In the power system, in order to increase the transmission range and quantity of electric energy, transformer has been widely used. However, the power system composed of power generation, transmission, distribution and electricity use generally requires 3 ~ 5 transformer processes, resulting in the total transformer capacity in the power system is far beyond the total capacity of the generator, resulting in serious loss. How to choose and control the operation mode of transformer is the key to realize the economic operation of power system. It not only brings considerable economic benefits by saving electric energy, but also has very important practical significance to the development of national economy.
Distribution transformer is an important electrical equipment for transmission and distribution of electric energy in the distribution network. Due to its large quantity and wide distribution, the total amount of active power loss and reactive power consumption generated by the transformer itself in the operation process accounts for about 30% of the electric power loss. Therefore, this paper combines the principle of distribution transformer economic operation, and several different situations of distribution transformer economic operation, design and implementation of a distribution transformer economic operation control system, the system combines the principle of distribution transformer economic operation with the practice, in order to reduce the power loss of distribution transformer.
First, the basic structure of the transformer
The basic components of power transformer are: core, winding, insulation sleeve, oil tank and other accessories. The core and winding are the main parts of the transformer, known as the body red.
The core of the power transformer is made of silicon steel sheet with high permeability. There are two kinds of silicon steel sheet, hot rolled and cold rolled. Because of its poor magnetic properties, the core of the power transformer has not used this material. Cold rolled silicon steel can be divided into two types: unoriented and oriented. Oriented cold rolled silicon steel has obvious directivity, that is, good magnetic properties along the rolling direction. Therefore, cold rolled oriented silicon steel is used in power transformers at present.
The winding is made of steel and aluminum wires with round and flat sections. The main forms of the winding are layer type and cake type. The turns of the layer type winding are arranged along the axial direction, while the turns of the cake type winding are arranged along the axial direction after forming a wire cake in the amplitude direction.
According to the configuration of the transformer core and winding to classify, can be divided into core type and shell type two. The characteristic of the core structure is that the winding is exposed outside, and the core is on the inside. Considering the insulation of the winding, this structure is suitable for high voltage. The characteristic of the shell structure is that the winding is placed on the inside of the core, and the core is visible from the outside.
Second, economic operation modeling of distribution transformation
The technical parameters of the distribution transformer characterize the characteristics of its own loss in the process of power supply operation: active power loss and reactive power consumption. The technical parameters of the double-winding distribution transformer include no-load current I0, no-load loss P0, short-circuit voltage UK and short-circuit loss PK.
The no-load current value I0 is closely related to the material of the iron core, the geometric size of the magnetic circuit and the fabrication process of the iron core.
I0%=I0/IN
Where: I0 -- no-load current, A; In - Rated current of the secondary winding of the distribution transformer, A.
No-load loss P0 refers to the loss in the core caused by periodic flux change due to excitation current under the condition that the distribution transformer is connected to the rated voltage, also known as core loss. Iron consumption includes basic iron consumption and additional iron consumption.
Short-circuit voltage UK refers to the voltage of rated frequency applied to the primary winding when the secondary winding is short-circuited. When the secondary winding passes the rated current, the voltage UK applied by the primary winding is called the short-circuit voltage of the distribution transformer, also known as the impedance voltage, which is: UK%=UK/UN100%
Where: UN -- Rated voltage of primary side winding of distribution transformer, kV.
The short-circuit loss Pk value given on the distribution transformer nameplate refers to the power loss generated by the rated load when the winding temperature is 75℃.
In the analysis and calculation of the economic operation of distribution transformers, in order to simplify the calculation, the active power loss is often approximated as:
Q0 material S0 = I0% * 10-2 SN
Reactive power consumption is approximately denoted as:
QK material SK = UK % * 10-2 SN
Comprehensive power loss includes active power loss, which is more systematic. The optimal operation of comprehensive power economy refers to the comprehensive optimal which considers both active and reactive power saving. The system that considers both user power saving and network loss reduction is the best.
Third, the economic operation analysis of transformer parallel operation
There are quite a number of substations in China, whose transformers use parallel operation mode, especially 22OKV, 110KV substations. For the parallel operation of transformers, the determination of its economic operation mode mainly refers to the parallel operation of all two or more transformers, under the condition of providing the same load, the optimal operation mode with the minimum power loss.
If the total number of transformers in the substation is N, there are (2n-l) combined operation modes. Take three transformers as an example, there are seven modes of operation, that is, A, B, C transformer single operation, AB, CB, two transformers parallel operation and ACB three transformers parallel operation "between these seven modes of operation, but also after 15 times of comparison to determine both to meet the load of electricity at the same time the transformer loss is also the smallest mode of operation. That is, three times between a single transformer, three times between two parallel ways, six times between one and two parallel, three times between two parallel and three sets. Only after 15 times of comparison can we determine the operation mode which not only meets the power consumption of load but also minimizes the loss of transformer. The others run side by side and so on.
1. The ideal situation of transformer parallel operation is:
First, when there is no load, there is no circulating current between the secondary side of each transformer, that is, each transformer is no-load. Because the circulation will not only cause additional losses, so that the transformer temperature, efficiency, but also occupy the capacity of the equipment.
Second, when the load, the load current borne by the transformer should be proportional to their rated capacity. Large capacity should be more load sharing, small capacity should be less load sharing, so that the transformer capacity is fully utilized. That is, the load coefficient of each transformer should be the same.
Third, the load side current of each transformer should be in the same phase. That is, the total load current is the sum of the load current of each transformer.
In order to achieve the above ideal juxtaposition requirements, the transformers running in parallel must meet the following four conditions, (1). The winding connection group must be the same; (2) the ratio should be equal; (3) The short-circuit impedance should be close to (the difference of short-circuit impedance should not exceed 10% when the transformer is running in parallel according to the transformer operation regulations); (4) The capacity can not be too big (for the parallel operation of the transformer, its capacity ratio can not be greater than 3:1).
2. Combined technical parameters of parallel running transformers
The parallel operation of N transformers is equivalent to the operation of a combined transformer, and the combined no-load loss P0N and excitation power Q0N are equal to the sum of the no-load loss and excitation power of each transformer respectively, which meets the requirements of economic operation of transformers.