10-11-2012, 01:54 PM
INTRODUCTION OF BAY EQUIPMENT
INTRODUCTION OF BAY EQUIPMENT.docx (Size: 118.48 KB / Downloads: 80)
CURRENT TRANSFORMER:-
Current transformer is similar in construction to single phase power transformer and obeys the same fundamental laws but primary current of C.T. is not controlled by the connected load in secondary circuit. In fact, it is governed by the current in the main circuit viz. line / transformer to which it is connected. A typical 400/220 KV CT has five cores which are used for following functions:-
1. Core1-Busbar I protection
2. Core2-Busbar II protection
3. Core3- Metering
4. Core4-Main I protection
5. Core5- Main II protection
The metering core of CT is of accuracy class of 0.5 whereas the other cores are having accuracy of PS class which is a special protection class for which knee-point voltage and max. exciting current is specified.
Knee-point voltage is that sinusoidal voltage of rated freq. applied to secondary winding of CT (all other windings open) which when increased by 10% causes the exciting current to increase by 50%.
Metering core of CT is required to maintain its accuracy up to 150% of the rated primary current and it is desirable that this core gets saturated beyond this current to prevent damage of the connected meters.
Protection core of CT is required to maintain its accuracy at fault current level which is very high compare to rated current. As such this core is not required to maintain accuracy at normal line / ICT currents. However this core must not saturate up to maximum short circuit level of the station, which is 40 KA for 400 / 220 KV substation otherwise this will cause mal-operation of the protection schemes employed.
POWER TRANSFORMER I.C.T:-
This is the costliest equipment of substation. ICT is used to step down the EHV transmission voltage (400 KV) to HV transmission voltage (220 KV). Normally 315 MVA, OFAF type auto transformer having 2 secondary winding is being used. The first secondary winding provides 220 KV voltage and the other 33 KV voltage (tertiary winding). Usually tertiary winding is connected in closed delta formation and can be used for auxiliary station supply purpose. In practice, it is preferred to installed three phase ICT as far as possible however in case of hilly terrain, where due to transportation limitation, three single phase units are installed. It is the most important as it is only
POWER LINE CARRIER COMMUNICATION
INTRODUCTION:
Communication is a vital area in Power System Operation. For better Operation of the grid, Communication between two adjacent Sub-stations and with the Load Dispatch center is a must. Similarly for acquiring real time data from different generating and major substations, communication link is a must for a Load dispatch center. Normally Generating stations and sub-stations are located at remote locations where Communication facilities will not be available or the efficiency of such facilities if available will be very poor. The Operation costs of such communication equipment are very high.
Power Line Carrier Communication (P.L.C.C.) is commonly used for Voice communication, telemetry, teleprotection and tele control / SCADA purpose. Carrier terminal equipments are installed at both ends of line. High frequency carrier current in the frequency range of 50 to 500 KHz is transmitted between two stations connected by transmission line. The carrier current equipments comprises of the following:-
1) Line Trap / Wave trap Unit
2) Coupling Capacitor (C.V.T.)
3) Line Tuning / matching Unit (L.M.U.)
4) Carrier Equipment (Transmitter, Receiver etc.)
This concept has resulted in the development of Power Line carrier Communication where High Voltage and Extra High Voltage Transmission lines themselves act as a medium for sending Communication Signals apart from Electric Power.
ADVANTAGES OF PLCC:
Its transmission reliability is high as that of Power lines themselves, whereas telephone lines are much weaker mechanically and will always fail under difficult conditions long before the overhead lines. As the channels are solely used by Power utilities only and not by general public, there will not be any traffic problem in communication. Carrier current transmission is not subjected to any variation from atmospheric or ionospheric conditions such as always the case with VHF and microwave systems. In general the capital cost of carrier current equipment is considerably less than the cost of installing VHF or Microwave equipment, while maintenance is very less.
DISADVANTAGES OF P.L.C.C.:-
As the medium used for communication is High Voltage Line, the cost of Insulation of communication equipment and Transmission Lines is very High Noise level will be high because of interference of Corona. High speed data transfer is not possible because of band width limitations.
BASIC PRINCIPLE OF OPERATION OF P.L.C.C.:-
A carrier frequency in the range of 50 to 500 KHz is generated by a high frequency oscillator. It is amplified and modulated by speech and other super–imposed signals like telemetry/ tele protection data. Then it passes through Coupling device and low impedance coupling capacitor of C.V.T. and transmitted over Power Lines.
At other end of Line, This high frequency signal is passed through low impedance coupling capacitor of C.V.T. and Coupling device and then it reaches to receiving unit comprising of Attenuator, Band pass filter etc.
Wave Trap is installed in series at both ends of Transmission Line which provide high impedance to carrier frequencies while offering negligible impedance to Power frequency current. This prevents the high frequency signals from entering the neighboring lines and Switchyard equipments through Station Bus. Line traps is designed to carry the Line Current continuously and, heavy short circuit current momentarily during system fault condition.