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Network Equipment. Presented By: Jock Moffat Network Protector Consulting Services, LLC. Network Equipment. Transformers Protectors Relays. Transformers Protectors Relays. Network Equipment. Network Transformers . General Information

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network equipment

Network Equipment

Presented By: Jock Moffat

Network Protector

Consulting Services, LLC

network equipment1
Network Equipment
  • Transformers
  • Protectors
  • Relays
network transformers
Network Transformers

General Information

  • Specifications
  • Voltage Ratings
  • KVA size
  • Liquid Cooled or Dry Type
  • Submersible
  • Padmounted
  • Network Protector Throat
  • ANSI C57.12.40 specifies the requirements for liquid immersed subway and vault type network transformers.
    • Subway type are suitable for frequent or continuous submerged operation.
    • Vault type are suitable for occasional submerged operation.
voltage ratings
Voltage Ratings


  • Ratings available from 4 kV to 34.5 kV
  • Connections available: grd-WYE or Delta

Grid Network applications

  • secondary – 208Y/120VAC or 216Y/125 VAC

Spot Network applications

  • secondary – 480Y/277 VAC or 600Y/347 VAC
kva ratings
kVA Ratings
  • Available kVA ratings:

Low Voltage 216Y/125V - 300, 500, 750 and 1000 kVA

Low Voltage 480Y/277V - 500, 750, 1000, 1500,

2000 and 2500 kVA

transformer fluids
Transformer Fluids

History – In 1932 polychlorinated biphenyl (PCB) was the insulating fluid of choice.

  • Very good insulating and thermal characteristics
  • High flash point
  • EPA prohibited use of PCB fluid in the 1970’s

Mineral Oil is the most common insulating fluid in use.

Buildings with inside vaults needed high fire point (non flammable) fluids. For a premium price the available choices are:

  • silicone based fluids
  • Ester based (vegetable oil) fluids
dry type transformers
Dry Type Transformers

Taller buildings required transformers to be installed inside the building many floors above ground level.

  • Good News - Dry Type transformers are non flammable due to the absence of insulating oil.
  • As technology improved higher voltages up to 35KV became available.
  • Not So Good News – Dry Type transformers are very expensive
  • Dry types typically have a fan cooled 80ºC rating.
  • Dry type transformers can be constructed with a secondary throat to support a network protector.
location location location
Location Location Location

Will the transformer be subjected to water?

  • Often – subway type equipment is more costly to purchase but will provide the longest service life.
  • Occasionally – vault type equipment is less costly to purchase and will perform well in the occasionally damp environment.
  • Not subjected to water – a padmounted network transformer or a dry type network transformer.
transformer low voltage side
Transformer – Low Voltage Side
  • Most common installations for network protectors is to be mounted directly to the transformer utilizing the bolted connection throat.
  • A standardized throat allows equipment to be interchangeable from different manufacturers.
  • Submersible units require the throat design to make a water tight equipment connection.
  • If a network protector is not attached, then a type CST transformer can be utilized having LV bushings.
transformer tank low voltage throat
Transformer Tank – Low Voltage Throat

Standardized throat design allows equipment to be interchangeable with different manufacturers.

transformer tank high voltage side
Transformer Tank – High Voltage Side
  • Primary terminal chamber provides a water tight cable entrance to HV terminals.
  • Commonly customers request a 3 phase primary rotary oil switch, constructed under the cable terminal chamber.
transformer tank
The main tank, terminal chamber and high voltage switch are three separate compartments.

The main tank and high voltage switch will be provided with sampling valves to assist in oil testing.

Usually the transformer nameplate will be attached to the outside of the high voltage chamber to allow for easy viewing.

Transformer Tank
high voltage disconnect switch
High Voltage Disconnect Switch

3 Pole, 3 Position Switch

  • CLOSED– connects high voltage feeder to transformer input.
  • OPEN – disconnects high voltage feeder from transformer input.
  • GROUND – connects high voltage feeder to ground.
high voltage disconnect switch con t
High Voltage Disconnect Switch(con’t)

Types of Switches available

  • Dead break switch
  • Electrical interlock on dead break switches are provided to keep switch handle from being moved while secondary of transformer is energized.
  • Mag- break switch
    • capable of interrupting transformer exciting current

Use 2 electrical interlocks.

transformer instrumentation
Transformer - Instrumentation

Top Oil temperature indicator

  • Displays current temperature
  • Drag hand displays the maximum temperature since the drag hand was manually reset
  • Must be submersible

Oil Level gauge

  • Magnetic style
  • Must be submersible
notes about transformer sizing
Notes about Transformer Sizing
  • Remember when sizing a network transformer allow enough kVA for contingency peak load periods.
  • Emergency overload capabilities for distribution transformers are published in ANSI C57.12.93.
ansi standards
ANSI Standards
  • No load taps are often specified as 2 – 2 1/2% taps above and below nominal voltage.
  • Customers have the option to specify 1 tap above and 3 taps below nominal voltage.
  • Impedances are specified by the standard.
  • Either 5% or 7% nominal by kVA rating
temperature rating
Temperature Rating
  • Network transformers temperature rise is specified at 55 deg C maximum rise in temperature over ambient.
  • The standard specifies the transformer shall be able to withstand a 65 deg C rise without loss of life, basically an overload capability of 12% over full load.
maintenance for network transformers
Maintenance for Network Transformers
  • Fluid in tank
  • Fluid in high voltage switch compartment
  • Fluid in cable termination chamber
  • Check for oil leaks
  • Check for corrosion

Most utilities establish annual vault inspection schedules and perform visual inspections and minor maintenance during this visit.

transformer fluid
Transformer Fluid

Perform dielectric and DGA testing

  • Greater than 30KV for new oil
  • Most common contaminants that affect the insulating properties of oil are:
    • Moisture – usually from water entering the tank
    • Hydrogen under partial discharge
    • Hydrogen, methane under heating
    • Acetylene and hydrogen under arcing
switch compartment fluid
Switch Compartment Fluid
  • The high voltage switch compartment contains oil.
  • This oil is contaminated with carbon every time the switch contacts close to energize the transformer.
  • A sample of oil is usually periodotically taken for moisture content and DGA.
notes about submersible equipment
Notes about Submersible Equipment
  • Regular inspections are a MUST.
  • An oil sheen on top of the water in the vault is not a good sign.
  • The source and size of the oil leak must be found.
  • Schrader valves can be helpful by allowing nitrogen to be used to pressurize the tank. The name plate usually has the maximum design pressure for the tank structure. A nominal pressure of 5 pounds can assist in finding a leaky bushing or gasket.
notes about submersible equipment1
Notes about Submersible Equipment
  • When the transformer load goes down, the transformer cools and the internal tank pressure will go negative with respect to the pressure outside the tank.
  • Negative tank pressure will pull water or moisture into the tank and contaminate the oil.
network equipment3
Network Equipment
  • Transformers
  • Protectors
  • Relays
network protector
Network Protector

What is a network protector ?

A network protector is an automatic air circuit breaker installed between the secondary side of the network transformer and the secondary network.

It must open when the feeder is opened or faulted.

reverse current
Reverse Current

The protector should open to prevent reverse current flow from the secondary network to the substation. Back-feed current is an abnormal condition and a safety hazard.

The protector should close when conditions allow power to flow from the transformer to the secondary network.

available enclosures
Available Enclosures
  • Submersible protectors maybe mounted to a network transformer with a throat or mounted to the vault wall.
  • Non-Submersible protectors maybe be mounted to a network transformer with a throat or mounted to the vault wall or mounted in a separate room.
  • Rain-tight enclosures can be used outdoors, but are not submersible.
load ratings
Load Ratings
  • Network protectors are available in three frame sizes:
    • Small - 800 to 2,000 amps
    • Medium - 2,500 to 3,000 amps
    • Large – 3,500-4,500 amps
  • Voltage Ratings (U.S. utility standards)
    • 216Y/125 VAC or 480Y/277 VAC
short circuit interrupting ratings
Short Circuit Interrupting Ratings

Interrupting rating must exceed the transformer maximum thru-fault value.

  • 800-2000A – 30,000 to 35,000 amps
  • 2500-3000A - 40,000 to 65,000 amps
  • 3500-4500A - 60,000- 85,000 amps

Note: (consult nameplate for actual ratings)

low voltage bushings
Low Voltage Bushings
  • Mounted on top of protector enclosure.
  • Solid copper, NEMA spade or stud
  • Have epoxy or porcelain insulators.
the circuit breaker
The Circuit Breaker
  • Protectors are not like typical LV circuit breakers, they do not have a forward over-current rating. The internal fuses are there to prevent equipment failure if the breaker malfunctions and fails to open during a primary feeder failure.
  • Modern protectors- spring close (motor charged) and spring open.
  • Legacy protectors- motor close (soft close) and spring open.
maintenance notes
Maintenance Notes
  • Regular inspections are a MUST.
  • Pay attention to the operations counter.
    • Too many operations could be pumping (multiple operations)
    • Too few operations must be verified to confirm functional unit.
  • Inspect mechanism for damage or loose parts.
  • Inspect arc chutes and arcing contacts.
  • Inspect main contacts
  • Inspect wiring for loose terminals.
maintenance notes submersible units
Maintenance Notes – Submersible Units
  • Keep the low voltage terminal connections free of debris.
  • Inspect enclosure for corrosion.
  • Water or oil inside a protector enclosure is not a good sign.
  • The source of the leak must be found.
  • Schrader valves can be helpful by allowing nitrogen to be used to pressurize the enclosure. A nominal pressure of 5 psig can assist in finding a leaky bushing or gasket.
  • Reduce the pressure to 3 psig to avoid over pressures once the internal temperature increases.
maintenance notes dust tight units
Maintenance Notes – Dust Tight Units
  • Keep the low voltage terminal connections free of debris
  • Inspect enclosure for dirt and debris.
test set and spare parts
Test Set and Spare Parts
  • Regular use of a network protector test set will verify the relay settings and confirm proper operation.
  • Excessive repair time can be reduced with a spare parts stocking strategy.
  • Older units removed from service can be an excellent source of spare parts for other units installed in the field.
network protector notes
Network Protector Notes
  • Network protectors are air insulated devices:
    • therefore the network protector has no overload capacity above nameplate rating.
    • A great amount of heat is generated inside the enclosure when operating near and at rated current carrying capacity.
    • The wiring harness must be properly secured to prevent damage during operation and racking out procedures.
network equipment4
Network Equipment
  • Transformers
  • Protectors
  • Relays
network protector relaying
Network Protector Relaying

History Lesson

  • 1934-1986- protectors used separate electro mechanical relays (master & phasing)
  • Next generation advanced to solid state based relaying. Tempo design
  • Present generation utilizes microprocessor based relaying.
relay function trip
Relay Function – TRIP
  • Makes its “Trip Contact” to trip the breaker when reverse current is sensed above a predetermined setpoint.
  • Reverse current is when current flows from the low voltage network system thru the protector and transformer and into the primary feeder cable.
relay function close
Relay Function - CLOSE
  • The relay makes its “Close” contact when:
    • The transformer secondary voltage is greater than the network system voltage, and--.
    • The phase angle of the transformer voltage compared to the network system voltage is leading.
master and phasing relay functions
Master and Phasing Relay Functions
  • Master relay function is essentially a three phase device with a directional watt or directional watt-var trip characteristic
    • When protector is closed it monitors current for tripping conditions.
    • When protector is open it monitors phasing voltage for closing conditions.
  • Phasing relay ensures that the phasing voltage phasor is leading.
time delay relay functions
Time delay Relay Functions
  • When elevators or other regenerative equipment is connected to the system, during light load conditions multiple reverse current events can occur
    • The user has the option to time delay the tripping function if only small amounts of reverse current are detected. This will eliminate numerous breaker mechanism operations that drastically increase maintenance.