Types of faults in three-phase systems

from Wikipedia, the free encyclopedia

The types of faults in an electrical power grid depend on the network structure and the treatment of the star points of the transformers feeding this power grid .

Network construction

An electrical network in a supply system is a galvanically connected structure of lines with a certain nominal voltage . A network is separated from other voltage levels using transformers, and networks with the same nominal voltage are separated using couplings . For the selection of the network protection and its settings, a distinction is essentially made between three types of network connection.

The radiation network is simple and clear. The lines run radially away from the feed point (e.g. generator ). There are clear real power and reactive power distributions . The selectivity is achieved by means of protective relays through simple current and time grading . On the other hand, the supply reliability is low, since in the event of a fault, all the systems and consumers behind this fault are disturbed. The voltage constancy is low because there is no load flow equalization. Double-beam networks improve the reliability of supply through a second parallel line between the stations under consideration. The expenditure on protection technology must be increased due to the additional difference in direction of the parallel lines.

The ring network increases the reliability of supply considerably. Since there is load balancing in the network, the voltage constancy improves. Sufficient selectivity is achieved through overcurrent protection.

The mesh network is a network structure made up of several intersecting lines that are connected to each other at the point of intersection. There they are protected by fuses. The crossing points are called nodes , the closed system between the nodes is called a mesh and each line section is called a mesh . Mesh networks are preferably used in low-voltage networks. They offer ideal security of supply. However, the financial outlay involved in setting up a mesh network is considerable. There are difficulties in rebuilding the supply system after a network breakdown. The network is protected with special mesh network relays for the feeds and with fuses for the mesh lines.

As meshed nets referred to historically grown networks that can be neither a radial network nor as ring network classify and still do not form a mesh network. These networks offer maximum security of supply. The voltage constancy is optimal, because by transverse and longitudinal regulation of the transformers the desired active and reactive power flow can be achieved with minimal network losses. Selectivity can only be achieved with distance protection relays . The disadvantages of these networks are the relatively expensive network protection and the high short-circuit power , which increases with the degree of meshing.

Neutral point treatment

Only certain types of errors require immediate shutdown of the faulty part of the system or the network. These types of errors include short circuits and short circuit-type errors. The neutral point treatment of the transformers supplying the network ( operational earthing) is decisive for the types of error . The type of neutral point treatment is specified for the protection technology. All protection systems are to be selected accordingly and adjusted accordingly.

  • With rigid neutral point earthing, all transformer neutral points are directly earthed. No resistance is connected between the star point and the earthing system. This is common, for example, in low-voltage networks such as the TT system and the TN systems .
  • With the semi-rigid star point earthing, at least one star point is directly earthed. However, not all other star points are earthed.
  • The low-ohmic star point grounding is the connection of a fixed resistor between the star point and earth . No star point may be directly earthed in this network. With this neutral point treatment, a resistance or a reactance determines the earth fault current. This neutral point treatment is called NOSPE and is usually used at the highest voltage level in the 220 kV or 400 kV transport network.
  • Without star point grounding, there is an isolated network in which all star points remain ungrounded, even if there is a high-resistance grounding via the star points of the voltage transformers. In the low-voltage area, referred to as IT network , in the case of a small area (industrial networks) also in the medium-voltage area.
  • The resonance star point grounding or deleted network is present when one or more star points are grounded via inductance ground-fault chokes, the so-called Petersen coil. The size of the earth fault reactors depends on the capacitive earth fault current of the network. The neutral point treatment of this compensated network is known as RESPE and is used in medium-voltage networks and at the 110 kV distribution network level.

Types of errors

  • Single pole short circuit (earth fault)

Short circuit between a conductor and earth with effective star point earthing. The short-circuit current flows back from the conductor via the arc resistor and via earth to the star point. With this resistance ratio in the fault circuit, about 75% of the voltage to earth drops due to the relatively high resistance and high step voltages and contact voltages arise at the fault location .

  • Two-pole short circuit

Short circuit between two conductors in a system. A short-circuit current flows in both affected conductors. The voltage at the point of failure is practically zero, but builds up towards the feed point. The impedance at the short-circuit point is also the smallest and increases in the direction of the feed point. The voltage and impedance of the fault loop are not linear, as they are dependent on the resistances at the short-circuit point (e.g. arc resistance). Furthermore, when the short circuit occurs, electrical power is drawn from the feed point , which is why the short circuit current is not a constant value either. The short circuit angle , which represents the phase shift between current and voltage at the fault location or in the short circuit, is always inductive. This means that the current lags behind the voltage.

  • Three-pole short circuit

Short circuit between three conductors in a system. This fault represents the greatest stress on networks and switchgear. For the most part, three-pole short circuits are initiated by a single-pole earth fault or earth fault , which expands to a two-pole or three-pole short circuit due to an arc migration and an increase in voltage in the healthy conductors. Multipole short circuits with earth contact can also occur. These processes are asymmetrical because not all conductors are affected equally. In the case of a three-pole short-circuit without earth contact, currents of practically the same size flow in all three conductors. These are driven by the star voltages of the feed point. The short-circuit angle is also inductive here, the current lags behind the voltage.

  • Two-pole short circuit with earth contact

Short circuit between two conductors and earth contact of one of these conductors in networks with ineffective neutral point treatment. The effects in terms of electricity correspond roughly to those of a two-pole short circuit without earth contact.

  • Earth fault

Conductive connection of a conductor with earth in networks with ineffective neutral point treatment. Ground faults endanger the electrical engineering systems to a high degree because in the inductively grounded network voltage increases occur in the conductors not affected by the ground fault, which often expand to two or three-pole short circuits. In addition, overvoltages can occur in the network due to the increase in voltage. Therefore, in the event of an earth fault, it is not only the current that is the cause of damage that is of interest, but above all the natural frequency of the network and its division into inductances and capacitances. In the compensated network, there is a ground-fault reactor between the transformer star point and earth. This has the task of compensating the capacitive current of the line in the event of a ground fault by means of an adjustable inductive current. A residual earth fault current remains, which is caused by the effective resistances in the circuit. Furthermore, the residual current to earth is dependent on the setting of the earth fault reactor, i.e. on the overcompensation or undercompensation of the network. Ground faults in the inductively grounded network can continue to operate for a maximum of two hours, depending on the capacity of the ground fault reactor. The detection of earth faults takes place via the earth fault relay .

  • Double earth fault

Conductive connection of two conductors (e.g. L1 & L2 & E) via earth impedance. This means the earth fault of two different conductors at different locations in networks with ineffective neutral point earthing. A short-circuit current flows depending on the impedance in the affected conductors and the earth impedance. The voltage drop between the conductors depends on the earth impedance between the fault locations.

  • Body connection (ground connection)

Conductive connection of a point in the winding of a rotating electrical machine to earth (housing). A ground fault current flows which is limited by the type of protective measure used so that an iron fire does not occur in the current path. There is an increase in the conductor-earth voltage in the unaffected conductors.

  • Rotor earth fault

Conductive connection of a point in the rotor winding of a rotating electrical machine to earth. This error has no effect at first, but carries the risk of expansion to the rotor double earth fault.

  • Rotor double earth fault

Connection of two different points of the rotor winding to earth. A part of the rotor winding is bridged. This leads to an asymmetrical weakening of the generator excitation and thus to the dropping out or destruction of the generator.

  • Interturn short-circuit

Bridging turns within a winding. As a result, the phase voltages shift as well as the conductor-conductor voltages. This results in an unbalanced load. With generators, an inverse rotating field is created in the rotor.

  • Winding short

Conductive connection between two different windings of an electrical machine. A partial short-circuit current occurs as well as a voltage drop in the affected conductors.

  • Broken conductor

Unintentional interruption of a conductor in the three-phase system. This error results in an interruption in the power supply and a voltage failure of the conductor concerned. The system goes into unbalanced load.

  • overload

A current flow in the equipment that is above the nominal value is called overload. Inadmissible heating and an increased voltage drop occur. Long-term overloading of equipment can lead to premature aging of the insulation .

  • Turbine return

Operation of a generator as a motor after failure of the prime mover (e.g. turbine ) without a network fault. The effect is the occurrence of damage to the drive machine (impermissible heating).

  • Reverse power

Feed back from a low-voltage network (mesh network) into a higher-level network. The infeed transformer is overloaded.

  • Operating frequency overvoltage

Significant increase in the operating voltage above the nominal voltage due to errors in the control system or load shedding. This error occurs within seconds and has no effect if the insulation is properly coordinated. This overvoltage also includes voltage increases as a result of line capacities in unloaded high-voltage lines, Ferranti effect . These can exceed the maximum operating voltage and seriously endanger the insulation. As countermeasures, charging current chokes as additional inductance or so-called driving circuits (simultaneous switching off of both circuit breakers on a line) are possible.

  • Switching and earth fault overvoltages

Increasing the operating voltage in the event of intermittent earth faults or when switching large inductances. This error occurs in the millisecond range and, depending on the network situation, leads to vibration phenomena in the network. These can especially endanger voltage converters . One therefore speaks of internal overvoltages.

  • Atmospheric surges

The effects of thunderstorms (lightning overvoltages) cause traveling waves with a steep rise in voltage and large amplitudes . These events occur in the microsecond range. The traveling waves emanating from the place of origin cause insulation breakthroughs. This is called external overvoltages.

Asymmetry in the three-phase system, which causes thermal overload and vibrations in rotating electrical machines.

Impermissible frequency drop due to network overload or insufficient generator use.

  • Oil fault

Deterioration in the quality of the insulating oil as a result of water or air inclusions or an unintentional lowering of the oil level in the corresponding equipment (e.g. oil transformer).

  • Swings

Oscillations are only partially viewed as an independent error. Rather, they are the result of short circuits or large load changes in certain network situations. Oscillation is a disturbance in the stability of the network. They occur in networks fed by multiple sources with relatively little meshing. Coupling lines between power plants are particularly affected. Synchronous oscillations are attributed to load fluctuations. Asynchronous oscillations are caused by short circuits, which equates to the fact that the power plants supplying the power plant cannot step out of the way. The protection technology must therefore distinguish whether short circuits turn into oscillations or oscillations into short circuits. In the first case the protection should be temporarily blocked, in the second case it must be tripped.

literature

  • G. Müller: Electrical machines . VEB Verlag Technik, Berlin.
  • A. Verduhn and W. Nell: Handbook of electrical engineering . Fachbuchverlag, Leipzig.
  • H. Koettnitz, G. Winkler and K. Weßnigk: Fundamentals of electrical operating processes in electrical energy systems. VEB German publishing house for basic industry, Leipzig.