There are two methods of arc extinction in circuit breakers. They are: 1. High Resistance Method 2. Low Resistance or Current Zero Interruption. 1. High Resistance Method: In this case the arc is controlled in such a way that its effective resistance increases with the time so that the current is reduced to such a value that the heat produced [...]
The circuit breakers may be subjected to widely varying stresses under different operating conditions. The current may vary from a few amperes due to no-load magnetizing current of a transformer to the heaviest short-circuit currents amounting to a hundred kilo-amperes. The circuit impedance may also change within the first 10–4 to 10–3 second because open lines or cables connected to [...]
In this article we will discus about:- 1. Introduction to Re-Striking Voltage Transient 2. Expression for Re-Striking Voltage Transient 3. Classification 4. Characteristics 5. Factors Affecting. Introduction to Re-Striking Voltage Transient: Electrically a power system is an oscillatory network so that it is logical to expect that the interruption of fault current will give rise to a transient whose frequency [...]
Generator faults can be considered under the following heads: 1. Stator Winding Faults 2. Field Winding or Rotor Circuit Faults 3. Abnormal Operating Conditions. 1. Stator Winding Faults: Such faults occur mainly due to the insulation failure of the stator coils. The main types of stator winding faults are: (i) Phase-to-earth faults (ii) Phase-to-phase faults and (iii) Inter-turn faults involving [...]
In this article we will discuss about:- 1. Stator Protection of Generators 2. Overload or Overcurrent Protection of Generators 3. Over-Voltage Protection 4. Over-Speed Protection 5. Protection against Motoring 6. Negative Sequence Protection 7. Protection against Vibration 8. Bearing Overheating Protection 9. External Fault Backup Protection 10. Direct Connected Protection. Stator Protection of Generators: The earth fault current is usually [...]
The following points highlight the top seven systems employed for stator protection of generators. The systems are: 1. Differential Protection for Generators 2. Modified Differential Protection for Generators 3. Biased Circulating Current Protection (Percentage Differential Relay Protection) 4. Self-Balance Protection System 5. Balanced Earth-Fault Protection 6. Stator Inter-Turn Protection 7. Stator Overheating Protection. System # 1. Differential Protection for Generators: [...]
In this article we will discuss about:- 1. Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors 2. Protection of Large Sized Motors 3. Protection for Exclusively Synchronous Motors. Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors: These are the most widely used industrial motors. Circuit diagram for a magnetic contactor starter for a low-voltage 3-phase induction motor is [...]
The different protection schemes for rotor protection of generators are described as follows: 1. Rotor Earth Fault Protection 2. Loss of Excitation (or Field Failure) Protection 3. Protection against Rotor Over-Heating Because of Over-Excitation 4. Rotor Temperature Alarm 5. Automatic Field Suppression and Use of Neutral Circuit Breaker. Scheme # 1. Rotor Earth Fault Protection: Field circuits are operated unearthed [...]
For power transformers, the protection is to be provided usually against dangerous overloads and excessive temperature rise. Dangerous overloads may be due to external faults or the internal ones. External faults, however, are cleared by the relay system outside the transformer within the shortest possible time in order to avoid any danger to the transformer due to these faults. Hence [...]
The differential protection used for power transformers is based on Merz-Price circulating current principle. Such protection provides protection against internal phase-to-phase and phase-to-earth faults and is generally used for transformers of rating exceeding 2 MVA. This protection system as applied to power transformers is fundamentally the same as that for generators but with certain complicating features not encountered in the [...]