Role of Marconite in T&D Field

This article presents a Case Study on the impact of high soil resistivity in lightning prone areas and mitigation measures being adopted in 400 kV D/C Silchar (Assam) - Byrnihat (Meghalaya) interstate transmission line of NETC by application of Bentonite & Marconite...

The earthing system has an important role in safe operation of Transmission Lines. In the hilly terrain / terrain of high soil resistivity or lightning prone areas, lightning over voltage causes flashovers and subsequent tripping in transmission lines. Reduction of grounding resistance is one of the efficient ways to prevent occurrence of such flashovers. The primary object of safe earthing design of a transmission line is to provide low resistance path to lightning current. 400 kV transmission lines of NETC traverses through areas having high soil resistivity (10–10,000 Ωm) and high isokeraunic levels. In North Eastern Region, soil is very acidic in nature in addition to other severe environmental conditions.


North East Transmission Company Limited (NETC) is a Joint Venture Company of POWERGRID, OTPC and Six states of North Eastern region. The main objective of the company is to evacuate 726.6 MW power from Gas Based Combined Cycle Power Project (GBCCPP) of OTPC at Palatana in Tripura. NETC is operating 400kV D/C Palatana-Silchar-Bongaigaon transmission line of 662.8kM traversing through the states of Tripura, Meghalaya and Assam. One circuit of 400kV Silchar –Bongaigaon double circuit interstate line is looped-in looped-out at Byrnihat substation in Meghalaya and another circuit is looped-in looped-out at Azara substation in Assam.

The condition

Major constraints in operation and maintenance of transmission line in north eastern region have been described hereafter. The operation and maintenance of transmission line in North Eastern Region is a big challenge due to the following major constraints:

  • Dense forest, Hilly terrain, fast growing vegetation, agricultural cultivation fire.
  • Frequent land sliding & road blockage due to heavy rainfall. The rain fall in NER is about 7-10 times more than all India average rainfall.
  • The acute ROW issues and theft prone area in some parts of Assam, river scoring.
  • Severe lightning in some portion of the line.
  • To minimize the impact of lightning effect, following options were analyzed.
  • Providing additional earthing of towers through PG clamp with separate earth pit.
  • Providing spike on tower top to capture lightning strikes.
  • Providing one more additional insulator disc to protect against Back flashover.
  • Installation of Transmission Line Arrestors (TLA) in locations  where  TFR value  is more than 50 ohms.
  • If  the  value  of  the  soil  resistivity is more  than 1500 ohms  metre – then need for  use of   counter  poise  earthing  up  to 70 metres.
  • The use  of  Morolite (Mixture  of  Bentonite with 7.5% salt) with 40 mm dia Galvanized pipe type  electrode.
  • Addition of Salt & Charcoal with Rod earthing.
  • Carbon  earthing.

Lightning is a massive electrostatic discharge caused by electric charges in the atmosphere and resulting in a strike from cloud to ground. Lightning is a very high frequency signal and high voltage /current, the impedance of the line changes with the frequency of lightning. Whenever lightning strikes the top of a transmission tower or earth wire, a lightning current flows towards bottom of the tower and causes voltage rise to a tower that results in back flashover across the insulators of transmission line or causes phase to ground fault. The probability of flashovers can be minimized if the lightning current gets the least resistance path to ground, which can be achieved by reducing TFR.

Types of chemical earthing

Following types of chemical earthing compounds are commonly used for reducing the soil resistivity:

  • Bentonite
  • Marconite.

Bentonite and Marcionite have certain similarities and both are used as Ground Improvement Materials (GIMs) to reduce earth resistance. NETC considered Bentonite due to its cost effectiveness over Marcionite for improvement of lightning performance of transmission lines.

Types of Bentonite: Two types of Bentonite are commonly available –

  • Sodium Bentonite.
  • Calcium Bentonite

Sodium Bentonite is having properties of swelling while calcium Bentonite is non-swelling in nature. NETC has used the sodium Bentonite as it can absorb water up to 5 times of its original weight and swells up to 13 times of its dry volume in 139 tower locations.

Characteristics of Bentonite

Bentonite usually forms from weathering of volcanic ash with high content of silica. It is well-known for its swelling  properties and moisture  retaining  properties.

Bentonite clay is a natural earth soil (clay) containing the mineral montmorillonite. Due to its high moisture characteristics, resistivity of soil decreases.

Marconite’s characteristic and advantage over the Bentonite

  • NETC has   explored   the alternative  option to  Bentonite  earthing  in  some  of  the  locations by  using  the  conductive  concrete earthing (cement  by using  the  Ground  Enhancement Material as  a Marconite). After application, we obtained  good  result.
  • The Marconite is Corrosion & Maintenance free product.
  • The life  of the Marconite   is  50  years  as compared  to  the  life of  bentonite,   which  is  4 to  5 years.
  • It  has  very  low  resistivity,  it dissipates the  fault current very fast.
  • It does not  leach  away by ground  water as  a conductive  concrete. It is chemically inert.
  • Marconite does not require the water to  flow of  the  electricity, as bentonite.  It conducts  the  electricity through  the  movement of  the  elctrons. It’s a pollution  free  green  product  used  by NETC as an innovative  measure in T/L  first  time  in  India for transmission  lines.
  • The soil  pollution  has  been  reduced by  the  application  of  the Marconite. It doesn’t  require  water  at regular  interval – so  there is saving in  the  water  resources.

The various  power  utilities /railways/companies  are  using  Marconite  in their  substations   and  proved  to be very  effective. For earth  pit  doesn’t  require  water  periodically, it is  saving  of  water resources.

Three types of earthing in   T/L

  • Pipe earthing
  • Counter poise earthing.
  • Pipe Type Earthing.

Bentonite treatment is done in counter poise earthing of transmission line of NETC.

While implementing chemical earthing, sodium bentonite was laid at a depth of 900 mm using new galvanized steel stranded wires (10.97mm) with proper compaction by means of ramming of soil after every 400 mm back filling. In each leg approximately 500Kg Sodium bentonite has been used –  length of galvanized steel wire was kept up to 40 metres depending upon soil characteristics.

The Graph showing the improvement of TFR before and after Bentonite Treatment.

(X- axis indicates tower locations & Y-axis indicates Resistance in OHMs)

Bentonite application work…

Marcionite conductive concreate implementation

Marconite earthing  execution: The Marconite  contains  the fixed carbon (98.5%), by product  of coal, very  low  resistivity  of 0.001 ohms, after  adding  with water  &  cement, it  will  be 0.19 ohms. The standard mixture  ratio is 75 kg marconite, 25 kg cement and 25 kg/liter water. Marconite is available in premix form also. It is a very challenging job of earthing maintaince in existing system line, various ROW  can  be  encountered during  excavation  work ,  but requires  no  outage. The Marconite has to comply with the  test  and  follow the  IEC62561-7  standard.

Procedure for Marconite application: Excavation of trench Length: 30 metres. Depth: 90 cms. Width: 30 cms. by manual means, removal of existing counterpoise earth wire, cleaning and rubbing (if any rusting found) of counterpoise earth wire, application of bituminous paint on the surface of counterpoise earth wire. (anti corrosive paint), setting of form box for putting of counterpoise earth wire and mixture, back filling of the trench with excavated soil (without stone mixed soil) and ramming of soil after every 40 cms back filling.

Application of Marconite

Mix Marconite with cement & water so as to form a mortar (Ratio of Marconite: cement is 2:1). Pour the mixture inside the form box say 25 to 50 mm (1-2 inch) thick. Lay the counterpoise earth wire on top of the mixture inside the form box. Apply another layer of the mixture say 25 to 50 mm (1-2 inch) thick. Ensure that the counterpoise wire is fully covered. Wait till the mixture gets set. Carefully remove the form box and backfill the trench with the excavated soil. Rammed the soil after every 40 cm to ensure that the soil is compact.

NETC  successfully  implemented  the  Pilot Project  of  the  Marconite  earthing,  where  the  tower  footing  value   improved. In  locations,  where the TFR  value  did not  improve,  the  value  is  more  than  50  ohms  the  idea of installation of Transmission Line Arrestors (TLA)  will  be  the  better  option. TLAs are installed on the conductors, electrically in parallel to insulator strings. In the event of lightning, instead of back flash over across insulator string, TLA conducts and provides path to the lightning current towards phase conductors. It not only avoids flashover but prevents tripping in the transmission line. It is worth mentioning that application of TLA nullifies any impact of higher tower footing resistance.

Marconite earthing execution work…

Conclusion and lessons learnt

Performance of transmission line depends on the performance of the earthing. It is essential to reduce the tower footing resistance to minimise the tripping of line during fault condition & lightning stroke on transmission line. This article mainly aimed to discuss various measures  to improve the Tower Footing Resistance by using Sodium Bentonite as a Ground Enhancement Material (GIM)   &  Marcionite  in 400 kV in  Transmission  lines for  improving the Tower Footing Resistance (TFR) However,  Marconite  is  better ground  enhancement  material  than  Bentonite.

In lightning prone areas, where reduction of tower footing resistance does not improve performance of transmission line, application of TLA may be a better option to  reduce  the  tripping.



Harshal Radheshaym Malewar
is a graduate in Electrical Engineering from the Govt. College of Engineering Chandrapur, Maharashtra. He also possesses an MBA degree in Power & Operation Management. He has diversified experience of more than 14 years in the Power Transmission & Distribution with various power sectors’ companies – such as RECTPCL, MSETCL, Sunflag Steel & Power, Jyoti Structures Ltd. and North East Transmission Company Ltd.

Leave a Reply