hvdc ground electrode

8/9/2019 HVDC Ground Electrode

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M.M. Babu Narayanan

[email protected]

GROUND CURRENT PROBLEMS

IN HVDC TRANSMISSION

Workshop on Power system Grounding Practices, August 2012, PRDC, Bangalore

Advantages of Ground return mode

HVDC Transmission system can be built i n two s tages

Operate first stage in Monopolar metallic return

(overhead or cable) or ground return in some cases

Second stage as Bipo lar with full power rating

Cost advantage

In case of outage of one pole of line or converter, DC

link can be operated at half power in Ground return



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Disadvantages

Design o f Ground electrode with low resistance and

high cost of O&M

Proper siting of ground electrodes

Screening of Ground electrode to cause negligib le

electrolytic corrosion of buried metallic structures

Step & tough voltages within the vicinity of electrodes

Interference with the operation o f other services


HVDC Transmission: Modes of operation



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HVDC Transmission: Modes of operation


Basic Ground electrode Site Selection

Parameters

Electrical Resistivi ty(Type of soil/ grain size/ chemical composition/ temperature)

Thermal Conductivi ty & Heat Capaciti vity

Ambient Temperature & Maximum operating temperature

Moisture



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Layering of Earth &

corresponding

generalized values of

Resistivities


Rated Current( Id )

(Allowable) Voltage Rise:

Volts

= Thermal Conductivity of soil (W/ m3 )

= Temperature rise of electrode, 0C

= Electrical Resistivity, Ohm-m

Re = Ve/ Id

Time Constant of soil

Rating of Ground electrode depends on:

2Ve



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Potential Gradient:

Volts/metre22

hx

xIE

Step Potential in a Land Electrode


Continuous ring electrode

Typical Ground Electrode installations



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Electrode section for shallow Horizontal Configuration


Star Electrode



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LOWER SILERU CONVERTER STATION

CASE STUDY:

NATIONAL HVDC PROJECT GROUND

ELECTRODE DESIGN

RATING: 100 MW, 100kV; BETWEEN LOWER SILERU & BARSOOR

MONOPOLAR GROUND RETURN

COMMISSIONED: 1989

OBJECTIVES:

EMPHASIS ON R&D

DEVELOP KNOW-HOW FOR DESIGN & CONSTRUCTION

OF HARDWARE

INDIGENOUS DEVELOPMENT OF CONTROL SYSTEM

EXPERTISE IN INSTALLATION , COMMISSIONING &

OPERATION

CONTINUOUS GROUND CURRENT OPERATION

NATIONAL HVDC PROJECT



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Barsoor Ground Electrode :Data

Data Unit Value

Surface Resistivity -m 85

Shallow level resistivity -m 50

Apparent resistivity -m 125

Thermal conductivity of soil W/m 0C 1.28

Heat capacitivity of soil J/m3 0C 3.5 x 106

Maximum natural soiltemperature

0C 35

Maximum electrode

temperature

0C < 100

Design criteria Anode or cathode

Courtesy: BHEL


Results

Data Unit Value

Permissible resistance 0.115

Design resistance 0.086

Tolerable ground potential V/m 7.55

Maximum ground potential

rise

V/m 7.41

Time taken to reachsteady state

Days 59.5

Current Density at Coke

surface

A/m2 0.424



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Case study:

Leakage currents in Talcher-Kolar HVDC

Mono polar operation

Talcher Kolar HVDC Transmission

Rating 2000 MW

Voltage/ Current 500 kV, 2000 A

Overload current 2250 A

Maximum step potential at

the surface at Id = 2250 A

6 V/m

Resistance to ground < 0.3

Touch voltage < 40 V

Ref: Cigre 2006


Presence of DC in the neutrals of transformers due to ground

potential rise

Accompanied by humming sound

Ground potential rise attributed to geological conditions away

from the Ground electrode at Kolar

DC neutral currents were proportional to the electrode current

Chinthamani 220 kV & Kolar 400 kV AC sub stations of KPTCL

identified as the most affected (being closest to HVDC electrode)

DC power in ground return mode restricted to 150 MW (300 A)

due to this phenomenon

Leakage currents in Kolar - Observations



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Phenomenon of leakage currents

DC flow in electrode causes a g round potential rise (GPR) at theHVDC earth electrode as well as in other sub stations

GPR at remote sub stations depends on geolog ical cond itions

and the distance of the sub stations wi th respect to HVDC

ground electrode

Difference in Ground potential between AC sub stations causes

DC current flow thru the neutrals of transformers in these sub

stations

Magnitude of leakage currents depend upon vo ltage between

grounding systems of two sub stations and zero sequence

resistance of AC lines connecting the sub stations

For higher magni tudes of DC, saturation of transformers in AC

sub stations nearby


DC Currents due to potential rise in remote sub stations



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Installation of Blocking device (BD) at Transformer Neutral

BD specifically designed to block flow of DC while allowingAC

Installed in series with power transformer neutral

A custom designed capacitor (0.8 Ohm at 60 Hz)

Operates whenever absolute voltage across device below a

threshold voltage blocking level

Blocking voltage decided based on field stud ies of DC

distri bution (GPR) at nearby sub s tations

A High cu rrent by pass path p rovided for so lid grounding ofneutral during AC side faults

No impact on protection settings of AC system


Schematic diagram of Blocking deviceWorkshop on Power system Grounding Practices, August 2012, PRDC, Bangalore


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Operational experience with Blocking Devices at

Kolar - end AC sub stations

Maximum value depends on maximum steady s tate DC

ground electrode current, location of AC sub stations & ac

system configuration

Abso lu te maximum limit fixed at 400 V (corresponds to a DC

electrode current of 2100 A (1050 MW)

Measured maximum b locking voltage at Chinthamani S/S:

257 V ( at 1350 A DC)

Ach ieved a DC power o f 650 MW (1300 A) wi th Blocking

device in Monopolar operation (Without BD: 150 MW at 200 A)


Electrode current during testing of Blocking Device



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hvdc ground electrode

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