Negative Sequence Relay Testing (46): Unbalance, Curves and Motor Protection

Negative sequence overcurrent testing proves that the 46 element calculates 3I2 correctly, picks up at the set unbalance, and trips on the selected inverse curve. It verifies the phase rotation, pickup, curve, time multiplier, alarm and trip outputs, and the reset behavior.

Key takeaways

• One current makes 3I2. Injecting a single phase current I with the other two phases at zero produces I1 = I2 = I0 = I/3, so 3I2 equals the injected current.
• Phase rotation matters. A reversed phase sequence changes the sign of the negative-sequence component and can disable the element or misoperate it.
• 46 is a heating element. The inverse curve models the rotor heating caused by the negative-sequence current; the time multiplier sets the trip time.
• Test the alarm and the trip. Many relays have a separate alarm stage with a lower pickup and a different delay.
• Coordinate with the motor thermal limit. The 46 curve must protect the motor before the rotor overheats, not after.

What does a negative sequence test prove?

A negative sequence element responds to the negative-sequence component of the three phase currents. Unbalanced voltages, single-phase loads, open-phase conditions, and faults produce negative-sequence current, which causes rotor heating in motors and generators.

The test proves that the relay calculates 3I2 correctly, picks up at the set value, times out on the selected curve, and operates the alarm and trip outputs. It also proves that the element does not pick up on a balanced load and that a reversed phase sequence is detected if the relay uses one.

How do you test a 46 element?

1. Record the settings. Note the pickup, curve type, time multiplier, alarm pickup, alarm delay, and trip output.
2. Inject a single phase current. Use IA with IB and IC at zero. The relay should display 3I2 equal to the injected current.
3. Verify the calculation. Compare the displayed 3I2 with the injected current at two values, for example 0.2 A and 0.5 A.
4. Find the pickup. Raise the current slowly and record the value at which the element starts. Compare with the setting.
5. Test the curve. Apply a current at 2x and 5x the pickup and measure the trip time. Compare with the calculated curve.
6. Test the alarm stage. Apply a current above the alarm pickup and confirm the alarm output and its delay.
7. Test the phase rotation. Reverse two phases and confirm the 3I2 reading and the element behavior.
8. Test the reset. Reduce the current below the drop-out and confirm that the element resets.

What settings and test points should you use?

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

For a three-phase unbalance test, inject a balanced set with one phase reduced or shifted. The relay’s 3I2 metering should match the symmetrical-component calculation. A single-phase injection is faster for the pickup and curve tests.

What is the pass criterion?

• 3I2 calculation: within 2 percent of the injected value.
• Pickup: within 3 percent of the setting.
• Time at 2x and 5x: within 5 percent or 30 ms of the calculated curve.
• Alarm: correct pickup, delay, and output.
• Phase rotation: correct 3I2 for a reversed sequence.
• Reset: within the published drop-out ratio.

Why does the element pick up on a balanced load, or fail to trip on an unbalance?

Pickup on a balanced load: the phase currents may not be balanced because of a test lead or a CT ratio error, or the relay may have a residual current that is not zero. Check the three phase currents and the neutral current.

No trip on an unbalance: the 3I2 calculation may use a different convention, the element may be disabled, or the output matrix may be wrong. Read the relay metering and the event log.

Trip too fast: the curve or time multiplier may be wrong, or the element may be operating on the alarm stage rather than the trip stage. Confirm which stage operated.

Reversed phase sequence: the relay may calculate 3I2 with the opposite sign or may block the element. Confirm the phase sequence setting and the actual wiring.

Frequently asked questions

How do I create negative sequence current with one current channel?

Inject a single phase current with the other two phases at zero. The symmetrical-component calculation gives I1 = I2 = I0 = I/3, so 3I2 equals the injected current. This is the fastest way to test the pickup and the curve.

Why is 3I2 equal to the injected current?

For a single-phase current I, the positive, negative, and zero sequence components are each I/3. Multiplying the negative-sequence component by three gives I. The relay displays 3I2, which is the quantity used for the setting.

What is the difference between 46 and 47?

The 46 element is a negative-sequence overcurrent element; it responds to the magnitude of 3I2. The 47 element is a negative-sequence overvoltage element; it responds to the negative-sequence voltage. They are related but use different inputs.

Why does a motor need negative sequence protection?

Negative-sequence current produces a reverse rotating field that induces double-frequency currents in the rotor. The rotor heats quickly, even when the phase currents are below the thermal limit. The 46 element models this heating and trips before the rotor is damaged.

Can I test the 46 element with a three-phase balanced source?

A balanced source produces no negative-sequence current, so it cannot test the element. Use a single-phase injection or an unbalanced three-phase set.

Where VA-TEK test sets fit

The VAE-430 provides three currents for the single-phase and three-phase unbalance tests. Start with the relay protection testing hub and use the IDMT curve calculator to predict the trip time.

Related reading: motor protection relay testing for the complete motor scheme, and overcurrent relay testing procedure for the phase elements.

Standards and references

• IEC 60255-151:2009 — functional requirements for over/under current protection.
• IEEE C37.96-2012 — guide for AC motor protection.
• IEEE C37.112-2018 — standard for inverse-time characteristic equations.
• Relay manufacturer’s technical manual — negative-sequence calculation and curves.