SUBSTATION PROTECTION

• Windings
  1.I.R (Insulation Resistance) & P.I (Polarisation Index) values.
  2.Tan Delta values.
  3.Winding Resistance.
  


• Condenser type Bushings .
  1. Tan Delta of bushings.
  2. Capacitance of bushings.
  


• Transformer Oil
  1. Break Down Voltage (B.D.V) – Reveals conductive
  contaminants & moisture present in oil.
  50KV Min-170KV & above.
  40KV Min-Between 72.5 & 170KV.
  30KV Min-Below 72.5KV.
  
  2. Resistivity – Indicative of conducting impurities.
  0.2x10¹²Ohm-Cm (Min) for all Voltage Classes.
  
  3. Dielectric Dissipation Factor or Tan
  Delta – Reveals presence of moisture, resins & their products of oxidation.
  0.2 Max-170KV & above.
  1.0 Max-Below 170KV.
  
  4. Moisture content – Reveals total water content.
  20ppm Max-170KV & above.
  40ppm Max-Below 170KV.
  
  5.Acidity - Amount of acid present in oil expressed in mg of koh - 0.3mg koh/gm Max.
  
  6.Flash point (degree centigrade)- 125°C Min.
  
  7.Intertfacial Tension – Reveals sludge present in oil - 15dynes/cm Min.
  
  8.Dissolved Gas Analysis – Reveals ppm of combustible gases dissolved in oil to assess the internal condition of the transformer.
  
  The gases are Methane, Ethane, Ethylene, Acetylene, Hydrogen, Carbon dioxide & Carbon monoxide. The values of the their concentration expressed in ppm depends on the serviced life of the Transformer.
  

• Current Transformers transform current from High Value to Low Value at a given ratio which is substantially constant for a given range of Primary current.
• The Rated Secondary Current of CTs are normally either 5Amp or 1Amp.
• CTs have 1 no. to 5no.s of secondary windings depending on the requirement.
• Types of current transformers secondaries
  1. Metering Core
  VA Burden, Accuracy, ISF
  e.g. 15 VA, 0.5 Cl., ISF < 10
  
  2. Protection Core
  VA Burden, Accuracy, ALF
  e.g. 15 VA, 5P20
  
  3. PS Core
  Vk, Io, Rct
  e.g. Vk > 400 V, Io < 50 mA at Vk/2, Rct < 5 Ohms
  
  
• Normally the secondary cores of any 5 core CT will be distributed as
  Core 1 – Primary Protection
  Core 2 – Metering.
  Core 3 – Back Up Protection.
  Core 4 – Bus Bar Protection (Main)
  Core 5 – Bus Bar Protection (Check Zone)
  
• Protection cores- Main characteristics
  a. Lower accuracy than measuring core
  b. High saturation voltage
  c. Little or no turns correction
  
• Protection cores
  Examples: 30VA, 5P20 40VA, 10P20 20VA, 10P10 20VA, 5P40 etc.
  30VA - rated burden at rated secondary current.
  5 - total error in % at accuracy limit and rated burden.
  P - protection class.
  20 - minimum multiple of rated secondary current at which the accuracy limit is reached with rated burden.
  
• Definition of knee-point voltage
  Vk = knee-point voltage
  Voltage at which an additional 10% increase in voltage requires an increase of 50% of magnetizing current.
  

• Voltage Transformers transform Voltage from High Value to Low Value at a given ratio, Volt -Ampere rating smaller compared to that of Power Transformer and the Secondary of which are used for Metering and Protection purposes.
• The Rated Secondary Voltage of VTs are normally 63.5 V (Ph-N) or 110V (Ph-Ph).
• VTs have 1 no. to 3no.s of secondary windings depending on the requirement.
• CONDITION MONITORING OF CURRENT / VOLTAGE TRANSFORMERS
  a. Insulation Resistance
  b. Tan Delta & capacitance
  c. Ratio & Accuracy
  

• Circuit Breaker is device which is capable of making, Breaking an electric circuit under normal and abnormal conditions.
• The different arc quenching media used in Circuit Breakers are Insulating Oil, Compressed Air, Vacuum, Sulphur hexa Floride Gas etc.
• The energy required for operation of a Circuit Breaker is generally derived from Tension or compression type Springs or Compressed Air or compressed Hydraulic Fluid and in some cases combination of the above.
• CONDITION MONITORING OF CIRCUIT BREAKERS
  Operation timings
  Time in milliseconds required for closing, Opening, close- open, P.I.R, Pole discrepancy operations of circuit breaker will be measured & adjusted if required.
  Contact Resistance
  The values of resistance in milliohms of contacts of all the three phases of circuit breaker will be measured. If the values are more than limits specified, decision regarding replacement of main contacts or overhauling of breaker can be taken.
  I.R values Insulation Resistance values in Mega Ohms w.r.t Earth with circuit breaker closed & across interrupters with circuit breaker in Open condition will be measured. If values are less than minimum values, the circuit breaker needs to be inspected & corrective action to be taken so as to restore good I.R values.
  Tan delta & capacitance of grading capacitors to be measured at regular intervals.

• Type of C & R Panels
  Simplex type wherein, the control panel and relay panel are placed separately. In recent times, panels with SCADA compatibility houses relays, BCU & control switches all in one panel.
  Duplex type wherein, the control panel and the relay panel are placed one behind the other and are separated by space for corridor.
  
• Control switches for Breaker control switch, Isolators Control, PT selection, Protection Transfer etc. are provided in the control panel.
• Meters for display of value of current, voltage, MW, MVAR etc. and Energy meter are fixed in the control panel.
• Indication lamps for display of status of Breaker, Isolators etc and annunciation scheme are fixed in the control panel.
• In recent times, C&R panels with SCADA compatibility will be fixed with Bay Control Unit (BCU) for all the controls, Indications, Annunciations, display of value of I,V.MW,MVAR etc. In addition, BCU does the functions of DR, Event Logger, Data Logging etc.
• Control switch for emergency operation of Breaker will be provided.

• Transmission Line
  Primary Protection: Distance Scheme Relay with carrier protection, Pilot wire protection for very short lines.
  Back Up Protection: Directional OCR/EFR, Local Breaker Backup (LBB).
  
• Transformer 400/220KV or 220/110KV or 220/66KV class.
  Primary Protection: Differential Relay, Restricted Earth Fault Relay (REFR) on HV & IV sides, Over Flux Relay, Transformer mounted relays like Bucholz Relay, PRV, OSR.
  Back Up Protection: Directional OCR/EFR on HV & IV sides, Non Dir OCR on LV side, High Oil Temp & Wdg Temp Trip (HV,IV,LV).


• Transformer 66/11KV or 110/11KV or 110/33KVclass.
  Primary Protection: Differential Relay, Restricted Earth Fault Relay (REFR) on LV side, Transformer mounted relays like Bucholz Relay, PRV, OSR.
  Back Up Protection: Directional OCR/EFR on HV & LV sides, High Oil Temp & Wdg Temp Trip (HV,LV).
  
• Fault types
  Transient faults
  
  • are common on transmission lines, approximately 80-85%
  • lightnings are the most common reason
  • can also be caused by birds, falling trees, swinging lines etc.
  • will disappear after a short dead interval
  Persistent faults
  
  • can be caused by a broken conductor fallen down
  • can be a tree falling on a line
  • must be located and repaired before normal service
• Overcurrent protection
  1. Normally used in radial networks with system voltage below 70 kV where relatively long operating time is acceptable.
  2. On transmission lines directional or Non-directional overcurrent relays are used as back-up protections.
  
• Directional Overcurrent Relays
  1. Relays on radial lines do not need directional element.
  2. Directional elements are useful on parallel lines, on looped lines, and in meshed networks.
  
• Pilot wire differential protection
  1. Pilot wires can be in soil or on towers.
  2. The resistance in the wires will limit the use on longer lines. The use is mostly restricted to distances up to 10 km.
  
• Transformer Internal Faults
  1.Earth faults
  2.Short-circuits
  3.Inter turn Faults
  4.Core Faults
  5.Tank Faults
  6. Reduced cooling
  
• Abnormal Conditions
  1. Overload
  2. Over voltage
  3. Reduced system voltage
  4. Overexcitation
  
• Differential Protection
  Classical application of differential protection.
  
  • Interposing current transformers for phase shift and transformation ratio correction
  • Protection zone, bushing CTs
  • Influence from OLTC
  • Harmonic restrained differential protections
• Earth Fault Protection
  Restricted earth fault protection
  
  • High-impedance restricted earth fault protections
  • Low-impedance restricted earth fault protections
• Protective and Monitoring equipment for transformers of 400kV and 220kV class
  
  • Transformer differential protection
  • Overfluxing protection
  • Restricted earth-fault protection
  • Back-up directional O / C + E / F protection on HV side
  • Back-up directional O / C + E / F protection on LV side
  • Protection and monitors built in to Transformer (Buchholz relay, Winding and Oil Temperature Indicators, Oil Level Indicator and Pressure Relief Device)
  • Protection for Tertiary winding
• BUS BAR PROTECTION:
  Bus bars 220KV and above class are provided with Bus Bar Protection with zone selection.
  
• ISOLATORS-WITH / WITHOUT EARTH SWITCH
  
  • Isolator is a device which can be opened or closed only under no current conditions and provides isolation of a circuit for maintenance purposes.
  • Earth switch is a device which connects a conductor to the Earth so as to discharge the charges on the conductor to the earth and generally mounted on the frames of an Isolator.
• INTERLOCKS-ISOLATORS
  
  • ISOLATORS ARE NO LOAD SWITCHES. THEY ARE NOT PERMITTED TO BREAK POWER CURRENT OR TO CONNECT DIFFERENT VOLTAGE SYSTEM.
  • MAKING OR BREAKING OF PARALLEL CIRCUITS IS PERMITTED.
  • AN ISOLATOR CONNECTED IN SERIES WITH A CB MAY BE OPERATED WHEN CB IS OPEN AND IF NONE OF ADJACENT EARTH SWITCHES IS CLOSED. EVEN IF ANOTHER ISOLATOR CONNECTED IN SERIES TO THE CB IS OPEN THE ISOLATOR IS NOT ALLOWED TO OPERATED UNLESS THE CB IS OPEN.
  • AN ISOLATOR CONNECTED TO A DUPLICATE BUSBAR OR AUXILIARY BUS MAY BE OPERATED IF
    i) NO LOAD IS CONNECTED AND NONE OF THE EARTH SWITCHES ARE CLOSED.
    ii) A PARALLELING CIRCUIT TO THE ISOLATOR IS PRESENT. (ONLY THROUGH THE BUSCOUPLER FOR THE ISOLATORS THAT ARE CONNECTED TO THE TRANSFER BUS.
    
• EARTH SWITCH
  
  • EARTH SWITCHES WHICH CAN EARTH AN INCOMING LINE DIRECTLY OR THROUGH A CB CAN BE OPERATED IF AND ONLY IF THE LINE IS DEAD.
  • CAN BE OPERATED ONLY IF THE ISOLATORS ON BOTH THE SIDES ARE OPEN.
• CONDITION MONITORING OF ISOLATORS
  Contact Resistance
  The values of resistance in milliohms of contacts of all the three phases of Isolator will be measured. If the values are more than limits specified, decision regarding replacement of main contacts can be planned / taken up.
  

The Bus Bar arrangement in any Sub Station is based on the following.

• Degree of flexibility desired.
• Importance of the Loads fed from the Sub Station, freedom of availing total shut down and it’s period.
• Economic consideration, availability and cost.
• Economic consideration, availability and cost.
• Technical considerations.
• Maintenance, Safety of Persons.
• Provision of extension.
• Simplicity.
• Protective zones.
• Single Bus Bar Arrangement.
  This arrangement consists of a single Bus Bar to which various feeders are connected. In case of a fault or maintenance of Bus Bar, total shut down occurs and hence provides least flexibility.
  
• Double Bus Bar Arrangement
  This arrangement provides additional flexibility, continuity of Power Supply, permits periodic maintenance without total shut down as there is redundant Bus Bar and Circuit Breaker (Bus Coupler).
  
• Double Bus Bar Arrangement with Transfer Bus.
  This arrangement provides more additional flexibility, continuity of Power Supply, permits periodic maintenance without total shut down as the two main buses can be operated independently with the same redundancy.
• One and a Half Breaker Arrangement.
  This arrangement provides three circuit breakers for every two circuits. It gives high security against loss of supply but higher cost is involved. Hence this is provided for important 400KV Sub Stations.

• Thyristor converters are used in case of HVDC Power Transmission system wherein, HVAC is converted to HVDC at the sending end and HVDC is inverted to HVAC at the receiving end.

Bipolar HVDC Power Transmission System is normally operated in the following modes.

• Bipolar Balanced Mode.
• Bipolar Un-Balanced Mode.
• Mono polar Metallic Return Mode.
• Mono polar Earth Return Mode.

Surge Arrestors are provided to discharge the Switching Over Voltage and Lightning Over Voltages to Earth and protect Line/equipments.
CONDITION MONITORING OF LIGHTNING ARRESTOR

• The third harmonic resistive component of the leakage current through the Lightning Arrestor has to be measured. The increasing trend in the above parameter is an indication of deterioration of the Lightning Arrestor and replacement can be planned.

Shunt capacitors will provide reactive power compensation to the low Lagging Power Factor loads and hence improve the overall Power Factor of the system.

Series Capacitors are provided in case of long EHV Lines to compensate for the series Inductive reactance of the line so as to enable transmission of higher power over the same line due to reduction in the impedance of the line.

Shunt Reactors are provided in case of long EHV Lines to compensate for the shunt capacitive reactance of the line so that shoot up of Voltage at the receiving end during switching and low load conditions are minimised.

11KV Switchgear is the vital component of any Sub Station feeding the auxiliary supply and power supply to local areas and Industries.

• DC Battery is the source of stored energy for the operation of the entire protective relaying and other functions in any sub station.
• The DC system voltage is 220V in case of 220KV and above sub stations, 110V in case of 110KV and below sub stations.
• The rating of the Battery is in Ampere Hours (AH) on 10 hour basis and is chosen based on the DC load of the Sub Station.
• The DC system voltage is achieved by connecting number of 2V cells in series.
• The specific gravity (SG) of the electrolyte in each cell is to be 1.200 at 27°C room temperature when the cell is fully charged.
• Normally, when auxiliary supply is available, Float Charger will feed the DC loads of the Sub Station and also keeps the Battery set in floating condition by charging it.
• The float charger output is to be adjusted to 118 to 121V for 110V DC System, 236 to 242V for 220V Dc System.
• Whenever the Specific Gravity in majority of cells falls below 1.185, then the Battery set needs to be Boost Charged from the Boost Charger. The amount of Boost charge in Amperes should be min of 7% of AH to max of 12% of AH rating of the Battery Set.
• Boost charging is to be continued until the specific gravity of the cells reaches 1.200 at 27°C and the applied voltage is 2.7V per cell ie. For a battery set having 55cells, around 148V (55x2.7).
• During Boost Charging DC loads of the Sub Station will be fed from Float charger.

LTAC system consists of 11KV/400V Auxiliary Transformer, LT Switchgear, LT distribution panels, Yard lighting control panels, DG set etc.

• Used for providing Power Line Carrier Communication to establish communication between sub stations.
• Used for providing Carrier Protection.
• It consists of
  a. Coupling Capacitor & LMU
  b. Wave Trap
  c. PLCC equipment
  d. Protection Coupler
  

• The non current carrying metallic parts in every electrical sub station are connected to the under ground earthing mesh called Earth Mat at earth potential for the safety of the personnel and for discharging fault currents. This system is called ‘Equipment Earthing’.
• The station earthing should have low earth resistance; low touch potential and low step potential.
• Touch Potential is the voltage between the fingers of raised hand touching a faulted structure and the feet of a person standing on the floor of the sub Station.
• Step Potential is the voltage between the feet of a person standing on the floor of the sub Station with 0.5m spacing between the two feet (one step) during the flow of Earth Fault current through the earthing system.

• Normally, the illuminance at the work plane height around the equipments area has to be 50 Lux and the average illuminance in the whole Sub Station area need to be 20 Lux.
• The fittings used for yard lighting will have to be arranged in such a way as to get the above illumination.

• Transformers are provided with system to sense the occurrence of fire by means of sensors, generate an alarm, to disconnect the power supply to that region by means of Relays and extinguish the fire by means of High Velocity Water Spray or Nitrogen depending on the type of system provided
• Portable Fire Extinguishers of Foam type, CO2 type, Dry Chemical Powder (DCP) type of different capacity are also provided for different types of fires in the Sub Station area.