Partial Discharges under HVDC stress

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NTNU Norwegian University of Science and Technology Faculty of Information Technology and Electrical Engineering Department of Electric Power Engineering

Georg Volden

Partial Discharges under HVDC stress

Master’s thesis in Energy and Environmental Engineering Supervisor: Frank Mauseth

Co-supervisor: Pål Keim Olsen May 2021

Master ’s thesis

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Georg Volden

Partial Discharges under HVDC stress

Master’s thesis in Energy and Environmental Engineering Supervisor: Frank Mauseth

Co-supervisor: Pål Keim Olsen May 2021

Norwegian University of Science and Technology

Faculty of Information Technology and Electrical Engineering

Department of Electric Power Engineering

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Abstract

To interconnect power grids across the North Sea is a necessity to balance a European power grid with large penetration of renewables. The connection is done by high voltage direct current power cables, that could be cheaper and better with the use of polymeric insulation. Extruded polymer cables have internal cavities in the insulation, filled with air and gasses from the production, that are subject to degradation from partial discharges. To investigate this phenomena, that is little researched, partial discharges in cavities in PET and HDPE were measured at a range of temperatures and test voltages that affects the time between discharges and discharge magnitude.

In measurements at75°C, the average apparent discharge magnitude was1.118 pCat8 kV,1.016 pCat 9 kVand0.974 pCat10 kV. When the test voltage was adjusted back down to8 kVafter this, the average went up to1.049 pC. The median showed a similar behaviour, only that the values were about0.15 pClower.

The time between discharges in the same series was5.399 sat8 kV,4.010 sat9 kV,2.929 sat10 kVand 4.637 swhen the test voltage was decreased to8 kVagain.

The effect of temperature is mainly on the time between discharges due to the increased conductivity in the insulation material and that the increased temperature increases the start electron generation rate. In one of the measurements at10 kV, the average time between discharges was125.199 sat50°C,32.787 sat 60°C,7.504 sat70°Cand105.899 swhen the temperature was adjusted back down to50°C. A decreased time between discharges implies more discharges per unit of time. An increase in number of discharges after exposure to higher temperature is an indication of degradation of the insulation material. Discrepancies between different tests at the same conditions, implies that the intrinsic conditions of the test object is of importance for the discharge activity.

Sammendrag

Sammenkobling av strømnett p ˚a tvers av Nordsjøen er en nødvendighet for ˚a balansere et europe- isk strømnett med stor grad av fornybare energikilder. Nettene kobles sammen ved hjelp av høypente likestrømskabler, som kan gjøres billigere og bedre ved bruk av polymerisolasjon. Ekstruderte polymerkab- ler har gassbobler i isolasjonen fra produksjonen, som gir opphav til partielle utladninger. For ˚a undersøke dette feltet som er lite forsket p ˚a, ble interne partielle utladninger i hulrom i polymerisolasjon m ˚alt ved en rekke temperaturer og spenninger, som p ˚avirker tid mellom utladninger og utladningsstørrelse.

M ˚aling ved75°Cresulterte i en gjennomsnittlig tilsynelatende utladningsstørrelse p ˚a1.118 pCved8 kV, 1.016 pCved 9 kV og 0.974 pCved 10 kV. Da testspenningen etter dette ble senket tilbake ned til 8 kV, gikk snittet opp til1.049 pC. Medianen fulgte dette forløpet, bare med en utladningsstørrelse som var rundt 0.15 pClavere. Tiden mellom utladninger i samme serie var5.399 sved8 kV,4.010 sved9 kV,2.929 sved 10 kVog4.637 sda testspenningen ble senket til8 kVigjen.

Effekten av temperatur er hovedsakelig p ˚a tid mellom utladninger p ˚a grunn av økt konduktivitet i isolasjonsmaterialet og økt generering av startelektroner ved økt temperatur. En av m ˚alingene ved10 kV, ga en gjennomsnittlig tid mellom utladninger p ˚a125.199 sved50°C,32.787 sved 60°C,7.504 sved70°Cog 105.899 sda temperaturen ble justert ned tilbake til50°C.

En redusert tid mellom utladninger impliserer flere utladninger per tidsenhet. En økning i antall utladninger etter isolasjonen har vært utsatt for en periode med økt temperatur, er en indikasjon p ˚a degradering av isolasjonsmaterialet. Inkonsistens mellom ulike tester ved like testforhold, impliserer at de indre forholdene i testobjektet er viktig for utladningsaktiviteten.

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Preface

During the spring semester of 2021, the present work was written as the denouncement of my MSc. in Electric Power Engineering at NTNU. After 18 consecutive years of schooling and studies, it is strange to finally reach the apex point. None of this could be possible without inspiring teachers and professors that I have connected with on a personal level.

Most of all I am thankful for the fantastic mentorship of my supervisor Frank Mauseth and co-supervisor P ˚al Keim Olsen. There is a lot of practical work behind every experiment, research to understand theory, discussing results and writing the master thesis itself. It has been helpful to have capable sparring partners in the process.

Special thanks should also be given to the service lab and workshop at the Department of Electric Power En- gineering for creating custom equipment and solutions to make high voltage experiments possible.

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1 Introduction

As connecting offshore windfarms and oil rigs to the power grid and connecting national power grids across oceans become increasingly relevant, long subsea HVDC cables become more and more important [1].

HVDC is deployed when the losses are important, or we want to connect asynchronous grids. In the case of very long cables, the reactive losses in the cables are severe.

The cost of maintaining said subsea cables as well as cost of power outages at oil rigs are astronomical.

Extruded cables are cheaper and require less maintenance than typical mass impregnated cables, and are of interest for this application, but little is yet known of the long term ageing of extruded HVDC cables.

An indicator of electrical ageing of the insulation system is the quantity and magnitude of partial discharges (PD) that occur in voids formed in the insulation during the extrusion process. In this master thesis, the partial discharge phenomenon at HVDC will be investigated with the goal of increasing the knowledge of the degradation due to internal partial discharges, which is of interest in cable design and choices, that are cost intensive investments.

The experimental setup in the thesis is based on previous works by other authors at NTNU [2, 3, 4], as well as the author of the present work’s initial studies [5] leading up to the present work. The initial studies will be referred to as “the specialization project”. The theory section of the present thesis is based on the specialization project [5]. The other authors looked into partial discharges at HVDC, but also HVDC with an superimposed AC ripple. Unlike this thesis, the previous authors also looked into statistical estimators and simulations.

The reason for looking into HVDC with a superimposed AC ripple, is because rectification of AC and power switching in the upstream power system causes a phenomenon as such. The present thesis will firstly focus on a HVDC signal input without an AC ripple. The partial discharge phenomena are different for AC and DC, so these should be considered separately to try to gain an understanding of each before imposing the ripple. As there was limited time to write the thesis, superimposed AC may be considered further work, or it might be read about in previous work [2]. The intention for the previous work by Olsen [2] was to look at PD phenomena both with and without the AC ripple, but as it turned out the AC signal was not completely disabled in his experiments. Considering this, investigating the PD activity without the ripple seems like a most interesting research gap to fill at the moment, as few others have researched PD at HVDC to this extent before. Other researchers have used DC sources that not necessarily are ripple free.

They have either commented on this, or assumed that this was of no importance. This thesis will do no such assumption.

Figure 1.1: Subsea cable installation [6].

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2 Theory

This section introduces the theory behind the topic of partial discharges, their behavior at DC voltages and relations affecting the discharge magnitude and time between discharges (tbd).

2.1 DC voltage

Direct current (DC) was the first invention deployed to deliver electrical power to consumers, but due to the advantages with transformation and distribution of alternating current (AC), this soon dominated power grids around the world. The advantage of using DC voltage is clearer with development of long power cables:

There is no reactive current components creating reactive power losses, making very long cable connections difficult, and less insulation effort is required [7].

2.1.1 Electric field

Electric potential (often referred to as voltage) and field are closely related. In electrostatics, the electric potential at a point in a static field is given by the line integral of the field [8]:

V =− Z

c

E·dl (2.1)

If the electric field is conservative and the curl is zero, the field is given by the gradient of the potential:

E =−∇V. (2.2)

At DC, the field distribution and breakdown mechanisms are different from at AC. The electric field at DC voltage,EDC, consists of parts determined by the permittivity and the conductivity of the dielectric insulation material [7]:

EDC =Eε+Eρ (2.3)

The permittivity caused field,Eε, is the same as an AC field, and the conductivity caused field,Eρ, is caused by the accumulation of space charge [7]. According to Kreuger [9], DC in it’s pure state does not occur, or is rare. The DC has to be switched on and off. At these events, the field distribution is like an AC field determined by permittivity. Only when the growth of internal charges is saturated, a pure DC field is established [9].

Figure 2.1: Switching on and off DC voltage. The dashed line representing growth of internal charges eventually saturates and a pure DC field is established [9].

Switching on the DC voltage, a capacitive current,ic, stresses the dielectric:

ic =CdV

dt (2.4)

Then comes a transient polarization current,i_p, generating internal charges in the dielectric before the material is stabilized and a constant leakage current,i_l, is established [9]. This is illustrated in Figure 2.2.

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Figure 2.2: Switching on the DC voltage, a capacitive current,ic, given by the derivative of the voltage runs, before a polarization current,i_p, eventually stabilizes the material and a constant leakage current,i_l, remains [9].

2.1.2 Polarization current transient

The slow polarization mechanism in the polymer at voltage application gives origin to a transient current [10]

I(t) =A⁰·t⁻ⁿ+I_∞, (2.5)

whereA⁰,nandI_∞are constants. Since the material conductivity is temperature dependent, the temperature increase will cause this increased current. The conductivity is also field dependent, meaning higher voltage application increases conductivity. After a time, the conductivity and current will stabilize. The stabilization time of the transient is given by [2]

tstabilize =

I_∞(k−1) A⁰

−¹_n

, (2.6)

where the factork >1.

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Figure 2.3: The polarization current. After the stabilization time, the current approaches a constant value [2].

2.2 PD

Partial discharges occur when applied voltage stress over insulation only makes part of the distance con- ducting [11]. Even if these discharges per definition do not cause a complete electric breakdown by them- selves, they may deteriorate the insulation until a breakdown occurs. At this point, the insulation is destroyed and needs to be replaced.

PD is as mentioned a known phenomena at AC, where the phenomena appears at least every half cycle [11], and therefore deteriorates the insulation quicker. At DC, thetime between discharges (tbd), is higher than for AC at the same voltage level.

In solid insulation, cracks and gas bubbles might form inside during manufacture. Thesecavitiesare weak- nesses, since the gas and contaminations, has a lower dielectric strength than the solid it is trapped in. I.e. it has lower capability to withstand voltages. PD may also occur at the surface of the insulation in the interface between gas and solid [12].

In dielectric bound cavities, Townsend discharges and streamer discharges are the most important discharge mechanisms. Both are observed during DC experiments, but the Townsend discharges seem to dominate [2]. In the case ofTownsend-like discharges, electrons are accelerated by the unipolar electric field from DC voltage and create electron avalanches when colliding with gas molecules and freeing new electrons.

Streamer dischargesare ionized narrow channels that occur at high overvoltages. This will look like one or several parallel miniature lightning bolts, whereas the Townsend discharge appears as a large glowing electron cloud with a narrower and a wider end because of the avalanche. In the streamers, space charge distorts the electric field. In high local fields, high energy photons are created and cause photon mitigated avalanches [2]. Unlike Townsend discharges, streamers do not necessarily leave a remnant electric potential and therefore has higher discharge magnitude.

For any of these phenomena to occur, stating electrons must be present at the cavity surface. The generation of free staring electrons is a stochastic process, meaning that it is governed by a collection of incidental variables and thereby considered random.

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2.2.1 Internal partial discharges at DC

At DC voltage, the partial discharge process is different from what would be expected at AC conditions. As mentioned in subsection 2.2, the discharge separation time, or time between discharges, is longer at DC.

The fact that the DC voltage does not change polarity very often, or not at all, also gives rise to unipolar discharges. The discharge magnitude is lower at DC voltage than corresponding rms AC voltage, because of the absence of voltage peaks. Then it is also high probability for Townsend-like discharges, because the overvoltages are likely lower before the discharge occurs. At DC, the conductivity of the surroundings govern the voltage rise rate of the cavity. This rate is lower at DC [2].

The test object in PD measurements can be modelled with the ABC equivalent [11] as depicted in Figure 2.4.

RcandCcrepresent the resistance and capacitance of the cavity,RbandCbrepresent the insulation in series with the cavity, while Ra andCa is the insulation in parallel. At AC conditions, the resistances are usually neglected, but at DC conditions they are important to include, since the voltage distribution very much will be determined by the conductivity.

+

Zi

C_a R_a

Cb

Cc

Rb

Rc

Figure 2.4: ABC equivalent circuit connected with a DC source and it’s internal impedance.Z_iis the source impedance. R_a,R_b,R_c,C_a,C_b andC_c respectively, are the resistances and capacitances of the insulation i parallel with the cavity, the insulation in series with the cavity and of the cavity itself.

[13] gives the DC voltage across the cavity without discharge as

V_c,DC = (1−e^−t/τ)·K_DC ·V_DC, (2.7)

where the resistive distribution factor is

K_DC = R_c Rc+Rb

(2.8) and the time constant is

τ =

RbRc

Rb+Rc

(Cb+Cc). (2.9)

It is very difficult to determine the nonlinear resistance,R_c. It is assumed to be infinite.R_c >>R_b, implying K_DC ≈1 andτ ≈ 1·(Cb+C_c). The voltage at which PD first occurs, i.e. the partial discharge inception voltage (PDIV) at DC is not well defined as the repetition rate goes to zero, and there will be no discharges at this limit [13]:

PDIVDC = Vpaschen

K_DC . (2.10)

The Paschen breakdown voltage,Vpaschen, is given inkVpeakby the empirically obtained formula [14]

V_paschen= 6.5h+ 24.5√

h (2.11)

wherehis the height of the cavity incm. For a75µmhigh cavity, this givesPDIVDC =Vpaschen= 2.17 kV.

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The mean time,∆t¯ ,between the discharges at DC voltage is [15]

∆t¯ =−τ·ln

1−V_PDIV,DC VDC

(2.12) for VDC >VPDIV,DC. In Figure 2.5, we see the cavity voltage without discharges like in Equation 2.7, the limit of this function, KDCVDC, the Paschen voltage,Vpaschenand the sawtooth form of the voltage because of discharges. For a discharge to occur, the voltage must surpass the Paschen voltage and there must be a starting electron present. The generation of such electrons is a stochastic process, and therefore the time of the next discharge is random.

Figure 2.5: The cavity voltage rises in periods with no discharges and falls after the Paschen voltage is passed and a starting electron is present, causing discharge [2].

2.2.2 PD Measurements

The PD measurement method is the same as for AC: A discharge in the test sample generates a high frequency pulse in the detection circuit [9]. Figure 2.6 shows the classicalstraight circuitfor PD measurements [7, 9, 11], i.e., the test object is connected in series with the measurement impedance. Alternatively, the measuring impedance can be connected in series with the coupling capacitanceC_k.

The coupling capacitance forms a path for the high frequency pulses and must be chosen in the same order of magnitude as the test sample [9]. This is determined by the expression for the pulse magnitude,vˆ [16]:

ˆ

v = q

a+C(1 +a/C_k), (2.13)

whereqis the discharge magnitude,ais the capacitance of the test object,Ck is the coupling capacitance andCis the capacitance of the measuring impedance. From this expression, it is seen that ifC_k approaches zero, thenvˆapproaches zero as well, and nothing is detected. I.e., the coupling capacitor is crucial.

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Z_i

Ck

T.O.

Zm

Figure 2.6: A general circuit for PD measurement. Z_i is the internal impedance of the voltage source,C_k is the coupling capacitance,Z_mis the measuring impedance and T.O. is the test object.

A discharge means that there is a drop in the apparent charge in the test object. This causes a detectable voltage drop over the measuring impedance Zm. If the measuring impedance is purely resistive, an exponentially decreasing transient voltage impulse will be the detected, but if the measuring impedance is a combination of resistive, capacitive and inductive, an oscillating response is obtained. The oscillating response is preferred, because a narrow band measuring instrument can be used, and some disturbances thereby excluded [11].

This measurement is acalibrated measurement. A calibrated measurement means in practice that a known pulse is injected into the test object to establish the relationship between the detected pulse and the discharge magnitude. The reason for having standardized calibrations, is that the results then will be compa- rable to other measurements with the same calibration and test conditions.

2.2.3 Discharge separation time distribution

The time lag from one discharge to another has been identified as the main stochastic parameter in the PD process. This time depends on time lag and recovery time;

∆t=tR +tL, (2.14)

where residual voltage governs the recovery time and stochastic behavior governs the time lag. Often, the residual voltage is assumed constant. This implies a constant recovery time as well.

Equation 2.12 gave only the mean time between discharges. However, the distribution of discharges of different sizes are not symmetric. The theoretical assumption, supported by empirical data, is that the time lag has an exponential probability distribution. The probability density function, pdf, for the time lag is given by [17]

pdft_L(tL) = 1

τ_se^−t^L^/τ^s, (2.15)

whereτs is the mean statistical waiting time at discharge inception. The exponentially decreasing function is illustrated in Figure 2.7 with an arbitraryτs.

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Time [h]

Probability density

1/T_s

Figure 2.7: Ideal time lag probability density function. Most discharges have low magnitude. This correlation decreases exponentially. To get total time between discharges, the recovery time is added and this curve moves to the right.

2.2.4 Start electron generation

The mean statistical waiting time, τ_s in Equation 2.15 is inverse of the mean start electron generation rate, N_el[18]. I.e.

τs = 1 Nel

(2.16) There are several start electron generation mechanisms that could be present in a cavity [19], which can be grouped in surface and volume generation mechanisms. In flat cavities as addressed in this thesis, the surface mechanisms are most important [19]. The generation follows the relation [20]

N˙e,surface =A eS

1−η α

exp−

"

Φ−p

eE/(4πε0) kT

#

(2.17) whereAis the surface area, e is the elementry charge,Φis an effective work function,εis absolute permittivity of vacuum,T is the temperature,k is the Boltzman constant,his the cavity height,E is the electric field out from the surface,αis the first Townsend coefficient,ηis the gas attachment coefficient andS is a function describing surface material, structure and state, and is given by [20]

S =ν₀eN_dt

A (2.18)

where ν0 is the fundamental phonon frequency and Ndt is the number of detrappable surface charges.

According to [21], the number of detrappable surface charges increase with deployment of discharges of opposite polarity. This is one factor increasing the parameterNdt from DC to AC. Empirical data [2] shows increased PD activity at increased level of superimposed AC on the DC signal.

2.2.5 Discharge magnitude

The other measured parameter in partial discharge measurements at DC, is the discharge magnitude. At AC, there would be additional information about phase of occurrence. The apparent charge mentioned in 2.2.2 is typically obtained by [7]

q_a=C_b∆Vc (2.19)

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from the classic ABC-equivalent, but this is neglecting space charge field due to breakdown charge move- ments. For a Townsend discharge, the expression becomes [22]

qa=αh·Cb∆VL, (2.20)

whereαis the first Townsend ionization coefficient,his the cavity height and∆VLis the overvoltage. In dry air,αis given by the cavity electric field strength,E_c, and pressurep, and fits the expression [23]:

α(Ec,p) =p·0.57e⁻²²²^Ec^/p

(Ec/p)^1/2 1 + 4·10⁻⁴·(Ec/p)

(2.21) valid for E_c/p ∈ h10, 1000i. The discharge magnitude follows the probability density of the time lag [2], meaning that it will be exponentially decreasing for increasing time as well. The function of time lag becomes [22]:

q_a(tL) =αh·C_b·(VDC−V_critical)t_L

τ (2.22)

valid fortL<< τ.

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3 Experimental setup

The experimental setup is based on previous works at NTNU [2, 3, 4, 5]. Previous works looked at the effect of cavity diameter and superimposed AC on the HVDC signal. Olsen [2] also intended to test at pure HVDC, but because the AC source used for superposition was only set to zero and not disabled completely, an AC ripple of10 Vwas indeed superimposed in the “pure DC” measurements.

Therefore, it will be shown in subsection 3.1 and subsection 3.2 how the test circuit and test objects were modified to achieve the aims of this thesis. I.e. to obtain reliable measurements on pure HVDC and considering the effects of field and temperature.

3.1 Test circuit

The test circuit is essentially a straight circuit as described in subsubsection 2.2.2. Compared to Olsen [2], the AC source was properly disabled in the experiments of the present work, to eliminate the aforementioned superimposed AC ripple and obtain pure DC. The DC source used, was a FuG Elektronik 35kV. This is a ultra stable source for the purest possible DC. In the specialization project [5], this was supplied from grid voltage through an isolation transformer, but this was removed from the test circuit for the present thesis, because it occasionally caused the fuse in the supply circuit to trip.

In the experiments, the PD detectors, Omicron MPD600, were connected in both branches. Each detector has a limited range and accuracy, so the pair of them was calibrated at different magnitudes. The detector in series with the test object is the most sensitive one, and was therefore calibrated at5 pC to detect the smaller discharges, while the detector in series with the coupling capacitance was calibrated to 50 pCto detect the discharges of higher magnitude. The detectors are connected in series with an optical cable and forth to a fiber optic bus controller, MCU502.

Rd

Rk

VDC

Ck Cobj

MPD600 MPD600

Ground filter MCU502 PC

VAC

Faraday cage

Figure 3.1: Line diagram of the test setup. R_d is the100 MΩblocking resistance in the DC source,R_k is the 500 MΩcoupling resistor,C_k is the coupling capacitance andC_obj is the capacitance of the test object. The orange line represents an optical signal cable between the Omicron measuring and data processing units MPD600 and MCU502.

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Table 3.1: Test circuit parameter values.

Circuit parameter Value

R_d 100 MΩ

R_k 500 MΩ

C_k 179 pF

Furthermore, in Figure 3.1,R_dis a blocking resistance, creating a high resistance path from the DC source to the test cell, preventing unwanted currents in the branch.Rk andCk are the coupling resistance and capacitance, andCobj represents the test object. The capacitances and resistances were placed in a solid metal box, comprising a Faraday cage to screen the circuit components from electrical interference. Additionally, the test setup was inside a lab cell with a courser Faraday cage.

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3.2 Test objects

The test objects that were to be placed between the electrodes, consisted of three 70 mmdiameter discs of PET and HDPE. The middle layer had a2 mmdiameter hole in the middle to represent a flat cylindrical cavity when the layers were glued together. The adhesive used was Loctite 435.

←−2 mm−→

←− 70 mm −→

←−3×75µm−→ ←−225µm−→

Figure 3.2: Cross section of a test object with a flat cylindrical cavity in the middle.

3.2.1 PET objects

Polyethylene terephthalate (PET) is a common plastic used in bottles and packaging. It is not used much in power cables, other than as sheath material, but is is easily available and recyclable. This is the reason for the choice of material, not for it’s dielectric properties. The material was commercially available as75µm film, making production of samples much faster than e.g. casting and cross binding a typical cable insulation material like XLPE.

For a stiff material like PET, it was sufficient to drill the hole through a number of samples and search for good samples as the sample holes shown in Figure 3.3a. The samples were sorted in class A (Figure 3.3a) and class B (Figure 3.3b), were the class B samples were discarded as there were imperfections due to the drilling, causing sharp wedges of material sticking out into the cavity. These sharp points at the end of the wedges, causes high field enhancements, increasing the chance of discharges.

(a) No sharp edges. (b) Fuzzy boundaries.

Figure 3.3: Microscopy imaging of PET cavity samples. A class A sample as shown in 3.3a, has no sharp edges along the cavity wall, while a class B sample as shown in 3.3b typically has sharp edges or material that was not properly removed.

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Figure 3.4: Cut through of a drilled through layer. The walls of the cavity should be as close to a cylinder as possible. In the red circle, some of the material has been displaced around the edge of the cylinder. In the blue circle, some material has not been successfully removed from the cavity sidewall.

PET sample production

1. Punch three circles with70 mmdiameter out of75µmsheets of PET insulation. Here, a custom made, fine honed circular copper blade as seen in Figure 3.5 was used in combination with a hydraulic press to punch out the discs. It is imperative to avoid splinters and uneven edges on the samples.

2. Drill out a 2 mm circle in the middle layer. This is done with a very sharp drill. Stacking several sheets on top of each other when drilling can be advantageous, because the middle layers will be less exposed to splintering.

3. Check the drilled cavity with microscopy to eliminate samples with splinters.

4. Find a work station with a flue (ventilator). Clean the surfaces of the work station and the sample layers with isopropanol, dry them with an anti static compressed air gun and keep the clean samples in an air tight plastic bag when moving.

5. Condition the sample layers in a vacuum chamber at80degrees Celsius.

6. Align the three layers making up a sample and put them in a hydraulic press. Use a piston with smaller diameter than the sample, so that the layers could be glued together whilst under pressure. The pressure is to avoid air cavities apart from the intentional one.

7. Glue the layers together to a sample with the use of a superglue like Loctite 435.

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Figure 3.5: The blade used to cut out the70 mmdics.

3.2.2 HDPE objects

High density polyethylene (HDPE), is a common material used in high voltage insulation systems [24]. Since this is a thermoplastic material, it may be melted and reshaped into a desired form. The following procedure was followed to create HDPE test samples with similar dimensions to those of PET:

1. Cut a10 cmdiameter disc out of commercial75µmPET to use as a mold.

2. Clean the mold and all surfaces on the work station with isopropanol.

3. Place the mold on smooth polished metal and another layer of PET to prevent damage of the smooth surface.

4. Weigh the material cut out to get an approximate estimate of how much HDPE material the mold should be filled with.

5. Measure up the appropriate ammount of HDPE granulate (ROC-2 was used here) and fill the mold with the granulate.

6. Place another smooth surface metal plate on top of the granulate, so this can be pressed into shape in a high pressure press at a temperature of140°C. At this temperature the HDPE will melt, but the PET will not.

7. The recipe for the press was12 minapplied pressure at140°C, followed by ten minutes of cooling.

8. Cut out70 mmdiameter discs out of the HDPE material.

9. Make a hole in one of three disc layers. Instead of using a regular drill, a spiral fluted router cutter was used instead to gain acceptable results seen in Figure 3.6.

10. Glue the layers together as described in subsection 3.2.

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Figure 3.6: HDPE sample drilled with the spiral fluted router cutter.

This revelation of using a spiral fluted router cutter to get smooth holes in HDPE layers, lead to more well defined cavities for the PET samples as well, seen in Figure 3.7. With this method, the surrounding material of the hole was not deformed as seen by the three dimensional shadows along the edge in Figure 3.3a.

Figure 3.7: Router cut PET sample. With this method, the surrounding material of the hole was not deformed as seen by the three dimensional shadows along the edge in Figure 3.3a and Figure 3.4.

3.2.3 Sample inception

It proved important to have good procedures for avoiding trapped air between the sample and electrode or under the test object sticking out from the lower electrode radially.

1. Clean the test cell and fill it with new oil until it covers the most of the epoxy part of the upper electrode, without reaching the metal part above of the epoxy.

2. Make sure there are no air bubbles in the oil.

3. Submerge the sample into the oil with an angle, and make sure that air is not trapped under the sample.

4. Place the sample in the center of the lower electrode.

5. Submerge the upper electrode with an angle into the oil to avoid trapped air.

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6. Push the upper electrode on top of the sample to create a vacuum. If the electrode is pulled now, the vacuum should hold it back if the oil level is high enough. The oil level must not be too high and reach the metal part of the electrode above the epoxy.

7. Make sure the electrodes and sample are center aligned.

3.3 Temperature control system

To regulate the temperature of the insulating oil in the test capsule efficiently, a commercial PID controller, Shimaden SR93, was deployed. This controller measures the temperature with a Pt100 element and delivers the appropriate power to the heating coil inside the test capsule according to control system theory. The reason for increasing the temperature in the first place, is to simulate operation conditions for power cables.

The temperature of the conductors in cables will depend on the loading and cooling from surroundings, and is limited by the capabilities of the insulation material.

3.4 Equipment and limitations

The main instrument used in the experiments was the Omicron MPD600 measuring unit as introduced in Figure 3.1. This equipment was calibrated with a certain range. The reason for using two MPD600 units, was to calibrate them at different ranges to increase the total range of the measurements. The most sensitive detector was connected in series with the test object,Cobj, and calibrated to5 pC, while the detector connected to the coupling capacitance,Ck, was calibrated to50 pC. The first mentioned, sensitive detector gives accurate discharge magnitudes from noise threshold, 500 fCto above calibrated magnitude of 5 pC. From initial tests in [5], it was shown that most discharges were below 15 pC. The other detector is more course as it is supposed to cover a greater range. Using the calibration tool, it was shown that the detector calibrated to50 pCreached saturation at200 pC, but not at100 pC. This means that it saturates at some value between 100 pCand200 pC. Discharges above100 pCwere sparse in [5], but could occur. At discharge magnitudes out of range, the detection accuracy is low.

The DC voltage source was accurate down to4.5digits inkV.

3.5 Experimental test schedule

To observe the effects of changing parameters, test schedules were planned to vary one parameter at the time. The parameters chosen were temperature, applied voltage level (field) and material. The aim of each test is described in the table caption.

Table 3.2: Trial plan 1. The aim of this test was to study the effect of temperature on a PET sample.

Parameter Period 1 Period 2 Period 3 Period 4

Temperature [deg C] 40 60 75 40

Voltage level [kV] 10 10 10 10

Material PET PET PET PET

Note Data lost

Table 3.3: Trial plan 2. The aim of this test was to study the effect of field on a PET sample. Unfortunately the data from period 3 was lost.

Note

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Table 3.4: Trial plan 3. The aim of this test was to study the effect of field on a PET sample when the temperature was somewhat decreased compared to Table 3.2.

Note Canceled

Table 3.5: Trial plan 4. The aim of this test was to study the effect of temperature on a HDPE sample.

Unfortunately this test failed, as explained in subsubsection 4.2.2.

Material HDPE HDPE HDPE HDPE

Note Failed Canceled Canceled Canceled

Table 3.6: Trial plan 5. The aim of this test was to study the effect of field on a HDPE sample. Unfortunately this test series was canceled due to problems portrayed in subsubsection 4.2.2.

Note Canceled Canceled Canceled Canceled

Table 3.7: Trial plan 6. The aim of this test was to study the effect of field on a HDPE sample at a lower temperature than Table 3.6. Unfortunately this test series was canceled due to problems portrayed in subsubsection 4.2.2.

Note Canceled Canceled Canceled Canceled

Table 3.8: Trial plan 7. Due to problems with the HDPE material tests, an additional temperature effect test was performed on PET, since the first test in Table 3.2 could have been better and lost some data.

Note

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4 Results and discussion

To gather research results, data was systematically collected in the test schemes given in subsection 3.5.

Even though some of the tests had to be aborted, the results in this section will be presented chronologically in the originally intended order to preserve logical structure.

4.1 Verification of new test setup

As compared to [5], the insulation transformer between the DC source and the AC grid was removed because it caused the fuse in the laboratory power supply to trip. The modified test setup is given in Figure 3.1.

Far below the partial discharge inception voltage (PDIV), there should be no realistic discharges. Although at DC there is no such limit. It an instead be given as discharges per minute. From a0 kVmeasurement on PET at75°C, the noise level was still below the detection threshold of500 fC.

In Figure 4.1, bursts or traces of bursts with magnitude lower than500 fCare what is observed. The presence of these bursts have been reported by [2] and by the present author in [5]. The reason for this finding, might be caused by that the present work experiments were performed in a cell divided lab with other active experiments, or by construction works that were conducted in a neighbouring lab. An alternative explanation is that there could be some issue with the measurement equipment.

0 5 10 15 20 25

Time [h]

0.5 1 1.5 2 2.5 3 3.5 4

q a [pC]

Log Discharge magnitude

Figure 4.1: Logarithmic plot of apparent discharge magnitude. No noise is present above the detection threshold of500 fC. The discharges visible here are burst discharges that probably are caused by a separate phenomena.

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4.2 Effect of temperature

4.2.1 Temperature effect on PET

In this section the results from the test scheme from Table 3.2 is discussed. The aim of the test was to investigate the effect of temperature on PET insulation material with a flat cylindrical cavity. Changing the temperature affects the conductivity of the insulation material. For PET, the discharge process is very slow at the lowest temperature level, here40°C. The series was supposed to include results from a 75°C measurement, but the data was unfortunately lost. Keeping in mind that the measurement was performed but not properly recorded, it has to be noted that the cavity and insulation material was exposed to75°Cat 10 kV, before the last40°Cperiod.

Unfiltered results

Firstly, unfiltered results are presented and commented. Discussion of results within the scope and goal of the test will then be done for thefiltered results only.

0 100 200 300

Time [h]

0 50 100 150 200 250 300

q a [pC]

Discharge magnitude (linear)

40 deg 60 deg 40 deg

(a) Linear plot of apparent discharge magnitude.

0 100 200 300

Time [h]

10⁰ 10¹ 10² 10³

q a [pC]

Log Discharge magnitude 60 deg

40 deg 40 deg

(b) Logarithmic apparent discharge magnitude.

Figure 4.2: Apparent discharge magnitude in the test series investigating the effect of temperature. Circled in Figure 4.2b, is burst discharges.

As observed in Figure 4.2, the same bursts of discharges as observed in [5], appeared several times during the test series. This phenomena occurring from time to another, might be expected if this has a reoccurring cause and the series lasted for over two weeks. The time between these incidents is irregular, but there are long periods where the phenomena does not occur as well. In [5], this was found to coincide with normal work hours, and the phenomena did not occur over the weekend. This pointed towards human disruption by construction works in neighbouring labs or other high voltage experiments in the same lab or surrounding labs. Olsen [2] found similar bursts of discharges, but only for AC ripple between50 %and100 %of the AC partial discharge inception voltage (PDIV). Since the test referred to lasted for6 h, this might have coincided by work hours as well.

A closer look into this will now be conducted before filtering out some of the worst bursts.

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0 100 200 300 Time [h]

10⁰ 10¹ 10²

q a [pC]

Moving average (magnitude)

(a) Moving average.

0 100 200 300

Time [h]

10⁰ 10¹ 10²

q a [pC]

Moving median (magnitude) 40 deg 40 deg 60 deg

(b) Moving median.

Figure 4.3: Moving average and median of apparent discharge magnitude. Averages and medians below 1 pC, indicate the presence of burst discharges if compared to Figure 4.2b.

The effect of the bursts are pehaps not the easiest to spot in Figure 4.3, but as seen in Figure 4.2b, the apparent discharge magnitude of these bursts are small. This means the moving average and median will be pulled down to less than1 pCat the time of the bursts. This will be commented again when the filtered results are presented.

(a) Combined probability density of apparent discharge magnitude in the three periods.

(b) Combined probability density of time between discharges in the three periods.

Figure 4.4: Combined probability densities of discharge magnitude and tbd. In Figure 4.4b it is apparent that the first bin in the histogram is unnaturally high, pointing towards noise or burst discharges.

Plotting the combined probability densities of apparent discharge magnitude and time between discharges is perhaps not the most interesting result, because the measurements are performed at different conditions,

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but for this quick comment before filtering out the bursts, the point is the same: In Figure 4.4a a strong overrepresentation of the lowest time between discharges is apparent.

0 100 200 300

Time of occurance [h]

10^-6 10^-4 10^-2 10⁰ 10² 10⁴

tbd [s]

Absolute time between dicharges

60 deg

40 deg 40 deg

(a) Absolute time between discharges.

0 100 200 300

10^-6 10^-4 10^-2 10⁰ 10² 10⁴

tbd [s]

Moving average (tbd)

60 deg

40 deg 40 deg

(b) Moving average time between discharges.

Figure 4.5: Absolute tbd and moving average time between discharges. Here the the bursts of discharges are noted by their short tbd and piling around orders of magnitude and time of occurrence. Circled in Figure 4.5a are an instance of burst discharges to the left (vertically) and how these discharges tend to pile around orders of magnitude to the right (horizontally).

In Figure 4.5, the time between discharges is visualized. Also here, the data points appear as bursts marked in Figure 4.5a and long vertical lines in Figure 4.5b. It seems that approximately all of the points where the time between discharges is less than one second are either these bursts or discharges that occurred during the startup of a new measurement period. The discharges in the first few minutes of a measurement is not of interest here, because pure DC is not yet established. According to Kreuger [9]: “... the test voltage is raised in steps, after a step the initial rush of capacitive discharges has to die out before the observations can be made.” The horizontal piling of discharges circled in Figure 4.5a could be a reflection of the resolution of the measurement.

It is more interesting to revisit the results after filtering out the discharges associated with a tbd less than one second, which will now be presented in next paragraph.

Filtered results

In Appendix D, a script is given that removes the bursts observed e.g. in Figure 4.2b, by removing discharges that are less than 1 sapart. The overall impression after filtering, is that this worked as intended. Now the intended discussion of the data apart from the bursts can be done in this section with clearer support in the dataset. As this is the same dataset only filtered, the test plan Table 3.2 still applies. The test voltage is 10 kVand the temperatures in the three periods are respectively40°C,60°Cand40°C.

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0 100 200 300 Time [h]

0 50 100 150 200 250 300

q a [pC]

(a) Linear plot of apparent discharge magnitude.

0 100 200 300

Time [h]

10⁰ 10¹ 10² 10³

q a [pC]

(b) Logarithmic apparent discharge magnitude.

Figure 4.6: Apparent discharge magnitude at10 kVand varying temperature. Compared to Figure 4.2, the burst discharges are filtered out if the time between them were less than1 s.

Due to the increased conductivity in the insulation material in the case of increased temperature, the discharge process is speed up. In Equation 2.17, it is noted that increased temperature increases the start electron generation rate. I.e. the data in Figure 4.6a is as expected. The apparent discharge magnitude in the last40°Cperiod of the series seems to be higher than in the first period at the same temperature. This is supported by looking at the moving average and median in Figure 4.7:

0 100 200 300

Time [h]

10⁰ 10¹ 10²

q a [pC]

Moving average (magnitude) 40 deg 60 deg

40 deg

(a) Moving average of apparent discharge magnitude.

0 100 200 300

Time [h]

10⁰ 10¹ 10²

q a [pC]

Moving median (magnitude) 40 deg 40 deg 60 deg

(b) Moving median of apparent discharge magnitude.

Figure 4.7: Moving average and median apparent discharge magnitude at10 kVand varying temperature.

Compared to Figure 4.3, it is observed that the moving average and median does not go down as often to magnitudes of1 pCor lower. Moving average of the ten last discharges are used throughout the thesis.

The reason the discharges stay high, might be caused by degradation of the cavity and insulation from the

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periods with higher temperature and higher discharge magnitude. Here it also is observed in the lower parts of the graph, that the moving average and median in Figure 4.7 is higher than in Figure 4.3 where the bursts originally were. The same is seen as was expected in Figure 4.6; that after exposure to higher temperature conditions, an increased discharge magnitude for at least a part of the discharges is observed also after the temperature goes down.

(a) Combined probability density of apparent discharge magnitude in the three periods.

(b) Combined probability density of time between discharges in the three periods.

Figure 4.8: Combined probability densities of discharge magnitude and tbd for the periods of 40°C and 60°C. The test was performed at10 kV. Comparing Figure 4.8b with Figure 4.4b, the overrepresentation of the shortest tbd’s is removed by the filter.

Combined probability densities of discharge magnitude and tbd may give an initial overview of the measurement, but since the periods in each series have different conditions, it is more accurate to look at distributions for each individual period. This is done in Figure 4.9 and Figure 4.10.

In subsubsection 2.2.3, it was recapped from Zuber [17], that the time lag’s probability density function followed an exponentially decreasing function. In Figure 4.8b, the function seems like a combination of this and a normal distribution. This is caused by the stochastic behavior of the start electron generation mechanism, which follows a strict normal distribution if enough data is gathered.

As seen in subsubsection 2.2.5, the discharge magnitude at stationary conditions, has a linear relation to the probability density of the time lag,pdft_L, and since thepdft_L is an exponentially decreasing function, the pdf of discharge magnitude will do so as well, as agrees with the results in Figure 4.8 and Figure 4.9.

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(a) Pdf of first 40 degree period. (b) Pdf of the 60 degree period. (c) Pdf of last 40 degree period.

Figure 4.9: Probability densities of discharge magnitude for each period in the filtered varying temperature measurement at10 kV. Increased temperature show increase in discharge magnitude. The magnitude stays high when the temperature is lowered again.

Looking at Figure 4.9, it is observed that the discharge magnitude probability density function is shifted towards higher discharge magnitudes in Figure 4.9b than in Figure 4.9a. This comes natural of the result that was already seen in Figure 4.6. Reviewing Figure 4.9c, it is also confirmed that the distribution in the last period is closer to the second period than the first period. I.e. the intrinsic conditions of the test sample is changed after exposure to the increased temperature. In Figure 4.9c it appears the discharge magnitudes are even higher than in Figure 4.9b. This is probably because of the lost data at75°C, where a shift towards higher magnitude than at60°Cwould be likely.

The result in Figure 4.9 match the observations Kreuger [9] made on cavity discharges, but if we look at the development in the later periods, the distribution seems to approach the distribution of surface discharges, although this might not be the case if more extensive tests are made. Olsen [2] measured a much flatter distribution, but he had unintentionally superimposed an AC ripple of10 V. With the ripple, a flatter distribution is expected. This is shown in his simulated and measured results where the ripple was with intent. His simulated result without a ripple, gave an exponentially decreasing probability density function close to the histogram in Figure 4.9a.

(a) Pdf of first 40 degree period. (b) Pdf of the 60 degree period. (c) Pdf of last 40 degree period.

Figure 4.10: Probability densities of time between discharges for each period of the measurement at10 kV and varying temperature.

The time between discharges distributions for each period in Figure 4.10 are shifted normal distributions.

The main difference between them, is that the tbd is higher at40°Cthan at60°C, because there are fewer discharges that occur at this temperature per unit of time. The reason the data last period, seen in Fig- ure 4.10c, shows even longer time between the discharges than the first period as seen in Figure 4.10a,

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is a consequence of the previous result that the (apparent) discharge magnitude is larger. I.e. if the tbd is short, little charge has built up, and vice versa. With high magnitude and few discharges, the time between discharges becomes long.

The distribution in Figure 4.10b is close to what Olsen [2] measured at10 kVand75°C, in both probability and the magnitude of tbd. Olsen’s [2] measured distribution is closer to a normal distribution than the present author’s, but that at a higher temperature, and his simulated results from an analytic model and a Monte Carlo model, predicts a shift to the left. It should also be mentioned that Olsen [2] had a superimposed AC ripple of 10 V that he failed to remove, and that he used a voltage division factor of ⁵₆ over the test object and the rest of the test circuit, that the present author found in [5] to be incorrect and actually be0.773. This implies that Olsen [2] tested at 9.276 kVand not10 kV. Fromm [7] also found a normal distribution testing with corona in oil and air.

0 100 200 300

10⁰ 10¹ 10² 10³ 10⁴

tbd [s]

Absolute time between dicharges 60 deg

40 deg

(a) Absolute time between discharges, filtered for burst discharges.

0 100 200 300

10⁰ 10¹ 10² 10³ 10⁴

tbd [s]

Moving average (tbd)

40 deg

60 deg

40 deg

(b) Moving average time between discharges, filtered for burst discharges.

Figure 4.11: Absolute tbd and moving average tbd at10 kVand varying temperature, portraying the effect of temperature variation on time between discharges, filtered for burst discharges less than1 sapart.

In Figure 4.11, the time between discharges for all the data points and as moving average is plotted. Here an increasing time between discharges in the transient period is observed. This coincides with what is found in Figure 4.7, where the moving average and moving median increase in the transient period. As expected, the temperature correlation is opposite for time between discharges and apparent discharge magnitude.

I.e. when the temperature is increased from40°Cto60°C, the average tbd decreases and there are more discharges per unit of time.

As mentioned, Equation 2.17 gives a higher start electron generation rate for higher temperatures. This causes more discharges to form per unit of time, and the time between the discharges becomes shorter as a consequence. [3] also found that a decrease in temperature causes an increase in time between discharges, because of the reduced conductivity of the material.

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Average value comparison

To compare the periods with each other more easily, the average and median discharge magnitudes and average tbd for each period as a whole is given in Table 4.1. This is also summarized graphically in Fig- ure 4.12.

Table 4.1: Periodical data for the temperature measurement, filtered for burst discharges. Green text indicates increase, while red indicates decrease from last period.

Period 1 2 3

Voltage level [kV] 10 10 10

Temperature [°C] 40 60 40

#Discharges 1965 7898 1281

Average [pC] 3.319 7.817 8.072 Median [pC] 1.399 3.206 1.324 Average tbd [s] ≈500 ≈40 ≈1000

From Table 4.1, much of the same results as before are seen, but more accurately: Average discharge magnitude, does indeed increase from increased temperature, and stays high after going down in temperature again, but what is seen here, it that the average is actually higher in third period than in second period. This sounds strange, but is caused by the75°Cperiod between period 2 and 3, that the data was lost from. The cavity is affected by previous discharges, so that when the discharge magnitude at75°Cwas even higher, this persisted to some degree when the temperature was decreased to40°C.

40 45 50 55 60

Temperature [°C]

1 2 3 4 5 6 7 8 9

q a [pC]

Median and average apparent discharge magnitudes

Average Median

Figure 4.12: 10 kV: Median and average as a function of temperature. The points are calculated for each measurement period in Table 4.1. The dotted line is a linear approximation. Note that the average is increased substantially in third period after having been exposed to higher temperature.

The median apparent discharge magnitude is substantially lower than the average. This implies that the discharges of low magnitude probably are more numerous, but the high magnitude of the higher discharges increases the average to surpass the median.

The average time between discharges decreases as mentioned when the temperature is increased, but more surprisingly, in Table 4.1, we see that the tbd in the last period is double of what of the first period with the same temperature. Again, this is because the cavity is affected by previous discharges and the discharges have a higher apparent magnitude on average.

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Added 75 degree measurement from earlier tests

As the data from the 75°Cperiod of the measurement series was lost, an attempt to cut in and compare with another75°Cmeasurement was made. Sadly, this data was not compatible with the data of the present series. The discharge magnitude was much lower, even if the temperature was higher. The reason for this is that the tests from different series had different test objects that had not gone through the same degree of ageing. The test object of the varying temperature series had gone through a much longer period of ageing, because it took a long time to gather enough data at40°C. Ideally, about ten thousand discharges at each level should be enough to get a statistically solid result of the stochastic process, but that would have taken months instead of weeks in the lab.

The reason for including this paragraph, is to support the conclusions that will be drawn later: The intrinsic conditions in the different test samples makes it difficult to compare different tests if the test objects and conditions are not fully equal.

4.2.2 Temperature effect on HDPE

In this section the results from the test scheme from Table 3.5 are considered. The goal of the test series was to investigate the effect of temperature on HDPE and compare this to the results with PET. Due to complications, the test was aborted after24 hat40°C. The surprising discharge pattern in Figure 4.13 being the deciding factor. Even if much effort was given to create HDPE samples with well defined cavities, a complex task because of the flexibility of the material, it was decided that due to the limited time, it would be safer to do some additional tests on PET instead.

Either way, it will now be presented what results was obtained from this, what went wrong and suggestions on improvements.

The test object was examined after seeing the initial results, and the material (HDPE) was clearly softened and disfigured, even if the temperature was relatively low. The cavity was no longer visible through the transparent material, but a somewhat larger ring of displaced material seemed to have formed. A dissection of the test object proved that the hole through the middle layer was still present. It could seem like the12 kg weight on top of the test object might have pressed the soft material together, collapsing the cavity. It was also found that the epoxy glue had not held all the way around the edge of the test object, and oil might have penetrated the sample and cavity.

0 5 10 15 20

Time [h]

0 50 100 150 200 250 300 350

q a [pC]

(a) Linear apparent discharge magnitude versus time.

0 5 10 15 20

Time [h]

10^-1 10⁰ 10¹ 10² 10³

q a [pC]

(b) Logarithmic apparent discharge magnitude versus time.

Figure 4.13: Apparent discharge magnitude of 10 kV, 40°C measurement with a HDPE test object. This result deviates from what was expected and the tests on PET (Figure 4.6).

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As the initial test at40°C,10 kVindicated that something was wrong, the rest of the test series was canceled.

A better production method for HDPE samples may be needed. If the material was pressed into the cavity, discharges might have formed along the edges of the cavity. The ingress of oil might also have given uncontrollable paths for discharges to form and changed the withstand voltage.

0 5 10 15 20

Time [h]

10⁰ 10¹ 10² 10³

q a [pC]

Moving average (magnitude)

(a) Moving average of apparent discharge magnitude.

0 5 10 15 20

Time [h]

10⁰ 10¹ 10² 10³

q a [pC]

Moving median (magnitude)

(b) Moving median of apparent discharge magnitude.

Figure 4.14: Moving average and moving median apparent discharge magnitude of 40°C,10 kVmeasure- ment with a HDPE test object.

Numerous discharges in the beginning, and very few discharges after15 h, seen in Figure 4.13, could indicate that the local conditions in the sample changed during the course of the24 htime period. In Figure 4.14, it is observed that the average and median apparent discharge magnitude decrease over time. One theory could be that as more insulating oil penetrated the test sample, the voltage withstand increased until there were no more discharges.

Partial Discharges under HVDC stress

Georg Volden

Partial Discharges under HVDC stress

Master ’s thesis

Georg Volden

Partial Discharges under HVDC stress

Master’s thesis in Energy and Environmental Engineering Supervisor: Frank Mauseth

Co-supervisor: Pål Keim Olsen May 2021

Norwegian University of Science and Technology

Faculty of Information Technology and Electrical Engineering

Department of Electric Power Engineering

Contents

Preface

1 Introduction

2 Theory

2.1 DC voltage

2.2 PD

3 Experimental setup

3.1 Test circuit

3.2 Test objects

3.3 Temperature control system

3.4 Equipment and limitations

3.5 Experimental test schedule

4 Results and discussion

4.1 Verification of new test setup

4.2 Effect of temperature