Motor differential protection (ANSI 87M, logical node PDIF in IEC 61850) compares the current entering each phase at the terminals with the current leaving on the neutral side. A difference means a fault in the stator winding or in the cables between the CTs, and the relay trips the breaker or contactor within a few cycles. It is used on large or critical medium-voltage motors with both ends of each phase brought out.
What 87M motor differential protection is
In a medium-voltage motor with all six leads brought out, each stator phase has a line end and a neutral end. Without a fault, the current entering one end leaves through the other, during starting as well. Motor differential protection measures both ends with current transformers (CTs) and trips if a difference appears: that current is flowing into a fault in the winding or in the cables inside the zone.
It is identified as follows:
- ANSI/IEEE C37.2: device 87, with the suffix M by custom (87M).
- IEC 61850: logical node PDIF.
The 87M clears internal faults within a few cycles and with high sensitivity, without waiting for the overcurrent elements to tell a fault from starting current. That limits damage to the stator iron, the most expensive part to repair.
Where it is applied
- Medium-voltage induction and synchronous motors in industry: pumps, compressors, fans, mills and crushers.
- Critical motors in power plants (boiler feed pumps, draft fans) and pumping stations.
- Motors where the phase fault current is close to the starting current and overcurrent 50/51 cannot separate them with good sensitivity.
IEEE C37.96 discusses when it is justified according to motor size, system grounding and how critical the motor is. The final decision belongs to the user and its protection philosophy. Complete motor protection is summarized in induction motor protection.
How it works
There are two common schemes:
Percentage differential. It uses six CTs, three on the line side (often in the switchgear) and three on the neutral side (in the motor terminal box). The relay calculates per phase a differential current, as the phasor sum of both ends, and a restraint current proportional to the through current. It trips when the differential current exceeds a characteristic with pickup and slope. Restraint provides stability during starting, when the current is several times rated and carries a DC component that can saturate the CTs unevenly.
Self-balancing (flux-balancing) differential. It uses one window (toroidal) CT per phase, mounted at the motor, through which both ends of the winding pass in opposite directions. Without a fault, the fluxes cancel and the secondary delivers no current. With a fault, the difference appears directly in the secondary and is detected by an instantaneous overcurrent element. Since both conductors pass through the same core, there is no ratio error between CTs and the setting can be very sensitive. Its zone covers only the winding, not the cable from the switchgear.
| Aspect | Percentage differential | Self-balancing differential |
|---|---|---|
| Current transformers | Six CTs: three on the line side, often in the switchgear, and three on the neutral side | One window CT per phase, with both ends of the winding passing through it |
| Measurement | Differential and restraint per phase, with pickup and slope | Instantaneous overcurrent on the secondary current |
| CT errors | Restraint provides stability at starting against uneven saturation | No ratio error between CTs; the setting can be very sensitive |
| Zone | Between the line-side and neutral-side CTs | Only the winding, not the cable from the switchgear |
In both cases, the trip opens the motor breaker or contactor. With contactors, the trip must be coordinated with their interrupting rating.
Setting criteria
- Rated and starting current of the motor, and the starting time, from the data sheet and from actual measurements.
- Ratio, class and burden of the CTs on the line and neutral sides. For the percentage scheme, both sets should be identical and able to carry the starting current without saturating unevenly (IEEE C37.110).
- Minimum fault to be detected, according to the system grounding.
- Slope and pickup according to the manufacturer’s recommendation and experience with starts of the same motor.
- Interruption with a contactor: avoid ordering it to break currents above its rating, by coordinating with fuses or the upstream breaker.
The actual values are defined in the protection coordination study.
How each manufacturer implements it
- Siemens SIPROTEC (DIGSI) includes the differential in motor relays with current inputs for both sides, or it is done with a dedicated differential relay.
- SEL (AcSELerator QuickSet) offers percentage differential and the self-balancing option in motor relays, depending on the model and the available inputs.
- ABB Relion (PCM600) offers it in certain variants of its motor relays.
- GE Vernova (Multilin, EnerVista) includes it in its motor relays with the additional set of current inputs.
- Schneider MiCOM/Easergy integrates it in high-end motor models.
In practice the differences are which model has the six current inputs, how restraint is defined, whether the self-balancing scheme is set as a differential or as an overcurrent element, and how it ties into the starting logic. Confirming the exact relay ordering code is the first step.
Why it trips or blocks
- Trip during starting: uneven saturation of CTs of different class or burden, very long secondary leads to the motor terminal box, or insufficient restraint.
- Trip with the motor running: reversed polarity in one CT set, phases crossed between the switchgear and the motor, or wrong ratio entered. With the motor loaded, the measured differential current should be almost zero.
- Genuine trip for an internal fault: moisture, contamination or insulation aging. Measure the winding insulation before restarting.
- Trip of the self-balancing scheme: window CT installed incorrectly (with only one conductor through it, or the cable shield handled incorrectly).
- Open CT circuit after maintenance in the terminal box.
Start by downloading the event oscillography and comparing the currents on both sides phase by phase.
How it is tested
With secondary injection, using a test set such as the Omicron CMC 356, the usual checks are the pickup, the percentage characteristic in all phases, stability with a through current similar to the starting current, operating times and the actual trip to the breaker or contactor. In the self-balancing scheme, primary current is injected through the window CT to confirm ratio and operation. During commissioning, the differential current is checked with the motor starting and under load.
Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.
Related guides
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms and Contact Designations.
- IEEE C37.96, Guide for AC Motor Protection.
- IEC 60255-187-1, Functional requirements for differential protection (motors, generators and transformers).
- IEC 61850-7-4, logical node models (PDIF).
- IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why did the 87M trip while the motor was starting?
Most often the line-side and neutral-side CTs saturated unevenly because of the DC component of the starting current. It also happens with CTs of different class or burden, reversed polarity or restraint too low for the starting current.
What is self-balancing motor differential protection?
It is a scheme with one window (toroidal) CT per phase, through which both ends of the winding pass in opposite directions. The CT only delivers current when there is a difference, which allows a very sensitive setting with an overcurrent relay.
Does the 87M replace the motor 50/51?
No. The 87M only sees faults inside its zone. Overcurrent, thermal overload, unbalance and ground fault protection are still needed for every other condition.