Some practical issues concerning MV distribution networks

Overhead networks

Weather conditions such as wind and frost may bring wires into contact and cause temporary (as opposed to permanent) short-circuits.
Ceramic or glass insulating materials may be broken by wind-borne debris or carelessly discharged firearms. Shorting to earth may also result when insulating material becomes heavily soiled.
Many of these faults are able to rectify themselves. For example, damaged insulating materials can continue functioning undetected in a dry environment, although heavy rain will probably cause flashover to earth (e.g. via a metallic support structure). Similarly, heavily soiled insulating material usually causes flashover to earth in damp conditions.
Almost invariably, fault current will take the form of an electric arc, whose intense heat dries the current’s path and, to some extent, re-establishes insulating properties. During this time, protection devices will normally have proved effective in eliminating the fault (fuses will blow or the circuit breaker will trip).
Experience has shown that, in the vast majority of cases, the supply can be restored by replacing fuses or reclosing the circuit breaker.
As such, it is possible to improve the service continuity of overhead networks significantly by using circuit breakers with an automated reclosing facility on the relevant feeders.
These automated facilities support a set number of reclosing operations if a first attempt proves unsuccessful. The interval between successive attempts can be adjusted (to allow time for the air near the fault to deionise) before the circuit breaker finally locks out after all the attempts (usually three) have failed.
Remote control switches can be used on cable segments within networks to further improve service continuity. Load-break switches can also be teamed with a reclosing circuit breaker to isolate individual sections.

(1) A medium-voltage loop is an underground distribution network based on cables from two MV substation feeders. The two feeders are the two ‘ends’ of the loop and each is protected by an MV circuit breaker. The loop is usually open, i.e. divided into two sections (half- loops), each of which is supplied by a feeder. To support this arrangement, the two incoming load-break switches on the substations in the loop are closed, allowing current to circulate around the loop. On one of the stations one switch is normally left open, determining the start of the loop. A fault on one of the half-loops will trigger the protection device on the associated feeder, de-energising all substations within that half loop. Once the fault on the affected cable segment (between two adjacent substations) has been located, the supply to these substations can be restored from the other feeder. This requires some reconfiguration of the loop, with the load-break switches being switched in order to move the start of the loop to the substation immediately downstream of the fault and open the switch on the substation immediately upstream of the fault on the loop. These measures isolate the cable segment where the fault has occurred and restore the supply to the whole loop, or to most of it if the switches that have been switched are not on substations on either side of the sole cable segment affected by the fault. Systems for fault location and loop reconfiguration with remote control switches allow these processes to be automated.

Underground networks

Cable faults on underground networks can sometimes be caused by poorly arranged cable boxes or badly laid cables. For the most part, however, faults are the result of damage caused by tools such as pickaxes and pneumatic drills or by earthmoving plant used by other public utilities.
Insulation faults sometimes occur in connection boxes as a result of overvoltage, particularly at locations where an MV network is connected to an underground cable network. In such cases, overvoltage is usually caused by atmospheric conditions, and the reflection effects of electromagnetic waves at the junction box (where circuit impedance changes sharply) may generate sufficient strain on the cable box insulation for a fault to occur.
Devices to protect against overvoltages, such as lightning arresters, are often installed at these locations.
Underground cable networks suffer from fewer faults than overhead networks, but those which do occur are invariably permanent and take longer to locate and resolve.
In the event of a fault affecting an MV loop cable, the supply can be quickly restored to users once the cable segment where the fault occurred has been located.
Having said this, if the fault occurs at a feeder for a radial supply, it can take several hours to locate and resolve the fault, and all the users connected in a single branch arrangement downstream of the fault will be affected.
In cases where service continuity is essential for all or part of the installation concerned, provision must be made for an auxiliary supply.

Remote control and monitoring for MV network

The use of centralised remote control and monitoring based on SCADA (Supervisory Control And Data Acquisition) systems and recent developments in digital communication technology is increasingly common in countries where the complexity associated with highly interconnected networks justifies the investment required.

Remote control and monitoring of MV feeders makes it possible to reduce loss of supply resulting from cable faults by supporting fast and effective loop reconfiguration. This facility relies on switches with electric controls which are fitted on a number of substations in the loop and linked to modified remote-control units. All stations containing this equipment can have their supply restored remotely, whereas other stations will require additional manual operations

Values of earth fault currents for MV power supply

The values of earth fault currents on distribution networks depend on the MV substation’s earthing system (or neutral earthing system). They must be limited to reduce their impact on the network and restrict possible increased potential on user substation frames caused by the coupling of earth switches (overhead networks), and to reduce flashover with the station’s LV circuits capable of generating dangerous levels of potential in the low voltage installation.
Where networks have both overhead and underground elements, an increased cable earthing capacitance value may cause the earth fault current value to rise and require measures to compensate this phenomenon. Earthing impedance will then involve reactance (a resistor in parallel with an inductor) in line with the leakage rate: the neutral earthing system is compensated. Compensatory impedance makes it possible to both:

• Control earth fault current values, regardless of the amount of cabling within the network, and
• Eliminate most temporary and semi-permanent single-phase faults naturally by facilitating self rectification, thereby avoiding many short-term losses