MV/LV Switchboards in general
Depending on the size of the building or factory site and whether the supply is high voltage or low voltage, there may be requirements for both a main high voltage switchboard and one or more low voltage switchboards or just a single low voltage switchboard. The preferred name for the switchboard unit is a “Switchgear and Controlgear Assembly” (SCA).
The basic aim of the switchboard is to take the electrical power from the main supply source and then to feed or distribute power to the appropriate circuits within the building. The switchboard has to perform this function in such a way that there is proper control of power flow and proper electrical protection against the damaging effects of faults.
This protection is necessary to prevent personnel hazards and also equipment hazards and possible fires. It should be able to operate to isolate a faulty section in the minimum possible time consistent with the fault severity.
In many cases, work is performed on the switchboard components while they are still live.
Components of MV and LV Switchboards
LV and MV are different despite they may look very similar. One thing they do the same is that they distribute electrical energy, but using different voltage levels. MV will always supply LV switchbord, never vice versa.
2. The major components of a LV switchboard:
1. MV switchboard
These may be either high voltage (HV) or medium or low voltage (MV or LV). For high voltage, they will normally be either impregnated paper insulation (unlikely these days), cross linked polyethylene (XLPE) or ethylene propylene rubber (EPR) insulated cable.
The last two types are the preferred types for new installations, with XLPE being the most common.
In some cases busbars can be used instead of cables. This option is far more expensive than cables, but it also offers better reliability. If you ask me, I prefer busbar systems, where applicable of course.
These may be any of the following types:
- Insulated cables,
- Insulated busbars,
- Busbar trunking systems
- Mineral insulated metal-sheathed (MIMS) cables
- Fire-resistant cables
Knife-switches are used in MV switchgear to isolate specific equipment or feeders for maintenance or other purposes such as earthing. They operate at no-load conditions by hand, or in remote-controlled installations, they are actuated by motor or compressed air.
Blades of knife-switch, mounted standing or suspended, must be prevented from moving spontaneously under their own weight. The physical size of this type of switches must be taken into consideration when deciding the dimensions of the switchgear.
Usually, the switchgear requires greater depth.
Load-break switches are increasingly being used in MV distribution systems. For instance, ring main units use load-break switches in the two incoming feeders that connect the consumer’s substation to the network.
Two mechanisms can be used for load-break switch operation:
- Snap-Action Mechanism: A spring is tensioned that is released shortly before the switching angle is completed. Its force is used to move the contacts. The procedure is employed for both closing and opening.
- Stored-Energy Mechanism: It has one spring for closing and another for opening. During closing operation, the opening spring is tensioned and latched. The stored energy for opening operation is released by means of a magnetic trip or fuse.
Mainly two types of load-break switches are used: knife-contact type with/without fuses and slide-in type with/without fuses.
- Switchgear: Switch-disconnector and earthing switch in an enclosure filled with SF6 and satisfying “sealed pressure system” requirements.
- Busbars: All in the same horizontal plane, thus enabling later switchboard extensions and connection to existing equipment.
- Connection: Accessible through front, connection to the lower switch-disconnector and earthing switch terminals or the lower fuse-holders. This compartment is also equipped with an earthing switch downstream from the MV fuses for the protection units.
- Operating mechanism: Contains the elements used to operate the switch-disconnector and earthing switch and actuate the corresponding indications (positive break).
- Low voltage: Installation of a terminal block (if motor option installed), LV fuses and compact relay devices. If more space is required, an additional enclosure may be added on top of the cubicle.
Earthing switches are commonly used and installed in switchgear. When isolating any of the feeders (incoming or outgoing) for maintenance, the feeder must be earthed by closing the earthing switch to discharge any static charge carried by the feeder.
The switchgear manufacturer must mechanically interlock the earthing switch with the circuit breaker or load-break switch to avoid severe symmetrical short circuit if they are closed simultaneously.
The basic function of a circuit breaker is to break/make the continuity of the circuit. It is the consideration of the effect on the circuit of doing this, which principally dictates the choice of breaker.
Therefore, circuit breakers are mechanical switching devices able to make, continuously carry and interrupt currents under normal circuit conditions and also within a limited time under abnormal conditions, such as short circuits.
The basic elements of circuit breakers are operating mechanism, insulators, interrupting chamber(s), capacitor and resistor.
The main types of circuit breakers include the following:
- Bulk oil
- Minimum oil
- Air blast
- Sulfur hexafluoride (SF6)
They protect apparatus and equipment against the thermal and dynamic effects of short-circuits. The outstanding features of MV fuse-links are:
- High breaking capacity
- High current limitation
- Low switching voltage
- Quick breaking
Medium voltage fuses generally fit into two categories: expulsion fuses and current limiting fuses. The definitions per ANSI C37.40 are explained in the following technical article:
Using simple words to describe: The ultimate goal of protective relay is to disconnect a faulty system element as quickly as possible. Sensitivity and selectivity are essential to assure that the proper circuit breakers will be tripped, but speed is the “pay-off.”
These are used for the higher voltages, together with their associated instrument transformers (current transformers (CTs) and voltage transformers (VTs)).
Overcurrent protection units are used to activate timing relays so as to provide proper fault protection operation.
There are many different relays used to protect various substation equipment, starting from transformers, incomers, feeders, capacitors, generators, motors, etc.
2. Low voltage switchboard
Same as at MV switchboard, incoming cables can be cross linked polyethylene (XLPE) or ethylene propylene rubber (EPR) insulated type.
Generally, busbars are more often used at low voltage level than at medium voltages.
This device allow segregation of the switchboard or its component parts to allow maintenance work. It is manually operated (some types are equipped with automatic close/open mechanism), and is a lockable, two-position (open/closed) device.
The isolator must be capable of withstanding the flow of short-circuit currents for a limited time (short-time withstand capability), usually 1 second. For operational overcurrent, the time is longer.
Therefore, LV isolator is essentially a dead system switching device to be operated with no voltage on either side of it, particularly when closing. This is because of the possibility of an unsuspected short circuit on the downstream side. Interlocking with an upstream switch or circuit breaker is frequently used.
These may be rigid copper (or aluminium) bars (insulated or uninsulated) in large switchboards or simply insulated single phase cables in small switchboards.
In large capacity switchboards each phase may have a number of conductor sections.
The resonant frequency must be calculated to ensure it is not close to 100 Hz.
Load break switch is a control switch, non-automatic, two-position (open/close), operated manually and sometimes provided with electrical tripping for operator convenience.
Its characteristics are determined by the frequency of switch operation (600 close/open cycles per hour maximum), mechanical and electrical endurance and current making and breaking capacity for normal and infrequent situations.
The circuit breaker is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation – isolation, control and protection.
For low voltage (less than 1000 V) units, the circuit breakers are invariably of the air-break type using the “de-ion” principle, with isolated metal splitter grids. Modern switchboards have molded-case circuit breakers (MCCBs) for the higher current ratings (more than about 100 Amps) and miniature circuit breakers (MCBs) for the lower rating levels (less than 100 Amps).
MCBs would normally be used in the smaller sub-main and local switchboards in a building.
Low voltage circuit breaker consists of the following principal parts to carry out four essential functions:
- Circuit-breaking components, comprising the fixed and moving contacts and the arc-dividing chamber.
- Latching mechanism that becomes unlatched by the tripping device on detection of abnormal current conditions. This mechanism is also linked to the operation handle of the breaker.
- Trip-mechanism actuating device (learn more here). It is either:
- Thermal-magnetic device in which a thermal-operated bimetal strip detects overload conditions, while an electromagnetic striker pin operates at current levels reached in short circuit conditions, or:
- Electronic relay operated from current transformers, one of which is installed on each phase.
Additional modules can be added to the circuit breaker to be adapted to provide further features such as sensitive detection (30 mA) of earth leakage current with CB tripping, remote control and indication (on-off fault) and heavy-duty industrial circuit breakers of large current ratings that have numerous built-in communication and electronic functions.
The contactor is very simple device. It is actually a solenoid-operated switching device that is generally held closed by reduced current through the closing solenoid. Different mechanically latched types can be used for specific applications (e.g., motor starting, switching capacitors).
The characteristics of contactors are specified by:
- The operating duration,
- The application in which to be used,
- The number of start/stop cycles per hour and
- Mechanical and electrical endurance.
These are also used in MV and LV switchboards for high level fault protection and, in many cases, there are combinations of HRC (high rupturing capacity) fuses and overload switches with limited interrupting capacity used (combined fuse-switch or CFS units) because of their economy.
It consists of three-switch blades, each constituting a double break per phase. These blades are not continuous throughout their length, but each has a gap in the center that is bridged by the fuse cartridge.
The metering of a switchboards usually include: line and phase voltage, line current in each phase, total power, power factor metering.
The current is monitored by a current transformer (CT): in SWBs there may be two CTs, one for protection and one for metering.
Modern switchboards will also have some overvoltage surge protection designed into both the MV and LV sides to protect equipment against the effects of any over-voltage transients that may be generated within the system or conducted in from external sources.
- UNSW Sydney (the University of New South Wales)
- Medium Voltage Technology Switchgear Application Guide by Siemens
- The art and science of protective relaying by C. Rusell Mason
- Electric distribution systems by Abdelhay A. Sallam and Om P. Malik