BBMB EXPERIENCE IN CONDITION BBMB EXPERIENCE IN CONDITION MONITORING OF HV SURGE MONITORING OF HV SURGE

ARRESTERSARRESTERS

WHAT IS A SURGE ARRESTERWHAT IS A SURGE ARRESTER Inexpensive device which allows passage of very small

leakage current through it under normal conditions, but diverts away high voltages inimical to insulation of equipment it is supposed to protect by giving them a path through itself, thereby prevents outage of costly equipment due to insulation failure

Reasons of High voltages – Switching, lightening Indispensable device in Insulation-coordination BIL for 400kV Sys. Around 1425kV.As per IEC highest

allowed is 1239kV The smartness of SA lies in limiting the voltage rise well

below 1239kV when a lightning impulse current of 10kA/20kA is impressed.

WHAT IS A SURGE ARRESTERWHAT IS A SURGE ARRESTER

WHAT IS A SURGE ARRESTERWHAT IS A SURGE ARRESTER

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WHAT IS A SURGE ARRESTERWHAT IS A SURGE ARRESTER

DEVELOPMENT HISTORY OF DEVELOPMENT HISTORY OF SURGE ARRESTERSSURGE ARRESTERS

Extremely nonlinear characteristic of MO type SAs thereby doing away with need of disconnection from mains through serial spark-gaps as in case of SiC type

Continuous flow of leakage current through gapless MO type SAs render them suitable for condition monitoring, which was earlier not possible in case of gapped SiC type SAs. This continuous flow of leakage current gives picture of inside of MOSA

Condition monitoring of SA required to pre-access performance of SA during system over voltages when it is supposed to intervene in addition to prevent consequential damages it may cause due to its own bursting

COMPARISON OF VI CHARACTERISTICS COMPARISON OF VI CHARACTERISTICS OF MOSA AND SiCOF MOSA AND SiC

Introduction of MOSA type Introduction of MOSA type Arresters in BBMBArresters in BBMB

• Being one of the oldest among power utilities was having vast population of conventional valve type SAs with SiC discs

• Self realization of disadvantages of SiC type arresters• Feedbacks from other utilities• Decision to replace convention type with MOSA in

phased manner taken in 1990s.• First priority Generating Stations• Rest in phased manner

Techniques for monitoring of Techniques for monitoring of MOSA type ArrestersMOSA type Arresters

• Realization developed that technique is insufficient and periodic recording of “Third harmonic resistive leakage current” is must for checking condition of MOSAs

• TransiNor make LCM-II procured in year 2002-03.• Centralized team formed to measure the above

parameter so as to collect sufficient data for developing some internal standards

Guidelines adopted in BBMB for Guidelines adopted in BBMB for condition monitoring of MOSA condition monitoring of MOSA

type Arresterstype ArrestersPeriodicity of testing

1.For third harmonic resistive leakage current less than 200 micro Ampere - After 2 years2.For third harmonic resistive leakage current more than 200 micro Ampere - After 6 months

Guidelines adopted in BBMB for Guidelines adopted in BBMB for condition monitoring of MOSA type condition monitoring of MOSA type

ArrestersArresters• For third harmonic resistive leakage current

more than 1000 micro Ampere—Surge arrester is to be replaced

• For third harmonic resistive leakage current more than 200 micro Ampere—Surge arrester on transformer is to be shifted to transmission line and surge arrester having less than 100 micro Ampere of resistive leakage current

Over all experience of BBMB Over all experience of BBMB • The adoption of MOSAs having better Volt-amp.

characteristics as compared to valve type SAs helped in saving costly equipments

• There have been instances where surge arresters having current of more than 800 micro ampere have got damaged.

• On the other hand surge arresters having current of range 300-400 micro ampere have also got damaged.

• Periodical condition monitoring of MOSAs also helped in preventing unpredictable outages and consequential damages resulting from a Surge arrester bursting

Shortcoming of THRM techniqueShortcoming of THRM technique• LCM for condition monitoring of MOSA quite popular but

the technique still suffers from certain drawbacks• Influence of weather conditions and poor repeatability of

readings either due to lack in training of LCM operating personals or uncontrolled electrical interference prevalent in HV substations.

• The methodology adopted to extract third harmonic resistive current is based on considerable assumptions.

• The actual temperature inside the MOSA is not known .• Absolute values of third harmonic resistive current can

not be used as criteria, only comparative analysis on time scale can be used.

Revision of guidelines Revision of guidelines

 

• The main basis of review is that condition of ZnO blocks is dependent on their manufacturing process including sintering temperature.

• It has therefore been proposed that analysis of third harmonic resistive leakage current should be made in reference to population of surge arrester of individual manufacturer and if possible, from single batch and comparison be made on time scale for assessing the healthiness of surge arrester.

• Another techniques like thermovision scanning of surge arrester be proactively be used to assess the condition of surge arrester.

Problems faced for procuring Leakage Problems faced for procuring Leakage current monitoring kitcurrent monitoring kit

• During specification preparation for procurement of new LCM, problem of selecting range of resistive leakage current

• One manufacturer says upto 900µA sufficient where as other stresses nothing less than 9mA will be suffice

• Convoluted explanations of manufacturers obligated us to refer the issue to CPRI

• Based on inputs from CPRI and references from IEC 60099-5, specification freezed