Overall Solution to High-and-Low-Voltage Distribution System
Traditionally, the power system is divided into power generation, transmission and distribution.
The electrical energy generated by the power generation system is transmitted through the power transmission system and finally distributed to various users by the power distribution system.
Generally, power distribution refers to the section of the power system from the outlet of the step-down distribution substation (high-voltage distribution substation) to the user side.
The power distribution system is a power network system composed of a variety of power distribution equipment, components, and facilities that can convert voltage and distribute power directly to end users.
Composition of a Power Distribution System:
In China, the power distribution system can be divided into three parts: high-voltage distribution system, medium-voltage distribution system and low-voltage distribution system.
As the last stage of the power system that directly faces end users, the improved power distribution system is directly related to the reliability and quality of electricity consumption.
Therefore, it is critical in the entire power system.
According to the “Code of Planning and Design of Urban Electric Network”, the transmission and transformation system has a voltage of 220kV and above, the high-voltage distribution system has a voltage of 35kV, 63kV and 110kV, the medium-voltage distribution system has a voltage of 10kV and 6kV, and the low-voltage distribution system has a voltage of 380V and 220V.
The power supply and distribution systems of regions and companies in China waste a lot of electricity for multi-faceted reasons.
The main problems and solutions are as follows:
●Mismatch between the capacity and the load of power grid: With the economic development and the improvement of people's living standards, electricity consumption has increased rapidly. The equipment and lines of the original distribution network do not match the electricity consumption, leading to overload in many places, which not only compromises the safety of power supply, but also greatly increases the losses of the power distribution system. The method of energy-saving reconstruction is to update the lines and equipment.
●The power supply voltage is inappropriate: In some areas and for some entities where the power consumption is considerable, the power supply voltage is too low. For instance, in the past the input voltage for enterprises was 6kV and multiple step-downs were needed during the process, requiring a large amount of construction fund and increasing the electricity loss of the system. Properly increasing the power supply voltage and reducing the number of step-downs from two or three to one can greatly reduce the equipment and line losses of the power supply system.
●The layout is unreasonable: Electricity users and enterprises in many areas are far away from the distribution center, which makes the low-voltage (0.4kV) transmission distance too long, causing great line loss and voltage drop.
This situation was common in large and medium-sized enterprises in old days, because the design then stipulated that the distribution center should be built at the end of the company's power input. The improvement measure is to shorten the distance between the distribution center and the electricity-consuming equipment as much as possible on the premise of ensuring safety, and turn the original long-distance low-voltage transmission to long-distance high-voltage and short-distance low-voltage transmission. In this way, it can greatly reduce the transmission line loss.
● Reactive power is in shortage: As the economy develops, the inductive load in the power supply and distribution system increases rapidly, and many distribution transformers and motors are in a non-economical operation state with a low load rate, resulting in a large demand for reactive power in the power supply and distribution system. If it is not replenished in time, it will cause a drop in the quality of the power supply voltage and an increase in system losses. It will not only waste electricity, but also compromise the utilization rate of power supply and distribution equipment, even cause accidents.
The technical measure to solve the above problem is to install compensation capacitors on the power supply side and the power consumption side. The former is called centralized compensation, and the direct beneficiary is the power supply department. The benefit of users comes from less penalty due to the noncompliance of power factor; the latter is called on-site compensation, and the direct beneficiary is the users. The main benefit is to reduce line losses. In addition to the above benefits of reactive power compensation, it can also increase the utilization rate of generators, transformers and other equipment, reduce power supply costs, and improve the safety of system operation.
● Outdated distribution equipment: Many of the distribution equipment used in China, such as distribution transformers and various switches, are outdated. They are not upgraded in time due to insufficient fund and poor energy-saving awareness of relevant departments, thus wasting a ton of electricity.
For example, as for the no-load loss of distribution transformers, the loss of ST transformer in early 1960s is 1.32 times that of the S1 transformer in 1970s, and that of S1 transformer is about 14% higher than that of the S6 transformer. The loss of S7 transformer used before late 1990s is 45% less than that of the S6 transformer. The no-load loss and load loss of the S9 transformer promoted by the nation in late 1990s is less. It is the same for electromagnetic switches, cable joints and connection fittings. If these old-fashioned and outdated power distribution equipment can be updated in time, the power distribution system can reduce a lot of unnecessary waste of electrical energy.