Protection systems for generators and motors

Protection plans

It is difficult to define precisely which protection systems should be adopted for an electricity distribution system, given a large number of alternatives valid for each situation, but some systems will be presented as alternatives. guide for the protection of the various elements that make up a power system.

Protection systems for generators and motors (photo credit: ABB)

However, any protection system should strike a balance between technical and economic aspects so that, for example, sophisticated guards are not used for small machines or for less important power system elements.

Protection systems for generators
Protection systems

Ok, this is the list of generators and motors that we are going to look at here.

  1. Generator protection schemes
    1. Small generators
  2. Large generators
  3. Motor protection systems
    1. Low power motors (less than 100 HP)
    1. Motors up to 1000 HP
    1. Engines over 1000 HP
    1. Additional protection for synchronous motors
    1. Protection for the field winding

1. Generator protection

Generator protection should take into account the importance of the generator set and its technical characteristics such as power, voltage, and grounding, as well as economic considerations. A complex protection system can ensure that the generator is protected against possible faults.

However, it is unlikely that such a cost can be justified for each plant, especially for small units.

It is therefore necessary to define a protection system adapted to the size of the machine. Two generator set protection systems are given below, based on manufacturers’ suggestions.

1.1 Small generators

For small generators, typically up to 5 MVA, it is considered necessary to have:

  • Protection against internal faults;
  • Emergency protection against external faults using overcurrent relays with voltage limitation;
  • Reverse power protection;
  • Protection against earth faults, using an overcurrent relay;
  • Overload protection by means of thermal relays.

This diagram is shown in Figure 1 (for relay identification, see the article ANSI Codes).

Large generators

For large generators, say more than 5 MVA, the protection, shown in Figure 2, should normally include:

  • Differential protection to cover internal faults;
  • Earth fault protection using high impedance relays;
  • Emergency protection by remote or overcurrent protection with voltage limitation
  • Reverse power protection;
  • Negative phase sequence protection;
  • Protection against loss of excitation;
  • Overload protection by thermal relays.

Generator protection (VIDEO SESSION)

2. Motor protection

The amount of protection and its type used for an engine is a compromise between factors such as the size of the engine, potential hazards, type of service, and protection coordination requirements versus the cost of the protection system.

The protection systems illustrated represent current practice and international recommendations for the protection of motors of different powers, and are divided into 4 categories: </ p>

  1. Protection of low power motors (less than 100 HP);
  2. Motor protection up to 1000 HP;
  3. Protection of motors over 1000 HP;
  4. Additional protection for synchronous motors.

In the protection systems, the starting equipment of the respective engines has not been shown.

2.1 Protection of low power motors

Low power motors are normally protected by fuses associated with thermal overload relays incorporating bimetallic elements (Figure 3) – the fuses protecting against short circuits.

Or low voltage circuit breakers and thermal overload relays (Figure 4) when the magnetic element circuit breaker trip instantly under short circuit conditions.


2.2 Motor protection up to 1000 HP

Protection devices should include thermal overload and short circuit protection (49/50), locked rotor protection (51), and earth fault protection (50G), as shown in Figure 5 below.

2.3 Protection or motors greater than 1000 HP

The protection scheme shown in figure 6 includes imbalance protection (46), thermal overload protection (49), locked rotor protection (51), differential protection for internal faults (87), short-circuit protection. circuits (50) and protection against earth faults (50 GRAMS).

2.4 Additional protection for synchronous motors over 1000 HP

In addition to the protection devices, a large synchronous motor requires protection for the field winding, plus a low power factor relay (55) and Undervoltage protection (27), as well. a high / low-frequency relay (81) to prevent motors from operating under low-frequency operating conditions.

2.5 Protection for winding in the field

This would require an earth protection relay (64) and a field relay (40) to deal with the loss of field current. Get free training by clicking here.

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