Instrumentation, Controls & Electrical
The capacitive divider in
medium-voltage switchgear
SPPA-E3000 Electrical Solutions
Voltage measurement using a capacitive
divider in medium-voltage switchgear
is a cost-saving alternative for medium-voltage
switchgear panels, e. g. if only information
about the applied operating voltage is required,
or if the installation of classical voltage
transformers is impossible.
In power distribution, current and voltage
values are among the most important measured
values. Above a certain level, the voltage can
no longer be measured directly, but must first
be reduced to a lower value using suitable
devices – converters. In medium-voltage
switchgear systems, this is mostly done using
classical voltage converters based on inductive
transformers.
In addition to this, capacitive voltage dividers
are installed in all panels of a switchgear
system for simple voltage checking.
For some applications, only information about
the applied voltage is required, with no need for
connecting further measurement or protective
equipment. For such applications, equipping
the switchgear panel with conventional voltage
converters (transformers) is very expensive.
Instead, the voltage measurement function can
also be achieved using a plug-in measurement
top-unit. This is attached to the capacitive
divider in the switchgear panel.
Advantages at a Glance
■ Simpler and quicker assembly and
commissioning of the switchgear
system due to simplification of the
switchgear panel
■ Exclusion of the risk of incorrect
connections through elimination
of the voltage transformers
Voltage display unit
■ Avoidance of errors due to the
simplified system engineering
■ Cost savings thanks to an efficient
display unit instead of a transformer
Measurement top unit
■ No special designs are required
in the switchgear system
■ Simple and cost-saving retrofitting,
e. g. during switchgear modernization
■ Increased functionality of the mediumvoltage switchgear system
■ Possibility of cost reductions and space
savings due to the elimination of the
metering panel
Measurement and display system
for capacitive voltage measurement
The voltage display unit which is integrated
into the switchgear panel and the plug-in
measurement top unit can be combined
into a measurement and display system.
For such situations, voltage measurement
using capacitive voltage dividers offers
an efficient and cost-reducing alternative,
and is ideally suited for selective deployment
in medium-voltage switchgear systems.
siemens.com/sppa
Voltage measurement using inductive voltage transformers
In medium-voltage switchgear technology, inductive voltage
converters are used for the purpose of measuring the alternating
voltage which is present across the equipment. Using a special
transformer, the high voltage which is to be measured is converted
proportionally to a lower value, which can then be measured and
further processed.
Voltage measurement using capacitive voltage dividers
For high voltages (over 100 kV), the design of inductive voltage
transformers is very expensive, and consequently not economically
sensible. For the purpose of measuring these high voltages, use is
made of capacitive voltage converters based on capacitive voltage
dividers.
Voltage measurement using a classical voltage transformer (above)
and an optional configuration using a capacitive divider (below)
These consist of capacitors connected in series. The voltage across
individual capacitors is divided in inverse proportion to the ratios
of their capacitances. The voltage is measured across one of the
capacitors. If the capacitances of the capacitors in the divider are
known, it is possible to calculate the applied voltage from the
measured voltage.
Application areas for capacitive voltage dividers
in medium-voltage switchgear systems
The relevant standards (e. g. IEC) require providing tests
for verifying the voltage-free conditions on the equipment
to guarantee the personal safety during operation and work
on the medium-voltage switchgear. This requirement can be
fulfilled, e. g., by using a voltage test system which is built
into the switchgear. Such testers are based on the principle
of a capacitive divider with a visual indication “voltage present”/
“no voltage present” and have now become established
as standard equipment.
However, the technology used offers much more than merely
a continual visual “yes”/“no” indication. If a suitable device
is connected onto the measurement capacitance of the divider,
the value of the voltage can also be continuously displayed.
If this cost-effective device is used directly at the interface
of the voltage display unit, already present in the switchgear
panel, the full available functionality of the voltage divider can
be obtained. In this way, one creates a type of voltage converter
which is also used in high-voltage switchgear. The essential
advantages here are the simplification of the primary part
of the switchgear system and the associated cost savings.
If further devices are connected, such as protective relays,
energy meters or measuring transducers, a voltage amplifier
is also installed. This method can be used in situations where
the installation of classical voltage transformers is either
impossible or is not economic.
If this capacitive interface is installed in the busbar compartment, it is possible if required to omit the metering panel.
Another area for the application of this type of voltage measurement is offered in the case of retrofits (functional enhancement)
to existing switchgear systems, or during modernization
of switchgear systems. It is then no longer necessary to make
design modifications to the primary part of the switchgear.
Published by and Copyright © 2014:
Siemens AG
Power and Gas Division
Freyeslebenstraße 1
D-91058 Erlangen
Siemens Energy, Inc.
Instrumentation, Controls, & Electrical
1345 Ridgeland Parkway, Suite 116
Alpharetta, GA 30004, USA
For more information contact
sppa-e3000.energy@siemens.com
www.siemens.com/energy/sppa-e3000
The method of voltage measurement using capacitive dividers represents
a complete alternative to voltage transformers.
E303_FP01_MedVSwg_e_V1-0
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the property of Siemens AG, its affiliates, or
their respective owners.
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.