The voltage regulating device of the transformer is divided into the transformer “off-excitation” voltage regulating device and the transformer “on-load” tap changer.
Both refer to the voltage regulating mode of the transformer tap changer, so what is the difference between the two?
① The “off-excitation” tap changer is to change the high-voltage side tap of the transformer to change the turns ratio of the winding for voltage regulation when both the primary and secondary sides of the transformer are disconnected from the power supply.
② “On-load” tap changer: Using the on-load tap changer, the tap of the transformer winding is changed to change the high-voltage turns for voltage regulation without cutting off the load current.
The difference between the two is that the off-excitation tap changer does not have the ability to switch gears with load, because this type of tap changer has a short-term disconnection process during the gear switching process. Disconnecting the load current will cause arcing between the contacts and damage the tap changer. The on-load tap changer has an excessive resistance transition during the gear switching process, so there is no short-term disconnection process. When switching from one gear to another, there is no arcing process when the load current is disconnected. It is generally used for transformers with strict voltage requirements that need to be frequently adjusted.
Since the transformer “on-load” tap changer can realize the voltage regulation function under the operation state of the transformer, why choose the “off-load” tap changer? Of course, the first reason is the price. Under normal circumstances, the price of the off-load tap changer transformer is 2/3 of the price of the on-load tap changer transformer; at the same time, the volume of the off-load tap changer transformer is much smaller because it does not have the on-load tap changer part. Therefore, in the absence of regulations or other circumstances, the off-excitation tap changer transformer will be selected.
Why choose the transformer on-load tap changer? What is the function?
① Improve the voltage qualification rate.
The power transmission in the power system distribution network generates losses, and the loss value is the smallest only near the rated voltage. Carrying out on-load voltage regulation, always keeping the substation bus voltage qualified, and making the electrical equipment run at the rated voltage state will reduce the loss, which is the most economical and reasonable. The voltage qualification rate is one of the important indicators of power supply quality. Timely on-load voltage regulation can ensure the voltage qualification rate, thereby meeting the needs of people’s lives and industrial and agricultural production.
② Improve the reactive power compensation capacity and increase the capacitor input rate.
As a reactive power compensation device, the reactive power output of power capacitors is proportional to the square of the operating voltage. When the operating voltage of the power system decreases, the compensation effect decreases, and when the operating voltage increases, the electrical equipment is overcompensated, causing the terminal voltage to increase, even exceeding the standard, which is easy to damage the insulation of the equipment and cause
equipment accidents. In order to prevent the reactive power from being fed back to the power system and the reactive power compensation equipment from being disabled, resulting in waste and increased loss of reactive power devices, the main transformer tap switch should be adjusted in time to adjust the bus voltage to the qualified range, so that there is no need to disable the capacitor compensation.
How to operate the on-load voltage regulation?
The on-load voltage regulation methods include electric voltage regulation and manual voltage regulation.
The essence of on-load voltage regulation is to adjust the voltage by adjusting the transformation ratio of the high-voltage side while the voltage on the low-voltage side remains unchanged. We all know that the high-voltage side is generally the system voltage, and the system voltage is generally constant. When the number of turns on the high-voltage side winding is increased (that is, the transformation ratio is increased), the voltage on the low-voltage side will decrease; on the contrary, when the number of turns on the high-voltage side winding is reduced (that is, the transformation ratio is reduced), the voltage on the low-voltage side will increase. That is:
Increase turns = downshift = voltage reduction Reduction turns = upshift = voltage increase
So, under what circumstances can the transformer not perform on-load tap changer?
① When the transformer is overloaded (except for special circumstances)
② When the light gas alarm of the on-load voltage regulation device is activated
③ When the oil pressure resistance of the on-load voltage regulation device is unqualified or there is no oil in the oil mark
④ When the number of voltage regulation exceeds the specified number
⑤ When the voltage regulation device is abnormal
Why does overload also lock the on-load tap changer?
This is because under normal circumstances, during the on-load voltage regulation process of the main transformer, there is a voltage difference between the main connector and the target tap, which generates a circulating current. Therefore, during the voltage regulation process, a resistor is connected in parallel to bypass the circulating current and load current. The parallel resistor needs to withstand a large current.
When the power transformer is overloaded, the operating current of the main transformer exceeds the rated current of the tap changer, which may burn the auxiliary connector of the tap changer.
Therefore, in order to prevent the arcing phenomenon of the tap changer, it is forbidden to perform on-load voltage regulation when the main transformer is overloaded. If the voltage regulation is forced, the on-load voltage regulation device may burn out, the load gas may be activated, and the main transformer switch may be tripped.
Post time: Sep-09-2024