MOBILE SYSTEM FOR DIAGNOSIS OF HIGH VOLTAGE CABLES (132KV/220KV)

VLF-200 HVCD
VERY LOW FREQUENCY (VLF) - PARTIAL DISCHARGES AND TANGENT DELTA
HV/EHV POWER CABLES DIAGNOSTIC AND ON-SITE FIELD TESTING WITH VLF+PD+TD
Electrical Testing Group INDUCOR INGENIERIA

Inducor Ingeniería S.A. – Buenos Aires Argentina – www.inducor.com.ar

SUMMARY KEYWORDS

The advantages of the utilization of VLF (Very Low VLF Very Low Frequency - PD Partial Discharges - Outline
Frequency) technology have positioned this testing mode (map) of PD - Tangent Delta.
to be the most efficient alternative for
installation/maintenance tests and diagnosis of VLF TECHNOLOGY IN THE VLF-200 HVCD SYSTEM:
underground cables with thermo-plastic insulation. More
than a dozen international regulations in force in Europe According to the definition of the IEEE, test equipment
and the USA encompass and govern testing procedures under the VLF system is any that can generate an
in VLF. alternating current signal, of a frequency on the order of
0.01Hz to 1Hz.

The principle of operation is achieved from a direct

current, which, after being conditioned by an
electromechanical/electronic system, goes on to form an
alternation through a systematic process of loading and
unloading of the cable under test.

It is clear that loading a cable with direct current is very

easy and requires a low power source, but the main
problem is in the intervals of discharge of substantial
energy accumulated in said cable, according to the
equation:

E (Joules) = 0.5 x C x V2

Presented here is the first Mobile System with VLF (0.1

Hz) technology, for testing of applied voltage and of
diagnosis by Partial Discharges/Tangent Delta, in HV and
EHV cables (HV/EHV), known as VLF-200 HVCD. (HIGH
VOLTAGE CABLE DIAGNOSTIX).

Developed and assembled in Argentina by INDUCOR

INGENIERIA S.A., in conjunction with its technology
partners: HIGH VOLTAGE Inc. (USA), & POWER
DIAGNOSTIX SYSTEM (Germany), the system has the
capacity to perform diagnostic tests on the state of HV
cables of up to 20 km of length, (3.75 uf of load),
occasionally identifying, the laying sites which produce
220 kv VLF-200 HVCD mobile unit, with
partial discharges (discharge map).
Tangent Delta/Partial Discharge test capacity (INDUCOR
INGENEIRIA S.A.)

This high energy variable at play, which must be

dissipated two times per cycle, was one of the more
complex parts to solve, especially when given such high
voltages (220 kV), and large capacities, as those of
extremely long cables(up to 20 km).
Taking the previous example, the maximum energy to be
dissipated in a full cycle would be nothing less than:

E = 2 x0.5 x 3.75x10-6 x 220,0002 = 150 K Joules.

 The latter include the partial discharge tests, in overall
form, as well as in the tracing of the “Discharge Map,”
and those of Tangent Delta or Dissipation Factor.
INNOVATION OF THE VLF-200 HVCD SYSTEM:
1- APPLIED VOLTAGE TEST (Withstand Test)
The advantage of VLF test equipment, is that of
approximating as far as possible a test performed with The IEEE/EPRI/CEA agencies, and other worldwide
alternating current at the network frequency, but the engineering and standardization entities set test levels for
substantial difference lies in the low power that is cable dielectrics, from 1.5 to 3 times the Uo voltage, for a
required in 0.1 Hz VLF mode, compared to the 50 Hz or minimum of 15 minutes.
60 Hz. At 50 Hz, a cable of 3.75uf, tested at 220 kV of
test voltage, would require 57 MVA of power (259 A), The update of the traditional IEC-60840-2004 rules, for
while at 0.1 Hz it would only require 0.52A. (114 KVA). A.C. testing, states in its TEST VOLTAGE section, that the
The variable at play is the capacitive reactance. level of test voltage for new cable systems must be
between 1.7 Uo and 2 Uo, for cables between 30 kV and
Many breakdowns in underground cables are due to an 150 kV nominal voltage.
inefficient execution of their splices, and for these cases,
VLF has been shown to be an easy and safe method for While for higher voltage cables, the test voltage
maintaining confidence in the energy distribution decreases from 1.4 Uo for those of (220-230 kV), to 1.3
systems. Uo (275-345 kV), 1.2 Uo for (380-500 KV), and to only 1
Uo for those of 500 kV.
Beyond detecting flaws, nowadays, the worldwide trend
is based on early anticipation of an electrical accident,
through the application of diagnostic techniques for
cables and accessories , allowing control starting from
their installation, the type of failure that they will have in
the future, when they constitute an integral part of an
underground layout, allowing for the analysis meter by
meter of the state of degradation, whether it be
increasing or stable.

VLF-200 HVCD voltage/current control panel

Additionally, IEC-62067 specifies tests at 1.7 Uo/1 hour

for all types of cables greater than 150 kV.
Both IEC standards accept tests at 1Uo/24hr. According
VERSATILITY OF THE VLF-200 HVCD SYSTEM: to the rule, the user and the manufacturer of the cables
may agree upon the test voltage and the testing
The new VLF-200 HVCD Mobile System allows for the procedure.
testing series of applied voltage (withstand
voltage//Withstand Test), in accordance with point 5.3 ACCORDING TO IEC, THE HIGHER THE NOMINAL
VLF testing with sinusoidal waveform; established VOLTAGE OF THE CABLE SYSTEM TO BE TESTED, THE
according to: LOWER THE COEFFICIENT OF THE VOLTAGE TEST

IEEE Std 400.3™-2006 The VLF-200 HVCD Mobile System, enables alternating
IEEE Guide for Partial Discharge Testing of Shielded current testing of up to 20km of cables, with only 20 KVA
Power Cable Systems in a Field Environment. of network power, and with a selectable frequency of 0.02
- 0.05 and 0.1 Hz.
As well as the diagnostic testing of the state
(degradation), according to the provisions of:

IEEE Std 400.2™-2004

IEEE Guide for Field Testing of Shielded Power
Cable Systems Using Very Low Frequency (VLF)
 Based on the results of the location, the diagram of the
PDs is that shown in the figure, which details the
activities according to the length of the cable.
2 – PARTIAL DISCHARGE TEST AND MAPPING
(Partial Discharge Test & Mapping)

The new VLF-200 HVCD Mobile System, in its

“DIAGNOSTIC” mode, goes beyond being able to quantify
an “overall” value of the partial discharges of high-
voltage wiring, according to IEC-60270, which would be
of very little use because it could not identify which
would be the accessory, splice, terminal or cable section
that produces more of them, it also allows a tracing or
mapping of partial discharges characteristic of a system
of installed cables according to its length in meters
(Coulombs-Peak vs. meter map).
Comparing this diagram with the cable installation plan,
The measurement principle for the tracing of this map, is and with positions of the existing splices (verified during
based on SIGNAL REFLECTOMETRY. Once the PDs have the calibration), the defective points of the system can
been produced through the application of a test voltage be identified.
(VLF in this case), they will travel toward both ends of
the cable; being reflected in the terminals and producing
a typical trace of three impulses for each discharge
event.

Start End

Origin of the
Time
discharge
PD Pulses

meters
Map of the discharges
Schematic reflectometry of a partial discharge signal

In this trace, the time between the first and the second
impulse corresponds to the distance between the origin
of the PD (location), and the final end of the cable;
likewise, the time between the first and the third impulse
reflects the time to move twice through the cable.

Schematic reflectometry of a partial discharge signal

PD measurement panel of the VLF-200 HVCD system
A simple calculation determines the position (origin) of
the discharge according to the length of the cable.
The VLF-200 HVCD Mobile System has a filtering stage VLF
coupled directly to the high voltage circuit, so as to OUTPUT
eliminate the high frequency interference coming from
the VLF type high voltage source and from the supply
network.

Thanks to a “T” type filter, consisting of two inductive

branches and a capacitive branch tuned to the
frequencies of about the bandwidth of the discharge
measurement system, according to IEC-60270, the
measurements are free of noise and interference,
allowing them to be more selective.

HV VLF INPUT

“T” FILTER In accordance with IEC - 60270, the PD measurement

system is complemented by an transfer impedance (four-
technical network), and a pre-amplifying stage with
OUTPUT selective passband filters in the range of 40 KHz to 20
MHz.
TO THE TESTED
CABLE

FOUR-
TECHNICAL
NETWORK
PRE-AMPLIFIER

The software of the PD measurement system makes it

possible to graph and follow on screen all the
simultaneously captured events, both by signal
reflectometry (DSO Mode), by tracing the Coulombs-Peak
discharges versus meters map. (Site Locator Mode).

PD measurement diagram in the VLF200-HVCD system

 VLF OUTPUT

3 - TANGENT DELTA TEST (Dissipation Factor)

In its “DIAGNOSTIC” mode, the new VLF-200 HVCD

Mobile System also enables a Tangent Delta (Dissipation
Factor) test, with a resolution of 1X10-4. according to
IEEE Std 400.2™-2004.
Its LCD type screen graphically shows voltage and
current waveforms The screen variables are C: Capacity;
V: Applied Voltage, and the Cos φ of the display.

The Tangent Delta measurement system simultaneously

accepts two current signals, one by means of a
capacitive shunt in series with the sample (CSX), and During the graphic analysis, the current angular phase-
another by means of a reference capacitor (CN), forming shift between the voltage and the current waveforms
a voltage divider (CSN) , in parallel with the sample. corresponds to the angle φ situated as a complement of
δ.
HV VLF INPUT
CSN signal
Converter 1

(Current) OUTPUT
TO TABLE CSX signal
BEING TESTED
Converter 2
Technically, an insulating system is constructed with low
loss materials, resulting in a small current Ir, in phase
with the applied voltage. This current can be interpreted
(voltage) as a resistor R in parallel with a capacity C.

TD measurement diagram in the VLF200-HVCD system

Both signals are captured directly on the high voltage

circuit. Then, two I/O converters transform these analog
signals into optic signals and send them to the Tang δ dissipation factor = IR / IC
measurement hardware, to be evaluated by means of a
microprocessor, finally obtaining the
capacity/tangent/Cos φ results, all in relation to the
applied VLF voltage.
The phase difference between actual current I, and the
ideal current Ic, is described as the angle δ. Given that P
= Q - tang δ, the losses are therefore proportional to the
tang δ, and yield an expression of the quality of the
insulating material.
The angle δ is described as the loss angle, and the tan δ
as the loss factor.

Applied particularly to a system of HV cables, the tang δ

refers to the quality and evolution of the installed
material as a whole (sole system composed of cable
/terminals/splices). VLF-200 HVCD mobile system during the conformance
As there are no prior tangent delta values establishing testing
comparative initial parameters, (evolution of defects),
the analysis of the result will be taken into consideration
in the following two variables:
1- Comparison of results between the different phases of
the same triad.
2- Stability of the tangent value in the face of increasing
test voltage.

Acceptance criterion:
T δ @ 2 Uo < 1.2 %o
T δ @2Uo -T δ @ Uo <≥ 0.6 %o.

Rejection Criterion:
T δ @ 2 Uo ≥ 2.2 %o
T δ @2Uo -T δ @ Uo ≥ 1.0 %o

Degrees 1 to 4 indicate the aging (aged condition) of the

Presentation of the VLF-200 HVCD mobile system at the
cable system in ascending order (degree of deterioration
BIEL 2009 show – Buenos Aires
or of degradation).
CONCLUSIONS:
DEGREE 4
The VLF-200 HVCD Mobile System, with VLF sinusoidal
technology, now makes it possible to perform installation
DEGREE 3 and diagnostic tests on the state of HV / EHV
underground cables, enabling precise measurements of
DEGREE 2 the longest cable lays.
DEGREE 1
GOOD Given the need to forego direct current tests, due to the
adverse effects that this produces on XLPE type
Applied Voltage V/Vo insulation, the versatility of the VLF-200 HVCD Mobile
System compared to conventional resonant equipment
Then, powerful software makes it possible to graph and marks a fundamental difference in both dynamism and in
keep on the screen all the measured variables , both in diagnostic capacity.
real time and simultaneously.
SOURCE: INDUCOR INGENIERIA S.A.

ELECTRICAL TESTING GROUP

H.V. PARTIAL DISCHARGE & VLF
Latin American Sales & Applications
www.inducor.com.ar