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DL/T 437-2012: Technical guide of HVDC earth electrode system
DL/T 437-2012
DL
ELECTRIC POWER INDUSTRY STANDARD
OF THE PEOPLE?€?S REPUBLIC OF CHINA
ICS 29.240
F 21
Record number: 35213-2012
Replacing DL/T 437-1991
Technical guide of HVDC earth electrode system
ISSUED ON: JANUARY 04, 2012
IMPLEMENTED ON: MARCH 01, 2012
Issued by: National Energy Administration
Table of Contents
Foreword ... 3??
1 Scope ... 4??
2 Normative references ... 4??
3 Terms and definitions ... 4??
4 Technical conditions ... 7??
5 Test ... 12??
6 Assessment and protection of the impact on surrounding facilities ... 16??
7 Operation and maintenance of DC earth electrode ... 16??
Appendix A (Informative) Unequal-distance quadrupole method for measuring
the resistivity of deep earth ... 19??
Appendix B (Normative) Thermal conductivity, heat capacity rate and coke
requirements ... 20??
Appendix C (Informative) Maximum allowable step potential on the ground .. 21??
Technical guide of HVDC earth electrode system
1 Scope
This Standard specifies the terminology terms and definitions of HVDC earth
electrode system, and proposes technical conditions, test items and methods,
and general technical principles of operation and maintenance.
This Standard applies to the earth electrode system at both ends of the
monopolar and bipolar HVDC transmission system; it does not apply to the
converter grounding grid.
2 Normative references
The following documents are indispensable for the application of this document.
For dated references, only the dated version applies to this document. For
undated references, the latest edition (including all amendments) applies to this
document.
GB/T 17949.1, Guide for measuring earth resistivity, ground impedance and
earth surface potentials of a ground system - Part 1: Normal measurements
GB/T 13498, Terminology for high-voltage direct current (HVDC)
transmission
DL/T 5224, Technical Rule for the Design of HVDC Earth Return Operation
System
DL/T 475, Guide for measurement of grounding connection parameters
3 Terms and definitions
Except for the terms which are specified in this Chapter, the rest shall be in
compliance with the relevant provisions of national and industry standards.
3.1
HVDC earth electrode system
The general term for a group of devices that are specially designed and
constructed to operate with earth or sea water as a current loop during normal
operation or failure, in the high-voltage direct current transmission system. It is
mainly composed of an electrode line, an earth electrode feeder line and an
earth electrode.
When the earth electrode is in operation, if a person stands on the ground near
the earth electrode and touches an earth conductor that is connected from a
remote place, or if a person stands on the distant ground and touches an earth
conductor that is drawn from a place near the electrode site grounding, the
touch potential it bears is the transfer potential. The maximum value of the
transfer potential is the earthing electrode potential rise.
4 Technical conditions
4.1 General technical guidelines
4.1.1 The design of the DC earth electrode shall consider the three working
conditions of rated current under monopolar mode, maximum overload current
and maximum transient overcurrents.
4.1.2 The design life of the DC earth electrode shall generally be no less than
30 years under the specified operation mode.
4.1.3 The DC earth electrode is generally composed of 2 or more separated
components.
4.1.4 In order to prevent the earth current of the HVDC earth electrode system
from corroding and interfering with the converter station, the straight-line
distance between the earth electrode and the converter station in the HVDC
transmission system should not be less than 10 km; it shall be ensured that the
grounding grid of the converter station is completely separated from the earth
electrode.
4.1.5 The DC earth electrode generally has ring, star, linear, ray, grid shapes,
which should be confirmed according to the conditions of the electrode site
topography, geology, hydrology, traffic conditions, from the two aspects of
convenient construction and reasonable technology and economy.
4.1.6 The buried depth of the DC earth electrode shall be determined, according
to the requirements for step potential in this Standard, by comprehensive
techno-economic comparison in combination with the soil climate
characteristics of the electrode site, the engineering excavation and the
external force factors; it is generally not less than 1.5 m.
4.1.7 The earth electrode design shall consider the changes of the groundwater
level; water injection devices shall be installed when necessary.
4.2 Electrode site selection
4.2.1 The DC earth electrode site should generally be far away from densely
populated cities and towns, and areas with more public facilities underground.
4.2.2 Geological and hydrological surveys must be carried out within 20 km of
the pre-selected electrode site. The content of the survey at least includes:
a) Geological structure and thickness of each layer. The depth from the
ground to the bedrock, the thickness of the bedrock.
b) Seawater erosion conditions, detailed geological map of pre-selected
electrode sites with contours (land electrodes) or isobaths (ocean
electrodes).
c) Surveys shall be conducted when the survey data is incomplete.
4.2.3 Before designing the DC earth electrode, it is necessary to evaluate the
influence of the earth electrode on the surrounding environment. Therefore, it
is necessary to investigate the existing and planned transmission lines and
important facilities around the electrode site. For the assessment of
environmental impact, refer to the relevant standards and regulations for
preventing corrosion of metal structures.
4.2.4 The selection of the DC earth electrode site shall consider the impact on
the surrounding environment. In principle, there should be no underground
metal pipelines, railways or effective grounding transmission and
transformation facilities within 10 km of the pre-selected electrode site.
4.2.5 The electrode site selection of the DC earth electrode should be
determined by the technical and economic comparison of no less than 3
different schemes.
4.3 Determination of earth parameters of the electrode site
4.3.1 Groundwater level
The groundwater level of the electrode site can be obtained through
hydrogeological maps or on-site detection.
4.3.2 Earth resistivity
4.3.2.1 The earth resistivity of the electrode site is generally measured by
injecting current on the spot.
4.3.2.2 The earth injection current that is used to measure the earth resistivity
on site shall be a direct current.
4.3.2.3 The test method can be any traditional earth resistivity test method, such
as Wenner quadrupole method, Schlumberger-Palmer method, or the unequal-
distance quadrupole method (see Appendix A) .
5 Test
5.1 General principles
5.1.1 The purpose of testing the HVDC earth electrode system is:
a) The various parameters of the earth electrode shall meet the design
standards.
b) Whether the step potential, touch potential and transfer potential, which
are caused by the HVDC earth electrode system under the maximum
operating voltage, meet the requirements of the guidelines.
c) Understand the interference and influence of the HVDC earth electrode
system on the public utility system (such as water supply, electricity, gas,
fishery administration) in the surrounding area; measure, if necessary, so
as to check whether it meets the design requirements.
5.1.2 The earth electrode shall complete the acceptance test before system
debugging.
5.1.3 The time sequence of the test items specified in this Standard can be
carried out at the same time or in crossover if there is no special description.
5.1.4 For the safety of test personnel, instruments and equipment, the earthing
current in the test shall be in several grades from small to large.
5.1.5 When repeating the same test item under different earthing currents, the
same instrument shall be used in the same position and direction.
5.1.6 All test items should be performed once at 70% ~ 80% and 100% rated
current under monopolar mode.
5.1.7 During the entire test process, measures must be taken to protect the
safety of the testers and the people and animals moving in the test area
according to the characteristics of the earth electrode site and its nearby electric
field.
5.2 Appearance inspection
5.2.1 The HVDC earth electrode system shall be visually inspected before the
formal power-on test, so as to confirm that the ground part of the HVDC earth
electrode system and its connection parts with the electrode line are intact, the
assembly and installation are correct, and the size meets the design
requirements, before the test.
5.2.2 Remove the electrode site and its nearby items that affect normal
operation and have nothing to do with the test; repair the natural damage (such
as washing or sinking) of the soil on the electrode surface before the test.
5.2.3 Check the detection device and the water seepage hole to prevent
blockage.
5.2.4 Check the safety signs and protective barriers to confirm that they are
intact and clearly marked.
5.3 Earthing current
5.3.1 After the earthing electrode is energized, the total current into the ground
shall be measured. This work shall be carried out during the whole process of
system debugging, so as to provide the most basic parameters for other various
test tasks.
5.3.2 In order to determine whether the current distribution of each segment of
component of the earthing electrode is balanced, the current distribution of each
segment of the feed component shall be measured. The measurement should
be carried out at the beginning of the test. The balance of the current distribution
of the earthing electrode components shall meet the design requirements.
5.3.3 Various DC current measuring instruments and meters, such as DC
transformers, DC clamp ammeters, and DC shunts, shall be used to measure
the earthing current. The accuracy of the DC current measuring instrument is
required to be grade 0.5 ~ 1.
5.4 Earthing resistance
5.4.1 The earthing resistance value of the DC earth electrode shall meet the
design requirements.
5.4.2 For the earthing resistance test of the DC earth electrode, adopt the
current injection method, that is, the ammeter-voltmeter method; do not adopt
the portable earthing resistance tester.
5.4.3 When measuring the DC earthing resistance, the injected earth current
shall be a DC current; no alternating current shall be used. This kind of DC
current can be provided by a DC power supply for separate test; the unbalance
current flowing through the earthing electrode during system operation, or the
earthing current during operation (or test) of the monopolar earth loop can also
be used.
5.4.4 When using the DC power supply for testing, the minimum distance
between the auxiliary current electrode and the earthing electrode shall be
greater than 10 times the maximum distance between any two points of the
5.7.5 Suitable temperature measuring instruments should be used to measure
the temperature in the ground.
6 Assessment and protection of the impact on
surrounding facilities
6.1 When the selected electrode site cannot avoid underground metal pipes,
underground cables, railways, and electrical equipment grounding devices and
other underground metal components within 10 km, the extent of the corrosion
and other adverse effects of the earthing electrode current on these
components shall be evaluated. The soil potential (electrode potential relative
to the copper-copper sulfate) of buried metals that have not been corroded or
disturbed is generally between -0.85 V ~ -1.50 V. This value can be used as a
control standard for the protected buried metals to the soil potential. If the
evaluation result shows that the earth potential rise that is caused by the
earthing electrode current and its corrosion to metal components will affect the
safe operation of these facilities, relocation shall be considered, or appropriate
protective measures shall be taken.
6.2 The earth potential rise, which is caused by the DC earth electrode current,
makes the neutral point of the AC power transformer that is effectively grounded
around it to have a DC current; its value is related to the location of the
transformer and its electrical parameters, system wiring, grid wiring. Analog
computation shall be carried out when selecting the electrode site; field
measurement shall be carried out after the earthing electrode is put into
operation. When the DC current flowing through the neutral point of the
transformer exceeds the allowable value, appropriate limiting measures can be
adopted.
6.3 The allowable DC current value of AC transformers (including power supply
transformers for electrified railways and locomotive traction transformers) is
related to their design, material, structure and manufacturing process. The
manufacturer should provide relevant technical requirements. If the
manufacturer cannot provide technical requirements, the allowable DC current
of each phase winding of the transformer is tentatively set as: 0.3% of the rated
current under monopolar mode for the single-phase transformer, 0.5% of the
rated current under monopolar mode for the three-phase five-leg transformer,
and 0.7% of the rated current under monopolar mode for the three-phase three-
leg transformer.
7 Operation and maintenance of DC earth electrode
7.1 Appearance inspection
After the land earthing electrode is put into operation, the following items shall
be inspected and disposed regularly. The cycle is once every 2 months in the
initial stage of operation, and once every 6 months after 1 year.
7.1.1 Subsidence of the backfill. If there is too much subsidence, continue
backfilling, to ensure the height of the earth electrode components from the
ground. However, th...