Provisions on the configuration of protective devices for small hydropower stations in rural areas

For units with a capacity of less than 750 kW, the overcurrent release of the automatic air circuit breaker can be used as overcurrent and short circuit protection. The setting value of the action can be adjusted by adjusting the tension of the armature spring. The setting value is generally 1.35 of the rated current of the generator. 1.7 times. In order to improve the reliability of protection, an overcurrent relay and an air circuit breaker undervoltage release device can be used for overcurrent and short circuit protection. The relay coil power supply is taken from a group (3) current transformer of the neutral point of the generator. The relay action value is also set according to the rated current of the generator from 1.35 to 1.7 times.

Principle: When the generator has a short circuit fault, the current through the overcurrent relay coil exceeds its action value, the overcurrent relay normally closed contact is disconnected, the air circuit breaker loss coil is de-energized and released, and the air circuit breaker main contact is tripped. , cut off the faulty component - the generator.

2 undervoltage protection

When the power grid is out of power, because the power load on the line is greater than the generator capacity, the voltage is greatly reduced at this time, the air circuit breaker undervoltage is released due to undervoltage, and the air circuit breaker is tripped to prevent the grid caller from causing parallelism.

3 water resistance protection

When the generator suddenly loads due to some reason (such as short circuit, long-term overload, power grid blackout, etc.), the unit speed will rise rapidly. This phenomenon is called Feiyi. Failure to turn off the governor and excitation in a timely manner may result in an accident. In rural small hydropower stations that do not use electric speed regulation, three-phase water resistors can be used as the protection load.

The capacity of the water resistor is considered to be about 70% to 80% of the rated power of the protected unit. If the water resistance capacity is too large, the unit will generate a large impact current and braking force at the moment of load shedding, which will affect the stability of the unit. In severe cases, the unit foundation may be loose. On the other hand, if the water resistance capacity is too small, the purpose of suppressing the flying speed of the unit cannot be achieved. The water resistance device is made of angle steel or steel plate and can be made into three-phase star type or triangle type.

For power stations with a single unit of 125 kW and below, the water resistance tank is empty, and the length is the number of units × (0.7 ~ 1) m, the width is (0.7 ~ 1) m, the depth is 0.6 ~ 0.8 m, and the capacity of the unit is considered. It should not boil the water in the pool in a short time (such as 3 to 5 minutes).

When debugging the water resistance load, the water resist should be gradually applied in the water to adjust the water resistance load until the requirements are met.

4 transformer overload, short circuit protection

The high-voltage side of the transformer uses a drop fuse (or SN10-10 type oil-less circuit breaker) for overload and short-circuit protection. Operating experience shows that drop-type fuses with a rated voltage of 6 to 10 kV can only be used with Transformers of 560 kVA and below, and drop-type fuses with a rated voltage of 10 kV can only be used for transformers of 750 kVA and below. When the transformer capacity exceeds 750 kVA, an oil circuit breaker should be used. The drop fuse fuse is selected according to the following formula:

When Se<100 kVA, the fuse rated current = (2 ~ 2.5) × high-voltage side rated current; when Se ≥ 100 kVA, the fuse rated current = (1.5 ~ 2) × high-voltage side rated current.

5 Transformer lightning protection

Since the overvoltage of the forward and reverse converted waves may occur on the transformer, in order to prevent lightning strikes and transformers, the transformers connected to Y/Y and Y/Y0 shall be equipped with one set of (3) valves on the high and low pressure sides. Type arresters FS-10 and FS-0.38. The closer the lightning arrester is to the transformer installation, the better the lightning protection effect is. The high and low voltage side arresters can be installed on the side of the transformer top cover, and then the transformer casing, the arrester down conductor and the transformer neutral point are connected together, and the three are grounded together. Generally, the grounding down conductor adopts the T-16 line or the GJ-25 line.