Star connection

General star
connection : Each connection is connected to the star point via a resistor.

A star connection is the interconnection of any number of connections via a resistor each to a common point, which is referred to as the star point.

Star connection in three-phase systems

Star connection
for three-phase circuit on a three-phase 230 V network with 120 ° phase shift

In the star connection, the connections of the three strings of a three-phase operating medium (e.g. three-phase motor , three-phase generator or three-phase transformer ) are connected to one another at their ends in the form of a star. The resulting union forms the center point, which is also called the star point or neutral point . This point is also the connection point for the neutral conductor . The beginnings of the windings then form the connections for the outer conductors (L1, L2 and L3) of a three-phase system . In the case of symmetrical loads , the currents of the individual strings add up to zero in the star point, so in some cases the connection of the star point and neutral conductor can be dispensed with.

With symmetrical loading (i.e. the three strings U, V, W have the same impedance ), the string voltage (also: star voltage) is applied across the windings U, V and W. This is lower than the line voltage by the concatenation factor . Based on the external conductor voltage of 400 volts that is common in Europe, the phase voltage between one of the external conductors (L1, L2 or L3) and the neutral conductor (N) is 230 volts.

The interlinking factor indicates the ratio of phase-to-phase voltage / line-to-line voltage (400 V) to phase voltage / star voltage (230 V). With three outer conductors, it corresponds to the square root of 3, rounded 1.732. For example, a star voltage of 230 V results in the linked voltage of:

{\ displaystyle \ mathrm {230 \, V} \ cdot {\ sqrt {3}} \ approx \ mathrm {400 \, V}}

Using suitable transformers it is possible to convert a four-wire star connection system into a three-wire triangle system and vice versa.

Individual phase strings of this circuit are used in households as the well-known 230 V connection (in Germany and Austria Schuko , in Switzerland SEV 1011 ) for so-called partial consumers.

The three phase strings are used jointly with electric motors ( three-phase motor ) and electric heating systems. Here the respective ends of the three phase strands are designated as follows:

u1 - u2
v1 - v2
w1 - w2

To z. B. to operate an electric motor in star connection, the outer conductors L1, L2 and L3 are connected to the string ends u1, v1, and w1 as follows:

L1 to u1
L2 to v1
L3 to w1

The remaining ends of the phase strands u2, v2 and w2 are connected to one another and form the star point.

The energy supply companies strive for an even load on the three phase strings. Since the three phase strings are unevenly loaded in practice, an equalizing current, which depends on the degree of asymmetry, flows in the neutral conductor.

Neutral point treatment

A differentiated neutral point treatment takes place in low and medium voltage networks. In this case, generally between the low impedance neutral grounding , the current limiting ground , the isolated neutral point , the resonant-ground and the active neutral earthing distinguished.

Low-resistance neutral point earthing (NOSPE)

The n ieder o hmige S tern p oint s ARTHING is intended to ensure in case of error, that a current flows that is sufficiently high, the protection devices of the grid protection to be responsive. At the same time, however, the current should not exceed a certain limit value in order to exclude damage to equipment or system parts. How the fault current is evaluated and which actions are triggered by the protective devices (immediate OFF, automatic restart ) ultimately depends on the existing network.

Short-term low-resistance star point earthing (BUD)

The k urzzeitig n ieder o hmige S tern p oint e works in principle as the resonant-ground (RESPE), see below ARTHING. In the event of a fault and only for its duration, the neutral point of the feeding transformer is briefly earthed with low resistance (NOSPE).

Current-limiting grounding

The star point of the three-wire system is connected via a resistance to the zero potential. There are the same advantages as with the NOSPE, namely that the fault is switched off quickly and selectively. Another advantage is that the fault currents do not reach the size of the fault currents in the NOSPE and so the circuit breakers that are necessary to carry out the short interruption can be dimensioned for lower fault currents.

A disadvantage here, however, is that in the case of extensive networks, the fault current could possibly be too small to be recognized as such by the protection.

Isolated star point

This type of earthing is mostly used in medium-voltage networks with small expansion, for example on the power plant generator or in the power plant's own requirements . Symmetrical loads and symmetrical generation are to be expected there, so that no impermissible neutral point displacement voltages ( neutral point displacement ) occur in normal operation . Since a possible fault current is determined by the earth capacities of the following lines or cables, their extension must not exceed a few kilometers so that the permissible fault current of 100 A cannot be exceeded.

One advantage of networks with an isolated star point is that immediate shutdown is not necessary in the event of a fault. If z. If, for example, a single-pole earth fault occurs, the earth fault current is so small that it does not have to be switched off. In the event of this fault, however, the voltage to earth of the other two phases is increased by a factor , which leads to greater stress on the insulation. This is why isolated star points are not possible in extra- high voltage networks. ${\ displaystyle {\ sqrt {3}}}$

Resonance neutral point earthing (RESPE)

With resonance star point earthing, the star point is connected to earth via a so-called Petersen choke . The size of the inductance is matched to the stray capacitances of the lines with respect to earth, so that a resonance occurs at just over 50 Hz in the event of an earth fault, i.e. the resistance of the Petersen choke and earth capacitances are almost the same. The principle is based on the fact that the predominantly capacitive residual current with a phase shift of + 90 ° is compensated for by a corresponding inductive current, caused by the Petersen choke, with an ideal phase shift of −90 °.

Usually, however, a detuning of the resonant circuit of 10-20% is provided so that in the event of uncontrolled shutdowns of long overhead lines, e.g. in the event of a fault, the resonance frequency is not exactly 50 Hz, but rather slightly higher. If the resonance frequency were exactly 50 Hz, there could be excessive resonance increases in the voltage in the star point and there could be a risk to operation. Another advantage of the slight overcompensation is that induced currents of parallel three-phase systems (e.g. two systems with different voltage levels on an overhead line pylon) are opposed by a defined resistance unequal to 0 and this cannot become impermissibly high.

The advantage of this method is that an uninterrupted supply of the consumers and subordinate network structures is possible. However, there is a voltage increase in the intact phases by the factor and in cable networks, high short-circuit currents are to be expected due to the high capacities to earth - this also means that the equipment will heat up considerably. The resonance star point earthing can no longer be used in high voltage networks. In addition to the capacitive earth current, an active current component flows via the conductance values of the lines. This is not compensated by the Petersen throttle. An active neutral point grounding is necessary, which impresses an active current phase shifted by 180 °, so that it complements the active residual current to zero. ${\ displaystyle {\ sqrt {3}}}$

Active neutral point earthing

In the case of active neutral grounding, a power electronic measuring device and a static converter are used to inject a component opposite to the active residual current into the zero system of the network. In this way, the residual current can be almost completely compensated, so that a self-extinguishing effect occurs on the arc due to the low remaining current. This neutral point treatment is still not fully ready for the market and is mostly only used in test systems or industrial networks. With this concept, however, it is also possible to continue to operate extra-high voltage networks without interruption in the event of a fault and thus to guarantee a permanent supply of the consumers without the use of short interruptions.

Conversion to polygon circuit

A star connection with n terminals can be converted into an equivalent polygon connection with resistors using the star-polygon transformation. The conversion applies to the conductance of the resistors ${\ displaystyle {\ tfrac {n \, (n-1)} {2}}}$

{\ displaystyle G_ {kr} = {\ frac {G_ {k0} \, G_ {r0}} {G_ {s}}}}

With

{\ displaystyle G_ {s} = \ sum \ limits _ {l = 1} ^ {n} G_ {l0}}

,

whereby . ${\ displaystyle k, r, l \ in \ left [1, n \ right] \ subset \ mathbb {N}}$

literature

Adolf J. Schwab: Electrical energy systems - generation, transport, transmission and distribution of electrical energy. Springer Verlag 2017, ISBN 978-3-662-55315-2 .