Principle and setting of diesel generator under-voltage alarm shutdown

Abstract: Undervoltage protection means that in a diesel generator set, the power supply is automatically cut off when the voltage drops to a certain level to protect the safe and stable operation of the diesel generator set. Undervoltage protection is an important protection measure in diesel generator sets, which can prevent diesel generator sets from being damaged due to low voltage. This article will explain in detail the principle and application of under-voltage protection of Cummins generator sets. At the same time, the article analyzes the significance of under-voltage protection of diesel generator sets, and through the analysis of various situations of diesel generator sets in transient states, it is concluded that The undervoltage protection setting value cannot be selected arbitrarily within the range of 80%-35% of the rated voltage. Most diesel generators should choose the conclusion of 80%-70% of the rated voltage.

1. Working principle of under-voltage protection

1. Principle of under-voltage protection

The principle of under-voltage protection is to use the voltage sensor in the diesel generator set to monitor the voltage. When the voltage drops to a certain level, the protection device is triggered and the power supply is cut off. Undervoltage protection devices usually use electrical components such as electromagnetic relays and circuit breakers to automatically cut off the circuit. The trigger value of undervoltage protection is generally set between 0.8-0.9 times the rated voltage to ensure that the diesel generator set can cut off the power supply in time after the voltage drops to a certain level to avoid equipment damage.

2. Undervoltage protection classification

Undervoltage protection can be classified according to its protection objects, protection methods, etc. According to different protection objects, undervoltage protection can be divided into generator undervoltage protection, transformer undervoltage protection, line undervoltage protection, etc. According to different protection methods, under-voltage protection can be divided into over-under voltage protection, under-voltage protection, under-frequency protection, etc. Overvoltage and undervoltage protection refers to automatically cutting off the power supply when the voltage exceeds or falls below a certain range to protect the safety of the diesel generator set. Undervoltage protection means that when the voltage drops to a certain level, the power supply is automatically cut off to protect the safety of the diesel generator set. Underfrequency protection refers to automatically cutting off the power supply when the voltage frequency drops to a certain level to protect the safety of the diesel generator set.

3. Undervoltage protection application

Undervoltage protection is widely used in diesel generator sets to protect the safe and stable operation of various diesel generator sets. In the generator set, undervoltage protection can protect the insulation of the generator rotor and avoid insulation damage caused by too low voltage.


Diesel generator undervoltage protection principle circuit diagram

2. Range of setting value

Regarding the under-voltage protection setting value of diesel generator sets, in actual implementation, the setting values of various switch factories, distribution cabinet factories, and generator factories are very different. Some are set at 160-190V, some are set at 190-210V, some are set at 240-260V, and some are set at 280-300V.

According to relevant regulations, undervoltage protection is an undervoltage protection device with delayed action characteristics. Its action value should be set to operate within the range of 70%-80% of the rated voltage, with a delay of 1-3 seconds. When the voltage is lower than 70% of the rated voltage, its delay characteristics should be coordinated with the delay characteristics of short-circuit protection. Undervoltage protection devices without delay characteristics should be adjusted to operate within the range of 40-75% of the rated voltage. It is also stipulated that generators running in parallel should be equipped with under-voltage protection and can meet the following requirements: to prevent the generator from generating electricity when closing the circuit breaker, it should act instantaneously; when the voltage drops to 70%-35% of the rated voltage, It should take action after the delay required by the system selective protection.




Synchronous generator voltage protection system composition

3. Research on pressure regulating system

The undervoltage protection of diesel generator sets has two meanings, namely generator protection and system protection. As far as generator protection is concerned, the requirements of various types of power stations are basically the same. The main function is to prevent power stations operating in parallel from instantaneous action when closing the circuit breaker when the generator is not generating electricity. But as far as system protection is concerned, the undervoltage protection setting value of the diesel generator set must be analyzed based on the specific situation. There are two states in the operation of the power station, the continuous steady-state operation state and the transient state. Generally speaking, when most diesel generator sets are currently designed, the system leaves a considerable margin for generator capacity after considering the system's load factor and simultaneity factor. Under continuous steady-state operation, the voltage regulation rate is not greater than 2.5%. If it is lower than 70% of the rated voltage, at about 65% of the rated voltage, due to the combined effect of the load and the reduction of reactance due to frequency decrease, the motor will have an armature reaction. Distortion will cause a continuous high current of more than 1.5 times. Long-term operation may be extremely detrimental to the motor. Due to the high power of the generator, it is likely that the overcurrent has not yet occurred at this time. If the undervoltage protection has not yet activated, some electrical equipment will be burned. Therefore, for this state, the generator undervoltage protection setting value is less than 70% of the rated voltage, which is meaningless.

It is worth discussing the various situations of diesel generator sets in transient conditions. The transient process of a diesel generator set is extremely complex, but it is mainly electromagnetic transient and motor speed and frequency transient. Compared with the latter, the duration of the electromagnetic transient transition process is much smaller, only 0.1 seconds to a few tenths of a second, completely within The undervoltage protection device with delay action characteristics is within the delay action range. Therefore, the voltage drop caused by electromagnetic transients will not cause action on the undervoltage protection device with delay action characteristics; while the speed, frequency and other mechanical Transient, except for a few special single maximum power motors, in extreme cases, the voltage drop caused will not exceed 10%-15% of the rated voltage. Therefore, when the voltage drops to 80%-35% of the rated voltage, It is appropriate to act after a delay according to the system selective protection requirements. Except for a few special products, the undervoltage protection action value of the diesel generator set should be set within the range of 70%-80% of the rated voltage, with a delay of 1-3 seconds. is beneficial.

In specific practice, for small and medium-sized generator sets below 400KW, the voltmeter shows that when 100% of the rated load is suddenly added, the instantaneous drop in voltage generally does not exceed 10% of the rated voltage. The instantaneous drop in voltage when starting a single maximum power motor generally does not exceed 10% of the rated voltage. Exceeding 5% of the rated voltage, the transition process takes about a few tenths of a second. For medium and large generating units above 400KW, the ratio of the power of a single maximum power motor to the rated power during normal operation of the power station is smaller than that of small and medium-sized power stations. Therefore, starting a single maximum power motor has a relatively small impact on grid voltage fluctuations. Small. Therefore, most power stations set the diesel generator undervoltage protection action value to operate within the range of 70%-80% of the rated voltage. A delay of 1-3 seconds is feasible. It will not affect the stability of the power system and has Conducive to the protection of generators and electrical equipment.

Summarize:

The undervoltage protection setting value of the diesel generator set is selected so that when the voltage drops to 80%-35% of the rated voltage, it should take action after the delay required by the system's selective protection. 35% of the rated voltage should be used as voltage loss protection. The undervoltage protection setting value cannot be selected arbitrarily within the range of 80%-35%. Most diesel generator sets should be selected at 80%-70% of the rated voltage.

To sum up, undervoltage protection is an important protection measure in diesel generator sets, which can protect the safe and stable operation of diesel generator sets. Undervoltage protection is classified according to its protection objects, protection methods, etc., and has a wide range of applications. In diesel generator sets, the role of undervoltage protection cannot be ignored, and it is one of the important means to ensure the safe operation of diesel generator sets.