Important considerations when design protection system



Important considerations when design protection system.

  1. Types of fault and abnormal Conditions to be protected against

  2. Quantities available for measurement

  3. Types of protection available

  4. Speed

  5. Fault position discrimination

  6. Dependability / reliability

  7. Security / stability

  8. Overlap of protections

  9. Phase discrimination / selectivity

  10. CT’s and VT’s ratio required

  11. Auxiliary supplies

  12. Back-up protection

  13. Cost

  14. Duplication of protection


Types of protection

A - Fuses

For LV Systems, Distribution Feeders and Transformers, VT’s, Auxiliary Supplies


B - Over current and earth fault

       Widely used in All Power Systems

  1. Non-Directional

  2. Directional.




For feeders, Bus-bars, Transformers, Generators etc

  1. High Impedance

  2. Low Impedance

  3. Restricted E/F

  4. Biased

  5. Pilot Wire




D - Distance

For transmission and sub-transmission lines and distribution feeders, also used as back-up protection for transformers and generators without signaling with signaling to provide unit protection e.g.:


  1. Time-stepped distance protection

  2. Permissive underreach protection (PUP)

  3. Permissive overreach protection (POP)

  4. Unblocking overreach protection (UOP)

  5. Blocking overreach protection (BOP)

  6. Power swing blocking

  7. Phase comparison for transmission lines

  8. Directional comparison for transmission lines


E - Miscellaneous:

  1. Under and over voltage

  2. Under and over frequency

  3. A special relay for generators, transformers, motors etc.

  4. Control relays: auto-reclose, tap change control, etc.

  5. tripping and auxiliary relays



Fast operation: minimizes damage and danger

Very fast operation: minimizes system instability discrimination and security can be costly to achieve.


  1. differential protection

  2. differential protection with digital signaling

  3. distance protection with signaling

  4. directional comparison with signaling


Fault position discrimination

Power system divided into protected zones must isolate only the faulty equipment or section


Dependability / reliability

Protection must operate when required to Failure to operate can be extremely damaging and disruptive Faults are rare. Protection must operate even after years of inactivity Improved by use of:

  1. Back-up Protection and

  2. duplicate Protection


Security / Stability

Protection must not operate when not required to e.g. due to:

  1. Load Switching

  2. Faults on other parts of the system

  3. Recoverable Power Swings


Overlap of protections

  1. No blind spots

  2. Where possible use overlapping CTs

Phase discrimination / selectivity

Correct indication of phases involved in the fault Important for Single Phase Tripping and auto-Reclosing applications


Current and voltage transformers

These are an essential part of the Protection Scheme. They must be suitably specified to meet the requirements of the protective relays.

1A and 5A secondary current ratings, Saturation of current transformers during heavy fault conditions should not exceed the limits laid down by the relay manufacturer.

Current transformers for fast operating protections must allow for any offset in the current waveform. Output rating under fault conditions must allow for maximum transient offset. This is a function of the system X/R ratio.

Current Transformer Standards/Classes:

British Standards: 10P, 5P, X


American: C, T.

Location of CTs should, if possible, provide for overlap of protections. Correct connection of CTs to the protection is important. In particular for directional, distance, phase comparison and differential protections. VT’s may be Electromagnetic or Capacitor types. Busbar VT’s: Special consideration needed when used for Line Protection.


Auxiliary supplies


Required for:

  1. Tripping circuit breakers

  2. Closing circuit breakers

  3. Protection and trip relays

    • AC. auxiliary supplies are only used on LV and MV systems.

    • DC. auxiliary supplies are more secure than ac supplies.

    •  Separately fused supplies used for each protection.

    • Duplicate batteries are occasionally provided for extra security.

    • Modern protection relays need a continuous auxiliary supply.

    • During operation, they draw a large current which increases due to operation of output elements.

Relays are given a rated auxiliary voltage and an operative auxiliary voltage range.

the rated value is marked on the relay. Refer to relay documentation for details of operative range. it is important to make sure that the range of voltages which can appear at the relay auxiliary supply terminals is within the operative range.

IEC recommended values (IEC 255-6):

Rated battery voltages:

12, 24, 48, 60, 11 0, 125, 220, 250, 440

Preferred operative range of relays:

80 to 10% of voltage rated

AC. component ripple in the dc supply:

<10% of voltage rated



The cost of protection is equivalent to insurance policy against damage to plant, and loss of supply and customer goodwill.

Acceptable cost is based on a balance of economics and technical factors. Cost of protection should be balanced against the cost of potential hazards there is an economic limit on what can be spent.


Minimum cost:

Must ensure that all faulty equipment is isolated by protection

Other factors:

  1. Speed

  2. Security/Stability

  3.  Sensitivity:
       Degree of risk in allowing a low level fault to develop into a more severe fault

  4. Reliability


Total cost should take account of:

  1. Relays, schemes and associated panels and panel wiring

  2. Setting studies

  3. Commissioning

  4. CT’s and VT’s

  5. Maintenance and repairs to relays

  6. Damage repair if protection fails to operate

  7. Lost revenue if protection operates unnecessarily


Distribution systems

  1. Large number of switching and distribution points, transformers and feeders.

  2. Economics often overrides technical issues

  3. Protection may be the minimum consistent with - statutory safety regulations

  4. Speed less important than on transmission systems

  5. Back-up protection can be simple and is often inherent in the main protection.

  6. Although important, the consequences of maloperation or failure to operate are less
        serious than for transmission systems.


Transmission systems

  1. Emphasis is on technical considerations rather than economics

  2. Economics cannot be ignored but is of secondary importance compared with the need  for  highly reliable, fully discriminative high speed protection

  3. Higher protection costs justifiable by high capital cost of power system elements protected.

  4. Risk of security of supply should be reduced to the lowest practical levels

  5. High speed protection requires unit protection

  6. Duplicate protections used to improve reliability

  7. Single phase tripping and auto-reclose may be required to maintain system stability