Differential Protection Simulator

Simulate 87T transformer and 87B bus differential protection — adjust slope, pickup, and harmonic restraint, then drive internal faults, external faults, and inrush to see the operate/restraint characteristic respond in real time.

AVAILABLEAUTOMATED CHECKS ONLYSTANDARDS-AWAREADVANCEDIEEENETA ATSWHAT DO THESE STATUSES MEAN?

87T / 87B DIFFERENTIALSLOPE + HARMONIC RESTRAINOPERATE vs RESTRAINTLIVE WAVEFORMS

Launch lab Sign in to bookmark Report an issue with this lab Try a guided scenario

Overview

Differential protection is the most sensitive and selective scheme available for transformers and buses, but the operate-vs-restraint characteristic at its heart is often poorly understood until you can see it move. This simulator gives you a live percentage-differential characteristic: set load, CT ratio, differential pickup, percentage slope, and harmonic restraint threshold, then inject currents to drive the operating point above or below the trip boundary. The Transformer tab applies 87T-specific logic including CT mismatch correction, magnetizing inrush harmonic restraint (2nd harmonic), and tap compensation. The Zone tab displays the protected zone boundary and shows which fault locations fall inside versus outside differential protection coverage. The Scenario tab provides guided fault conditions including through-faults, internal faults, CT mismatch, and inrush. Current waveforms, an operate-vs-restraint chart, and a current balance breakdown panel are updated in real time. An essential tool for engineers designing protection schemes and technicians verifying differential relay commissioning test results.

App preview

A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

Inside the lab

Example data — not a real field report

Overview with system diagram and live current metrics — Overview · Zone diagram, entering vs leaving currents, and the operate/restraint summary at a glance.

Operate vs Restraint chart with internal fault above the trip line — Operate / Restraint · The percentage-slope characteristic with a live operating point — see exactly why an internal fault trips and a through-fault does not.

Protection Zone view showing inside vs outside fault coverage — Protection Zone · Visualize the CT-bounded zone and compare faults inside vs outside the differential coverage.

Scenario Lab with guided fault presets — Scenario Lab · Guided cases — normal load, internal fault, external through-fault, CT mismatch, inrush, and borderline conditions.

Transformer Differential view with tap compensation and inrush restraint — Transformer Diff · 87T-specific logic — CT mismatch correction, tap compensation, and 2nd-harmonic inrush restraint.

Learn Mode covering slope rationale and inrush behavior — Learn Mode · Why percentage slope beats a fixed pickup, and why inrush looks like a fault but shouldn't trip.

What you'll do

01Explain the operate-vs-restraint characteristic and why a percentage slope is used instead of a fixed pickup
02Distinguish an external through-fault from an internal fault using differential current and the operating-point chart
03Describe why magnetizing inrush produces large differential current but should not trip the 87T relay
04Set harmonic restraint percentage and explain the trade-off between security and dependability
05Identify the protected zone boundary for transformer and bus differential schemes
06Interpret CT mismatch as false differential current and explain how tap compensation corrects it

Try a guided scenario

Apply faults inside and outside the differential zone.

Differential protection · Protection & Relay Coordination

No sign-in required. Progress is saved on this device.

What this lab is not

Not a design document, stamped calculation, or approval record.
Not a substitute for engineering analysis by a licensed professional.
Not an accredited test report — it is a NETA-aligned educational field workflow, and acceptance decisions stay with the responsible test organization.
Not a relay setting file and not a coordination study of record.
Not connected to live equipment and not a safety device.

Calculation basis

References IEEE, NETA ATS. Results are computed from the entered inputs and cited back to the referencing clause or table number — no code text or table is reproduced verbatim.

Simplifying assumptions are stated in the lab itself; where a model is intentionally conceptual, the lab says so rather than implying study-grade accuracy.

Standards basis: Standards-aware. Validation state: Automated checks only (last checked 2026-08-15).