Electrical Power System Substation Components

Electrical Power System Substation Components

Substation Overview

• A substation is a crucial part of the electrical system, integrating generation, transmission, distribution, and consumption.
• It transforms voltage levels, provides switching facilities, and allows for the installation of safety devices to disconnect circuits during faults.

Substation Components

1. Lightning Arrester

   • Protects insulation and conductors from lightning damage.

   • Types:

     • Station Class: Used in power stations and high-voltage structures; protects against lightning and overvoltages; designed for equipment above 20MVA.
     • Intermediate Class: Protects against surges from lightning and overvoltages in medium voltage equipment areas such as electrical utility stations and substation transformers; designed for equipment in the range of 1-20MVA.
     • Distribution Class: Commonly found in transformers (dry and liquid-filled); used on equipment rated 1000kVA or less.

   • Specifications:

     • Rated Voltage

     • Power Frequency Sparkover Voltage

     • Standard Lightning Impulse Sparkover Voltage

     • Front-of-wave Impulse Sparkover Voltage

     • Max Residual Voltage at Nominal Discharge Current

     • Example Values:

       Arrester Rated Voltage	Power Frequency Sparkover Voltage	Standard Lightning Impulse Sparkover Voltage	Front-of-wave impulse sparkover voltage	Maximum residual voltage current impulse 8/20μS @ 10kA	@ 20kA
       60	78	108	120	126	138
       105	137	190	218	230	254
       171	207	297	330	347	380
       200	247	356	396	416	456
       180	234	301	360	378	414
       195	254	351	390	410	449
       216	410	626	726	648	703

2. Current Transformer (CT)

   • Transforms high currents to lower, manageable values for relays and instruments.

   • Insulates metering circuits from the high voltage primary system.

   • Primary Side: Where input power is received, connected to the high-voltage side (utility power or power generation system).

     • Example: In medium voltage, the primary side senses current flowing in the conductor/busbar/line.

   • Secondary Side: Where output power is delivered, connected to the lower voltage side.

     • The secondary side provides stepped-down current and voltage.
     • Example: A CT with a 2000/1 Amp ratio in a 380kV system means 2000 amps on the primary side results in 1 amp on the secondary side, with a low voltage.

   • Specifications:

     • Standard (IEC, NEMA/ANSI IEEE, BS)
     • Rated Insulation Level (service voltage)
     • Altitude above sea level (if >1000m)
     • Ambient temperature
     • Rated Primary Current
     • Rated Secondary Current
     • Rated Short-time Current
     • Number of Cores
     • Burdens (outputs) and Accuracies
     • Pollution Level

   • Example Values:

     Rated Nominal Voltage	Rated Maximum Voltage	Short Time Current Rating	Number of Cores	Ratio	Class	Standard	Installation
     230kV	245kV	40kA	1	400-300-200-100/5A	Metering: Class 0.2 30 VA, Protection: 5P10, 5P20	IEC-186	Installed in Pedestal
     115kV	123kV	40kA	1				Incorporated in Transformer or GCB (BCT)
     69kV	72kV	25kA	1

   • Burdens and Accuracies:

     • Characterization of CTs:
       • Rated primary current, e.g., 200 A
       • Rated secondary current, e.g., 5 A
       • Accuracy load: P_n = 15 VA
       • Accuracy limit factor (ALF), e.g., ALF = 10
       • For I = ALF \cdot I_n, accuracy is 5% (5P).
     • Example: 15 VA 5P 10 indicates accuracy limit factor = 10, accuracy class = 5P, accuracy power = 15 VA.

   • Metering:

     • 0.1 accuracy: laboratory use
     • 0.2 accuracy: precision revenue metering
     • IEEE C57.13: B-0.1, B-0.2, B-0.5, B-0.9, B-1.8
     • High-accuracy metering: C100 T100, C200 T200, C400 T400, C800 T800
     • 3-10% error at 20X rated current
     • Equivalent in IEC is 10P20

   • Accuracy Class of Protection CT:

     • Applications:
       • Instantaneous overcurrent relays & trip coils: 2.5VA (Class 10P5)
       • Thermal inverse time relays: 7.5VA (Class 10P10)
       • Low consumption Relay: 2.5VA (Class 10P10)
       • Inverse definite min. time relays (IDMT) overcurrent: 10P10/5
       • IDMT Earth fault relays with approximate time grading: 15VA (Class 10P10)
       • IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA (Class 5P10)

   • Metering Accuracy Class as per IEEE C37.20.2b-1994

     Ratio	B0.1	B0.2	B0.5	B0.9	B1.8	Relaying Accuracy
     50:5	1.2	2.4	–	–	–	C or T10
     75:5	1.2	2.4	–	–	–	C or T10
     100:5	1.2	2.4	–	–	–	C or T10
     150:5	0.6	1.2	2.4	–	–	C or T20
     200:5	0.6	1.2	2.4	–	–	C or T20
     300:5	0.6	1.2	2.4	2.4	–	C or T20
     400:5	0.3	0.6	1.2	1.2	2.4	C or T50
     600:5	0.3	0.3	0.3	1.2	2.4	C or T50
     800:5	0.3	0.3	0.3	0.3	1.2	C or T50
     1200:5	0.3	0.3	0.3	0.3	0.3	C100
     1500:5	0.3	0.3	0.3	0.3	0.3	C100
     2000:5	0.3	0.3	0.3	0.3	0.3	C100
     3000:5	0.3	0.3	0.3	0.3	0.3	C100
     4000:5	0.3	0.3	0.3	0.3	0.3	C100

   • Sample Current Transformer Nameplate:

     • Includes CT serial number, year of manufacture, CT type, applicable CT standard, network current characteristics, and rated insulation voltage.
     • Shows network voltage and current characteristics such as I{th} (thermal current) and I{dyn} (dynamic current).
     • Indicates ratio, accuracy, safety factor, and accuracy limit factor (ALF).

3. Potential Transformer (PT)

   • Transforms high voltages to lower, manageable values for relays and instruments.
   • Insulates metering circuits from the high voltage primary system.

• Specifications:

  • Rated Nominal Voltage

  • Rated Maximum Voltage

  • Frequency

  • Number of Windings

  • Primary Voltage

  • Secondary Voltage

  • Class (metering and protection)

  • Standard (IEC-186)

  • Example Values:

    Rated Nominal Voltage	Rated Maximum Voltage	Frequency	Number of Windings	Primary Voltage	Secondary Voltage	Class	Standard
    230kV	245kV	60Hz	2	230/\sqrt{3} kV	115/\sqrt{3} V	0.2 for metering, 3P for Protection	IEC-186
    115kV	123kV	60Hz	2	115/\sqrt{3} kV	115/\sqrt{3} V	0.2 for metering, 3P for Protection	(INDUCTION)
    69kV	72kV	60Hz	2	69/\sqrt{3} kV	115/\sqrt{3} V	0.2 for metering, 3P for Protection

• Altitude Correction Factor:

  • According to IEEE, dielectric strength that depends on air should be multiplied by an altitude correction factor.

    Altitude above sea level (m)	Altitude correction factor for dielectric strength
    1000	1.00
    1200	0.98
    1500	0.95
    1800	0.92
    2100	0.89
    2400	0.86
    2700	0.83
    3000	0.80
    3600	0.75
    4200	0.70
    4500	0.67

• Rated Insulation Levels:

  • According to IEC 61869-1:

    Max. System Voltage kV	Power frequency withstand voltage kV	Lightning impulse withstand voltage kV	Switching impulse withstand voltage kV	RIV test voltage kV	Max. RIV μV	PD test voltage kV	Max. PD pC
    36	70	170	43	52	10		10
    52	95	250	78
    72.5	140	325	129
    123	230	550	148	145	10		10
    145	275	650	174
    170	325	750	204
    245	460	1050	300
    362	510	1175	360
    420	630	1425	434
    420*	680	1550	485
    550	975	2100	800*

  • Basic Insulation Levels:

    • According to IEEE C57.13 -2008:

      Max. System Voltage kV	Power frequency withstand voltage kV	Lightning impulse withstand voltage kV	Chopped wave test voltage kV	RIV test voltage kV	Max. RIV level μV
      36.5	70	200	230
      48.3	85	250	290
      72.5	140	350	400
      123	230	550	630
      145	275	650	750
      170	325	750	865
      245	460	1050	1210
      362	575	1300	1500
      420	630	1425	1630
      550	800	1800	2070
      800	920	2050	2360

1. Disconnect Switch (DS)

   • A mechanical switching device that provides an isolating distance in the open position for safety.
   • Allows isolation of apparatus (transformers, circuit breakers) for maintenance, repairs, or adjustments.
   • Rules for rating (BS 5463):
     • Rated voltage (e.g., 72.5kV, 123kV, 145kV, 245kV)
     • Rated insulation level
     • Rated frequency
     • Rated normal current
     • Rated short time current
     • Rated peak withstand current
   • Capable of opening and closing a circuit when negligible current is broken or made.
   • Capable of making, carrying, and breaking currents under normal circuit conditions.
   • Disconnect is always in series with Gas Circuit Breaker (SF6).

2. Circuit Breaker

   • A mechanical switching device capable of making, carrying, and breaking currents under normal and specified fault conditions (e.g., short circuit).
   • Types:
     • Gas Circuit Breaker (SF6):
       • Uses Sulphur hexafluoride (SF6) gas as the arc-quenching medium.
       • Commonly used for voltages 110 kV and above.
       • SF6 is an electro-negative gas with a strong tendency to absorb free electrons.
       • Contacts open in a high-pressure flow of SF6 gas, capturing conducting free electrons to form immobile negative ions, building up insulation strength to extinguish the arc.
     • Oil Circuit Breaker:
       • Used to protect medium and high voltage systems, using oil as a dielectric or insulating medium to extinguish the arc.
       • Types: Bulk Oil Circuit Breaker (large quantity of oil for arc quenching and insulation) and Minimum Oil Circuit Breaker (small amount of oil only for arc quenching).
     • Vacuum Circuit Breaker:
       • Used to protect medium and high voltage systems where arc quenching takes place in a vacuum medium.
       • Advantages: High insulating strength, superior arc quenching properties, no exhaust of gas, and noiseless operation.
       • Disadvantages: Uneconomical at voltages exceeding 38 kV, and if produced in small quantities.
   • Technical Terms to Specify Circuit Breaker:
     • Rated Maximum Voltage: The highest RMS phase-to-phase voltage for which the circuit breaker is designed.
     • Rated Power Frequency: The frequency at which the circuit breaker is designed to operate.
     • Rated Dry Withstand Voltage: The RMS voltage that the circuit breaker can withstand for 1 minute under specified conditions.
     • Rated Wet Withstand Voltage: The RMS voltage that an outdoor circuit breaker can withstand for 10s.
     • Rated Lightning Impulse Withstand Voltage: The peak value of a standard 1.2 \times 50\mu s wave that a circuit breaker can withstand.
     • Rated Continuous Current: The current in RMS symmetrical amperes that the circuit breaker is designed to carry continuously.
     • Rated Interrupting Time: The maximum permissible interval between energizing the trip circuit and interrupting the current.

• Basic Insulation Levels:

  • According to IEEE C57.13-2008:

    Max. System Voltage kV	Power frequency with-stand voltage kV	Lightning impulse withstand voltage kV	Chopped wave test voltage kV	RIV test voltage kV	Max. RIV level μV
    36	70	200	230

  • Rated insulation levels according to IEC 61869-1:

    Max. System Voltage kV	Power frequency withstand voltage kV	Lightning impulse withstand voltage kV	Switching impulse voltage kV	RIV test	Max. RIV level μV	PD test voltage kV	Max. PD pC
    36	70	170	43				10

  • Rated Short Circuit Current (Required Symmetrical Interrupting Capability) - The value of the symmetrical component of the short-circuit current in RMS amperes at the instant of arcing contact separation

  • Required Asymmetrical Interrupting Capability - The value of the total RMS short-circuit current at the instant of arcing contact separation

  • Rated closing and latching capability - The circuit breaker shall be capable of closing and latching any power frequency making current whose maximum peak is equal to or less than 2.6 (for 60 Hz power frequency; 2.5 for 50 Hz power frequency) times the rated short-circuit current.

  • Rated Short-Time Current - The maximum short-circuit current that the circuit breaker can carry without tripping for a specified period of time.

  • Maximum Permissible Tripping Delay - The maximum delay time for protective relaying to trip the circuit breaker during short-circuit conditions.

  • Rated Transient Recovery Voltage (TRV) - At its rated maximum voltage, a circuit breaker is capable of interrupting three-phase grounded and ungrounded terminal faults at the rated short-circuit current in any circuit in which the TRV does not exceed the rated TRV envelope.

  • Normal Rating of (SF6) GAS CIRCUIT BREAKER:

    Rated Voltage	Rated SC Breaking Current (kA)	Rated Current (Ampere)
    72.5kV	12.5	800, 1250, 1600

  • (SF6) GAS CIRCUIT BREAKER Operating Duty

    • Rated Frequency : 60 Hz
    • Rated Operating Sequence: O – t – CO – t’ – CO
    • t = 3 mins for CB not intended for auto-reclosing
    • t = 3 sec for CB intended for auto-reclosing
    • t’ = 3mins
    • O – t” – CO
    • t”= 15 sec for CB not intended for auto-reclosing

1. Power Transformer

   • Transfers electric energy between the generator and the distribution primary circuit.
   • Indoor Transformer: Must be protected from weather.
   • Outdoor Transformer: Weather-resistant construction.
   • Transformer Parts:
     • Primary Bushing
     • Secondary Bushing
     • Tank Diagrammatic Nameplate
     • Drain/Sampling valve
     • Skid Base
     • Fan
     • Radiator
     • Conservator
     • Magnetic oil gauge
     • Buchholz Relay
     • Fan control box
     • Oil filling valve
     • Earthing terminal
     • Pressure relief device
   • Transformer Internal Parts
     • Core: high-permeability, grain-oriented, silicon electrical steel
     • Windings: paper insulated copper conductors wound around the core
     • Tap Changer: on-load, or off-load
   • Pole Type Transformer: Suitable for mounting on a pole or similar structure.
     • Accessories for self-protected units
   • TRANSFORMER SPECIFICATIONS
     • Preferred continuous KVA rating 3750; 5000; 7500; 10000; 12000; 15000; 20000; 25000;30000; 37500; 50000; 60000; 75000; 100000
     • RATED KVA: Output that can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature rise limitations.
     • PERFORMANCE CHARACTERISTICS: Impedance, Losses, Di-electric test levels, Temperature rise, Sound level
     • TOTAL LOSSES: The sum of no load and load losses, excluding losses due to accessories.
     • LOAD LOSS: Those losses which are incident to the carrying of a specified load. Load loss includes I^2R loss in the windings due to load and eddy currents; stray loss due to leakage fluxes in windings, core clamps and other parts.
     • NO-LOAD (EXCITATION) LOSSES: Includes core loss, di-electric loss, conductor loss due to excitation current , conductor loss due to circulation current in parallel windings.

• CORE LOSSES: The power dissipated in a magnetic core subjected to a time varying magnetizing force.
• LIQUID IMMERSED TRANSFORMER: immersed in insulation liquid
• INSULATION DRY TYPE TRANSFORMER: gaseous or dry compound insulation medium
  * Transformer Cooling
  * ONAN cooling: Oil Natural (thermo-syphon) circulation with Air Naturally circulated for cooling.
  * ONAF: Oil Natural Air Forced cooling.
  * OFAF: forced (pumped) oil circulation and forced (fan) air-cooled radiators.
  * OFW: oil forced cooling, where the oil is, in turn, cooled by water.
• Typical cooling requirements for transformers equipped with circulating oil pumps and air-cooled radiators are usually listed as:
  * 1ST LETTER 2ND LETTER 3RD LETTER 4TH LETTER
  * Cooling medium in contact with the windings, Type of Medium, Type of Circulation
  * Cooling medium in contact with the external cooling system, Type of Medium Type, of Circulation
  * O N A N, Natural Oil Cooling Natural Air Cooling Radiators
  * O N A F, Natural Oil Cooling Forced Air Cooling Fans
  * O F A F, Forced Oil Cooling Forced Air Cooling Oil Pumps Fans
  * 450/600/750 MVA 65°C
  * ONAN/ONAF/OFAF
  * 450 MVA with normal oil circulation, no fans or oil pumps on.
  * 600 MVA with normal oil circulation, fans on.
  * 750 MVA with both fans and oil pumps on.
• Winding taps adjust the turns ratio between input and output to adjust output voltage.
  • Off-load taps are for transformers that rarely require output voltage adjustments.
  • On-load taps require frequent operations when customer or system load fluctuates.
  • Off-Load Tap Changers: Most transformers associated with medium level voltage distribution for nuclear generating stations have off-circuit tap changers
  • On-Load Tap Changers: permit tap changing and hence voltage regulation with the transformer on-load.
    • BASIC INSULATION LEVEL (BIL): kV of the crest value of standard lightning impulse
    • WITHSTAND VOLTAGE: The voltage that an electric equipment is capable of withstanding without failure or disruptive discharge when tested under specified conditions.
    • INSULATION LEVEL expressed in terms of withstand voltage.
      • ROUTINE TEST: Test made for quality control by the manufacturer on every device to verify during production that the product meets the design specifications. Test performed on every transformer
        • Resistance Test at rated tap setting
        • Turns Ratio
        • Polarity and Phase Relationship
        • Excitation (No Load) Loss at 100% voltage
        • Impedance and Load Loss
        • Applied Potential
        • Induced Potential
        • Leak Test (Positive Pressure)
        • Production line full wave impulse
      • ADDITIONAL TEST that may be applicabe depending upon how and where the transformer will be used:
        • Impulse (where lightning and switching surges are prevalent)
        • Sound (important for applications in residential and office areas and that can be used as comparison with future sound tests to reveal any core problems)
        • Temperature rise of coils, which helps ensure that design limits will not be exceeded
        • Corona for medium voltage (MV) and hign voltage (HV) units, which help determine if the insulation system is functioning properly.
        • Insulation resistance (determines dryness of insulation and is done after delivery to serve as benchmark for comparison agains future readings.)
        • Insulation power factor, which is done at initial installation and every few years thereafter to help determine the ageing process of the insulation.
        • TRANSFORMER SPECIFICATIONS
          • Capacity 100MVA
          • Phase 3PH
          • Frequency 60Hz
          • Primary Voltage 115kV
          • Secondary Voltage 13.8kV

1. Secondary Switchgear

   • Switchgear includes switching devices, controls, measuring equipment, protective gear, and regulating equipment with associated connections, enclosures, and structures.
   • IEC 298 specifies requirements for factory-assembled metal enclosed switchgear and controlgear for alternating current at rated voltage above 1kV and up to and including 52kV for indoor and outdoor installation and for service frequencies up to and including 60Hz.
   • Metal-enclosed Switchgear and Controlgear: Switchgear and controlgear with an external metal enclosure intended to be earthed and complete except for external connections.
     • Note: Metal enclosed switchgear and controlgear subdivided into three types
       • Metal-clad switchgear and controlgear
       • Compartmented switchgear and controlgear (with one or more non-metallic partition)
       • Cubicle switchgear and controlgear
         • DEGREE OF PROTECTION
           • Protection against approach to live parts and contacts with moving parts IP2X
     • By fingers of similar objects of diameter greater than 12mm IP3X
     • By tools, wires etc. of diameter or thickness greater than 2.5mm IP4X
     • By wires of diameter or strips of thickness greater than 1.0mm
     • Metal-clad Switchgear and ControlgearMetal enclosed switchgear and controlgear in which components are arranged in separate compartments with metal partition intended to be earthed
     • Note 1: This applies to metal-enclosed switchgear and controlgear with metal partitions providing the degree of protection in table below (or higher) and having separate compartment
       • Cubicle Switchgear are Metal enclosed switchgear and controlgear, other than metal-clad and compartmented switchgear and controlgear.
       • Note: This term applies to switchgear and controlgear having metal enclosure and having either
         • Comparmented Switchgears are Metal enclosed switchgear and controlgear in which components are arranged in separate compartments as for metal- clad switchgears and controlgears but with one or more non-metallic partitions providing the degree of protection included in table 1 (or higher).
           • Particular of Service Conditions: Minimum & Maximum Ambient Air Temperature Altitude Unusual exposure to vapour, fumes, moisture, explosive gas Excessive Dust or Salt Risk of Earth Tremors or other vibration.
             • Particular of Installation and its Components: Indoor/Outdoor Number of phases and busbars Rated Voltage Rated Insulation Level Rated Normal Current of main bus and feeder circuits Rated Short Time Withstand Current (Isc)
             • Particular of the System: Nominal Voltage, Highest Voltage, Frequency
         • Rated Values of Components, Degree of Protection for Enclosure and Partition

• Type of Metal-enclosed Switchgear and Contro
  * IEC PUB 56-2 Rated normal current, 630; 800; 1250; 1600; 2000; 2500; 3150; 4000; 50000; 6300A
  * Gas Insulated Switchgear (GIS) is utilized in locations where space is
• SF6 is the most potent greenhouse gas despite being an inert, harmless

1. Protection Relays

   • A PROTECTIVE RELAY whose function is to detect defective lines or apparatus or other power system conditions of abnormal or dangerous nature and to initiate appropriate action in the control circuit.

• TYPES
  • ELECTRO-MAGNETIC (MECHANICAL)
• STATIC TYPE: ELECTRONIC (DIGITAL)