature
geometry
pulse rise time
Since PD is the worst ageing factor, in organic materials the PDIV is very often assumed as the end-of-life parameter The ability of materials to withstand PD is a fundamental condition but is not the only parameter to be considered The effects of ASD on ageing acceleration is quite wide The problem is to verify whether fixing the voltage below the PDIV, PD occurs due to other parameter modification
Voltage Amplitude and PD Erosion The PDIV for the inter-turn insulation can be evaluated using a twisted pair model PD occurs in the air gap (see the electric-field intensity curves around the magnet wires Voltages below PDIV do not lead to any PD
Breakdown strength, PDIV and pulse repetition rate are also related to the dielectric material and its thickness)
Unipolar vs Bipolar Voltages Life curves of the inter-turn insulation obtained applying impulse voltages of different polarity and different pulse amplitude (fixing the RT=100ns, PW=5s, RR=5kHz) are compared. Considering the IPL, let Vb: the pulse amplitude tb: time to breakdown (at 63.2%) nb: number of voltage pulses to breakdown kb: a constant 1 n: VEC
Vb kb nb
n
These results were drawn as an area of possible lognormal distribution functions considering the scatter of specimen and PD ignition
Assuming
nb (V ) pnb PPD (V )
as the probability to PD inception (ratio of number of pulses where PD occur and the total number of voltage pulses) 3 different ranges can be derived: 1: each pulse is able to trigger at least 1 PD per pulse (it follows the IPL) 2: the number of pulses to breakdown is larger due to the reduction of pnb 3: no PD inception
Contrasting results were obtained by comparing the same materials subjected by unipolar and bipolare square waves Charges accumulated by PD on the surface generates a local electric field If the charge diffusion rate is lower than the polarity reversal speed, the two fields can be added thus increasing the local electric stress (W.Yin, “Failure Mechanisms of Winding Insulation in Inverter-Feed Motors”, IEEE El.Ins.Mag., Vol.13, pp.18-23, November 1997)
Different aging phenomena at high and low stresses must be considered Further considerations after PD and space charge measurements will be drawn
PD and Slew Rate Besides over-voltages, dielectric heating, uneven voltage distribution between turns, high slew rate affects also the space charge formation If the RT is shorter than the time constant of the surfacecharge build up, the max electric field in air can be enhanced thus reducing the PD inception voltage
PD and Repetition Rate The number of pulses to breakdown: is independent of their repetition rate if PD occur less that 1xpulse (voltage range type 2) is linearly dependent to the inverse of the RR in voltage range 1 With bipolar square waves, up about 5 kHz, the charge accumulation affects the PD intensity thus decreasing the number of pulse-tofailure (W.Yin et al., “Critical Factors for Early Failures of Magnet Wires in Inverter Fed Motors”, Proc. of IEEE CEIDP, pp.258-261, October 1995)
PD and Thermal and Mechanical Stresses Higher values of temperature can promote PD thus reducing the PDIV due to an increased permittivity of the polymer (the specimen
capacitance increases leading to higher electric field intensity in the airgap)
the decreased breakdown strength of air because of its lower density
Besides the thermal ageing, higher temperatures lead to a thermally accelerated electrical ageing of the low voltage interturn insulation
Form-Wound Windings (MV, HV) Enamelled wires are protected by strand insulation based on mica tapes Inorganic materials withstand PD and the life of the insulation system when subjected by rectangular wave shapes, is almost comparable with that supplied by sinusoidal voltages Strand Insulation Enamelled wire Ground Wall Insulation Conductive Tape
Form-Wound Windings: End-Arm Stress Grading The performance of the end-arm stress grading decreases increasing the frequency content of the supply voltage V [V] 36000 34000
30 kV f variabile
32000 30000 28000 26000 24000 22000 20000
50 Hz
18000
250 kHz
16000
1 kHz
14000 12000
20 kHz
10000 8000 6000 4000 2000 0 -80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150
Distanza x dal termine del ricoprimento conduttivo [mm]
GEN.3.1
GEN.3.2
GEN.3.3
GEN.3.4
The stress grading for ASD applications must be designed properly to avoid the inception of PD and its rapid deterioration
Discussion PD is the dominant ageing factor in organic insulation while longer life if found when organic/inorganic materials were considered Type 1: insulation based only on organic materials that does not withstand PD Type 2: insulation based on combination of organic/inorganic materials able to withstand PD Inter-turn insulation breakdown is the most important failure in type-1 random wound motors due to PD in the air-gaps of enameled wires that are touching
High frequencies, short rise times and fast oscillating pulses shorten the lifetime. However, if no PDs occurred no premature breakdown was observed
Satisfactory lifetime of inverter-fed low voltage motors
can be achieved if PDs in the winding insulation are avoided This can be done by an appropriate limitation of the rise time and amplitude of the terminal voltage using short cables, appropriate filters, or lower dc voltages
Great care should be taken with proper insulation design to avoid a low PD inception voltage
A short description of PD and Space Charge measurements is provided before to discuss new solutions for insulation systems for ASD applications
