Neutral grounding
Studies on the selection and optimization of transformer neutral grounding in
electrical networks
At a glance
During the normal operation of an electrical
network, the method of neutral treatment is not
decisive. In the case of a ground fault, however,
it is of crucial importance. More than 80 per
cent of all faults in electrical networks start as
ground faults. Siemens Power Technologies
International (Siemens PTI), your provider of
network consulting, network planning software
and training on the Siemens T&D portfolio,
offers explicit solutions for the review and optimization of neutral grounding, which is an essential task in sustainable network planning.
Due to the topological, demographical and
political conditions and historical developments
different types of neutral grounding were established for public transmission and distribution
networks. Also for industrial networks different
methods are applied.
In many regions and networks the preferred
method of neutral treatment has remained unchanged for decades. Due to increasing demands on the networks and changes to the
network structures in many cases the selected
method is no longer the optimum solution.
The Siemens PTI portfolio includes:
„ Measurements in the network for analyzing
the present situation, data collection and verification
„ Evaluation and optimization of the present
method of neutral treatment
„ Selection and sizing of neutral grounding
equipment
„ Analysis of concepts for fault localization and
protection, protection coordination
„ Recommendations for future operation
„ Detailed analysis of documented faults
The challenge
The selection of the optimum neutral grounding
method for an individual network strongly
Power Technologies International (Siemens PTI)
Network Consulting
Answers for infrastructure and cities.
depends on its size and structure, shares of
cables and overhead lines, and also on quality
requirements regarding the energy supplied.
A technical and economical optimization of the
neutral treatment is particularly required in the
following situations:
„ Erection of a new (partial) network, e. g. in
the industrial sector
„ Enlargement of a network due to expansion
projects or integration of neighboring networks
„ Replacement of overhead lines by cables or
other measures which increase the line-toground capacities
„ Increasing level of network automation
„ High investment costs for keeping the present type of neutral grounding
„ Unsatisfactory level of supply reliability due
to the number of outages, poor ground fault
detection or localization
Our solution
In close cooperation with the client the optimum
method of neutral grounding is developed for an
individual network. The following describes a
typical roadmap for optimizing the neutral
grounding of a distribution network:
„ Technical comparison of the pros and cons
of the different types of neutral grounding
„ Measurements in the network including zerosequence impedances, reduction factors, inductive coupling, ground potential rise and
touch voltages as well as ground fault tests
„ Review of the neutral grounding equipment
and dimensioning of new equipment
„ Calculation of the ground fault currents
„ Analysis of the existing protection concept
and requirements for new devices
„ Evaluation of selectivity in case of
ground faults (primary and backup
protection)
2,0
1,8
1,6
R0
„ Selection of devices for ground fault
localization
„ Evaluation of the grounding systems
of substations and ring main units
„ Assessment of inductive coupling on
telecommunication lines
Impedance in Ohm
1,4
1,2
1,0
0,8
X0
0,6
It was found that the installed Petersen
coil controller was not capable of dealing
with the present conditions and thus an
automatic control process was not initiated. Therefore, the installation of an
appropriate new Petersen coil controller
was recommended.
0,4
Resonance curve
0,2
0,0
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
Measurement current in A
„ Compilation and comparison of equipment costs for the considered variants
„ Proposals for network conversion
actions
Application example 1
In this project, the network operator had
to decide whether to invest in the adaptation of resonant grounding (RNG) or in
the installation of low-impedance neutral
grounding (LNG). The assessed 20 kV
partial networks mainly were of urban
character with only a small share of
overhead lines (figure 1).
Figure 2: Measured zero-sequence impedance
of a selected cable type
After a detailed assessment of both
variants it was recommended to convert
one partial network immediately and to
temporarily operate two partial networks
with short-term LNG. These were to be
converted in medium term after adequate measures behead been taken
(figure 3).
Petersen coil +
grounding transformer
UW-West
RNG
3 resistances, coil+
grounding transformer
UW-West
short term
LNG
earthing of pole
substations with
overhead lines
LNG
3 resistances
earthing of pole
substations with
overhead lines
Figure 3: Recommendations for optimization
Figure 1: Present status of the three partial
networks (schematic view)
First, the zero-sequence impedances of
the decisive cable types were measured
(figure 2) and the inductive coupling on
telecommunication lines and the grounding conditions at typical and critical ring
main units were determined.
Secondly, appropriate equipment for
neutral grounding was selected and
network calculations were performed for
both options.
Published by and copyright © 2012:
Siemens AG
Infrastructure & Cities Sector
Smart Grid Division, Services
IC SG SE PTI BD
Freyeslebenstr. 1
91058 Erlangen, Deutschland
www.siemens.com/power-technologies
www.siemens.com/smartgrid
Application example 2
Figure 4: Resonance curve analysis
Application example 3
For a planned offshore wind farm the
suitable method for neutral grounding for
the 33kV level had to be determined.
Both the damping of ground fault currents and the proposed protection concept had to be considered. It was recommended to operate the 33 kV level
with low-impedance neutral grounding
and limit the single-phase short-circuit
current to max. 500 A. For this concept
location and parameters of the required
grounding transformers were determined. Further, guidelines for normal
operation and emergency operation were
developed. Figure 5 shows the recommended approach.
In this project the effectiveness of the
Petersen coil controller for optimum
setting of arc suppression coils had to be
investigated for an industrial network
operated with resonant grounding.
At the beginning the phase-to-ground
voltages and the neutral-to-ground voltage were measured. Then the resonance curve of the network was recorded (figure 4). Finally, the control
process was verified by partial shutdown
of cable sections in the network.
Siemens Industry, Inc.
Siemens Power Technologies International
400 State Street
P.O. Box 1058
Schenectady, NY 12301-1058 USA
132 kV
33 kV
Figure 5: Recommended concept.
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Subject to change without prior notice.
The information in this document contains general
descriptions of the technical options available,
which may not apply in all cases.
The required technical options should therefore be
specified in the contract.