The section of the power system from the step-down distribution substation (high-voltage distribution substation) to the user end is called the power distribution system. The power distribution system is a power network system composed of various power distribution equipment (or components) and power distribution facilities that transforms voltage and directly distributes power to end users.
1. Introduction to power distribution system
Traditionally, the power system is divided into three major components: generation, transmission, and distribution.
The electric energy generated by the power generation system is transported through the power transmission system, and finally distributed to various users by the power distribution system.
Generally, the section of the power system from the step-down distribution substation (high-voltage distribution substation) to the user end is called the power distribution system.
The power distribution system is a power network system composed of various power distribution equipment (or components) and power distribution facilities that transforms voltage and directly distributes power to end users.
2. Composition of power distribution system
In my country, the power distribution system can be divided into three parts: high voltage power distribution system, medium voltage power distribution system and low voltage power distribution system.
As the last link of the power system, the power distribution system directly faces the end users, and its perfection is directly related to the reliability and quality of power consumption of the majority of users, so it plays an important role in the power system.
The voltage level of my country's power distribution system, according to the "Guidelines for Planning and Design of Urban Power Grids", 220kV and above are power transmission and transformation systems, 35, 63, and 110kV are high-voltage power distribution systems, and 10 and 6kV are medium-voltage power distribution systems. System, 380, 220V is a low-voltage power distribution system.
3. Operation mode of power distribution system
The safe operation and management of transformers is the focus of our daily work. Through the experience summarization of abnormal operation of transformers and analysis of common faults, it will help to judge the cause and nature of faults in a timely and accurate manner, and take effective measures in time to ensure the safe operation of equipment. .
The transformer is an extremely important electrical equipment in the power transmission and distribution system. According to the operation and maintenance management regulations, the transformer must be inspected regularly, so as to understand and grasp the operation of the transformer in time, and take effective measures in time, and strive to eliminate the fault in the bud, so that Ensure the safe operation of the transformer. Transformer abnormal operation and common faults are as follows:
3.1 The sound of the transformer is abnormal.
3.2 Under normal load and normal cooling mode, the oil temperature of the transformer keeps rising.
3.3 The color of transformer insulating oil changes significantly.
3.4 The oil conservator or the explosion-proof pipe is sprayed.
3.5 Three-phase voltage imbalance occurs.
3.6 The situation where the relay protection operates.
3.7 Flashover and explosion of insulating porcelain bushing.
3.8 In case of tap-changer failure.
4. Basic methods of power distribution system
According to various protection methods and term concepts stipulated by IEC, low-voltage power distribution systems are divided into three categories according to different grounding methods, namely TT, TN and IT systems, which are described below.
4.1 TT mode power supply system The TT mode refers to the protection system that directly grounds the metal casing of the electrical equipment, which is called the protective grounding system, also known as the TT system. The first symbol T means that the neutral point of the power system is directly grounded; the second symbol T means that the exposed metal conductive part of the load equipment that is not in contact with the charged body is directly connected to the earth, regardless of how the system is grounded. All grounding of the load in the TT system is called protective grounding, and the characteristics of this power supply system are as follows.
4.1.1 When the metal casing of the electrical equipment is charged (the phase wire touches the casing or the insulation of the equipment is damaged and the leakage occurs), the risk of electric shock can be greatly reduced due to the grounding protection. However, the low-voltage circuit breaker (automatic switch) may not be able to trip, causing the voltage of the earth leakage device to be higher than the safe voltage, which is a dangerous voltage.
4.1.2 When the leakage current is relatively small, even if there is a fuse, it may not be blown, so a leakage protector is needed for protection, so it is difficult to popularize the TT system.
4.1.3 The grounding device of the TT system consumes a lot of steel, and it is difficult to recycle, time-consuming, and material-consuming.
Some construction units now use the TT system. When the construction unit borrows its power supply for temporary power consumption, a special protection line is used to reduce the amount of steel required for the grounding device.
Separate the newly added special protection line PE line from the working neutral line N, and its characteristics are: a. There is no electrical connection between the common grounding line and the working neutral line; b. During normal operation, the working neutral line can have current, while the dedicated protection line There is no current in the line; c. The TT system is suitable for places where the grounding protection is scattered.
4.2 TN mode power supply system This power supply system is a protection system that connects the metal casing of electrical equipment with the working zero line, which is called a zero connection protection system and is represented by TN. Its features are as follows.
4.2.1 Once the shell of the equipment is electrified, the zero-connection protection system can increase the leakage current to a short-circuit current. This current is very large, 5.3 times that of the TT system. In fact, it is a single-phase short-circuit fault, and the fuse of the fuse will blow , The release of the low-voltage circuit breaker will trip immediately, so that the faulty equipment will be powered off, which is safer.
4.2.2 The TN system saves materials and man-hours, and is widely used in my country and many other countries. It can be seen that it has more advantages than the TT system. In the TN mode power supply system, it is divided into two types, TN-C and TN-S, according to whether the protective zero line is separated from the working zero line.
4.3 TN-C power supply system It uses the working neutral line as the zero protection line, which can be called the protective neutral line and can be represented by NPE
4.4 TN-S power supply system It is a power supply system that strictly separates the working zero line N from the special protection line PE, called TN-S power supply system, and the characteristics of TN-S power supply system are as follows.
4.4.1 When the system is running normally, there is no current on the dedicated protection line, but there is unbalanced current on the working zero line. The PE line has no voltage to the ground, so the zero protection of the metal shell of the electrical equipment is connected to the special protective line PE, which is safe and reliable.
4.4.2 The working zero line is only used as a single-phase lighting load circuit.
4.4.3 The special protection line PE must not be disconnected, and must not enter the leakage switch.
4.4.4 A leakage protector is used on the main line. The working zero line must not be repeatedly grounded, and the PE line is repeatedly grounded, but it does not pass through the leakage protector. Therefore, a leakage protector can also be installed on the TN-S system power supply main line.
4.4.5 The TN-S power supply system is safe and reliable, and is suitable for low-voltage power supply systems such as industrial and civil buildings. "Three links and one leveling" before the start of the construction project (electricity, water, road and ground - must use TN-S power supply system).
4.5 TN-C-S mode power supply system In the temporary power supply of building construction, if the front part is powered by TN-C mode, and the construction code stipulates that the construction site must adopt the TN-S mode power supply system, then the main distribution box on the site at the back part of the system can The characteristics of the TN-C-S system are as follows.
4.5.1 The working neutral line N is connected to the special protection line PE. When the unbalanced current of this line is relatively large, the zero connection protection of electrical equipment is affected by the potential of the neutral line. There is no current on the PE line from point D to the back, that is, there is no voltage drop on this section of the line. Therefore, the TN-C-S system can reduce the voltage of the motor shell to the ground, but it cannot completely eliminate this voltage. The size of this voltage depends on the ND line The condition of unbalanced load and the length of this line of ND. The more unbalanced the load and the longer the ND line, the greater the voltage offset from the equipment case to ground. Therefore, it is required that the load unbalanced current should not be too large, and repeated grounding should be done on the PE line.
4.5.2 Under no circumstances should the PE line enter the earth leakage protector, because the action of the earth leakage protector at the end of the line will cause the tripping of the previous stage earth leakage protector and cause a large-scale power outage.
4.5.3 Except that the PE line must be connected with the N line at the main box, the N line and the PE line must not be connected at each sub-box. No switch or fuse is allowed to be installed on the PE line, and no large gu Doubles as PE wire.
Through the above analysis, the TN-C-S power supply system is a temporary modification of the TN-C system. When the three-phase power transformer is well grounded and the three-phase load is relatively balanced, the effect of the TN-C-S system in the construction power practice is still feasible. However, when the three-phase load is unbalanced and the construction site has a dedicated power transformer, the TN-S power supply system must be used.
4.6 IT power supply system I means that the power supply side has no working ground, or is grounded through high impedance. The second letter T indicates that the electrical equipment on the load side is grounded for protection.
When the power supply distance of the IT mode power supply system is not very long, the power supply reliability is high and the security is good. It is generally used in places that do not allow power outages, or places that require strict continuous power supply, such as electric steelmaking, operating rooms of large hospitals, underground mines, etc. The power supply conditions in underground mines are relatively poor, and cables are prone to moisture. Using the IT power supply system, even if the neutral point of the power supply is not grounded, once the equipment leaks, the leakage current of the single phase to ground is still small and will not destroy the balance of the power supply voltage, so it is safer than the system with the neutral point of the power supply grounded.
However, if it is used for a long power supply distance, the distributed capacitance of the power supply line to the ground cannot be ignored. When the load has a short-circuit fault or leakage current electrifies the equipment casing, the leakage current forms a bridge through the ground, and the protection equipment does not necessarily operate, which is dangerous. It is only safer if the power supply distance is not too long. This type of power supply is rarely seen on the construction site.
5. Transformation of power distribution system
The power supply and distribution systems in China's regions and enterprises have a lot of waste of electric energy, and the problems are multifaceted. The main problems and solutions are as follows.
5.1 The current grid capacity does not match the load
With the development of the economy and the improvement of people's living standards, the power consumption has increased rapidly. The equipment and wires of the original distribution network do not match the power consumption. Many places are overloaded, which not only affects the safety of power supply, but also greatly Increased power distribution system loss. The way to energy-saving transformation is to update lines and equipment.
5.2 Unreasonable power supply voltage
In some areas and many large power-consuming units, the power supply voltage is low. For example, in the past, it was stipulated that the incoming line voltage of the enterprise should be 6 kV, and it needs to be stepped down several times in the middle, which requires more construction funds and increases the power of the system. loss. Appropriately increasing the power supply voltage, reducing the original secondary or even tertiary step-down to one, can greatly reduce the equipment and line losses of the power supply system.
5.3 Unreasonable layout
In many areas, the electricity consumers and enterprises' electricity equipment are far away from the power distribution center, which makes the low-voltage (0.4 kV) power transmission distance too long, causing great line loss and voltage drop. This kind of situation is common in the old large and medium-sized enterprises, because the design stipulated that the power distribution center should be built at the end of the imported power supply of the enterprise. The improvement measure is to move the distance between the power distribution center and the electrical equipment as close as possible under the premise of ensuring safety, and change the original low-voltage long-distance power transmission to high-voltage long-distance and low-voltage short-distance power transmission, which can greatly reduce the power transmission line. loss.
5.4 Reactive power shortage
With the development of the economy, the inductive load in the power supply and distribution system increases rapidly, and many distribution transformers and motors are in a non-economic operating 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 the quality of the power supply voltage to decline and the system loss to increase. It will not only waste electric energy, but also affect the utilization rate of power supply and distribution equipment, and even cause accidents. The technical measure to solve the above problems 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 user's benefits come from less fines for power factor failure; the latter is called on-site Compensation, the direct beneficiary is the user, mainly to reduce line loss. 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.
5.5 Outdated power distribution equipment
Many of the power distribution equipment in use in my country, such as distribution transformers and various switches, are outdated and backward. Due to insufficient funds and insufficient awareness of energy conservation in relevant departments, they cannot be updated in time, resulting in a waste of a lot of electricity. For example, the no-load loss of distribution transformers in the early 1960s was 1.32 times that of the S1-type transformers in the early 1970s, and the S1 series was about 14% higher than that of the S6 series. The S6 series is 45% smaller, and the no-load loss and load loss of the S9 series transformers promoted by the country in the late 1990s are even smaller. Others such as electromagnetic switches, cable joints and connecting fittings are similar. If these old and backward power distribution equipment can be updated in time, the power distribution system can reduce a lot of unnecessary waste of electric energy.
6. Equipment for power distribution system
6.1 DC screen
DC screen is the abbreviation of DC power operating system. The general name is intelligent maintenance-free DC power supply screen, referred to as DC screen, and the general model is GZDW. Simply put, a DC screen is a device that provides a stable DC power supply. (When the input has 380V power supply, it is directly converted into 220V, and when there is no input of mains power and backup power, it is directly converted into battery power supply - DC 220V: in fact, it can also be said to be an industrial special emergency power supply). The power operating power supplies in power plants and substations are all DC power supplies today, which provide power for control loads, power loads, and DC accident lighting loads, and are the basis for control and protection of contemporary power systems. The DC screen is composed of a power distribution unit, a charging module unit, a step-down silicon chain unit, a DC feed unit, a power distribution monitoring unit, a monitoring module unit and an insulation monitoring unit. It is mainly used in small and medium-sized power plants, hydropower stations, various substations, and other users using DC equipment (such as petrochemical, mining, railway, etc.) in the power system. Electrical protection and fault lighting and other occasions.
The DC screen is a new type of DC system with digital control, protection, management and measurement. The monitoring host part is highly integrated and adopts a single-board structure (All in one), which includes functions such as insulation monitoring, battery inspection, grounding line selection, battery activation, silicon chain voltage stabilization, and microcomputer central signal. The host is equipped with a large LCD touch screen, and various operating states and parameters are displayed in Chinese characters. The overall design is convenient and concise, and the man-machine interface is friendly, which is in line with user habits. The DC screen system provides powerful functions for remote detection and control, and has remote control, remote adjustment, telemetry, remote signaling functions and remote communication interfaces. Through the remote communication interface, the operating parameters of the DC power system can be obtained remotely, and the operating status and fixed values can also be set and modified through this interface to meet the requirements of power automation and power system unattended substations; equipped with standard RS232/485 serial Line interface and Ethernet interface, which can be easily incorporated into the power station automation system.
6.2 Microcomputer protection
The microcomputer protection device is a relay protection composed of a microcomputer. It is the development direction of the power system relay protection (it has been basically realized, and it still needs to be developed). It has high reliability, high selectivity, and high sensitivity. The hardware of the microcomputer protection device includes a microprocessor (single chip microcomputer) as the core, equipped with input and output channels, man-machine interface and communication interface, etc. The system is widely used in electric power, petrochemical, mining smelting, railway and civil construction. The hardware of the microcomputer is universal, but the performance and function of the protection are determined by the software.
The digital core of the microcomputer protection device is generally composed of CPU, memory, timer/counter, Watchdog and so on. At present, the mainstream of the digital core is the embedded microcontroller (MCU), which is commonly referred to as a single-chip microcomputer; the input and output channels include analog input channels (analog input conversion loops (convert the quantities measured by CT and PT into lower suitable Internal A/D conversion voltage, ±2.5V, ±5V or ±10V), low-pass filter and sampling, A/D conversion) and digital input and output channels (man-machine interface and various alarm signals, trip signals and electrical pulses, etc.).