Zero-Sequence Directional Relay Testing: 3U0, Sensitivity Angle and Boundaries

Zero-sequence directional relay testing proves that a 67N element detects the direction of an earth fault from 3I0 and 3U0. It verifies the current and voltage pickup, sensitivity angle, forward and reverse boundaries, and the polarity of the zero-sequence filters.

Key takeaways

• 3I0 is a sum. Inject the three phase currents in phase, or use the relay’s dedicated 3I0 input. A single current in one phase is not 3I0.
• 3U0 is a sum. Use a broken-delta voltage input or inject three phase voltages in phase. Verify the zero-sequence value in the relay metering.
• The angle is the test. A 67N element can pick up on current and voltage and still choose the wrong direction if the polarizing quantity is reversed.
• Test both boundaries. Sweep the angle through the forward boundary from each side and prove the reverse block at the opposite angle.
• Check the filters. A broken-delta VT or a zero-sequence CT filter can invert the polarity; confirm the phase at the relay terminals, not at the source.

What does a zero-sequence directional test prove?

A zero-sequence directional element uses the phase relationship between zero-sequence current 3I0 and zero-sequence voltage 3U0 to decide whether an earth fault is in front of or behind the relay. The element is common on solidly earthed and resistance-earthed systems, where the zero-sequence quantities are large enough to polarize the element.

The test proves the pickup of 3I0 and 3U0, the sensitivity angle, the forward and reverse boundaries, and the integrity of the zero-sequence filters. It does not prove the phase overcurrent elements; test those separately.

How do you produce 3I0 and 3U0?

There are two practical methods:

1. Three-phase method. Inject IA, IB and IC at the same angle (0 degrees) into the relay’s three phase current inputs. The relay calculates 3I0 = IA + IB + IC. Inject UA, UB and UC at the same angle into the three voltage inputs to produce 3U0.
2. Dedicated input method. If the relay has a separate 3I0 current input and a broken-delta 3U0 voltage input, inject one current into 3I0 and one voltage into 3U0. This is simpler, but it does not prove the phase summation inside the relay.

Use the three-phase method for a commissioning test and the dedicated-input method for a quick functional check. Record which method you used.

How do you test a 67N element?

1. Isolate the relay and record the settings. Note the 3I0 pickup, 3U0 pickup, sensitivity angle, time delay, and the polarizing quantity.
2. Wire the current and voltage circuits. Connect the three currents in phase and the three voltages in phase. Connect the trip contact to DI A and COM+KM.
3. Verify the zero-sequence measurement. Apply a known 3I0 and 3U0 and confirm that the relay metering shows the expected values. If the relay shows zero, the phase angles or the return path are wrong.
4. Measure the 3I0 pickup. Hold 3U0 at a value above its minimum and raise 3I0 until the element picks up. Record the pickup and drop-out.
5. Measure the 3U0 pickup. Hold 3I0 above its minimum and raise 3U0 until the element picks up. Record the pickup and drop-out.
6. Measure the sensitivity angle. Set 3I0 and 3U0 to twice pickup and sweep the angle around the expected sensitivity angle. Record the operating point.
7. Sweep the forward boundary. Move the angle in 5-degree steps through the boundary. Record the last no-trip and first trip angles.
8. Prove the reverse block. Apply the same quantities at the reverse angle and confirm no trip for the full supervision time.

What settings and test points should you use?

How do you wire the VAE-430 to the relay?

If the relay has a dedicated 3I0 input, use one current channel for 3I0 and one voltage channel for 3U0. Confirm the input burden and the relay’s rated current before connecting.

What is the pass criterion?

• 3I0 pickup: within 5 percent of the setting.
• 3U0 pickup: within 5 percent of the setting.
• Sensitivity angle: within 5 degrees of the setting.
• Boundaries: within 5 degrees of the expected characteristic.
• Reverse: no trip at the reverse angle.
• Time: within 5 percent or 30 ms at the tested point.

Why is 3I0 zero, or why does the relay trip in reverse?

3I0 is zero: the three currents are not in phase, one current lead is reversed, or the relay is measuring phase current instead of the sum. Check the phase angles and the neutral return first.

3U0 is zero: the broken-delta winding is not connected, the voltage inputs are not in phase, or the relay’s zero-sequence filter is disabled. Verify the voltage metering before changing the settings.

Reverse trip: the CT or VT polarity is reversed, the sensitivity angle is set to the wrong sign, or the polarizing quantity is taken from the wrong side of the transformer. A 180-degree error in the polarizing voltage is the most common cause.

Frequently asked questions

Can I test 67N with a single current channel?

Yes, if the relay has a dedicated 3I0 input. If it does not, use three phase currents in phase; a single phase current will not produce the correct 3I0.

What is the difference between 67N and 51N?

67N is directional and uses 3U0 to decide the direction. 51N is non-directional and responds to 3I0 magnitude and time only. The test for 51N does not need the angle sweep.

Why does the relay need a broken-delta VT?

A broken-delta (open-delta) winding produces a low-impedance zero-sequence voltage that the relay can measure. During a test, a three-phase voltage source connected in phase can produce the same 3U0 without a broken-delta winding.

How do I know which angle is forward?

Use the relay manual’s connection diagram and the sensitivity angle. For a typical -45-degree characteristic, the forward region is between -135 and +45 degrees. Confirm the sign convention with a known forward test before you sweep the boundary.

Can I use the VAE-430 for a 67N test?

Yes. The VAE-430 has three current channels and four voltage channels, so it can produce 3I0 and 3U0 with the three-phase method. Use the independent UX channel if the relay needs a separate polarizing voltage.

Where VA-TEK test sets fit

The VAE-430 covers the three currents and four voltages needed for a 67N test, including the independent UX channel. Start with the relay protection testing hub and use the relay testing whitepaper for the record sheet.

Related reading: overcurrent relay testing procedure for the non-directional elements, and CT and PT testing for the polarity and ratio checks that precede this test.

Standards and references

• IEEE C37.112-2018 — inverse-time characteristic equations.
• IEC 60255-151:2009 — functional requirements for over/under current protection.
• IEEE C37.90-2005 — surge withstand capability of relay systems.
• Relay manufacturer’s technical manual — zero-sequence connection and angle convention.