Motor Protection Relay for High Voltage Induction Motor

A motor protection relay for a high-voltage induction motors feeder trips the breaker when current, stall or earth-fault measurements show a winding or driven-load fault. IEEE Std C37.96 is the usual AC motor protection guide. Plants often choose high voltage for ratings above about 250 HP so operating current and cable size stay lower. That 250 HP cut-off is a site heuristic, not an IEC voltage-class limit. IEA motor-system work treats asynchronous induction machines as the usual industrial type; the page’s “over 90% of industrial motors” figure is not an IEEE statistic.

Why We Require Protection of Motors?

A failed high-voltage motor has these costs:

• Loss of production (Cost of production)
• Replacement of motor (Replacement cost)
• Cost of repair
• Cost of man hours due to this emergency

The primary function of a protective relay is to detect a fault and isolate the damaged branch so the rest of the network can stay in service. This improves the reliability of the power system.
For protection of motor, match each failure mode below to a relay element on the motor feeder.

• Thermal stress on winding
• Single phasing
• Earth fault
• Short circuit
• Locked rotor
• Number of hot starts
• Bearing failure

Brief descriptions of the different failures are given below:

• Thermal Stress on Winding –
  Current above rating overheats copper and insulation and shortens winding life. IEEE C37.96 treats that heat as the usual life-limiting mechanism. Low voltage at rated shaft load also raises current and heat.
• Single Phasing –
  Loss of one supply phase on a 3-phase motor is single phasing. The remaining phases carry extra current. Starting or running on load in that state can burn the winding.
• Earth Fault –
  If winding insulation contacts earth, residual or core-balance current appears. Running or starting into that fault can destroy the stator.
• Short Circuit –
  Contact between two phases of a three-phase winding, or between turns of one phase, is a short circuit. Phase overcurrent, and on larger machines differential current, is used to clear it.
• Locked Rotor –
  A jammed driven machine or shaft is a locked-rotor (stall) condition. Starting into stall produces near locked-rotor current with little cooling, so the relay must trip before the hot stall time on the motor data sheet.
• Number of Hot Start –
  Each motor has a limited number of hot starts. A hot start is a restart before the winding has cooled. The thermal model and starts-per-hour limits in a numerical relay must follow the motor’s allowed start sequence.
• Bearing Failure –
  A failed bearing can let the rotor rub the stator and tear insulation. A Bearing Temperature Detector (BTD) or RTD trips the motor when bearing temperature leaves the set band.

Most motor protection relays use stator current as the main input. A motor protection relay on a high voltage motor typically includes:

• Thermal overload protection
• Short circuit protection
• Single phasing protection
• Earth fault protection
• Locked rotor protection
• Number of start protection

Settings need the CT ratio and the motor full-load current, plus start time and stall time from the data sheet. The numbers below come from one plant setting sheet, not from IEEE C37.96 defaults. Confirm each pickup against the motor curves and the chosen relay manual.

• Thermal over Load Element –
  Set the thermal pickup from continuous load as a fraction of full-load current, using the service factor and the relay thermal model.
  
• Short circuit Element –
  The original sheet used 1 to 5 times starting current, often 2 times starting current with 0.1 s delay. Numerical relays such as the GE Multilin 469 set phase short-circuit pickup in multiples of CT primary (brochure range 2.0 to 20.0 × CT) so the element sits above start current. Check locked-rotor current and CT saturation before copying the sheet values.
• Single Phasing Element –
  This element trips on current unbalance among the three phases. IEEE practice uses negative-sequence or percent unbalance (GE 469 example pickup 4 to 40% UB), not a fraction of starting current. The original 1/3 starting-current pickup, raised to 1/2 if it trips during start, is a local workaround. Recalculate from the relay’s unbalance definition.
• Earth Fault Protection –
  This element measures residual current from star-connected CT secondaries, or current from a core-balance CT. GE 469 lists residual or core-balance inputs and an instantaneous ground range of 0.1 to 1.0 × CT primary. The original 0.02 to 2 × CT primary band, 0.1 × with 0.2 s, and 0.5 s if it trips at start, is one setting sheet. Sensitive earth-fault pickup must also respect charging current and CT errors.
• Locked rotor protection –
  Stall pickup is typically a multiple of full-load current (GE 469 mechanical jam 1.01 to 3.00 × FLA, blocked during start). Time delay must exceed documented start time. “Starting time means the time require by the motor to reach its full speed.” The original 1 to 5 × FLC and 2 × FLC example still needs the hot stall time from the motor curve.
• Number of hot start protection –
  Enter the allowed starts in a set interval so the rotor is not restarted while it is still hot.

The schematic diagram to connect a motor protection relay is as below

Numerical relays add undercurrent (no-load / load-loss) and stator or bearing thermal protection from RTDs.
Undercurrent trips if running current stays below a set fraction of full load after start. RTD or BTD probes trip if bearing or winding temperature exceeds the set point.