INVESTIGATION OF TRANSFORMER AND MOTOR INSULATION UNDER REPETITIVE SURGE VOLTAGE Arijit Basuray
[email protected] Neo Tele-Tronix Pvt. Ltd.(NTPL), Kolkata, India
ABSTRACT: Recurrent Surge Generator (RSG) can be used for testing various parameters of Motor or Transformer windings including inter turn insulation. Dry type or solid insulation in transformer has many interfaces and undesirable defects and these defects can be exposed under this nondestructive testing methodology. Since insulation system used nowadays are shifting more and more to high frequency application such as in power electronics, variable frequency drives (VFD) etc. authors have used the recurrent surge generator to a PD measuring instrument at fast rising voltage enabling PD measurement at closer situation under which the insulation system is supposed to work. Author have discussed test results on different insulation system with recurrent surge voltages of different rise time. Introduction: Over the past 20 years pulse width modulation (PWM) type voltage source converters have become widely accepted for Low Voltage motors. In recent time PWM and step voltage source converters have been developed for medium voltage converter drives to reduce stress on the electrical power system during motor starting and energy savings associated with running a motor at a lower speed yet producing adequate torque for acceleration. VF Motor control being initially the most demanding requirement, high frequency converter technology developed immensely during these days and serving a lot more application like PWM based power supplies High Voltage Modulators and Pulse Power technology. The introduction of these types of high frequency switching application, however, raised a number of issues. One of the major
concerns is the effect of repetitive fast risetime, relatively high voltage surges on the insulation system of windings of the transformers and inductors used in the PWM converter as well as motor stator causing premature stator winding failure. The main reason of this premature failure is PD which gradually erodes the insulation at a rate that depends on the nature, amplitude of PD and the type of insulating material used. Authors here studied two situations in insulation system 1) PD within turn Insulation tested in the form of a twisted pair and 2) PD in ground-conductor interface in the form of insulation film between two electrodes. As for windings, the repetitive, short rise time voltage impulses may cause a high percentage of the voltage step appear between the turns as well as in insulation between winding and ground potential. Theoretical Basis: To start with, let us consider a simple configuration of interface of two different insulating materials of different dielectric constant confined between two electrodes parallel to each other.
Fig. 1 We know D E ……………………(1) where E= Electric Field ε= absolute permittivity D= Displacement vector
The effective gap distance may be written as
deff d1 d 2 1 . 2 If 2 1 , 1 1 hence deff d 2 d , 1 , 2 being constant for V 1 the field stress E1 will always increase if the thickness of layer with higher permittivity is increased. That’s why in designing insulation system compatibility of insulating components is of extreme importance.For this reason it is found that in many situation partial replacement of the gas with solid dielectric material does not improve the dielectric strength of oil or gas insulated system as the stress within the gas will then be even more than the original system. Let us look at some practical situation where interfaces of dielectrics of different permittivity are obvious(Fig.2 a,b,c). The electric field at these interfaces or in void is much higher than within the rest of insulation system which can be easily understood from Fig. 3
Fig. 3 Because of repetitive & prolonged partial discharge voids get carbonised in course of time and the effective gap between electrodes increases which may lead to a possible breakdown(fig.4).
Conductor
+ + + + + + + + Layered Film Insulat ion Gaseous Dielect ric Oil/ Solid polymeric Insulation
+ + + + + + + +
Fig. 4: Voids with traces of discharge within a filled cast resin transformer
(a) : Winding layers of Transformer We know that nature of typical PWM waveform is as shown in Fig 5(a) where a real PWM waveform captured in DSO in a three state converter is shown in Fig 6(a) and Fig 6(b)
(b) : Air pocket formed in winding
A typical waveform of PWM converter Fig. 5 (c) : Motor winding within the core slot Fig. 2
(a) : 3 State PWM waveform
(b): PET film between two Electrodes Fig. 7 In the first phase we measured the response of step input voltage across the samples by DSO (Fig 8) and obtained the following wave forms (Fig 9,a,b,c)
(b) : 3 State PWM waveform, resolved Fig. 6 Experimental Set-up and results: In our experiment we used a High Voltage Recurrent Surge Generator where peak impulse voltage of sharp rise time where level can be varied from as low as 100volts to 5000volts with a repetition rate same as power frequency. On each cycle a capacitor is charged to the set voltage and then discharged to the object under test in the form of impulse by means of solid state HV switching circuit. We have measured HV impulse response as well as PD on the following samples as shown in Fig 7(a) and Fig 7(b).
Fig. 8
(a): Reponse obtained at 600 Volts(similar for both the samples)
(a): Enameled wire in the form of twisted pair.
lower cut Off frequency of 500KHz but could not get proper PD signals but at 1MHz Lower cut off we got the following responses (Fig 11 a,b)
(b)Response obtained at 3KV for twisted pair sample.
(a): At 3KV Impulse voltage for twisted pair test of Magnet wire
(c)Response obtained at 2.8KV for PET Film. Fig. 9
In the second phase we incorporated arrangement for partial discharge measurement during surge testing. We modified the test setup as shown in Fig. 10
(b): At 2.8KV Impulse voltage for PET Film Fig. 11. The amplified PD Pulses observed in by DSO
Fig-10
A coupling capacitor (CC) of 100pF along with matched impedance (Z) was connected parallel to the sample under test and the PD pulses were decoupled to the input of signal processor. Initially we used
Conclusion: So far what we have done is more of a qualitative study and could detect PD signals at relatively higher discharge level. We could not collect PD signals at lower voltages efficiently. Improving the efficiency of our filter and amplifier circuit the same can be achieved. We are making an elaborate experimental arrangement to make further study on this.
References: 1.
Basuray, A, “Compatibility of Insulating materials in insulating System, The Physics and Engineering”, accepted in 8th. International Seminar on Electrical & Electronic insulating materials & Systems, Insulec-2009, 19-20 February 2009 Bangalore, India.
2.
“Recurrent surge generator as a versatile diagnostic tool”. Arijit Basuray and Sujata Chakraborty. INSUCON 2009,25-28 May 2009 in Birmingham,UK.
3.
Lindell, E & others, “Measurement of Partial Discharges at Rapidly Changing Voltages, IEEE Transactions on Dielectrics and Electrical Insulation”, Volume 15, No. 3, 823-831(June 08).
