Research On Principles And Applications Of On Load Tap Changing Voltage With Connectivity Topology Assembling State

The traditional method used to regulating voltage is of the On-load tap changers of mechanism for power transformers. Because of their structure simple and cost low, the on-load tap changers of mechanism are wildly used in power systems inland and overseas currently. With the development of society and sharp increase of electricity consumption, the electric power networks become much huger and more complex. The on-load tap changers of mechanism cannot be satisfied by the requirements of power systems'security and economical operation in the case. Many significant problems of the on-load tap changers of mechanism emerge. Three of them are inherently inevitable. Firstly, there is the electric arc that is produced during the switching over transformer taps and causes mechanical contacts coking and transformer oil medium polarizing. Secondly, switch on-off act is very slow and the response time of tap changer is long. They are cannot regulate voltage dynamically and synchronously. All of these increase the risk of operation failure and the difficulties of device maintenance. Thirdly, because of the dispersedness of mechanical switches'acting time, the time of voltage regulation can not be controlled exactly, the over-voltage and over-current are inevitable in the network, which harm the safety of power systems. There are two improved schemes: a thyristor assisted mechanical on-load tap changer and a solid-state on-load tap changer. The first one can suppress the arc between contacts, but response time and operation accuracy are not improved, and the fault rate of devices driven mechanically is still a problem, besides its high expense, this scheme's application is restricted. The second one has effective action on the aspects of response time and fault rate, but at the same time it needs amounts of thyristor switches, which added the volume and price of device, and moreover, single high power thyristor can't stand the voltage in high voltage-level or medium voltage-level network. There are many problems should be resolved before application, such as simplifying the structure of on-load tap changers, decreasing the number of thyristor switches, reducing the voltage on electronic devices, and lowering the cost. Aiming at the problems above, this paper dissertates and summarizes main innovational and distinctive work made by author in distribution systems following: (1) A novel solid-state on-load tap changer scheme based on the principles of on-Load tap changing voltage with connectivity topology assembling state is proposed. The scheme adopts the coding rule considered on both falling devices'withstanding voltage and reducing number of tap changing windings to set up combination relationship between tap changing voltage state and tap changing windings. It changes the existing method that a tap changing winding includes a voltage step and a tap changing voltage state is corresponding to into the new method that a tap changing winding includes more voltage steps and a tap changing voltage state is combination of more tap changing windings. Therefore attains the aim of less transformer taps and simpler system structure. Additionally, the paper also accepts the technique of connected-topology link of tap changing windings, which makes it possible to avoid circumfluence on tap-change windings. (2) The frameworks of "arm-bridge"and "source-terminal"based on the principles of on-load tap changing voltage with connectivity topology assembling state are presented. They can assure the appropriate working condition and withstanding voltage of solid-state switches and can reduce switches number effectively. (3) The strategy of controlling thyristor switches restricting short-current in tap changing windings is proposed that when the current of primary winding is at zeros triggering the thyristor pair at the instant is validated. By this means the over-current in short circuit can not engender in on-load tap changing process and dispense with resistance and reactance restricting short-current in transformer, so the process of on-load tap changing voltage decrease from more steps into one step. (4) The non-transient conditions of transformer no-load switching on or off and on-Load tap changing voltage are deduced. The non-transient conditions and the signal of on-load tap changing voltage work together to trigger corresponding thyristor switches, so the over-voltage and over-current can be avoided.Two subsidiary breakers and series resistances are used in transformer switching-on procedure to supply working voltage of thyristor switches and decrease inrush current a certain extent during transformer no-load switching on. (5) A united radiator structure is proposed, that the thyristor pair and its adjacent non-working thyristor switches share a common radiator sink to improve radiating efficiency and decrease volume. (6) The formulae of optimizing tap changing voltage with minimum transformer synthesis loss are deduced. If the transformer loss of on-load and off-load ,average load coefficient, line loss are known, an optimizing tap changing voltage with minimum transformer synthesis loss and a value of transformer synthesis loss can be reckon out. (7) The method of optimizing tap changing voltage with minimum network loss based on meliorate genetic arithmetic is presented. With the method, multi-transformers'synchronization controlling with minimum network loss can be actualized. (8) This paper introduces the applications of the principles of solid-state on-Load tap changing voltage with connectivity topology assembling state in four aspects: â‘ the dynamic smoothing on-load tap changing control in distribution network: maintaining the voltage level ,restraining the voltage fluctuation; â‘¡the voltage and reactive power collaborated control considering increment of reactive power and voltage adjusting: correcting conventional algorithm with adjusting increment, making it possible to avoid tap changing overregulation,reactive-load compensation equipment overcompensation and adjusting vibration; â‘¢the synchronal on-load tap changing on transformers parallel operation: controlling voltage ratio synchronically,identically and uniformly to make the parallel transformers refrain from ratio disaccord causing high unbalanced current in tap changing process; â‘£the optional tap changing control in minimum network loss based on meliorate genetic arithmetic: adjusting transformer output voltage dynamically in response to the requirement of operation mode and load variation, making each node in distribution network have an available voltage in normal voltage extent and maintaining the network loss minimal. The paper also provides the results of key techniques at laboratory experiment and computer simulation. Compared with the topical structure of "direct substitution scheme", the topical structure of this novel scheme can decrease number of transformer taps by 56.25%, reduce the number of thyristor switches by 58.06%, and improve the response time by 3 times.