Power factor relay 55 for synchronous motors

What the 55 protects

In IEEE C37.2, device number 55 is the power factor relay: it operates when the power factor of an AC circuit rises above or falls below a predetermined value.

Its application as a protection is the synchronous motor. In normal operation it runs at unity or leading power factor, because its field is overexcited. If it loses synchronism (pull-out) because of a mechanical overload, a voltage dip or a reduction of the field, the rotor slips with respect to the stator field and the machine:

• absorbs reactive power from the system, at a lagging power factor;
• draws high, pulsating currents;
• subjects the shaft to oscillating torques;
• heats the amortisseur winding, which is not designed for continuous duty;
• induces voltages in the field winding.

Total loss of excitation produces the same symptom: the machine starts absorbing reactive power. The 55 recognizes that change and trips the motor, or lets it try to resynchronize for a limited time.

It is a function for large synchronous motors, such as those driving mills in mining and cement, compressors and large pumps. It should not be confused with the power factor control of a capacitor bank, which is regulation, not protection; that case is covered in automatic capacitor bank control.

Where it is applied

Operating principle

The relay receives voltages from its voltage transformers (VTs) and currents from its current transformers (CTs) and calculates the angle between them, that is, the motor power factor. It needs CTs and VTs.

To avoid acting at low currents, where the angle is unreliable, the function is supervised by a minimum current. It is enabled only with the field applied: during starting the synchronous motor runs as an induction motor, at a lagging power factor, and the 55 must be blocked.

Why the power factor gives the problem away. In a synchronous motor the field current sets the reactive power: overexcited, it delivers reactive power to the system; underexcited, it absorbs it. That is what the manufacturer’s V curves show, relating stator current to field current for each load. While the motor is in synchronism and properly excited, the power factor stays at its design value even when the load changes. If the field is lost or the rotor slips, the machine can no longer sustain that reactive power and the power factor moves toward lagging. That change is much clearer than the change in current, which rises and falls with each slipped pole.

Some controllers combine the 55 with a fast overcurrent detector to recognize pole slipping by the current pulsations.

The characteristic

It is drawn on the P–Q plane, using the motor convention:

• The normal operating point lies in the quadrant of active power consumed and reactive power delivered (leading), or close to the unity power factor axis.
• A straight line drawn at the setting angle separates the normal region from the trip region, on the side of reactive power absorbed (lagging).
• If the operating point crosses that line and stays there longer than the time delay, the function operates.

When the motor loses synchronism, the operating point does not stay still: it rotates and oscillates with each pole slip, moving in and out of the trip region. That is why the time delay and the way entries are counted matter as much as the angle.

Operating modes. Some controllers offer two modes: remove the field for a time so that the motor resynchronizes when it is reapplied, and trip if it does not succeed; or trip directly. The mode is defined by the process and the motor manufacturer.

Setting criteria

• Angle: on the lagging side, with margin from the normal operating point, including the lowest expected excitation.
• Time delay: long enough not to act on transient oscillations caused by load impacts, as in mills; shorter than the time the amortisseur winding tolerates while slipping.
• Minimum supervision current and starting block until the field is applied.
• Resynchronization or trip mode, agreed with the motor manufacturer.

Interaction with other functions

• Loss of excitation 40: monitors impedance or field current. It complements the 55 when the loss of field is partial.
• Out-of-step 78: on large machines, impedance-based pole-slip protection can replace it.
• Field application: the motor excitation controller, which applies the field near synchronous speed, enables the 55.
• Undervoltage 27: a deep voltage dip can pull the motor out of synchronism; the 27 and the 55 are coordinated so that the right one acts.
• Overload 49 and overcurrent 50/51: cover stator heating and faults; the 55 covers slipping.
• Outputs: the trip goes to the motor breaker and, depending on the mode, to field removal.

How each manufacturer implements it

• Synchronous motor controllers: the GE Vernova SPM monitors the power factor with a time delay and offers resynchronization and ride-through modes, as well as loss of field current or voltage.
• Motor relays: several manufacturers include power factor elements in their motor relays; the IEC 61850 conformance statement of the SEL-849 includes the POPF power factor node.

The practical differences are the sign convention (lagging and leading, motor or generator), how the function is enabled after starting, and the operating modes.

IEC 61850 logical node

IEC 61850-7-4 includes the POPF node, for power factor, which motor relays such as the SEL-849 declare in their conformance documentation. Other manufacturers implement the function with generic or proprietary nodes. As with any protection function, it publishes the start (Str) and operate (Op) signals, and the trip goes through PTRC to the breaker (XCBR).

How it is tested

In general, secondary injection is used to verify:

• The operating angle, rotating the current with respect to the voltage at constant magnitudes.
• The sign convention: the normal, leading operating point must not operate.
• The time delay and reset.
• The minimum supervision current.
• The block during starting and its release by the field-applied signal.
• With a state sequence, the response to an oscillating operating point.

During commissioning, the actual motor power factor is recorded at different loads and excitations.

Typical mistakes

• Wrong sign convention: the relay reads normal leading operation as lagging and trips a healthy motor.
• Reversed CT or VT polarity, with the same effect.
• No starting block: a trip on every start.
• Time delay too short on loads with impacts, such as mills.
• Resynchronization mode not coordinated with the excitation controller.
• Angle copied from another machine with a different rated excitation.

We configure your synchronous motor relay

Is your synchronous motor tripping on power factor with no clear cause? We review the events, sign convention, blocking and settings of the relay or controller remotely, worldwide. Tell us the brand and model through remote relay configuration or contact us.

Related guides

• Loss of excitation 40
• Out-of-step 78
• Undervoltage 27

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (device 55).
• IEEE C37.96, Guide for AC Motor Protection: protection of induction and synchronous motors.
• IEC 61850-7-4, Compatible logical node classes and data object classes: POPF and PTRC nodes.
• GE Vernova, SPM Synchronous Motor Protection System: pull-out protection by power factor.
• SEL, IEC 61850 conformance statement (MICS) of the SEL-849.

Frequently asked questions