Skip to main content

What is VLF Hipot Testing

VLF-60 KEP Product catalogue

KEP Product Catalogue

What is VLF Hipot Testing

Withstand or hipot testing is used to evaluate the condition of a cable insulation during installation, acceptance, or maintenance testing. As a result of a hipot test, at the point of an insulation defect an electrical tree will start to progress, create the insulation breakdown and make it possible for the technician to pinpoint the faulted place.

As a result of a hipot test, at the point of an insulation defect an electrical tree will start to progress, create the insulation breakdown and make it possible for the technician to pinpoint the faulted place.

The main focus of this article is the very low frequency (VLF) withstand cable test, which essentially is a type of AC hipot test conducted at a frequency rate of 0.01 to 1.00 Hz. Due to a significant drop in the operation frequency compared to AC hipot testing, done at 50 or 60 Hz, a VLF hipot tester is much smaller than a typical AC hipot test set, and thus is widely applicable for field usage.

Main VLF Hipot Testing Parameters: Waveform, Voltage, Duration

The typical waveforms for VLF testing include the sinewave, squarewave, and cosine-rectangular (trapezoidal) wave. The sinewave, or sinusoidal voltage, is the same as the voltage used by AC power systems. Out of the three mentioned waveforms, the sinewave is found to be the most efficient for cable testing as it causes the electrical tree to progress at the fastest rate, which results in shorter test times.

IEEE  400.2  guide stipulates that the test voltage should be 2.0 to 3.0 times the cables’ normal line to ground voltage. Since maintenance testing is done on service age cables, the recommended voltage is 80 per cent of that used for installation/acceptance testing. If there is a need to perform several test cycles, it may be reasonable to decrease the testing voltage by a further 20 per cent. The table below reflects the IEEE 400.2 guidelines as to the optimal testing sinewave voltage for 5 kV to 35 kV cables.

Table 1. VLF test voltage

Cable

rms voltage
Installation

rms / peak voltage
Acceptance

rms / peak voltage
Maintenance

rms / peak voltage
5 kV
9 / 13 kV
10 / 14 kV
7 / 10 kV
8 kV
11 / 16 kV
13 / 18 kV
10 / 14 kV
15 kV
18 / 25 kV
20 / 28 kV
16 / 22 kV
25 kV
27 / 38 kV
31 / 44 kV
23 / 33 kV
35 kV
39 / 55 kV
44 / 62 kV
33 / 47 kV

The advised testing time is 60 minutes at 0.1 Hz on new cables, and 30 minutes at 0.1 Hz on service age cables; it is recommended that the withstand testing time during maintenance testing should be extended to 60 minutes at 0.1 Hz if the circuit in question is of special importance.

VLF Testing Method

VLF testing is performed on offline cables, that is, the cable under test should be disconnected from the power system, and properly grounded. The VLF tester is then connected to the cable under test, and the required test voltage is set.

If the cable under test passes the VLF testing, the voltage should be decreased to zero, the cable under test and the VLF instrument should be discharged, and the cable should be grounded.
If the cable under test fails the test, the test voltage drops down to zero on its own. The VLF tester should be switched off for the cable and the tester to discharge, and the cable should be grounded. After that, the fault or faults can be found with fault locating instruments and repaired, or the respective cable sections are replaced.

VLF testing

Connection Setup for a VLF Cable


Advantages and Disadvantages of VLF Cable Testing

The strong points of VLF cable testing include the following:
  • VLF cable testing is applicable for extruded polyethylene, laminated paper, and mixed insulation;
  • the small size of the VLF test set is perfect for field testing;
  • VLF hipot testing can be done on both new and service age cables, which means that it can be used for installation, acceptance, and maintenance testing;
  • With the right voltage and testing time, VLF testing is not destructive for less severe insulation defects that could remain dormant for a long time and not impede the cable performance. Yet, it is efficient for locating more severe insulation defects that are likely to cause cable faults in the nearest future;
  • Combined with standard cable diagnostics equipment, the VLF test set can be used for diagnostic testing.
At the same time, the difficulties associated with VLF hipot testing are these:
  • VLF withstand testing may not provide conclusive results if the insulation is significantly damaged by water treeing or has partial discharges. In this situation, it is recommended that VLF testing should be combined with other diagnostic testing methods;
  • VLF testing with very high testing voltage at a frequency below 0.01 Hz may cause space charges in extruded polyethylene insulation;
  • VLF cable testing is done on offline cables only.
To sum up, although VLF withstand testing is attributed to destructive cable testing as it causes severe defects to initiate treeing and break down the insulation, the VLF test nevertheless does not harm good insulation. So, VLF hipot testing performed with alternating sinusoidal voltage best meets the testing purpose of locating and removing several defects in insulation that is mostly of good quality.



Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://www.kep.ua

http://news.chivindo.com/375/vlf-withstand-test-introduction-to-hipot-cable-testing.html

Comments

Post a Comment

Popular posts from this blog

BT-BASIC commands used

8.10     Some of the most frequently used BT-BASIC commands used are: msi                               Changes default working directory. Mass storage is            Same as “msi” cat                               Catalogs (list)the node names in the specified directory.             get                               Brings the contents of a file into the system workspace.        ...
1 comment
Read more

Perhitungan & Cara Merubah Kumparan Blender Dari 220 V Menjadi 12 V

          Seperti yang telah dijelaskan pada buku “menggulung motor listrik arus bolak-balik, servis peralatan listrik rumah tangga kelompok penggerak dan perbaikan peralatan listrik pertukangan”, bahwa motor penggerak yang digunakan pada perlatan listrik rumah tangga dan pertukangan seperti blender, mixer, bor tembak, gerinda dsb menggunakan jenis motor universal. Motor universal adalah jenis motor listrik yang dapat disuplai dengan sumber listrik arus bolak-balik (AC) dan arus searah (DC). Jadi peralatan-peralatan listrik rumah tangga dan pertukangan tersebut yang biasanya kita suplai dengan sumber listrik AC dari PLN atau Genset sebesar 220 V sebenarnya dapat juga kita suplai dengan sumber listrik DC yang tentunya tegangan juga harus sama yakni 220 V.           Yang menjadi permasalahan bagaimana kalau peralatan listrik rumah tangga atau pertukangan tersebut, sebagai contoh misalkan blender yang ingin digunakan atau dioper...
1 comment
Read more

BT-BASIC command line

8.8       At the BT-BASIC command line type the command  msi  and the directory path, then press the “ENTER” key on the keyboard.  Example:                   msi ‘/hp3070/boards/aspect/main’ 8.9       At the BT-BASIC command line type the command  get ‘testplan’ and press the   ENTER” key on the keyboard.  You should now see the body of the testplan file displayed in the work space of the BT-BASIC window. 8.10     Some of the most frequently used BT-BASIC commands used are:
Post a Comment
Read more

Autodesk SketchBook Pro 2021 Full Version

BAGAS31 – Sesuai dengan namanya, Autodesk SketchBook Pro 2021 Full Version ini merupakan software digital sketching atau drawing terbaik yang bisa kamu gunakan. Pada versi terbaru kali ini, ada beberapa penambahan fitur yang sangat efektif. Dengan fitur baru tersebut, diharapkan mampu meningkatkan proses sketching maupun drawing kamu. Autodesk SketchBook sendiri sudah bisa kamu dapatkan secara gratis melalui website resminya. Namun untuk kamu yang mau download versi Autodesk Sketchbook Pro, maka bisa langsung download melalui link yang sudah saya sediakan di bawah ini. Download Autodesk SketchBook Pro 2021 Full Version Screenshot: System Requirements: Windows 10 2.5 – 2.9 GHz of Intel or AMD CPU 4 GB of Memory 256 MB Graphics card with OpenGL 2.0 support We recommend that you use a pressure-sensitive tablet and pen for basic features Download: Autodesk SketchBook Pro 2021 Full Version [ FileUp ][ Uptobox ][ UsersDrive ] Jamu Only [ File...
2 comments
Read more

Testhead

4.3         Testhead The testhead is that portion of the tester that supports the PIN, ASRU and Controller cards.   The testhead is divided into two BANKS and each BANK is divided into two MODULES, see figure 2 below.  Bank 1 contains modules 0 and 1, bank 2 contains modules 2 and 3.  The test fixtures are placed on the banks of the tester and locked down for board testing.  The testhead cards interface to the test fixture through the spring loaded pogo pin “nails” at the top edge on each of these card types. 4.4       Support Bay The support bay is a stand-alone cabinet that houses the power supplies for the Unit Under Test.  This bay also houses the test station power distribution unit and test station controller on earlier models. 4.5       Emergency Shutdown Switch The emergency shutdown switch is the large red button located at the lower left corne...
Post a Comment
Read more

Kelebihan dan Kekurangan Saluran Listrik Jenis Saluran Udara dan Saluran Bawah Tanah

Berdasarkan pemasangannya,   saluran distribusi dibagi menjadi dua kategori, yaitu : saluran udara (overhead line) merupakan sistem penyaluran tenaga listrik melalui kawat penghantar yang ditompang pada tiang listrik. Sedangkan saluran bawah tanah (underground cable) merupakan sistem penyaluran tenaga listrik melalui kabel-kabel yang ditanamkan di dalam tanah. 1.    Saluran Bawah Tanah (Underground Lines) Saluran distribusi yang menyalurkan energi listrik melalui kabel yang ditanam didalam tanah. Kategori saluran distribusi seperti ini adalah yang favorite untuk pemasangan di dalam kota, karena berada didalam tanah, maka tidak mengganggu keindahan kota dan juga tidak mudah terjadi gangguan akibat kondisi cuaca atau kondisi alam. Namun juga memilik kekurangan, yaitu mahalnya biaya investasi dan sulitnya menentukan titik gangguan dan perbaikannya. Kedua cara penyaluran memiliki keuntungan dan kerugian masing-masing. Keuntungan yang dapat diperoleh dari suatu jaringan bawah...
Post a Comment
Read more