Page 32 - Analysis Report of Global Market in the Submarine Cable Field (2017)

Wire and Cable

Talk about the partial discharge and detection of high

voltage power system
Keming Jiao Dayong Chen
Baosheng High Voltage Cable Co.,Ltd. Baoying Jiangsu province 225800

Abstract:The development of partial discharge in high voltage cable detection system relates to the highvoltage insulation and high voltage test,partial
discharge detection is throughcondition monitoring faultfound hidden dangers,to take effective means of treatment,reduce the power supply system
operationrisk,improve the operation efficiency of power supply equipment,ensure the security and stability of powersystem.
Keywords:Power systems ;High voltage cable;Partial discharge;Detection method
1 Introduction is difficult to touch the charged conductor; 2.the sensing points are
Nowadays the ultra high voltage XLPE cable is widely used in the distributed on the long term, and the various signals willbe distortion
world,theelectric properties are superior, with low dielectric loss, high during transmission; 3.external interference signals. The sensor used for
intensity of breakdown electric field and high current-carrying capacity. measuring partial discharge is normally placed on the semi-conductive
However after the cable put into operation, due to the insulation layer near the cable joint, copper or aluminum conductors are usually
overheating, aging metamorphism, mechanical damage and other factors, used as internal sensors. Online PD detection methods are usually
can cause thequality ofcable insulation is reduced.To prevent the cable below methods: High frequency current transformer method, Very high
operation accident from above causing,we must monitor the running frequency method, Ultra high frequency, Acoustic emission method,
state of the cable line in real time.The monitoring system can reflect the Capacitive coupling method,etc.
insulation of the cable and accessories all the time.The main monitoring
indicators include changes in load and temperaturevariation,the partial 3.1 High frequency current transformer method
discharge is the most important defect among them. When the insulation When partial discharge occurs in the cable, a high frequency pulse
is discharged, a local signal is generated within the cable.Through test the current is produced at the same time, and flow into the metal sheath
discharge capacity, the insulation level of the cable can be fully mapped. like the leakage current, then flowing through the cable intermediate or
And the cable accessories is the weakest part in the high voltage line, a terminal connector to the earth. The HFCT is installed on the grounding
great probability of cable breakdown occurs in the joints. Therefore, we line of the connector or terminal, collect partial discharge signals by
need maintainthe cable linesat regular intervals, in particular, it is very current coupling, the frequency band range is 100kHz～200MHz, it is an
important to monitor the insulation of the cable accessories.This paper external type transformer, so the external disturbance is obvious, the PD
briefly introduces the basic principle and detection method about partial test sensitivity of XLPE cable is generally 10pC, but near the outdoor
discharge, evaluate the quality of high voltage cable insulation, ensure the sensor, the noise will more than 1000 PC.The interference is mainly
powersystemsecurity and stability. from corona discharge, leakage current and the poor connection of high
voltage equipment.
2 The principle of high voltage cable partial discharge Pic 1 HFCT
The XLPE power cable has excellent electrical performance and heat
resistance, it is widely used in the area of industrial transmission and
transformation, but due to the process of production and the method
of construction, some impurities may be mixed into the cable insulation
material, can cause the surface of the cable insulation or the internal area
bear the non-uniform electric field, thus can cause the partial discharge
easily.

The time when the air gap in the insulation is discharged, the gas in the
air gap will be Ionization,it can causes neutral molecules to be separated
into positive ions and negative ions, and under the presence of external
electric field, the positive ions move along the electric field, the electrons
and negative ions move along the opposite direction of the electric field.
These space charges are formed into new electric fields, the direction
is opposite to the external electric field, then the electric field intensity
in the air gap isEc=Eouter—Einner.The partial discharge process of
high voltage cable is an extremely complicated process, it needs a variety 3.2 Very high frequency method
of parameter to fully describe its states, the main parameters include The monitoring frequency of VHF method focuses on 1 ~ 300MHz. The
discharge energy, discharge power, apparent discharge charge, discharge technical core of this method is Rogowski coil. In the known online PD
repetition rate and discharge average current and so on.The long - term detection, the sensitivity of Rogowski coil method and capacitive sensor
partial discharge has irreversible damage to the insulation of the cable, method is widely recognized by people. However the capacitive sensor
the main performances are local heat, chemical activity products, impact needs to be placed on the right place, not portable. Rogowski coil can
from charged particles and radiation, etc. The above factors infringe solve this problem, because what they’re measuring is the closed current,
on the insulating media is a fairly slow process, and it starts from a so the Rogowski coil does not depend on the geometry of the device.
small part of the area and eventually leads to the insulation breakdown. The signals whichRogowski coil collected also contain interference
Therefore, the partial discharge in the high voltage cable system is signals. The discharge current pulse of PD signal is steep and very short.
extremely harmful to the operating power system, which is one of the Generally, the rise time of discharge pulse is measured by ns. The PD
main reasons for the ultimate insulation breakdown. pulse is a broadband signal, has a wide frequency range, the spectrum is
hundreds of megahertz. The frequency range of noise is mainly in the
3 The principle and common methods of partial discharge and lower frequency band, we can set the cut-off frequency between 500kHz
detection ~ 1MHz, so it can filter out the interference signals.For the sensors which
There are three main difficulties in online PD detection: 1.the sensor collect the PD signals, bandwidth and sensitivity are considered, as well
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