Portable Installation for Space Charge Measurements on Full-size High Voltage Cable Loops based on a Multicell Power Converter

A. Darkawi, P. Notingher, T. Martiré, J.-J. Huselstein, F. Forest

Institut d’Electronique du Sud, Université Montpellier 2 / CNRS CC 079, Place E. Bataillon, 34095 Montpellier Cdx 5, France

Abstract— The development and the qualification of polymer-insulated high voltage dc cables require the measurement of the space charge in order to ensure that charge accumulation does not result in high internal fields leading to breakdown.

The aim of this work is to set up a portable installation to be used for space charge measurements on long cable loops (up to 100 m) and high conductor sections (up to 2500 mm2). Its principle is based on a transient heating of the cable core or of the outer conducting screen by Joule effect for generating a thermal pulse of several degrees, which, by crossing the insulation, generate a capacitive current measured by a current amplifier (inner heating thermal method). This requires the ability to obtain and to control a high current (up to 12 kA) during a given period of time. The use of ac current heating requiring significant apparent power, it can hardly be used for such a goal. We propose a set-up based on a coupled inductor parallel multicell power buck converter, with a topology especially designed for generating and controlling high currents during short periods of time. The design and the manufacturing of the converter are described. The validation of a base cell delivering 600 A through tests on power cable is presented.

I. INTRODUCTION

The increasing need of submarine and underground transmission of electric power across long distances pushes forward the development of high voltage direct current (HVDC) cable links. HVDC cables allow power transport toward remote locations, as they are not subjected to capacitive currents that limit the length of ac cables to several dozens of kilometres. Asynchronous systems or networks operating under different frequencies can also be connected by HVDC links associated to power converters. HVDC cables equally provide increased efficiency with respect to their HVAC counterparts, as there are no induced losses in the conductor, screen(s), armoring, neighboring cables and metal structures, and the electric field which can be applied to the insulation is usually higher.

To avoid the shortcomings of oil and paper-insulation, a significant effort is being made towards the development of polymer-insulated HVDC cables and accessories. In the case of DC cables with polymeric insulation, the problem of reliability is related to space charge accumulation in the dielectric, as a result of electrical stress and thermal gradient. Such charge accumulation can prematurely age or break down inadvertently the dielectric. The development and the

qualification of this type of cables require the study of space charge accumulation dynamics in order to ensure that the accumulated charge does not give birth to high internal fields leading to early breakdown. In this context, space charge measurements have been recently included in the CIGRE recommendations for testing dc extruded cable systems for power transmission at a rated voltage up to 500 kV [1].

There are today several methods allowing to measure space charge on flat insulating samples, but in order to assess the ability of the cable insulation to store and evacuate space charge in conditions close to the service ones, it is preferable to measure the charge directly on cable loops during long-term testing. Indeed, the charge dynamics in the insulation of a dc power cable could differ significantly from that of a flat sample due to the differences in the manufacturing process and to the thermal gradient which establishes across the insulation, which may affect significantly the electric field repartition and charge trapping/detrapping. Setting up measurement apparatus for non-destructive space charge measurements, able to be used directly on important lengths of full size high section power cables during type tests, is therefore of significant importance for manufacturers and utilities.

The non-destructive charge measurement in a solid dielectric is based on the use of an external stimulus (pressure or temperature) that causes a low temporary imbalance in the material electrical state. The response analysis allows to quantify and locate the electrical charges. Accordingly, the Thermal Step Method (TSM) [2-5] consists to apply a temperature step and to measure a capacitive current due to the displacement of space charges in the cable insulation. In order to measure these charges, a low temperature rise is necessary and can be obtained by injecting, during a short period of time, a high current in the cable core (Joule effect). The resulting thermal wave diffuses into the insulation, and the induced displacement current through the dielectric is measured after the heating current has been turned off.

The use of this technique requires high values of “heating current” during several seconds. This can be achieved with the aid of a heating transformer [4]. Such a setup has been successfully used to assess long-term ac aged cable loops [6]. However, for long cables of strong section, the inductive reactance of the conductor becomes very important, and a huge apparent power is needed to produce the required heating

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current. Thus, the inductance of the conducting core does not allow to reasonably considering the use of such a principle on more than several meters of high voltage cable.

Another way to produce a high current during a short period of time is be the use of a DC/DC converter in transient regime. Therefore, we aim to set up a measurement installation including a robust, efficient and compact DC/DC converter which must be able to inject and regulate a sufficiently high current with a rising time as short as possible. Designing and integrating such a converter is a challenging task, both in terms of power electronics and low signal instrumentation. Indeed, the system must be able to convey a high energy (up to 1 MJ) during several seconds in order to supply a current of several thousands of amperes to the cable core, than to measure thermal step responses of the order of 10 pA to 1 nA. This system needs to be portable and usable on-site.

This paper presents an original solution based on compact and modular power electronics converters for generating very high current (heating current) to allow on-site power cable diagnostic by the TSM. It is a modular system that will finally consist of ten “low voltage/high current DC/DC converters”, supplied separately by super-capacitors. Each module is a 12 phase interleaved buck converter with magnetic coupling; each module will provide up to 1200 A during two seconds. A current control loop will impose the total current waveform in the power cable conductive core. The principle of the system is first addressed, than the validation of a 600 A base cell on a dc-conditioned power cable loop is presented.

II. APPLICATION DESCRIPTION AND PRINCIPLE

A. Space charge measurement using a heating transformer

This method, also called “Inner Heating” [4-6] and illustrated in Fig. 1, is based on the Joule effect and consists to make a high ac current to flow across the cable core to generate the thermal wave. The dielectric response in measured just after the heating current has been turned off. This technique gives a global insulation evaluation.

II((tt))Heating transformer

Fig. 1 Space charge measurment by TSM using a heating transformer [5]

The measured signal I(t) (see equation (1)) is a function of: - the electric capacitance of the measured region C, - the electric field repartition within the material in the radius

direction E(r), - the temperature distribution across the insulation: T(r, t) = T(r, t) - T0 (T0 is the cable temperature before applying the thermal step):

e

i

R

R

drt

trTrECtI

, (1)

where is a material constant related to the sample contraction (or expansion) and to the permittivity variation

with temperature, Re and Ri are the external and internal radii of the insulator. The thermal wave is generated from the core toward the external semicon by a 50 Hz ac current of the order of 400 A to 2400 A induced by the heating transformer through the cable core. As asserted before, due to the high value of the core inductance, the use of this method for long cables of high section may require very high apparent power (i.e., up to several MVA in the case of testing 100 m of a 500 kV cable). This makes it hardly usable on cable loops as long as those used in type tests.

B. High current injection using a DC/DC converter

To avoid the drawbacks of the heating transformer set up, a DC/DC converter can be used for generating very high DC current pulses needed for core heating. In order to achieve this objective, it is necessary to design a low voltage and high current converter. The main objective is to allow energy transfer from the energy source (Ultracapacitors) to the power cable conducting core and to generate the thermal step by Joule effect. As the final converter will be used for space charge measurement on cable with lengths up to a hundred meter and conductors sections up to 2500 mm², the heating current injected will be close to 12 kA during one or two seconds. The elementary module should produce up to 1200 A and will be used for testing HVDC cable with a 250 mm² conductor section. Each module consists of twelve commutation cells and will be supplied by ultracapacitors (165F, 48V), which provide the necessary energy to produce the thermal step. A current control loop can be used to maintain a constant heating current (Fig. 2-3).

This kind of application could be implemented using interleaved converters, since they are an interesting alternative for many applications due to their advantage in terms of power density, dynamic response, output ripple cancellation, thermal management and optimized design. However, the drawback of this technique is the current ripple in each channel that increases with the number of parallel phases. Recent work [7] shows that to overcome this drawback, coupled inductors can be used and the converters effectiveness can be improved. The magnetic coupling can be achieved by means of single monolithic transformer or separate transformers. The use of InterCell Transformers topology greatly improves input/output waveform quality, and increases converter compactness [8-9].

The InterCell Transformers (ICT) are modeled in order to obtain an analytical expression of the magnetic flux density; this is done by calculating the transformers windings voltage matrix. Then, a magnetic field matrix can be determined in order to find out the peak value that can saturate ferromagnetic-core. It has been demonstrated that the value of the flux density Bmax obtained for a duty cycle of 50% increases with commutation cells number. Accordingly, care must be taken in the Interphase converter design parameters definition. If the number of phases is high (k>4), a significant reduction of the induction peak value can be obtained by judiciously modifying the command sequence of the k cells and by optimising the coupled inductors ICT design. This mode is then called permuted mode [10]. The reduction ratio depends on switching cells number (even or odd). Modelling the ICT allows to determine the transformers voltage matrix, the flux and the magnetic field for any number of cells (even or odd). It also leads to predict the inductors behaviour. Then, the coupler parameters such as the switching frequency, the magnetic core geometry and the number of turns of the windings can be correctly calculated.

500

Fig. 2 Power electronics based technique (1200 A module)

Fig. 3 k-cell Interleaved Multi-cell converter using InterCell Transformer

Fig. 4. Structure of the set up including a 600 A DC/DC power converter and Ultracapacitors for generating transient heating of power cable core [11]

(the cable is not shown)

Fig. 5. Waveform of the output (heating) current (200A/div, 0.5 s/div) [11]

An analytical model of cyclic ICT for coupled inductors multiphase converters design applicable for any number of cells has been developed and is described in [11]. We have shown that a significant magnetic field reduction can be obtained for k>4 by applying a judicious permutation of the command of phases. The proposed model has been validated by simulations and experimental results. Based on the

developed analytical model, we have built a 4-phases 600 A power converter.

As it can be seen in Fig. 4, the result is a very compact setup including an Ultracapacitors module (165 F, 48 V) connected to the power part of the converter, which is placed on the top of the super-capacitor. The command signals are generated by an FPGA kit connected to the power part via an interface card, which allows setting the duty cycle, switching frequency and impulsion duration. Fig. 5 presents an output current waveform measured during tests carried out with, as charge, a 3 m power cable loop with a 95 mm2 conductor. For this test, the Ultracapacitors have been charged at 24 V, and the duration of the current pulse was set to 2 s. As the test has been made without a current control loop, during the discharge of the super-capacitor the output current decreases from 600 A to 500 A. It is to note that the current filtering is naturally made by the coupler and the charge, without additional passive elements.

III. RESULTS AND DISCUSSION

In order to assess the usability of the new set up for space charge measurement by thermal methods, a dc-conditioned 12/20 kV cable loop was used. The two ends of a 3 m long, 95 mm2 cable piece have been connected to constitute a single turn secondary of a heating transformer. A 30 kV dc voltage has been applied to the cable core. In the same time, a thermal gradient of 40°C was applied between the cable core and the outer semicon (90°C on the core and 50°C on the external semicon). This thermal gradient has been obtained by generating an RMS ac current of 220 A through the core of the cable with the heating transformer. After one week, the ac current has been switched off and the cable has been left to come back to room temperature with the dc voltage still applied, in order to keep as much as possible of the charge accumulated in the insulation and to “freeze” the polarization gradient.

After poling, the cable loop has been kept as a one-turn secondary of the heating transformer and submitted to a thermal step measurement. The heat wave was generated by a 600 A RMS 50 Hz ac current that has been established in the cable core during 2 s with the aid of the heating transformer. The thermal step current induced by the space charge in the insulation was then immediately measured by the current amplifier (Fig. 1). The cable has then been removed from the heating transformer and transferred to the DC/DC converter bench from Fig. 4. The converter command was set to provide a 600 A current during 2 s through the cable core. After the transient heating of 2 seconds, the thermal step current issued by the crossing of the insulation by the thermal wave was recorded.

pAHeating current

Thermal step current

Conducting core of the cable

External semicon

0

( , )( ) ( )

e

i

R

R

T r tI t C E r dr k t T

t

DC/DC Converter

Ultracapacitor

Fig. 6. Thermal step measurement using the DC/DC converter.

The heating current is provided by the controlled discharge of the ultracapacitor to generate a thermal wave across the insulation, and then the

thermal step current is measured by the current amplifier.

501

Fig. 7. Comparison of the thermal step responses

obtained by using the AC heating transformer and the new DC/DC set up

Fig. 8. Thermal step currents recorded on the dc-poled cable

using the DC/DC converter with various heating times

TABLE I. HEATING TIMES, RMS CURRENTS AND TEMPERATURE ELEVATIONS CORRESPONDING TO THE THERMAL STEP RESPONSES FROM FIG. 8

Heating time [s] 0,25 0,5 0,75 1 1,25 1,5

Heating current [A RMS] 590 575 560 551 546 532

Temperature raise [K] 0,10 0,20 0,29 0,37 0,46 0,52

The thermal step currents measured with the two heating set-ups (AC transformer and DC/DC converter) are shown in Fig. 7. As it can be seen, the acquired responses are almost superposed. The slight differences are due to the fact that the true RMS value of the current supplied by the DC/DC converter was 551 A instead of 600 A, because of the decrease shown in Fig. 5. This decrease can be easily eliminated by providing the converter with a current control loop.

In order to further check the validity of the signals measured by the current amplifier inserted in the DC/DC bench, tests have been made by varying the heating time. Indeed, the thermal step current amplitude is fairly proportional to the temperature rise [4]. Table 1 is concerned with the heating times, the RMS values of the heating currents and the resulting temperature raises at the origin of the thermal step currents from Fig. 8. It comes out that the ratios of the amplitudes of the thermal step currents are in accordance with the ratios of the temperature raises from Table 1.

The presented results demonstrate the capabilities of a 600 A cell for setting up a high current portable bench for

space charge measurements on long cable loops. We deal with an easily portable set up of dimensions comparable to those of an oscilloscope. Higher heating currents can be easily generated by putting into parallel such elementary cells while maintaining a good compacity of the resulting bench by increasing the number of phases. Indeed, a 12-phases 1200 A setup which is currently under test has roughly the same dimensions as the presented 600 A setup, also because the super-capacitor represents more than 60% of the volume of the bench. This should allow setting up a 12000 A bench easy to be handled and displaced, allowing on-site measurements.

IV. CONCLUSIONS AND PROSPECTS

The issue of space charge measurements in full-size HVDC cables has been addressed in this paper. An original experimental set up based on a high power density multi-cell buck converter is proposed for allowing space charge measurements on long cable loops. A 600 A DC elementary cell using a 4-phases DC/DC power converter has been tested and validated to be used with the principle of the thermal step method for space charge measurements. The presented results demonstrate the capabilities of the proposed type of DC/DC cells for setting up, via a multi-cell converter, a high current portable bench for space charge measurements on long cable loops with high conductor sections.

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