Voltage converter (power engineering)

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Capacitive voltage converter from 150 kV to 100 V in an outdoor switchgear

A voltage converter in the field of electrical power engineering is a measuring converter for measuring AC voltage . The function of a voltage converter is to proportionally transfer the high voltage to be measured to low voltage values . This lower voltage, common values are 100 V, to voltage measuring devices , energy meters transmitted and similar devices, including voltage transformers for measuring purposes are provided, or electrical protection and control facilities, which is voltage converter for protective purposesgives. They are manufactured as inductive and capacitive voltage converters, the latter for primary rated voltages (nominal values) up to over 1  MV .

These voltage converters are used for electrical energy distribution in substations and power stations .

Executions

Inductive voltage converters

Inductive voltage converters are basically constructed like transformers . They consist of a primary winding, which is electrically connected to the voltage to be measured, and a secondary winding, which is galvanically separated , but for safety reasons leads to the connected devices at one end. For the metrological application, voltage transformers have some special design features in order to maintain small deviations in the transmission ratio and small error angles in the phase shift between primary voltage and secondary voltage. The in-phase transmission is important in order to be able to correctly record the active power and reactive power together with current transformers . The secondary side can be terminated with a burden , the value of which depends on the power of the converter and ranges from a few 10  Ω to a few 10 kΩ.

Voltage transformers are designed on the primary side for measurements either between two live conductors (outer conductors) or between a conductor and earth . In the first case, the primary winding is completely isolated from earth. For the voltage measurement on a single outer conductor to earth, one end of the primary winding is earthed and the converter has only one high-voltage connection. There are single-phase versions and three-phase versions for the three-phase alternating current network .

In both versions, the secondary winding is earthed on one side for safety reasons and because of capacitive leakage currents. Depending on the specific model, further secondary windings are provided, which are used, for example, to monitor against earth faults in the three-phase network. For this purpose, these auxiliary windings of the three transformers for the three outer conductors are connected in a triangle so that the voltages add up to zero if there are no faults.

Capacitive voltage converters

Connection of a capacitive voltage converter with a downstream inductive converter

Capacitive voltage converters are used in the range of high and extra high voltages above 100 kV. The reason is that the insulation resistance at high voltages can only be ensured with great effort in the transformer or at high costs. Therefore, as shown in the adjacent circuit, a capacitive voltage divider is provided on the high-voltage side. The high-voltage- resistant capacitor C 1 shown in the circuit is accommodated in the interior of the insulating body and connected to the voltage U p to be measured . This capacitor can also consist of a series connection of several individual capacitors in order to increase the dielectric strength of the entire arrangement. C 2 can also be accommodated outside of the converter body.

At the connection point of the capacitors, a small, more processable voltage U t is available

However, the divided voltage contains the overvoltage pulses that occur in the network. For this reason and also because of possible insulation faults, in addition to an optional transformation (see picture), surge protection is provided beforehand for network applications. The coil L 1 is used to protect the transformer and to decouple the TFH signals. In addition to measuring purposes, capacitive voltage converters are also used to determine whether there is no voltage. Conversely, such simple, cost-saving capacitive converters can be retrofitted for voltage measurement.

More converters

In addition to exclusive voltage and current transformers, there are also combined versions which combine voltage and current transformers in one housing and allow a compact system structure.

Identification for the company

Rated voltage

For each selectable primary rated voltage, the transformation ratio of the voltage transformer is fixed so that the secondary rated voltage assumes a uniform value. This depends on common practice and is often 100 or 110 V in Europe. For single-phase converters for use between conductor and earth in three-phase systems, values ​​that are smaller by a factor are also provided.

Rated power

The load capacity of the voltage transformers is given by the rated power. Values ​​between 1 and 10  VA are standardized for ohmic loads and between 10 and 100 VA for inductive loads with a power factor of 0.8. With the switchgear's own power supply, it can be up to several 100 VA. The rated power, up to which the accuracy requirements of the converter are guaranteed, must be distinguished from the thermal rated limit power, up to which it can be loaded without being damaged. In contrast to current transformers, the secondary connections must never be short-circuited.

Accuracy requirements

The voltage measurement deviation is defined as a relative quantity by

with = rated translation according to the specification, = actual primary voltage, actual = secondary voltage when applied under measurement conditions.

The standard provides accuracy classes for information on error limits . In the case of a single-phase inductive converter for measuring purposes, these are classes 0.1 - 0.2 - 0.5 - 1.0 and 3.0. For example, in class 0.5, a voltage measurement deviation of up to 0.5% and an error angle of up to 20 'are permissible, namely for each voltage between 80 and 120% of the rated voltage. In addition, the burden must be in a specified range.

Classes 3P and 6P are specified for a converter for protection purposes. For example, in class 3P, a voltage measurement deviation of up to 3% and an error angle of up to 120 'are permitted, namely from 5 to 120% of the rated voltage, depending on the network and grounding conditions up to 190%. At 2% of the rated voltage, the permissible limit values ​​are twice as high. In addition, these converters must be assigned to an accuracy class for measurement purposes.

literature

  • Andreas Küchler: High voltage technology: Basics - Technology - Applications . 2nd Edition. Springer, 2004, ISBN 978-3-540-21411-3 .

Individual evidence

  1. ↑ a b DIN EN 61869-1: 2010-04 (also VDE 0414-9-1) measuring transducers - Part 1: General requirements (IEC 61869-1: 2007, modified)
  2. ↑ a b DIN EN 61869-3: 2012-05 (also VDE 0414-9-3) Instrument transformers - Part 3: Additional requirements for inductive voltage transformers (IEC 61869-3: 2011)
    There was a transition period for the previous standard DIN EN 60044-2 until Aug. 2014.
  3. ↑ PS-E-15 - Provisional Specifications for Approval of Electronic Voltage Transformers. Retrieved June 17, 2013 .
  4. ↑ a b http://ritz-international.com/wp-content/uploads/2015/11/RITZ-Mittelspannungswandler_Standard_GER_2014_01.pdf page 10
  5. ↑ http://www.home.hs-karlsruhe.de/~lagu0001/lehre_exponate_messwandler_allgemeines_konven2.htm
  6. ↑ https://www.energy.siemens.com/hq/pool/hq/automation/automation-control-pg/sppa-e3000/Switchgear_Control_Protection_Upgrade/mittelspannungsschaltanlagen-kapazitiver-teiler.pdf page 2
  7. ↑ DIN EN 60038: 2012-04 (also VDE 0175-1) CENELEC standard voltages (IEC 60038: 2009, modified)

Web links

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