Some practical issues concerning MV distribution networks

Overhead networks

Weather conditions such as wind may bring overhead wires into contact and cause phase to phase short-circuits.

Over voltages due to lightning strokes may generate flash-over across ceramic or glass insulators and cause phase to earth faults

Temporary contacts of vegetation such as trees with live overhead conductors may also generate phase to earth faults.

Most of these faults are temporary. They disappear naturally with the interruption of the voltage. This means that the supply can be restored after a short delay following the tripping. This delay is usually named "dead time".

Hence the sequence of fault clearing and voltage restoration in an overhead network is as follows:

• Fault detection by phase to phase or phase to earth protection
• Circuit breaker opening, the faulty over-head line is de-energized
• Dead time
• Circuit breaker reclosing. Following the reclosing two situations are possible:
  • The fault has been cleared by the interruption of the voltage, the reclosing is successful
  • The line is still faulty, a new tripping is initiated followed again by a reclosing sequence.
• Several sequences of tripping-reclosing may be activated depending on the rules of operation of the network adopted by the utility
• If after the execution of the preselected number of reclosing sequences the fault is still present, the circuit breaker is automatically locked and consequently the faulty part of the network remains out of service until the fault is localized and eliminated.

As such, it is possible to improve significantly the service continuity of overhead networks by using automatic reclosing facilities. Generally a reclosing circuit breaker is associated to each overhead line.

Underground networks

Cable Faults on underground MV cables may have several causes such as:

• Poor quality of cable laying, absence of mechanical protection
• Bad quality of cable terminations confection
• Damages caused by excavators or tools such as pneumatic drills
• Over voltages generated by lightning strokes occurring on overhead line connected to underground cables. The over voltages can be amplified at the levels of the junctions between overhead lines and underground cables causing the destruction of the cable terminations. Lightning arresters, are often installed at these locations to limit the risks of damages.

The experience shows that the rate of fault occurring on underground cables is lower than the one registered for overhead lines. But faults on underground cables are invariably permanent and take longer time to locate and repair.

A loop architecture (see Fig. B10) correctly instrumented with fault detectors and motorized load break switches allow within a short period of time to identify a faulty cable, to disconnect it and to restore the supply to the whole substations included in the loop.

These procedures of faults detection, cables disconnection and supply restoration can be automatically performed in less than one minute by dedicated functions commonly integrated in remote control and monitoring systems of MV networks.

Remote control and monitoring for MV networks

(see Fig. B7)

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 make it possible to reduce loss of supply resulting from cable faults by supporting fast and effective loop reconfiguration.

This facility relies on motorized switches associated with fault detectors on a number of substations in the loop and controlled by remote control units. All stations containing this equipment can have their supply restored remotely, whereas other stations will require additional manual operations.

Fig. B7 – Supervisory Control And Data Acquisition System SCADA

Values of earth fault currents in MV networks

(see Fig. B8 and Fig. B9)

The values of earth fault currents in MV distribution networks depend on the MV neutral earthing system. These values must be limited to reduce their effects, mainly:

• Damages to equipment
• Temporary over voltages
• Transient over voltages
• Touch and step voltages.

The neutral of an MV network can be earthed by five different methods, according to type (resistive, inductive) and the value (zero to infinity) of the impedance Z_n connected between the neutral and the earth:

• Z_n = ∞ isolated neutral, no intentional neutral earthing connection
• Z_n is related to a resistance with a fairly high value,
• Z_n is related to a reactance, with a generally low value,
• Z_n is related to a compensation reactance, designed to compensate the capacitance of the network
• Z_n = 0: the neutral is solidly earthed.

Fig. B8 – Effects of the phase to earth fault current

	Methods of Neutral Earthing
	Isolated	Resistance	Reactance	Compensated	Solid
Damages	Very low	Low	Low	Very low	Very high
Temporary over voltages	High	Medium	Medium	Medium	Low
Transient over voltages	High	Low	High	High	Low
Touch and step voltages	Very low	Low	Low	Low	High

The fault current I_K1 is the sum of two components:

• The neutral current through the impedance Z_n
• The capacitive current through the phase to earth capacitors.

When Z_n is a reactance these two currents are opposite, which means that the reactance compensate the capacitive current. If the compensation is perfect, the fault current value is zero.

Fig. B9 – Circulation of the phase to earth fault current

Medium voltage loop

(see Fig. B10)

A medium voltage loop is generally supplied from two separate primary substations. It supplies secondary MV/LV substations dedicated to the LV public distribution and private electrical installations.

The MV/LV secondary substations of the distribution operators and those dedicated to the connection of private electrical installations are sequentially organized on the same grid by means of underground cables.

Two load break switches are used for the connection of each secondary substation.

The loop is normally open, all the load break switches are closed except one.

In case of fault between A and B, the breaker C trips clearing the fault. The two substations S1 and S2 are de-energized. The restoration of the supply to all substations is then realized as follow:

1 - Isolation of the faulty cable by opening load break switches A and B
2 - Closing open point D
3 - Reclosing circuit breaker C. The open point is now between S1 and S2.

This sequence of faulty cable disconnection followed by the restoration of the supply can be executed either manually by the operators of the MV network or automatically by means of dedicated functions integrated in remote control and monitoring systems of MV networks.

Manual operations are generally long whereas automatic supply restoration can be executed within less than one minute by the remote control system. These automatism now available in any control system require a suitable instrumentation of the loop:

• Fault detectors at both ends of the underground cables
• Motorized load break switches
• Remote Terminal Unit (RTU) in each secondary substation. The RTU performes:
  • The monitoring of the fault detectors and load break switches
  • Local automatism
  • The command of load break switches
  • The communication with the remote control and monitoring center
• DC auxiliary supply in every substation.

As described above, most of the loops are historically equipped with load break switches and protected at each end only by circuit breakers located in the HV/ MV primary substations. In case of fault, all the customers supplied by a faulty feeder are disconnected. But in fact the customers upstream from the fault could have not been disconnected.

The addition of circuit breakers, adequately located and associated with appropriate protection relays may reduce the number of customers disconnected in case of fault.

As an example a loop including two additional circuit breakers is divided in four independent sections. Assume the open point located between the two additional circuit breakers. In case of fault in the section delimited by these two circuit breakers only a part of the secondary substations of the section will be disconnected, all the other remaining energized.

Fig. B10 – Open loop configuration and operation