At the most basic level, lost power equals lost money. Utility networks have a duty to keep the lights on, and their performance is regulated by measures such as Customer Minutes Lost/Customer Interruptions. In the same way, industrial networks demand continuity and security of power supply to keep their processes running. A halted process can soon result in massive losses. For example, if an oil refinery loses its supply, it can be two weeks before it is back up and running. Automation schemes can help to keep power interruptions to a minimum.
Protection is used to detect power system faults and other abnormal conditions. It also protects human life and properties closer to the power network. In the home, this protection comes in the form of fuses in plugs, and fuses and MCBs (miniature circuit breakers) in the consumer unit that measure the current flowing. There are also (RCD) residual current devices, which essentially use Kirchoff’s current law to detect an earth fault, which could be a current about to pass through a human, and so trip the circuit before any serious harm occurs.
Power Network Protection
The protection used on power networks is essentially the same, just on a much larger scale. The quality of a protection scheme tends to be measured in terms of reliability, speed and selectivity:
· A reliable protection relay operates correctly when there is a power system fault, but doesn’t make an incorrect operation when no fault is present
· Speed is the minimum operating time to clear a fault – to avoid damaging equipment and causing system instability
· Selectivity means only disconnecting the faulted section of the network or plant – this helps maintain continuity of the rest of the supply or system
Main and Back-up Protection
In general, main and back-up protection is applied. Main protection operates every time a fault is detected. Back-up protection is set to operate should the main protection fail to operate.
Overcurrent
Overcurrent is most the basic form of protection and is used at all voltage levels. To achieve selectivity, the protection is graded according to time and/or current (higher fault current = faster operating time, lower fault current = slower operating time). If circuits are in parallel, or if there are multiple sources, the direction of the current needs to be considered, so directional overcurrent protection is used.
Differential
Differential (unit) protection is applied to lines, transformers, motors, generators and bus-bars. This is absolutely discriminative / selective protection, but it requires communication either via copper pilots or other more advanced forms such as telephone circuits or optical fibres.
Distance
Distance (non-unit) protection discriminates between faults by measuring the impedance of the line. The line generally has a constant impedance (independent of current and voltage levels). It doesn’t require communications but can use them to help increase speed, or selectivity/security.
Signalling
Teleprotection signalling can be used in conjunction with distance protection to provide increased selectivity and faster tripping times, using communications infrastructure such as pilot wires or SDH/PDH networks.
Intertripping uses similar communications channels to pass trip signals from one substation to another to ensure that both ends of a faulty circuit are disconnected from the healthy power system.
Loadshedding
When generation capacity cannot support the load then the balance between generation and load needs to be addressed. The indication that generation is not matching the load is a drop in voltage and/or frequency. Loadshedding schemes are used to disconnect less important loads to help redress the balance. Of course, measuring power in and out would also indicate the loss of balance.