Power Transformer: Learn the Purpose, Cost, and Lead Time to Procure

Purpose of transformer

Transformers are designed to do one of the following:

• Step-up voltage; consequently step-down current – enables long-distance transmission by reducing I^2\times R copper losses. Step-ups are found near generating stations.

• Step-down voltage; consequently step-up current – enables distribution of power to customers at a safe voltage magnitude. Step-downs are found near load centers and in sub-transmission systems.

• On distribution transformers, when equipped, the load tap changer keeps the secondary voltage constant as the load increases or decreases.

• The transformer is also designed for special applications such as
  • phase-shifting transformer (pushes more power on the transmission line by varying power-angle \sin\delta – learn more)
  • HVDC converter transformer
  • Static Var Compensator (SVC) transformer (VARs generated by capacitive and inductive elements at medium voltage, you still need a transformer to step-up when tying these elements to the high-voltage grid)
  • grounding zig-zag transformer (provide a path for zero-sequence current in an ungrounded system)

Cost of transformer

The cost of a power transformer varies significantly based on BIL rating, MVA rating, core design, guaranteed losses requirement, tank design, etc. The prices listed below are for a standard design transformer (i.e., standard sound – core type transformer).

• Small power transformer with LTC – 10MVA or lower: ~ $600,000
• Medium power transformer with LTC – 10MVA to 50MVA: ~$800,000
• Large power transformer with LTC – 50MVA to 100MVA: ~$1,500,000
• Large power transformer – 100MVA or larger: ~ $2,500,000
• Specialty phase shifting transformer – 100MVA+: ~$4,000,000

Lead time to procure transformer

~ 1 year, regardless of MVA rating.

Information on cost and lead-time are rough estimates — contact vendor with your equipment specifications for actual figures.

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Substation Major Equipment

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You are building an interconnect station, tying wind farm generation to the power grid. At the substation, the sources are tied through a tie-breaker. Which device would you install to make sure the sources are synchronized?

Current transformer

Voltage transformer

Ideal transformer

Power transformer

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

A synchronization check is done using a relay. It needs low voltage input (from both sides), obtained using a voltage transformer a.k.a. instrument transformer. Learn more.

An industrial facility is plagued with voltage flicker (due to harmonics) and poor power factor at the service entrance. Which device would you install to fix both issues?

Voltage regulator

Install capacitor bank

Harmonic filter bank

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

Cap bank fixes the poor power factor only. It does provide a low impedance path for harmonics but it is not tuned to filter them out. A regulator does not remove the harmonics and neither does it fix the power factor. A filter bank addresses both - it is made up of inductors and capacitors therefore tunable and also capable of releasing VARs. (Note: one manifestation of harmonics is voltage flicker). Learn more.

In a distribution substation, the available ground-fault current on the low side of the step-down transformer is excessive, over-dutying the switching equipment. What is your cost-effective solution to reducing the fault current?

Buy new transformer with higher impedance (%Z)

Add current limiting fuses in series on the switchgear

Add inductor in the transformer neutral

Add series inductors

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

All methods mentioned help with limiting the fault current. The cheapest though is a neutral inductor, especially when mounted directly on the transformer. Learn more.

Current limiting fuses directly on the transformer secondary are rare. Mostly because of the continuous current requirement (1000amps or more) - fuses are rarely fabricated this large. This was a trap answer.

Which combination of devices would you use to implement power line carrier?

Wavetrap, capacitor coupled voltage transformer, and line tuner

Wave trap, capacitor voltage transformer and line tuner

Wave trap and line tuner

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

You need a device that allows a certain frequency to pass (CCVT and line tuner) and a device that stops it (wavetrap). Note a capacitor voltage transformer does not come with coupling accessories as such it does not work for transmitting carrier frequencies.

Click here for more detail on wavetrap and here for CCVT.

A transformer develops an internal fault. Which device would you install on the transformer high-side to interrupt fault current?

Either a breaker or a circuit switcher

Circuit switcher

Disconnect switch

Circuit breaker

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

Both the breaker and the switcher have an interrupting chamber that can safely interrupt the fault current. A switch does not. A switch will draw an arc, that is damaging and a safety hazard. Learn more about switches here and circuit switchers here.

For which event should a surge arrester operate? Check all that apply.

Phase-to-ground short-circuit

Lightning strike

Temporary overvoltage

Switching surge

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

Surge arresters are specified to discharge lightning strikes and switching surges. The equipment/system insulation can handle temporary overvoltage. Learn more.

What's the purpose of a conventional power transformer?

Transform voltage and current

Transform voltage only

Transform power, voltage, and current

Transforms voltage and current only. Power remains the same on both sides. Learn more.

Transforms voltage and current only. Power remains the same on both sides. Learn more.

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