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2.1.1 HVDC earth return system
The generic term of a group of devices expressly designed and built to operate with earth or seawater as the return circuit in HVDC power transmission system.It mainly includes earth electrode line,earth electrode,current guiding system and auxiliary facilities.
2.1.2 Earth electrode
An earthing device that can continuously transfer direct current for DC system,consisting of a number of earthing conductors and active fillers.Land embedded earth electrodes are known as land earth electrodes and those immersed in sea or embedded in beach are known as sea or beach earth electrodes.
2.1.3 Earth electrode site
Sites where the earth electrode is located.
2.1.4 Earth electrode line
The overhead line or underground cable connecting the neutral bus in the converter station and the current guiding system of the earth electrode.
2.1.5 Separated earth electrode
Earth electrode consisting of two or more earth electrodes which are connected in parallel and connected to the neutral point of a converter station via the earth electrode line.
2.1.6 Common earth electrode
A single earth electrode or separated earth electrode that is shared by two or more converter stations.
2.1.7 Compact earth electrode
Earth electrode that improves current distribution features and occupies less land by external technical measures.
2.1.8 Vertical earth electrode
Earth electrode whose sub-electrodes are arranged vertically to the ground.
2.1.9 Feeding rod(component)
Earthing conductors embedded in the active fillers of an earth electrode.
2.1.10 Guiding line
Main branch connecting the bus and the feeding cable,which may be overhead line or underground cable.
2.1.11 Feeding cable
The generic term of cables that connect guiding lines and feeding rods,including current guiding cables(or guiding rod) and distributing cables.
2.1.12 Current guiding system
Devices that guide the current from the earth electrode line to the feeding rod,composed of bus,guiding line,gantry structure,disconnector,feeding cable and fittings.
2.1.13 Jumper cable
Interconnecting cables between feeding rods.
2.1.14 Rated current under monopolar mode
Working current at rated power under monopolar earth return operation mode.
2.1.15 Maximum overload current
The maximum overload current at which the converter valve can continuously operate under monopolar earth return mode.
2.1.16 Maximum transient current
The maximum average current that flows through the earth electrode in several seconds during a system disturbance.
2.1.17 Unbalance current
Current difference between two DC poles in bipolar DC system operation.Under balanced bipolar operation mode,the difference in firing angles and equipment parameters may also lead to unbalance current,the amplitude of which may be automatically controlled within 1% of the rated current by the control system.Under unbalanced bipolar operation mode,the current flowing through the earth electrode is the difference between the current of two poles.
2.1.18 Composited current
In common earth electrode,current that flows through earth electrode when monopolar earth return operation occurs at two or more converter stations at the same time.
2.1.19 Cathode
The polarity of the earth electrode when current flows from the earth to the earth electrode.
2.1.20 Anode
The polarity of the earth electrode when current flows from the earth electrode to the earth.
2.1.21 Current releasing density
Current released into the earth from a per unit length of the feeding rod.
2.1.22 Design life
The designed service life of an earth electrode,generally being the same as that of converter station.
2.1.23 Corrosion life
The integration of product of current and time when the earth electrode serves as anode.
2.1.24 Thermal time constant
The time required for the temperature of the earth electrode rising to the steady state temperature at the initial rate of rise of temperature under continuous rated current.
2.1.25 Continuous time under rated current
The maximum allowable operating duration under rated current.
2.1.26 Earthing resistance
The resistance of the earth electrode to the infinity of ground.
2.1.27 Step potential difference
The voltage between a person s two feet on the ground(1m apart)under the DC earth electrode operation.
2.1.28 Touch potential difference
The potential difference between a point where the horizontal distance is 1m to a conductive metal object on the ground and a point where the vertical distance is 1.8m from the energized object to the ground,under the DC earth electrode operation.
2.1.29 Transfer potential
The potential difference applied to a person when he touches an earthing conductor originating from far away while standing on the ground near the earth electrode,or when he touches an earthing conductor originating from a point nearby the electrode while standing on the ground far away,under the DC earth electrode operation.
2.1.30 Electro-osmosis
The effect that the DC field drives the water molecules in the soil nearby the earth electrode away from the anode.
2.1.31 Deviation coefficient
Physical quantity used to measure the balance degree of current releasing density of an earth electrode.
2.1.32 Equivalent earthing current
The ratio of total ampere-hour (A·h)when the earth electrode serves as cathode or anode to design life.It is used to analyze the corrosion of earth electrode to the nearby buried metal.
2.2.1 System parameters
I d ——earthing current of earth electrode;
I N ——rated current;
I eq ——equivalent earthing current of earth electrode;
I L ——current flowing through earth electrode line;
T 0 ——continuous time under rated current.
2.2.2 Parameters of soil and material
ρ s ——equivalent resistivity of surface soil;
ρ m ——equivalent resistivity of soil in electrode burial layer;
λ m ——equivalent thermal conductivity of soil in electrode burial layer;
ρ P ——soil resistivity at point P;
C P ——soil thermal capacity at point P;
θ c ——maximum natural temperature of soil;
ρ ——resistivity of coke;
C ——thermal capacity of coke;
g ——material density of feeding rod.
2.2.3 Calculation coefficient
k ——coordination coefficient for soil temperature rise calculation;
k 1 ——protection coefficient,the ratio of the ion current per unit area of coke to the total current;
k 2 ——electrocorrosion cumulative effect coefficient;
k er ——current releasing density deviation coefficient;
η ——coordination coefficient of length and interval of vertical sub-electrodes.
2.2.4 Geometric parameters
S p ——side length of the coke section at point P;
Φ p ——equivalent diameter of the feeding rod at point P;
Φ ——residual equivalent diameter of the feeding rod upon expiry of the design life of earth electrode;
D 1 ——interval of vertical sub-electrodes;
L ——total length of electrode;
L 0 ——length of vertical sub-electrode;
h ——minimum burial depth of earth electrode;
d ——minimum distance between electrode and underground metallic components;
D ——horizontal distance between pole conductors of earth electrode line;
f c ——maximum sag of conductor;
A ——incremental constant of distance between iced conductors.
2.2.5 Characteristic parameters
V ——potential of earth electrode;
ρ eq ——equivalent ground resistivity of electrode site;
R e ——earthing resistance of earth electrode;
T ——thermal time constant of earth electrode;
θ pm ——design maximum allowable temperature of electrode;
τ——current releasing density of earth electrode;
τ P ——current releasing density at point P ;
U pm ——maximum allowable step potential difference on the ground;
F ——service life of anode;
V f ——electrocorrosion rate of feeding rod in soil;
U p ——voltage at point P on earth electrode line;
R o ——resistance per unit length of earth electrode line;
L P ——distance of point P from the earth electrode.