25-05-2012, 04:29 PM
vast increase in power system
vast increase in power system .doc (Size: 3.79 MB / Downloads: 111)
INTRODUCTION
SWITCHYARD
What is a switchyard?
A switchyard is fundamentally an electrical utility used to step down voltage and distribute power to the various facilities. It consists of an indoor substation and an outdoor yard. The outdoor parts is mainly a yard requiring transformers, circuit breakers, relay protection devices etc and is used for stepping down high voltages of the order of 132 kV or more.
Functions of an electrical switchyard:
The electrical switchyard is the most important part of a consumer’s electrical system. It performs a number of important functions that ensure the smooth and uninterrupted supply of electrical power to the consumer:
1) It transforms the voltage supplied by the transmission lines to a value which is suitable for use by the consumer. The transmission voltage is generally of the order of 66kV or more and hence is unfit for use by most consumers. The usual working voltage at most factory locations ranges from around 1.1kV to 6.6kV. In this case (Tata Motors) the client has requested a voltage of 415V at the secondary side. Hence the switchyard performs the task of stepping down/up the voltage to the required level.
2) It isolates faults in either the transmission or distribution systems. The high voltage sides of the transmission lines are prone to faults(phase to ground or phase-phase) and thus may endanger the sensitive equipment in the installation. Hence the switchyard contains several protective equipments (relays, circuit breakers, isolators etc) to isolate the load side in the event of a major fault.
3) An important function performed by a switchyard is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned". A transmission line or other component may need to be de-energized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.
Switchyard
Steps taken in switchyard implementation:
The first step in the switchyard design is the preparation of an initial single line diagram (SLD). This is prepared in accordance with the requirements of the client as specified in his Bill of Quantity (BOQ).
The client specifies his specifications and also releases a preliminary arrangement of what the switchyard looks like. Henceforth the design teams sits together and decides whether what the client has asked is feasible or otherwise offers their own suggestions for the same. This initial SLD is based on a general layout followed while designing a switchyard consisting of lightening arrestors, CT/PT’s, circuit breakers, relays etc. The ratings are decided by the design team which looks through the specifications of the plant.
The next step is the switchyard layout, which is then decided, based on a single bus/double bus or a ring bus system. The layouts are modeled on AutoCAD and the relevant distances are maintained between the equipments e.g. creep age distances between conductors, adequate clearances are maintained between the various components. The switchyard elevations are also decided. The outdoor bus bars are flexible and are aluminum tubes supported on post insulators or flexible ACSR conductors supported on strain insulators. Each bay has several equipment connected in certain well-defined patterns. Circuit breakers are connected between the bus bar and each outgoing and incoming circuit. Isolator is provided for measurement and protection. Circuit transformers are provided for measurement and protection. The protection zones should overlap and cover the circuit breaker. Voltage transformers are connected generally to bus bars and incoming line side. Surge arrestors are connected phase to ground, at incoming feeder, as the first apparatus and also at the terminal of transformer etc for diverting lightening surges to the ground.