The Effect of Static Converters on Field Grading Materials in Rotating Machines

INTRODUCTION DISCUSSIONModeling

Figure 1. Label in 24pt Arial.

ABSTRACT

Silicon Carbide Microvaristors

Some of objectives

CONTACT

Silicon carbide microvaristors are the mostcommon type of stress grading materialsbeing used for the design of coils and bars.the main objective of this project isdetermination of the microvaristor’sresponse over a wide range of electric field,frequency and temperature. Unfortunately,in spite of previous research in this area, anaccurate measurement method, whichclears the performance of these materials inmentioned conditions, has not beenaddressed effectively, owing to problemsarising during practical measurement suchas thermal run away and responsedetermination in high frequency. On theother hand, such types of insulations aredistinctively different from conventionalinsulations because of Maxwell-Wagnerpolarisation process, originating fromcomposite mixture of the microvaristors andmatrix of insulation. This project aims toprovide a viable solution for aboveaddressed issues.

Two major challenges in this project are thenonlinearity and transient response of fieldgrading materials. The transient response oflinear insulation layers (including strand, micaand slot corona protection layer) can be foundby frequency domain analysis andimplementation of fast Fourier transform for dataconversion to the time domain . However, for thenonlinear regime of insulation materials, suchcorrelations are usually not available requiringconsideration of permittivity harmonics as well.Furthermore, in order to obtain the fast responseof stress grading layer, the responses of theother insulation layers are also required.

With the introduction of transformer-lessconverters and their implementation in hydro-power generator systems, electric fielddistribution of insulation systems is drasticallychanged. Concerning consequential issues ofconverters, there are two important agents,namely voltage and temperature according toIEC standards for Type II insulation systems.These effects are summarized in Table I.

Reza SargaziNorwegian University of Science and Technology (NTNU)5/27/2019

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764011.

Insulationcomponent

Fundementalfrequency

Impulserepetitionfrequency

peak to peakvoltage of

fundementalfrequency

peak to peakvoltage of twoconsecutive

impulses

Impulse rise time

Turn to turn insulation

Mainwallinsulation

(Mica)

Corona (CAT) and stress

grading Layer(SG)

Note: High Importance Low Importance

Figure 1. insulation system in Roebel bars

VoltageCurrent

Voltage source

Series Resistor

Sample V

HF and HV Transformer

Amplifier V

Matching Impedance

Stator core

Mica

Copper

SGOverlapCAT

Traditionally, before the introduction ofmicrovaristors, slot corona protection layer wasused both in the slot as well as the end windingarea for rotating machines above 4kV. With Theadvent of microvaristors in the 1970s,manufacturers could cut the corona protectionlayer just a few centimeters outside the slot withoutextension into the endwinding region. The resultinglocal electric field enhancment at this point wasalso reduced by introducing stress grading layer.this layer has a electric field dependent resistancereducing the high fields and pushing it out into thelower field areas. However, under impulse voltagestresses, its field controlling ability is influenced byfactors summerised in table I. The insulationsystem of Roebel bars for rotating machines isshown in figure 1.

Table I. different types of stress on insulation system fed by static converters

SampleAC U I

Voltage measurement Current measurement

The effect of pressure finger on performance of SG

The effect of mutual capacitance of bars on performacne of SG

The effect of overlapp methods and resin insulation on SG layer

Dielectric spectroscopy

Dielectric spectroscopy of insulation is aneffective method for insulation modeling. Thismethod can be implemented both in time andfrequency domain with possibility of dataconversion between both domains in terms oflinear dielectric materials. Generally, conductivityof dielectrics can be expressed as followingequation.

𝐽𝐽 𝑡𝑡 = 𝜎𝜎0𝐸𝐸 𝑡𝑡 +𝜕𝜕

𝜕𝜕(𝑡𝑡)(𝜀𝜀0 𝐸𝐸(𝑡𝑡) + 𝑃𝑃 𝑡𝑡 )

Where E(t) is applied electric field, 𝜎𝜎0 is DCconductivity and P(t) is polarization process.General method for frequency domainspectroscopy is shown in below hierarchy.