8.3 Three-Phase Relay Test Set, Phase-Angle Meter & Induction Disc

8.3.1 Three-Phase Relay Test Set

A three-phase relay test set is a portable, programmable source that injects controlled AC voltages and currents — typically three independent voltage channels and three (or six) independent current channels — each with adjustable magnitude, phase angle, and frequency. It is the workhorse for testing any multi-input relay: distance (21), directional overcurrent (67), differential (87), synchronizing (25), and bus or breaker-failure protection.

Common models: Doble F6150 / F6050, Omicron CMC 356 / 256plus, Megger SMRT362. All can:

• Inject prefault and fault states with one keystroke (playback of COMTRADE files).
• Sweep phase angle to find the relay's characteristic angle (RCA / MTA).
• Simulate CT saturation, DC offset, and residual current for realistic testing.
• Test communications-assisted schemes (pilot wire, 821, IEC 61850 GOOSE).

For a 67 directional overcurrent element, the test set applies load current, then a fault current at a swept phase angle, and the technician records the angle at which the element picks up — the operating zone must match the relay's characteristic-angle setting. For an 87 differential element, the test set injects matching currents on the two restraining inputs and then unbalances them to verify the percentage-differential slope.

8.3.2 Phase-Angle Meter

A phase-angle meter measures the angular difference (in degrees) between two signals — typically one voltage and one current, or two voltages. Field uses:

• CT polarity verification: compare the angle of the primary current to the secondary current; 0 degrees (in-phase) means polarity is correct, 180 degrees means reversed.
• Generator synchronizing (25): verify the angle between the incoming generator and the bus before closing the breaker.
• Load-flow direction: confirm real-power direction (kW sign) from the V-I angle.
• Phase-sequence (rotation) check: ABC rotation means phase A leads B by 120 degrees and B leads C by 120 degrees; incorrect rotation can damage motors and misoperate protective relays.

Modern power quality analyzers (Fluke 435, Dranetz HDPQ) display all phase angles automatically; older electrodynamic meters use a crossed-coil movement. A reversed CT shows up immediately as a 180-degree angle error versus the expected reference.

8.3.3 Induction-Disc Relay Principle

The classic induction-disc overcurrent relay (GE IAC, Westinghouse CO, ABB CD) operates on the same electromagnetic principle as a watthour meter. An aluminum disc is driven by flux from a CT-supplied coil; a second, shaded-pole flux is phase-shifted in time, and the interaction of the two fluxes produces a net torque on the disc. A permanent magnet across the disc provides drag (damping), and a spiral spring provides reset force.

The result is the IDMT (Inverse Definite Minimum Time) characteristic:

• Low current (near pickup): disc turns slowly, long trip time.
• High current (many times pickup): disc turns fast, short trip time.
• Very high current: the curve flattens to a definite minimum time (the disc can only spin so fast).

The time dial setting adjusts the disc's rotation distance to the trip contact — a higher time dial means more rotation and a longer trip time at the same current. Common IDMT curves per IEC 60255 and IEEE C37.112: Standard Inverse, Very Inverse, Extremely Inverse (IEC); Moderately, Very, and Extremely Inverse (IEEE). Solid-state and digital relays emulate these same curves in firmware, but the characteristic shape is the same.

8.3.4 Differential Relay (87) Principle

ANSI device 87 is differential protection. It compares the currents entering and leaving a protected zone (bus, transformer, generator, or motor). When the zone is healthy, the currents are equal and the differential current is approximately zero. When an internal fault occurs, the currents differ and the relay operates.

For a two-terminal zone (e.g., a motor or a simple bus):

• I_diff = |I_in - I_out|
• Restraint = (I_in + I_out) / 2
• The relay trips when I_diff > k x Restraint (percentage-differential characteristic, k typically 10 to 30%).

Why CT polarity and ratio matter so much for 87: through-faults should produce near-zero differential current only if the two CTs are matched, on the same ratio, and correctly polarized. A reversed polarity on one CT makes I_diff = 2 x I_through on every external fault — the relay cannot distinguish external from internal and will trip on load. A saturated CT does the same thing, which is why 87 schemes use matching CT classes (often C800 or dedicated class PS/PX) and may add harmonic restraint (2nd harmonic for transformer inrush, 5th harmonic for overexcitation).

ANSI/IEEE C37.2 device numbers you should know for this section: 50 = instantaneous overcurrent, 51 = time overcurrent, 67 = directional overcurrent, 87 = differential, 21 = distance, 25 = synchronizing-check, 79 = reclosing, 27 = undervoltage.

8.3.5 Connection to Coordination

The three tools work together in a coordination study. The TCC curve (current on X, log scale; time on Y, log scale) for every downstream device must sit below (faster than) every upstream device in the worst-case fault region. The induction-disc characteristic and its digital equivalent are what give the 51 element its inverse shape; the phase-angle meter verifies that CTs feeding directional or differential elements are polarized correctly so the protection operates in the intended direction; and the three-phase test set injects the fault currents that prove the curves and the polarities in the field. Selective coordination — the downstream feeder trips before the upstream main for a feeder fault — is only credible after all three are verified. Zone-selective interlocking (ZSI) or instantaneous restraint on the main can improve coordination where curves would otherwise overlap.