EE 313: Power Distribution and Utilization (3 Credit Hours) During the operation of power system, it is often desirable and necessary to switch on or off the various circuits (e.g., transmission lines, distributors, generating plants etc.) under both normal and abnormal conditions. With the advancement of power system, the lines and other equipment operate at very high voltages and carry large currents. A circuit breaker can make or break a circuit either manually or automatically under all conditions viz., no-load, full-load and shortcircuit conditions. Circuit Breaker A circuit breaker can • Make or break a circuit either manually or by remote control under normal conditions • Break a circuit automatically under fault conditions • Make a circuit either manually or by remote control under fault conditions Thus, a circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions. The latter control employs relays and operates only under fault conditions. Operating principle. A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. The contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart, thus opening the circuit. Isolator It is one type of mechanical switch used to isolate a fraction of the electrical circuit when it is required. Isolator switches are used for opening an electrical circuit in the no-load condition. It is not proposed to be opened while current flows through the line. Generally, these are employed on circuit breaker both the ends thus the circuit breaker repair can be done easily without any risk. Earthing Switch Earthing switch is used to ground the residual charge in power lines after disconnecting the line from source. When a circuit is open by the circuit breaker and Isolator, there is a residual charge remaining in the circuit. So to discharge the charge Earthing switch is used. This ensures the safety of the maintenance personnel during maintenance and routine checking. In normal condition, the isolators and circuit breakers are closed but the earthing switch is opened. During the maintenance period, the circuit breakers and isolators are kept open and earthing switch kept closed. In the Substations circuit breakers, isolators and earthing switches are connected. For the maintenance or any other purpose if we need to open the circuit then we should follow a proper sequence for the operations of circuit breakers, isolators and earthing switches. If we follow the wrong sequence then it will be a danger for us as well as equipment and circuits. The sequence of operation of Isolator, Circuit Breaker, and Earthing Switch: While opening a circuit we should follow the below sequence While closing a circuit we should follow the below sequence What happened if we open the isolator before the circuit breaker? We know that the Isolator is designed to operate under no load condition, so if we open the isolator before the circuit breaker, that means we open the isolator under live condition. So there will be huge sparking between the contacts of the Isolator which is very dangerous for us Here a circuit diagram of 11KV/400V substation is shown. Here is one incoming line and two outgoing line. Suppose we need to the maintenance of the Transformer of the second outgoing line. So for the maintenance, first the transformer is to be disconnected from the circuit. So below sequence should be followed, 1. First circuit breakers A, B should be open. 2. Then the isolators C and D should be open. 3. Then the earthing switches E and F should be closed. Now after complete the maintenance, to connect again the Transformer to the line we should follow the below sequence, 1. First Open the earthing switches E and F 2. Then close the isolators C and D 3. Then close the circuit breakers A and B When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. Classification of Circuit Breakers The most general way of classification is on the basis of medium used for arc extinction. The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6) or vacuum. Accordingly, circuit breakers may be classified into : • Oil circuit breakers which employ some insulating oil (e.g., transformer oil) for arc extinction. • Air-blast circuit breakers in which high pressure air-blast is used for extinguishing the arc. • Sulphur hexafluoride circuit breakers in which Sulphur hexafluoride (SF6) gas is used for arc extinction. • Vacuum circuit breakers in which vacuum is used for arc extinction. Fuses A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit. Battery Banks A battery bank is the result of joining two or more batteries together for a single application By linking batteries together, we can increase the voltage, or the Capacity (AH / Wh), or both There are two primary ways to successfully connect two or more batteries: • Series • Parallel To connect batteries in a series, A wire is used to connect the negative terminal of the first battery to the positive terminal of the second battery Connecting batteries in series adds the voltage of the two batteries, but it keeps the same AH rating. For example, these two 6-volt batteries joined in series now produce 12 volts, but they still have a total capacity of 10 amps. To join batteries in parallel, A wire is used to connect both the positive terminals, and another wire is used to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive. Parallel connections will increase capacity rating, but the voltage will stay the same. In the “Parallel” diagram, voltage is 6 volts, but the amps increase to 20 AH. It is also possible to connect batteries in series and parallel configuration If you have two sets of batteries already connected in parallel, you can join them together to form a series to create a series parallel battery bank. In the diagram above, we have a battery bank that produces 12 volts and has 20 amp hours.
