Jun 20, 2019

Neutral Point Treatment | The Solidly Grounded Network


ELCOME dear friends of protection and control engineering. In the previous post we looked at electrical networks with isolated star point grounding, today we look at the solidly grounded network. Have fun and let's go!

The solidly grounded network

We always speak of a solidly grounded network if the star point of one or more generators, transformers or grounding transformers is effectively grounded and the ground connection is carried out with virtually no resistance. The word "solidly" here means that the earthing resistance is almost zero. Strictly speaking, the solidly grounded network is thus a special form of the effectively grounded or activ network, which also includes the low-impedance neutral grounding and is characterized by an grounding factor of less than 1.4.

The solidly grounded network

Once again to remember

The grounding factor is the rise ratio of the values ​​of the phase-to-ground voltages of the healthy phases in the ground fault case to the phase-to-ground voltage in faultless condition. In the isolated network, we had shown that the voltage of the healthy phase is raised stationary to 1.73 times the value before an earth fault occurs.

Grounding factor of the isolated network

We defined the magical limit of the grounding factor at 1.4 and said: If the grounding factor is greater than 1.4 we are talking about a network that is not grounded activ. In our solidly earthed network and in all effectively grounded networks, on the other hand, a grounding factor of 1.4 is not exceeded at any point.

Networks and grounding factor

Now when a single-pole ground fault occurs, we are talking about a short circuit, it is not the voltage but the current that literally goes through the roof. Due to the conductive connection between star point and ground, the circuit can effectively close.

The solidly grounded network with ground short circuit

The amount of the incoming short-circuit current thus depends decisively on the positive and zero impedance of the network. This, in turn, is formed by all the components lying in the short circuit, such as generators, transformers, lines and also by the impedance of the star point, which is virtually zero in our present solidly grounded system.

The short circuit in the component network

Another decisive factor for the level of the expected short-circuit current is the resistance at the fault location, which we also traditionally refer to as:

Fault Resistance.

Our single-pole earth short-circuit current is thus calculated as follows:

Formula for single-pole ground short circuits in solidly grounded networks

The positive-impedance is doubled in the sum, since the positive and negative impedance are the same size and Z2 was simplified substituted.

Other important features

Solidly grounded systems have the great advantage that transient oscillations are greatly reduced and the ground fault can be switched off quickly and also automatically. This results in a lower insulation stress than in compensated or isolated networks. Above all, the high demands on voltage isolation in the area of ​​the maximum voltage mean that solidly grounding is the first choice of our transmission system operators and that the 220 kV and 380 kV networks are preferably grounded solidly.

Special features of the solidly grounded system

However, since the short-circuit currents in the solidly grounded network can amount to a few 1000 amperes, higher contact voltages also occur then in networks with isolated neutral point or with ground fault compensation. On the other hand, the very fast turn-off times, which effectively limit the risk of consequential health damage due to longer-flowing ground short-circuit currents, are advantageous.

Of course, since every ground short circuit leads to a shutdown of the affected component, always a supply interruption is the logical consequence. Due to the short-circuit current, there is an additional voltage drop in the medium and low-voltage network, which continues even in fault-free outlets until the faulty outgoing feeder is switched off by the protective device.

In the sense of our bipolar world, there are advantages and disadvantages as always.
Special features of the solidly grounded system

In our next post, we will stick to the effectively earthed networks and look in detail at the peculiarities of low-impedance grounded systems.

Next time: Low Impedance Grounding

Best regards