Operational grounding

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The operational earthing is the earthing of a point in the operating circuit. All grounding measures that are absolutely necessary for the operation of an electrical power system are referred to as operational grounding. The primary task of operational earthing is therefore to ensure the trouble-free operation of a system or device.

Designation and tasks of the operational earthing

In electrical engineering , operational earthing is referred to as either direct or indirect , depending on the design . Operational earthing is referred to as immediate if, apart from the earthing impedance , there are no other resistances . If the operational earthing is established via additional inductive, capacitive or ohmic resistors, it is referred to as indirect earthing.

The operational earthing should achieve the following:

  • Quiet potential in a circuit
  • Preventing interference from external systems or devices
  • Compliance with limit values ​​in the event of radio interference

Execution of operational earthing

The operational earthing is carried out in such a way that it meets the operational requirements and withstands any equalizing currents. It is connected to the potential equalization of the earthing system and is separated from the lightning protection earthing. In electrical energy systems, the neutral points of transformers are usually earthed via the operational earth (with the exception of IT networks ) . In switchgear, the top-side star points of voltage converters , but also compensation capacitors, compensation coils and earth fault extinguishing coils are connected to the operational earth. In power plants , the generator protection devices are also connected to the operational earth. The earthing resistance of all connected plant earths in the distribution network must be sufficiently low; a value of two ohms is generally considered sufficient.

In many cases, combined operating and protective earths (BSE) are also installed. If the earth voltage exceeds the value of 50 volts alternating voltage or 120 volts direct voltage , combined operating and protective earths are no longer possible.

Neutral point grounding

In the three-phase networks of the energy supply company (EVU), it is predominantly the star points that are earthed for operational earthing. With regard to the design, a distinction is made between five variants of the star point earthing:

  • Resonance star point grounding
  • Low-resistance neutral point earthing (NOSPE)
  • Rigid neutral point earthing
  • Central star point earthing
  • Partly rigid star point earthing
Resonance star point grounding

The resonance neutral point earthing is used in distribution networks in the medium-voltage range , mainly in overhead line networks . The star point is earthed via a Petersen coil. This form of indirect star point earthing is a special case of earthing.

Star point earthing:
a) rigid
b) low resistance
c) inductive

The low-resistance star point earthing is used in extensive networks with nominal voltages of up to 1 kV and also in systems with higher nominal voltages. In systems with nominal voltages of up to 1 kV, this grounding variant is mainly used for power generating sets with unsolicited winding in parallel operation. The star point is grounded via a choke coil or an ohmic resistor. This measure serves to reduce equalizing currents. Short-term low-ohmic neutral point earthing is used to locate permanent earth faults in deleted networks, in which only one neutral point is earthed via a resistor. The star point is earthed for a short period of time via a switching device. Even in distribution networks with a high proportion of cables, predominantly in the medium-voltage range, only one star point is earthed via a fixed resistor. In the event of a fault, this resistance limits the earth fault current to a defined value. The level of the fault current depends on the impedance at the fault location and on the ohmic resistance of the neutral point resistor. In the event of an earth fault near the transformer, the maximum earth fault current can flow.

The rigid star point earthing is mainly used in high-voltage networks and in the low-voltage range in TT networks and TN networks . All star points are directly earthed, i.e. without resistance. With rigid star point earthing, a single-pole earth fault automatically causes a single-pole short circuit. The resulting short-circuit current is only limited by the impedance at the fault location.

Central star point earthing

The central star point earthing is mainly used in TN networks with protection through disconnection with overcurrent devices ( zeroing ), this in turn mainly in buildings in which a high proportion of information technology systems are present. With this earthing variant, only one star point per galvanically connected network is directly earthed.

Partly rigid star point earthing

With the partially rigid star point grounding , not all star points are grounded directly, only a part. This grounding variant is preferred for use in high-voltage networks.

The earthing system

With operational earths, the earth current diverted into the earth is greater than with protective earths . The earthing systems are therefore designed so that the main voltage drop occurs in the ground. For this reason, the operating earths are dimensioned correspondingly large . Particularly in switchgear with very high short-circuit currents , high ground voltages and difficult grounding conditions often arise. The protective earthing and the operational earthing are constructed spatially separated from each other if the earthing voltage exceeds certain values. In this case, the earthing systems must also be connected via insulated cables.

In transformer stations it is common for the operational grounding for the low-voltage range and the operational grounding for the high-voltage range to be spatially combined. This means that the zero systems are coupled with one another. Due to the two-sided earthing of the star points, there is a mutual influence on the networks in the event of earth faults. If a ground fault occurs on the high-voltage side, a considerable voltage drop of several hundred volts occurs on the ground due to the ground resistance. However, for safety reasons, these voltages to earth must not exceed certain values ​​if the operational earths are installed spatially connected. For ground voltages that are greater than 50 volts alternating voltage or 120 volts direct voltage, the switchgear must be switched off in less than 5 seconds. If this is not technically possible or if the earth voltage exceeds the value of 600 volts alternating voltage or 1200 volts direct voltage, the operating earths for high voltage and low voltage must and will be installed spatially separate from one another. According to DIN VDE 0141, the distance must be at least 20 meters for high-voltage systems with rated voltages below 50 kV.

Statutory provisions and other regulations

  • DIN VDE 0100 part 540 "Earthing, protective conductor, equipotential bonding conductor"
  • DIN VDE 0141 "Earthing for special high-voltage systems with rated voltages above 1 kV"
  • DIN VDE 0101 "Power systems with nominal alternating voltages above 1 kV"

literature

  • Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply. 7th edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden, 2007, ISBN 978-3-8348-0217-0

Individual evidence

  1. ^ Friedhelm Noack: Introduction to electrical energy technology. Carl Hanser Verlag, Munich As 2003, ISBN 3-446-21527-1 .
  2. ^ Georg Flegel, Karl Birnstiel, Wolfgang Nerreter: Electrical engineering for mechanical engineering and mechatronics. Carl Hanser Verlag, Munich 2009, ISBN 978-3-446-41906-3 .
  3. ^ Gerhard Kiefer: VDE 0100 and the practice. 1st edition, VDE-Verlag GmbH, Berlin and Offenbach, 1984, ISBN 3-8007-1359-4 .
  4. Deutsche Telekom instruction sheets for earthing in telecommunications systems online ( Memento from February 27, 2012 in the Internet Archive ) (PDF file; 2.18 MB).
  5. Réne Flosdorff, Günther Hilgarth: Electrical Distribution. 4th edition, Verlag BG Teubner, 1982, ISBN 3-519-36411-5 .
  6. Österreichischer Verband für Elektrotechnik, Österreichisches Normungsinstitut (Ed.): Construction of electrical systems with nominal voltages of up to 1000 V ~ and 1500 V -. Part 1: Terms and protection against electric shock (protective measures). (ÖVE / ÖNORM E 8001-1).
  7. a b Amir M. Miri: Balancing processes in electrical energy systems, mathematical introduction, electromagnetic and electrochemical processes. Springer Verlag Berlin-Heidelberg-New York, Berlin 2000, ISBN 3-540-67735-6 .
  8. Gino Else: On permanent or temporary, low resistance. Neutral grounding in medium voltage networks. Online ( Memento of July 21, 2013 in the Internet Archive ) (last accessed on January 14, 2013; PDF file; 1.20 MB).
  9. TÜV Süddeutschland: Protection against electromagnetic interference through low-interference neutral point earthing. (last accessed on January 14, 2013; PDF file; 352 kB).
  10. ^ A b Gregor Häberle, Heinz Häberle, Armin Schonard: Protection by VDE 0100. 12th edition, Verlag Europa-Lehrmittel, Nourney Vollmer GmbH, Haan Gruiten, 2007, ISBN 3-8085-3006-5 .
  11. Hartmut Kiank, Wolfgang Fruth: Planning Guide for Energy Distribution Systems . Siemens Aktiengesellschaft Berlin and Munich (ed.), Publicis KommunikationAgentur GmbH, Erlangen 2011, ISBN 978-3-89578-359-3 .

See also