Menu
Search
Distribution of the MV neutral conductor right to the loads
Distribution of the MV neutral conductor right to the loads (on photo: Premset MV secondary distribution switchgear; credit: Schneider Electric)

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.

Due to the direct earthing, the current for a directly earthed fault is mainly limited by the impedance of the network segment between the HV/MV transformer and the location of the fault. This situation calls for the use of “decentralized” protection, capable of managing increasingly low thresholds as the distance increases, and nonetheless capable of being coordinated.

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.

North-American overhead MV distribution layout (each phase shown)
North-American overhead MV distribution layout (each phase shown)


Protection system

// 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.

For rather long lines, a phase-earth fault a long way away can cause a lower current than the unbalanced current permitted at the substation feeder. In this case, a protection device placed in the substation will not be capable of detecting this fault. An additional protection device with lower thresholds is then necessary to extend the part of the network which is actually monitored, known as the “protection zone”.

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).

Example of a North American distribution network
Figure 1 – Example of a North American distribution network comprising a number of protection devices placed in cascade: Note how the protection zones overlap

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.

As far as the underground parts (using cables) of networks are concerned, they generally cover a smaller area than an overhead network and have lower impedance, therefore the value of a phase-earth fault current is only slightly affected by the distance from the fault to the substation.

Nonetheless, in order to serve limited zones from a trunk cable, these networks also include single-phase junctions protected by fuses.


Operation

// of networks with distributed neutral (“4-wire”)

Operation of this type of network may be characterized by two major difficulties:

  1. Electrical risks due to possible high-impedance faults which are difficult to detect easily
  2. 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.

Protection of a power supply loop against earth faults
Figure 2 – For complete protection of a power supply loop against earth faults, this loop should be entirely contained in the protection zone for the HV/MV substation devices

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.

HV/MV substation devices which protect the main loop
Figure 3 – These are the HV/MV substation devices which protect the main loop, and both ends of each junction organized as a sub-loop incorporate protection devices to ensure the safety of the various possible configurations.

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.

For similar reasons, any upgrading of the topology or the network load level involves checking the compatibility of the protection devices in place. Modifying this type of network is therefore a very delicate and expensive operation. Such a system can only be considered mediocre in terms of upgradability.

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

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

3 Comments


  1. Manuel Bolotinha
    Dec 19, 2015

    What is the advantage of supllying MV (Medium Voltage!) loads with only one phase and the neutral?
    Do you know any MV load working only with one phase? And do you know MV circuit breakers with single pole operating mechanism? I don’t and I have 42 years of experience!
    Can you show specific exemples of this MV distribution? In Europe, Africa, Asia and even in South America this system is not used.
    I can see no advatange of this system.


  2. Roberto Guerra
    Dec 16, 2015

    Thanks for your article.
    Can you make a contribution about Transmission Lines, 60, 138, 220 and 500 kV regarding the requirements for Resistance values of Grounding at towers, self supported, Steel towers, with EHS overhead ground wires and OPGW overhead ground wires, (2 ground wires), with Direct neutral connection at each end of the line. Ground wires are attached directly to towers at each tower.
    At each tower 4 grounding rods , one at each leg, Counterpoises installed in addition when high resistivity soils.
    Protection system for the line: Differential Current Relay, Distance relay, Overcurrent Relay, with redundancy. so Clearing time for single phase faults less than one cycle, 60 hz, plus circuit breaker single phase operation.
    Accordingly NEC multigrounded systems need 4 grounding points at each mile, without requirements of máximum value of grounding resistance, more dependent of number of grounding points than requierements of values. Comment this last “Note” in the Code.


  3. amine
    Dec 16, 2015

    thanks for this articles
    can you write article about bus bar trunking ?

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Time limit is exhausted. Please reload CAPTCHA.

Get PDF