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Protection requirements; protective system vs scheme

Short answer

Basic requirements: selectivity, speed, sensitivity, reliability, stability, simplicity, economy, adequateness. A protective system performs one function using a single principle (e.g., differential), while a protective scheme is the coordinated arrangement for an element, comprising multiple systems plus breakers.

What are the basic requirements of protection scheme? Differentiate between a protective system and protective scheme.

Basic requirements of a protection scheme

  1. Selectivity: only the faulty section is disconnected by the nearest breakers.
  2. Speed: the fault is cleared quickly to limit damage and preserve stability.
  3. Sensitivity: operates for the minimum fault current (high-resistance faults, minimum generation).
  4. Reliability: operates whenever required (dependability) and never wrongly (security).
  5. Stability: stays inactive for through-faults, load swings and transformer inrush.
  6. Simplicity and economy: cost proportional to the importance of the equipment; easy to maintain.
  7. Adequateness: covers all likely faults and abnormal conditions of the equipment.

Protective system vs protective scheme

A protective system is a complete arrangement of protection equipment and associated devices that performs one specified protective function, based on one protection principle. It includes the relay(s), CTs and VTs, wiring, trip circuit, auxiliary DC supply and, where used, the communication channel (by the IEC definition the circuit breaker itself is excluded). Example: the distance protection system of a line, or the differential protection system of a transformer. If any of its components fails, that protection fails.

A protective scheme is the coordinated arrangement of protection for one or more elements of the power system. It includes all the equipment needed to make the protection work (relays, CTs, VTs, circuit breakers, batteries) and may comprise several protective systems. Example: the protection scheme of a power transformer = differential system + restricted earth-fault system + Buchholz and temperature devices + back-up overcurrent/earth-fault system, all tripping the HV and LV breakers.

PointProtective systemProtective scheme
MeaningArrangement that performs one protection functionCoordinated arrangement protecting a whole element (or elements)
Based onOne protection principle (distance, differential, overcurrent)Several principles combined (main + back-up)
ContentsRelay, CT/VT, wiring, trip circuit, DC supply, communication channelSeveral protective systems plus circuit breakers and all supporting equipment
ScopeNarrower; a part of a schemeWider; may comprise several systems
ExampleDistance relay + CTs + CVT + DC supply + trip circuit of a lineComplete line protection: main-1 and main-2 distance, back-up overcurrent, breaker-failure, auto-reclose
 Protective scheme of a transformer
 ┌───────────────────────────────────────┐
 │ Differential system (CTs + 87T relay) │
 │ REF system          (CTs + 64R relay) │
 │ Buchholz / temperature devices        │──► trip HV & LV CBs
 │ Back-up O/C & E/F   (CTs + 51/51N)    │
 │ Common DC supply and trip circuits    │
 └───────────────────────────────────────┘
   each row = one protective system

✓ Answer checked by Claude Opus 5.5 (high effort)

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