Earth fault compensation
The earth fault compensation , also known as resonance neutral point earthing (RESPE) or as deleted network , is used in electrical power supply networks to compensate for the fault current in the event of unintentional earth faults in an external conductor . Earth fault compensation is limited to AC voltage systems such as three-phase systems as a form of neutral point treatment and, in a modified form, to single-phase three-wire networks , such as occur in traction current networks . It uses a coil which , after its inventor, is also referred to as a Petersen coil or an earth fault extinguishing coil , which compensates for the capacitive earth fault current and thus reduces the fault voltage at the fault location. The process was patented by Waldemar Petersen in 1917 .
The earth fault compensation is typically used in the high voltage area in overhead line networks up to 110 kV . It is an effective method to increase the security of supply with electrical energy.
motivation
A significant proportion of the disturbances in electrical power supply networks can be traced back to an earth fault. Earth faults can be caused by cable damage, a tree growing into an overhead line , wind damage or by faulty insulation of the system.
The grounding of the star point could be done via a low-ohmic resistor (low-ohmic star-point grounding - NOSPE; short-term low-ohmic star-point grounding - BUD), as in TN systems in the low-voltage range. The disadvantage of this method in medium / high voltage networks for large-scale energy supply without redundancy in the form of additional lines is that, in the event of an earth fault, the line can be switched off , similar to the residual current circuit breaker in the low voltage range, and only be put back into operation after the error has been rectified . Since a large number of energy customers may be affected by these failures, the earth fault compensation is used in order to be able to continue operating the earth fault network. This leaves the network operator time to determine the point of failure and to activate it by switching the network without a supply failure.
In the case of single-pole earth faults, the earth fault compensation prevents a high fault current and the need to switch off the affected line immediately.
function
When carrying out the earth fault compensation, the neutral points of one or more power transformers are connected to the earth potential via the earth fault extinguishing coil L E , as shown in the adjacent circuit diagram for transformer 1. In the absence of an error, there is practically no voltage at the earth fault extinguishing coil. In the simplified case, the resistances of the line can be neglected; in real, large-scale systems it may be necessary to set up the earth fault coils in separate locations to minimize the ohmic components.
If there is a low-resistance earth fault, as indicated in the sketch on the outer conductor L 3 , the earth potential shifts to that of the conductor L 3 . The star point is shifted by the star voltage , which means that the triangular voltage to earth, which is a factor larger, occurs on the two remaining outer conductors L 1 and L 2 . This voltage increase must be taken into account when designing the insulation.
In the event of a fault, the earth fault extinguishing coil forms a parallel resonant circuit with the conductor capacitances C E of the transmission line . The inductance of the coil is set so that the reactive current through the coil L E is equal in magnitude to the sum of the two reactive currents through the conductor capacitances C E to the external conductors L 1 and L 2 . The three conductor capacitances C E per outer conductor to earth are almost identical, which is ensured, among other things, by the use of twisting masts along the overhead line.
Since the inductive reactive current is phase shifted by 180 ° (π) compared to the capacitive reactive current, the potential of the neutral point of transformer 1 is raised to the external conductor potential with earth fault, in the example to the potential of the external conductor L 3 when the coil is matched . Since the neutral point of transformer 1 has the same potential as L 3 , the conductor current on L 3 is ideally 0 A, which means that the current at the earth fault is ideally 0 A. If active components are neglected, this is precisely the case when the inductance of the coil L E is matched to the conductor capacitance:
The angular frequency ω is a factor for the network frequency . So that the earth fault extinguishing coil can be adapted to different line capacities - these values are different depending on the switching status and length of the lines - it typically has devices for changing the air gap by means of a plunger core. The mechanical adjustment of the plunger core is carried out by an external motor drive via a shaft.
The voltage conditions on the transformer 2, shown in the circuit diagram in delta connection, remain unchanged even in the event of an earth fault in an external conductor. Likewise, the currents on the two intact outer conductors, in the example on L 1 and L 2 , remain unchanged. The power that is transmitted also remains unchanged: It is compensated for by the excess voltage on the two intact outer conductors, since the outer conductor with earth fault - L 3 in that case - fails for transmission.
In real electrical systems, due to the ohmic active components, both in the cable and due to the low conductance of the insulators, a small residual current flows through the earth fault. Due to the phase position, this active current component cannot be compensated by the earth fault extinguishing coil. The active residual current can be 5% to 10% of the earth fault current and should be as low as possible in order not to prevent the arc from being extinguished automatically at the earth fault point.
Arc failure
Earth fault compensation also serves to avoid arcing faults to earth and to extinguish the arc without interrupting the voltage. The arc fault is particularly dangerous because of its extreme heat generation. It is not characterized by a full electrical short circuit. An appropriately balanced earth fault compensation draws the necessary current from the arc to maintain the ionization of the surrounding air, which means that it goes out automatically. The designation earth-fault extinguishing coil results from this property.
Alternative procedures
At voltage levels with maximum voltage from 220 kV upwards, in some cases even at 110 kV, the Petersen coil can no longer be used because of the residual residual current to earth, consisting of the ohmic active component and the detuning. Furthermore, because of the voltage increase in the healthy conductors in the event of an earth fault, increased insulation is not economical. In the upper network levels, the transformer star point is therefore solidly earthed (SSPE). In the event of a fault, the line protection is switched off within 100 ms. An arc fault, such as an earth fault, then disappears in the de-energized pause of about 400 ms. A Autoreclosure (AWE) provides for reclosing the line after the arcing fault. If there is a metallic short circuit, the line is definitively switched off (unsuccessful AR) and the distance protection relay can automatically determine the position of the fault location on the line.
The security of supply is ensured in the area of the upper voltage levels and in meshed networks through redundancy in the form of the N-1 rule . In simplified terms, this means that electrical equipment such as a transformer or line may fail at any time without overloading other equipment or interrupting the power supply.
Networks
According to Schossig, the joint 16.7 Hz 110 kV traction network of DB and ÖBB , raised slightly to the frequency of 16.7 Hz in 1995 and reunited with the network of the former GDR in the same year , is worldwide in terms of length and area largest extinguished operated high voltage network.
literature
- Réne Flosdorff, Günther Hilgarth: Electrical energy distribution . 8th edition. Teubner, 2003, ISBN 3-519-26424-2 .
- Clemens Obkircher: Expansion limits deleted operated networks . Dissertation, Graz University of Technology, 2008 ( ifea.tugraz.at [PDF; 1,2 MB ]).
- R. Willheim: The earth fault problem in high voltage networks . Published by Julius Springer, Berlin 1936
- Guide to the use of protection systems in electrical networks. VDE-FNN / VEÖ, edition September 2009, vde.de and appendix for Switzerland. VSE / AES. Edition of November 17, 2011 strom.ch ( Memento of February 26, 2015 in the Internet Archive ) (PDF)
Individual evidence
- ↑ Lecture notes on star point earthing, Lecture Electrical Energy Supply II, University of Hanover
- ↑ SfB arbitration board for questions of influence (ed.): Guideline for protective measures on PBXs against influence by networks of electrical power transmission and distribution as well as AC railways . 2005, p. 52-55 ( sfb-emv.de [PDF]).
- ↑ Patent DE304823A with the title "Device for suppressing the connection current of high-voltage networks", patent holder: Allgemeine Elektrizitäts-Gesellschaft (AEG), Berlin, patented in the German Reich from January 24, 1917, patent specification issued on April 12, 1918.
- ↑ The coupling between DB and ÖBB network is made galvanically at 2 points ( Murnau / Kochel am See ( Walchensee power plant ) - Zirl and Traunstein - Steindorf ) , so both together can technically be viewed as one network. The Swiss railway network, on the other hand, has a voltage of 132 kV and is therefore only coupled via transformers.
- ^ Walter Schossig: 40-year interruption ended: 10 years of electrical reunification of Germany . ( Memento of the original from October 8, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF) In: EW , vol. 104, 2005, issue 21–22, pp. 80–83 - "With the connection of the Austrian rail network, the 110 kV network DB / ÖBB provides due to the circuit length of 19 100 km and the In terms of area, it represents the largest, extinguished high-voltage network in the world. "
- ↑ Other sources cite 7900 or 7959 km for Germany and about 2100 km for Austria.
