Zero-Sequence Overcurrent Relay Testing: 51N and 51G Inverse Curves

Zero-sequence overcurrent relay testing proves that the 51N or 51G element picks up at the set earth-fault current and trips on the selected inverse curve. It verifies the residual connection, pickup, time multiplier, curve type, and coordination with the upstream relay.

Key takeaways

• Know the connection. A residual connection sums the three phase currents; a core-balance CT measures the earth current directly. The test injection is different for each.
• 3I0 is three times the residual. The relay may display 3I0 while the test set injects one current. Confirm the convention before comparing values.
• Test the curve, not only the pickup. The time multiplier and the curve type determine the trip time at every current.
• Check the high-set element. Many 51N elements have a separate instantaneous or definite-time high-set stage for a solid earth fault.
• Coordinate at the same current. Prove the time difference between the downstream and upstream relays at the maximum earth-fault current.

What does a zero-sequence overcurrent test prove?

A zero-sequence overcurrent element responds to the earth-fault current. It is used on solidly earthed, impedance-earthed, and isolated systems. The element may be inverse-time (51N), definite-time (51G), or a combination with a high-set instantaneous stage.

The test proves the pickup, the curve and time multiplier, the high-set stage, the reset, and the coordination with the upstream relay. It does not prove the directional element; test that separately with a polarizing quantity.

How do you test a 51N or 51G element?

1. Record the settings. Note the pickup, curve type, time multiplier, high-set current and time, and the residual or core-balance connection.
2. Confirm the connection. For a residual connection, inject current into one phase and confirm the relay displays 3I0 equal to the injected current. For a core-balance CT, inject through the window.
3. Find the pickup. Raise the current slowly and record the value at which the element starts. Compare with the setting.
4. Test the time at 2x and 5x. Apply a current above the pickup for a known time and measure the trip. Repeat at a second current to prove the curve shape.
5. Test the high-set stage. Apply a current above the high-set setting and confirm the instantaneous or definite-time trip.
6. Test the reset. Reduce the current below the drop-out and confirm that the element resets.
7. Coordinate. Compare the measured time with the upstream relay at the same current. The difference should match the coordination interval.

What settings and test points should you use?

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

For a core-balance CT, pass the test lead through the CT window and return it through the window in the opposite direction, or use the manufacturer’s primary injection method. Never inject into the secondary of a core-balance CT with the primary circuit energized.

What is the pass criterion?

• Pickup: within 3 percent of the setting, or the relay manual tolerance.
• Time at 2x and 5x: within 5 percent or 30 ms of the calculated curve.
• High-set: within 5 percent of the current and 10 ms of the time.
• Reset: within the published drop-out ratio.
• Coordination: the measured interval matches the study at the maximum earth-fault current.

Why does the relay trip too fast or too slow?

Wrong curve or time multiplier. A 51N element set to IEC SI trips at a different time from an IEC VI element at the same multiple. Confirm the curve name, not only the dial.

Residual sum error. If one phase current is reversed, the residual current is wrong and the element may pick up on a balanced load. Check the polarity of all three phase CTs.

High-set stage active. The element may have tripped on the high-set stage, not the inverse stage. Read the event log to see which stage operated.

Core-balance CT ratio. A core-balance CT has its own ratio; the relay setting must be in terms of the secondary current. Confirm the ratio before comparing the test current with the setting.

Frequently asked questions

What is the difference between 51N and 51G?

Both are earth-fault overcurrent elements. The suffix N usually means a residual connection from three phase CTs; G usually means a core-balance or ground CT. The function is similar, but the measured current and the ratio are different.

Why does the relay show 3I0 instead of I0?

The relay may display the zero-sequence current as three times the zero-sequence component. If the test set injects 1 A into one phase, 3I0 is 1 A in a residual connection. Confirm the convention in the relay manual.

Can I test a zero-sequence element with one current channel?

Yes for a residual connection, because one phase current produces a residual current equal to the injected current. For a core-balance CT, you need to pass the conductor through the window or use a primary injection set.

Why does the element pick up on a balanced load?

The residual sum should be zero on a balanced load. A pickup indicates a reversed CT, a wiring error, or a relay setting that is too sensitive. Check the measured 3I0 with a balanced injection.

Do I need to test the directional element at the same time?

No. The directional element needs a polarizing voltage or current; test it separately with the appropriate source. This procedure covers the non-directional overcurrent element.

Where VA-TEK test sets fit

The VAE-430 provides three currents and a neutral return, enough for a residual connection and a single-phase earth-fault injection. Start with the relay protection testing hub and use the IDMT curve calculator to predict the trip time before the test.

Related reading: zero-sequence directional relay testing for the directional element, and overcurrent relay testing procedure for the phase elements.

Standards and references

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