Relay Protection Academy Module 10 of 25
Module 10 Advanced

Unit Protection
of Feeders

⌛ ~2.5 hours 📚 Merz-Price / IEC 61850 📑 12 slides

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Module 10 / 10.1

Unit Protection Principle

Merz-Price Principle
Current entering the zone must equal current leaving. A non-zero vector sum means fault current is flowing inside the zone.
Zone Selectivity
No coordination with adjacent zones. Operates in 20-60 ms regardless of fault severity or location within the zone.
Immune to Load
Only differential current trips the relay. Through-load and through-fault current produce zero differential.
Communication Channel
A dedicated channel between zone boundaries is mandatory. This is the key trade-off versus overcurrent protection.
Convention of direction Current flowing into the zone is positive. Through-fault: one end positive, one negative - sum zero, relay restrains. Internal fault: both ends positive - sum non-zero, relay operates.

Module 10 / 10.2

Circulating Current vs. Balanced Voltage

Circulating Current
Through-fault current circulates in the pilot loop, bypassing the relay. Internal faults produce "spill current" that flows through the relay. Normal CTs can be used.
Balanced Voltage
CTs act as voltage sources; through-fault voltages cancel - no pilot current flows. Internal faults upset the balance. Requires transactors (gapped-core CTs) to avoid open-circuit saturation.
High-impedance principle A large stabilising resistor in series with the relay limits voltage during CT saturation on through-faults, while internal faults drive the voltage well above the setting - simple, stable threshold.

Module 10 / 10.3

Spill Current and Percentage Bias

CT Mismatch
Nominally identical CTs draw slightly different exciting currents, producing unequal secondary outputs.
Unequal Burden
Different pilot loop resistances alter knee-point and saturation behaviour at each end.
Transient Saturation
One CT may saturate during a severe through-fault while the other does not, producing large transient spill.
Tap Changer Variance
OLTC deviation from nominal tap creates a standing differential proportional to tap position error.
Percentage bias - operating criterion
As through-fault current rises, the trip threshold rises with it - stable against spill at high current, sensitive at low current.

Module 10 / 10.4

Dual-Slope Bias

Trip criterion - lower region ()
Trip criterion - upper region ()
Is1 - sensitivity floor
Minimum differential to trip at zero bias. Set above line charging current (typically 2.5x).
k1 - lower slope (20-30%)
Covers steady-state mismatch: tap changer variance, CT ratio error, unequal burden.
Is2 - slope crossover
Transition from k1 to k2. Typically 1.0-2.0 p.u. of rated current.
k2 - upper slope (80-150%)
Stability against transient CT saturation - spill can reach 30-40% of through-fault current.
⎓
Diagram needed

Dual-slope bias characteristic graph. X-axis: Ibias (p.u.), Y-axis: Idiff (p.u.). Show: horizontal line at Is1 from origin to left, then slope k1 from Is1 breakpoint, steeper slope k2 starting at Is2. Shade operate region (above the line) and restrain region (below). Label: Is1, Is2, k1 slope, k2 slope, Operate, Restrain. Dark background, clean coordinate graph.

Module 10 / 10.5

Digital Differential: Time Sync

Ping-pong round-trip equation
Symmetric path: one-way delay = half round-trip minus remote processing delay .
GPS Synchronisation
Both relays lock to GPS (or IEEE 1588 PTP). Absolute time-stamps on each sample - no path symmetry assumption needed.
SONET/SDH - ping-pong fails Transmit and receive paths may route through different nodes. - false time offset creates apparent differential current and risks false trip or failure to operate.
Numerical Relay Channel
Fibre or multiplexed digital link. Samples tagged with timestamps; differential computed on matched pairs from both ends.

Module 10 / 10.6

Transformer Feeder: Vector Group Compensation

The Problem
Delta-star winding introduces a 30-degree phase shift. Without compensation, a standing differential current exists under healthy load - continuous false trip.
Software Compensation
A transformation matrix rotates digitised samples by 30 degrees. Replaces hardware interposing CTs with no additional error sources.
Zero-sequence trap - select the correct option Two software settings both correct the 30-degree shift. Only one traps LV zero-sequence:
  • Option A (wrong): HV = Yd1, LV = Yy0 - corrects phase, does NOT trap LV zero-seq
  • Option B (correct): HV = Yy0, LV = Yd11 - corrects phase AND traps LV zero-sequence
Option A causes false trips on external LV earth faults.

Module 10 / 10.7

Ratio Correction Factors

Scaling formula - both ends must present 1.0 p.u. at full load
HV Side (350 A rated, CT 400/1)
Secondary at full load = 350/400 = 0.875 A
Correction = 1/0.875 = 1.14
LV Side (1050 A rated, CT 1250/1)
Secondary at full load = 1050/1250 = 0.84 A
Correction = 1/0.84 = 1.19
OLTC variance Correction is set for nominal tap. OLTC deviation creates standing spill - k1 slope must absorb the maximum tap departure without tripping.

Module 10 / 10.8

Phase Comparison Schemes

Through-fault / Load
Phase difference ~180 deg (current in at one end, out at the other). Restrain.
Internal Fault
Infeed from both ends - phase difference ~0 deg. Trip.
Modulating quantity
With , NPS (fault) current dominates PPS (load) - correct operation under heavy load.
Keyhole problem Line capacitance leads by 90 deg. At light load, apparent phase shift can exceed the trip threshold - false trip. Keyhole characteristic restricts operation below a minimum current where the measurement is unreliable.

Module 10 / 10.9

Charging Current and Is1

Charging current
= line length (km), = specific charging current (A/km).
Is1 minimum setting rule
Factor of 2.5 covers asymmetric capacitance distribution and line data tolerances.
Worked example
25 km, 33 kV line; = 0.065 A/km; CT 400/1:
In practice Minimum relay setting is typically 0.20 p.u. - far above the calculated requirement. Overhead line charging current is rarely the limiting factor.

Module 10 / Worked Example

Transformer Feeder: 33/11 kV Settings

Given data 33/11 kV delta-star transformer. HV rated 350 A, LV rated 1050 A. HV CT 400/1, LV CT 1250/1. 100 m cable (negligible charging). Direct fibre link.
Step 1 - Vector compensation
Delta-star = 30 deg shift. Select: HV = Yy0, LV = Yd11. Yd11 corrects phase and traps LV zero-sequence.
Step 2 - Ratio correction
Step 3 - Bias settings
, , ,
Step 4 - Synchronisation
Direct fibre path confirmed symmetric. Use ping-pong method.
Final settings HV = Yy0, LV = Yd11 | k_HV = 1.14, k_LV = 1.19 | Is1 = 0.20 p.u., k1 = 20%, Is2 = 2.0 p.u., k2 = 150% | Sync: ping-pong

Module 10

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