Complex networks

Selectivity applied to ring and meshed networks

Classical radial distribution networks have a big drawback: the path of energy is linear from the source to the load, meaning that in the event of a fault on the equipment along the path, the rest of the line from that point onwards is de-energized (see Fig. 1).

To overcome this problem, more than one feed point must be supplied to the load.
Technical solutions include ring networks and meshed networks. With these solutions, which are quite often used on ships and smart grids in low voltage, the critical loads are still energized after a fault on the electrical bus. The main drawback of this solution is its complexity, which makes it difficult to quickly find and safely exclude the faulty zone. In fact, if compared with classical radial networks, the power can flow from different directions to feed the fault, thus involving more devices (see Fig. 2).

Selectivity in such a complex situation can be guaranteed only by using special techniques, one of which is directional zone selectivity (hereinafter SdZ D).

Implementing the SdZ D function is simple. No special external devices are required: it is available on Ekip Hi-Touch and Ekip G Hi-Touch trip units, and can only be set to “on” when zone selectivity for S and G protections is set to off and an auxiliary power supply is available.

How does it work?

To define the zone and power flow, each relay has two input contacts (DFin and DBin: i.e. Directional Forward in and Directional Backward in) and two output contacts (DFout and DBout: i.e. Directional Forward out and Directional Backward out), which must be suitably connected to the other relays. Each output can send a “lock” signal. The breaker that receives the signal will open within the backward or forward time prescribed; the breaker that does not receive a lock signal will open within a set time t7s. Thus, the relays behave in two different ways depending on the direction of the power flowing across them and whether they are receiving a lock signal or not.

With reference to the image on the right:

if a fault is detected in one of the sections of the system (Load A), the end circuit-breakers of the section concerned (relay 1 and relay 2) communicate the presence of a fault to the connected circuit-breakers (relay 3 and relay 4) by setting the output signals DFout or DBout, depending on the direction of the power with respect to a preset direction (in our case, DFout of relay 1 and DBout of relay 2 are on).

The circuit-breaker relay 1 and relay 2, on determining the section affected by the fault, trip after the selectivity time t7s set has passed, while the circuit-breakers further away from the fault count down the delay time set, t7FW (relay 4) and t7BW (relay 3), before opening. In this way, the system is isolated within the time t7s, and only the part affected by the fault is excluded.

For detailed information, please read the Technical Application Paper No.1 (Low voltage selectivity with ABB circuit-breakers) and the White Paper No.2 (Directional protection and directional zone selectivity). You can find both documents in the Technical documentation page, Tools & Support section.

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