Wave Trap: Learn the Purpose, Cost, and Lead Time to Procure

Purpose of wave trap

A wave trap, also known as line trap, is a band-stop or notch filter. It blocks the high-frequency carrier signal and allows 60hz power frequency. A wave trap, CCVT, line tuner, and carrier relays – work hand-in-hand implementing Power Line Carrier pilot protection.

The trap is mounted either horizontally or vertically. It is sized to handle the continuous load current and the mechanical stress generated by the high magnitude fault current for a short time period.

The GIF below shows how the carrier frequency is contained using the wave traps. The frequency that travels on the line is chosen from a range of 30kHz to 500kHz. So, while the trap is tuned to block this frequency, it allows the 60hz power frequency through.

One additional item factored into the trap design is how the inductance of the trap reacts with the capacitive elements in the substation. Because of capacitance in various equipment, shown in the image below, a series resonant circuit is formed with the inductance in wavetrap. This mini-circuit diminishes the inductive impedance leading to more carrier signal leaking through the trap.

One way to combat this issue begins by introducing a resistor into the wavetrap’s tuning hardware (one that contains the capacitor). Although adding this resistance (typically 400 ohms, matching t-line characteristic impedance) decreases the peak impedance offered to the carrier signal, it limits the resonance. This logic, however counter-intuitive, is a worthwhile trade-off.

Cost of wave trap

• 69kV 1200Amp wave trap: ~$12,000 each
• 138kV 2000Amp wave trap: ~$12,000 each
• 230kV 2000Amp wave trap: ~$25,000 each
• 345kV 3000Amp wave trap: ~$30,000 each

Note: The cost of any support structure is not included.

Lead time to procure wave trap

~20 weeks

Information on cost and lead time is for your general knowledge only. Contact vendor with your equipment specifications for actual figures.

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Substation Major Equipment

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What's the purpose of a conventional power transformer?

Transform voltage only

Transform voltage and current

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.

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

Disconnect switch

Circuit breaker

Circuit switcher

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.

Temporary overvoltage

Phase-to-ground short-circuit

Lightning strike

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.

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

Wave trap and line tuner

Wavetrap, capacitor coupled voltage transformer, and line tuner

Wave trap, capacitor voltage transformer 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.

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?

Harmonic filter bank

Install capacitor bank

Voltage regulator

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.

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?

Ideal transformer

Voltage transformer

Current 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.

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?

Add current limiting fuses in series on the switchgear

Add inductor in the transformer neutral

Buy new transformer with higher impedance (%Z)

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.

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