1. 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
499978-1-4673-1252-3/12/$31.00 ©2012 IEEE

2. 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
dr
t
trT
rECtI
,
(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

3. 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 t
I 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

4. 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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