Supply of power at medium voltage (full page)
From Electrical Installation GuideThe term «medium voltage» is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV(1). For technical and economic reasons, the nominal voltage of medium-voltage distribution networks rarely exceeds 35 kV.
In this chapter, networks which operate at 1000 V or less are referred to as low-voltage (LV) networks, whereas networks requiring a step-down transformer to feed LV networks are referred to as medium voltage (MV) networks.
| (1) According to the IEC there is no clear boundary between medium and high voltage; local and historical factors play a part, and limits are usually between 30 and 100 kV (see IEV 601-01-28).The publication IEC 62271-1 "High-voltage switchgear and controlgear; common specifications" incorporates a note in its scope: "For the use of this standard, high voltage (see IEV 601-01-27) is the rated voltage above 1 000 V. However, the term medium voltage (see IEV 601-01-28) is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV." |
Power supply characteristics of medium-voltage networks
The main characteristics of an MV power supply are:
The characteristics of the MV network determine which switchgear is used in the MV or MV/LV substation and are specific to individual countries. Familiarity with these characteristics is essential when defining and implementing connections.
Different types of MV power supply
The following power supply methods may be used as appropriate for the type of medium-voltage network.
Connection to an MV radial network: Single-line service
The substation is supplied by a tee-off from the MV radial network (overhead or cable), also known as a spur network. This type of network supports a single supply for loads (see Fig. B1).
The substation usually consists of an incoming panel, and overall protection is provided by a load-break switch and fuses with earthing switches as shown in Figure B1.
In some countries, the “substation” comprises a pole-mounted transformer without a load-break switch or fuses (installed on the pole). This type of distribution is very common in rural areas. Protection and switching devices are located remotely from the transformer. These usually control a main overhead line to which secondary overhead lines are connected.
Fig. B1: Single-line service (single supply)
Connection to an MV loop: Ring-main service
The power supply for the substation is connected in series to the power line of the medium-voltage distribution network to form a loop(1). This allows the line current to pass through a busbar, making it possible for loads to have two different power supplies (see Fig. B2).
The substation has three medium-voltage modular units or an integrated ring-main unit supporting the following functions:
- 2 incoming panels, each with a load-break switch. These are part of the loop and are connected to a busbar.
- 1 transformer feeder connected to the busbar. General protection is provided by load-break switches, a combined load-break/isolating switch or a circuit breaker.
All these types of switchgear are fitted with earthing switches.
All switches and earthing switches have a making capacity which enables them to close at the network’s short-circuit current. Under this arrangement, the user benefits from a reliable power supply based on two MV feeders, with downtime kept to a minimum in the event of faults or work on the supplier network(1).
This method is used for the underground MV distribution networks found in urban areas.
Fig. B2: Ring-main service (double supply). The transformer is protected, in accordance with the applicable standards, by a circuit breaker or load-break switch as shown in Figure B1.
Connection to two parallel MV cables: Parallel feeders service
If two parallel underground cables can be used to supply a substation, an MV switchboard similar to that of a ring-main station can be used (see Fig. B3).
The main difference to the ring-main station is that both load-break switches are interlocked. This means that only one of them can be closed at any one time (if one is closed, the other must be open).
In the event of the loss of supply, the associated incoming load-break switch must be open and the interlocking system must enable the switch which was open to close. This sequence can be implemented either manually or automatically.
This method is used for networks in some densely-populated or expanding urban areas supplied by underground cables.
Fig. B3: Parallel feeders service (double supply). The transformer is protected, in accordance with local standards, by a circuit breaker or load-break switch as shown in Figure B1.
Some practical issues concerning MV distribution 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.
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