OUT DOOR SUBSTATION....

             There are several equipment's in outdoor substations

1. Transformer

2.Breaker

3. Isolator

4.Earth switch

5.Auto re closer

6.Wave trap

7.Pole discremency

8.Current transformer

9.Voltage transformer or capacitive voltage transformer

10.Protection relays

                                             

                                                  BUSBAR erection at outdoor substation
  The control voltage we are using for protection was dc voltage as we can store easily and protect the system under no power due to fault and in order to reduce voltage drop.

1. Transformer:

                           It is used to step up or step down the voltage levels with maintaining same power and frequency.This process is done because of reducing copper losses.

                                                                P =V' * I' = V" * i"

2. Breaker:

                          It is used to interrupt the main power supply at normal and abnormal condition as well.

3.Isolator:

                          It should be opened after opening the breaker which was connected before. The need of isolator is  for maintenance purpose of breaker and in order to protect the persons from mal-operation of breaker.

4. Earth switch:

                         For high amount of voltages,the equipment are charged and possess the charge in shutdown period also.In order to discharge the charge the charge on equipment's when maintenance work is going to do . When the line beside the dead line is in running condition then there is a emf will induce in dead line also due to induction so it is necessary to discharge the charge induced in that line with earth switch

5. Auto re closer:

                         In order to serve the load after clearing the temporary faults.We should design the scheme as 86 should not pickup for temporary faults.Then the auto re closer will check for every time period and close the breaker.

6.wave trap:
                           It is a LC low pass filter which used to allow only power frequencies and block the high frequencies like communication frequencies ( which are using for PLCC ). It is placed at entering of line and leaving of line after CVT only. And in so many substations they will be placed in only two phases out of three phases, because of we are using only two channel communication. By calculating line parameters they will decide the phases which are suitable to place wave trap.

7.Pole discrepancy:

                        For small substations we have one three phase breaker.But when going to large substations we will find three single phase breakers. when any fault occur a phase then its respective breaker is going to trip,then our auto re closer come in to picture and check the faults was temporary or permanent,if fault was temporary then breaker will get closed by using auto re closer,in case of permanent fault auto re closer will open that phase permanently.Then there is unbalance in load,so we will set time in pole discrepancy relay (ex. 2sec),then it will give trip command to remaining healthy phases also.

8.Current transformer:

                       Current transformer is also same as the power transformer with slight difference,current transformer is not used for step up or step down  of voltage levels as in case of power transformers.It is mainly used for step down the current levels. Generally in transmitting of power is done at high currents and voltages,but our ammeters and energy meters are does not with stand such high currents, on other hand protective devices like relays are also unable to with stand the high currents.

         So by step down the current with certain ratio and that value is used for protection and metering,then by multiplying with that factor we can get the real values that in line.

         So generally secondary current range is 1A or 5A. And primary current was is in order of 1000 amps.

 MAIN DIFFERENCE between POWER TRANSFORMER and CURRENT TRANSFORMER

  In power transformer primary current depend upon secondary current, but in current transformer primary current will depend upon on only load not on secondary burden or secondary current.

   note: CT secondary circuit should always in closed condition only,we should short spare core.

9.Voltage transformer or capacitive voltage transformer:
                         Potential transformer is almost same as power transformer. But used for only step down of  high voltages to 110 V (line to line).It was very low power rating which is used for only burden. As our voltmeters and energy meters and relays are unable to work with very high voltages, we step down the voltage range and use that for meters and relays and multiply with factor.
     generally secondary voltage is 110 V (line-line) where as primary voltage is of order of kilo volts.

                  capacitive voltage transformer is also similarly voltage transformer but it has construction like below  figure.The construction is like this because to measure high voltages which are not measured by normal voltage transformers. some of voltage will be dropped across capacitor C1 and remaining voltage was stepped down.The combination of capacitors and inductor acts as filter which was used to catch only communication signals and leave the power waves towards load.

                                            One of the most important factor is voltage transformer is used for only voltage measuring purpose and ordinary protection,but capacitive voltage transformer has advantage that it can also used for communication purpose between substations (as like wave traps).

  note:never short the spare cores of VT or CVT.

10.Protection relays:
                               Protection is very important for electrical machines and equipment's as they are cost very high, in order to protect those we will use the electrical quantities and estimate the abnormal and normal condition of a system,based on those values we will design our protection scheme.Some of major protections that are employed in electrical system.
                            1.over current protection.
                            2.over voltage protection.
                            3.over flux protection.
                            4.under voltage protection.
                            5.under and over frequency protection.
                            6.distance protection.


          NECESSARY INTER LOCKS FOR OUTDOOR SUBSTATION

Need of interlocks : in order to protect the system from maloperation  of skilled and unskilled labour persons.

ISOLATOR AND BREAKER:
                     Isolator should not open and close when breaker is in closed condition.Why. because  seviour amount of arc will struck between the isolator poles as there is no arc extinguisher medium and as time of opening and closing of isolator are high.

ISOLATOR AND EARTH SWITCH:
                    Isolator should close when earth switch is connected,because total charge will get down to earth as earth fault.so in order to protect from that situation we have to design interlock

EARTH SWITCH AND BREAKER:
                   Earth switch should not be closed when breaker is in closed position (for remote operation). For local operation and manual operation no necessary of this interlock (depend upon customer requirement).

DIFFERENT PROTECTIONS IN OUTDOOR SUBSTATIONS....

TRANSFORMER DIFFERENTIAL PROTECTION and RESTRICTED EARTH FAULT PROTECTION:
                                       Differential protection,protects the transformer from its internal faults only,it will compare entering and leaving currents of transformer and operate on its setting
                                       REF protection operates if any earth fault occurs inside the transformer.The principle behind this is to sum the neutral current and phase currents.

TEE DIFFERENTIAL PROTECTION:
                                        In one and half scheme substations we have three CTs connected as T section,if any fault occurs in this T section then this protection will operate.