Protections

In the base and distribution grid can to develop faults, short circuits etc. They can damage the high value grid and the devices. We spend 10% more for a substation investing and we protect our devices from the faults, high currents. In your home are fuses, small breakerts too, iy they would not there and you make a fault, the wires will burn off in you walls and the home fire is ready. You spend $1000 to your electricity installing and you will spend other $100 for the protection: fuses, breakers, relays. That is cheaper than a home fire. In every substations are any type of protections. We speak there morte Kilovolts and 200-1000 Ampers. We can't measure it directly, we have to use measuring transformers. If we have a 400 kV transmission line with 1000 A nominal current, we will use 1000/1 A current and 400/0,1 kV voltage measuring transformer. The 100 V and 1 A is confortable to measuring, and these values are proportional with the primary values. The job of the protections: -Measuring -After the limit passing: Decision -Break command giving It must be very fast, mostly under 100 ms (milliseconds). The standard writes, 3 seconds must be tolerated of the fault values on the primary devices. Can you imagine, if there is 120 kA (120,000 Amps) peak value in the first period of the sinus wave by a fault, that means more 1000 Newton hit to the post Insulators, that means a big truck's ipact.. The protection relays have 3 generations: Electromechanic, Electronic, Digital. The electromechanic relay includes transformers, coils, relays, clocks inside, like the analgoue instruments or the watthour meter. Deprez relays measure the values. The Deprez relays is similar than a measuring instrument, but it don't have pointer, it has a contacor. If the deprez relay measure above the adjusted value, it will close the contactor. After the mesuring the decisive logic will make the decision by comparator, compaundor coils and relay logic. At the end the output relay will gives the break command. Obsolete technology. The electronic relay includes transistors, ICs, operational amplifier comparators etc inside. The working logic is similar than the electromechanic relays. The digital protection is the newest technology. They are similar than your PC, that is a computer. It includes different cards: microprocessor, measuring, input and output cards, bus system and memory. The microprocessor does all job. The analog input cards convert the measured current and voltage to digital value, the processor comparates and decisies, the output (relay) card gives the break command and the signals.

Fuse The fuse is the easyest and cheapest protection type. That is one person with function of protection relay and breaker. The fuse is from a porcelain pipe with copper fusible thread, the pipe is filled up with quartz sand. The fuse is an overcurrent-time protection. The heating of the thread needs time, it depends from the current value: bigger current, faster fusing. If the fuse works, the fusible thread melts, after the arc between the two pole melts the sand and it breaks the circuit. After the fuse working you have to change it to a new one. They are used on the protection of small distributor lines, networks and middle voltage transformers. The fuses have 5 types: very fast, fast, normal, slow and very slow. The type depends from the protected devices. You will use slow fuse for fransformers, motors because the fuse must survive the ON switching currents. These devices suck up 10-12x of nominal current in the firtst sinus periods. You will use fast fuse for the sensitive and measuring circuits.

MCB The MCB is a small self-work breakes. It includes a coil, a bimetal release gear and arc grafter chambler. The coil works if it get very high current at the moment and it let to work the release gear at the moment, it protects from the high currents very fast. The bimetal generates heat by the current, by the expansion it bends and let to work the release gear. The bimetal is the currrent limiter and overload protection. The bimetal works similar than the fuse, faster with bigger currents, slower with lower currents. In the substations the MCBs are used for supply circuits, supply of logic and measuric ones: operating, signal, latcher voltages, etc, 220V DC mostly.

Overcurrent-time protection The overcurrent protection mostly a Deprez relay with amplifier relay. When it pass le limit current value it gives break command. But there are not ideal protected devises, the overcurrent protection includes a time relay. When you swich ON an inductive device, high starting currents will flow for a short time. The time relay delays and break command. When the current lomit passed, the time relay begins to count, if the limit passing fall back, the time relay will fall back too and it will sart the counting again art the next limit passing. It can filter out lot of flase breaking commands. This protection is not egough alone, it is only an overload protection, it is with other protections.

Electromechanic, Electronic, Digital, Mechanic Fuse, Overcurrent, Undervoltage, Differential-Section-Busbar, Breaker, Impedancy, Distance, Frequency, Power, Back-Watt, Back-Reactive, Buholz, Tempenature, Backswitcher,