MV distributed neutral conductor
4-wire systems are characterized by distribution of the MV neutral conductor right to the loads. This type of distribution is used in the USA and in certain countries influenced by North America, and is always subject to ANSI regulations. It is only used in a “directly earthed” neutral plan, and applies a global earthing concept consisting of earthing the neutral conductor at multiple points on the network, approximately every 200 meters.
The neutral-earth voltage is therefore fully controlled.
Distribution of the neutral conductor enables power to be supplied to the loads between the neutral and one phase (to the single voltage). A significant part of the energy is therefore consumed in single-phase.
In a normal operating situation, this single-phase use, whose proliferation is not totally controlled by the distributor, results in the presence of a current in the neutral conductor or the earth. It is generally acknowledged that the load unbalance between the various phases can be as much as 40% of the rated current for a feeder.
The resulting protection system is complex and poorly suited to network reconfiguration, in the event of an incident. This system should also be adapted to each significant modification for a feeder, whether in terms of impedance or topology, which constitutes a major constraint in terms of upgradability.
// for networks with distributed neutral conductor (4-wire)
In these networks, the unbalanced current due to single-phase loads can “mask” an earth fault current. In fact, protection cannot discriminate between the current of a phase-neutral load and the current of a phase-earth fault if they have comparable values.
The value of the phase-earth fault current is linked on the one hand to the expected impedance of the fault itself, and on the other hand to the network impedance between the HV/MV power supply transformer and the location of the fault.
It therefore varies according to the distance from the fault to the substation.
In a network, the higher the impedance of the faults to be eliminated, the smaller the protection zone for each device.
Therefore, in order to have adequate detection of faults on this type of network, where the normal load currents diminish the greater the distance from the substation, a number of protection devices should be placed in cascade (see Figure 1).
When the distributed power on the last segment is low, the protection furthest away from the substation is often in the form of fuses, for reasons of cost.
Nonetheless, in order to serve limited zones from a trunk cable, these networks also include single-phase junctions protected by fuses.
// of networks with distributed neutral (“4-wire”)
Operation of this type of network may be characterized by two major difficulties:
- Electrical risks due to possible high-impedance faults which are difficult to detect easily
- When a loop is required for good continuity of service, it should be of sufficiently low impedance to be in the protection zone.
Recent American publications note the fact that, in more than half of the operations to re-erect conductors which had fallen to the ground, the conductors on the ground were still energized when the technicians arrived. These situations represent high risks for both people and equipment (electrocution or fire).
When the neutral is distributed, two network structures can be distinguished according to whether or not a loopable connection which does not incorporate decentralized protection is present.
Presence of a loopable connection which does not incorporate decentralized protection
If such a loop exists, it is necessarily at low impedance so that it can be entirely in the protection zone of the HV/MV substation devices (see Figure 2). This is typically the case for dense urban geographical areas with underground distribution.
The loop can be used according to the open loop principle in order to benefit from the capacity to return to service associated with this layout, if an incident concerning the cable occurs on the loop itself. From this loop, junctions equipped with protection devices can be created in single-phase or three-phase (see Figure 3).
They may be organized into sub-loops if necessary, to benefit from the same operating mode, but these sub-loops should be entirely in the protection zone of the junction devices.
Due to the limited impedances of the cable segments, and the existence of only two levels of protection to be managed, such a system may be considered to be satisfactory in terms of upgradability. Any geographical extension is nonetheless limited by the need to respect the protection zones.
Presence of connections which are physically capable of being looped, but incorporate decentralized protection devices
When the network is structured around radial feeders, with cascaded protection devices, “emergency” type layouts are not permitted, although the topology itself would allow it.
In fact, a load reconnection, even temporary, which occurs at the end of a tree structure would necessitate redefining the protection thresholds and the discrimination stages of the various devices concerned.
Since these settings are the result of fairly complex calculations, taking into account the lengths and the types of the various segments, there is no chance that incidents could be handled by modifying the settings. Operation is therefore limited to radial mode, and the incident situations may entail long periods without power until they are repaired.
Whatever the network structure, faults which cause the current protection devices to work can be located easily using detectors which react to overcurrents. These fault detectors, placed on the phase conductors, work with both faults between phases and earth faults.
Reference // Cahier technique no. 203 – Basic selection of MV public distribution networks by Schneider Electric