供配电系统

POWER SUPPLY AND DISTRIBUTION SYSTEM

1. Composition of the distribution network system

It is composed of total step-down substation, high voltage distribution line, substation, low-voltage distribution line and electrical equipment.

Total step-down substation: substation is the transformer output voltage below the input voltage.

High voltage distribution line: power transmission refers to the voltage level above 35kV

Distribution refers to the voltage level of 220V to 10kV and sometimes 35kV Electricity distribution means to distribute electricity to users

The power distribution from 110kV (or 35kV) substation is reduced to 10kV distribution line, and the transformer is converted to 220V at the user's level to be used by users.Due to 10 kv line after the user space, in order to ensure the conductor is not with the surrounding trees, buildings, so often used insulated conductor, high voltage class and transmission line, transmission distance, more than in the high mountain with tower erection, all use bare wires.

Because power distribution is to distribute power to different users, it is necessary to use the branch box, while transmission line is transported directly from one substation to another substation, and there is no branch box.

Some distribution lines take underground cables, while transmission lines are all overhead lines.

Substation (substation), as the name implies, is the place where voltage is changed.It is the place where the voltage and current of the power system are transformed, concentrated and distributed.In order to guarantee the electrical energy quality and the safety of equipment, still need to adjust voltage in the substation, tide (in the power system voltage, current and power of each node and branch) flow and distribution of the control and power transmission and distribution lines and the protection of the main electrical equipment.The utility can be divided into power substation and traction substation (electric railway and trolley).In the national standard GB 50053-50053 "under 10 kv substation design specification" which sets the term is defined as "10 kv and below the ac power for electric power equipment after a power transformer transformer", in accordance with the substation.

The revolution of electric power system has brought a new big round construction,which is pushing the greater revolution of electric power technique along with the application of new technique and advanced equipment. Especially, the combination of the in formation technique and electric power technique, to great ex- tent, has improved reliability on electric quality and electric supply. The technical development decreases the cost on electric construction and drives innovation of electric network. On the basis of national and internatio- nal advanced electric knowledge, the dissertation introduces the research hotspot for present electric power sy- etem as following.

Firstly, This dissertation introduces the building condition of distribution

automation(DA), and brings forward two typical construction modes on DA construction, integrative mode and fission mode .Itemphasize the DA structure under the condition of the fission mode and presents the system configuration, the main station scheme, the feeder scheme, the optimized communication scheme etc., which is for DA research reference.

Secondly, as for the (DA) trouble measurement, position, isolation and resume, Thisdissertation analyzes the changes of pressure and current for line problem, gets math equation by educing phase short circuit and problem position under the condition of single-phase and works out equation and several parameter s U& , s I& and e I& table on problem . It brings out optimized isolation and resume plan, realizes auto isolation and network reconstruction, reduces the power off range and time and improves the reliability of electric power supply through problem self- diagnoses and self-analysis. It also introduces software flow and use for problem judgement and sets a model on network reconstruction and computer flow.

Thirdly, electricity system state is estimated to be one of the key techniques in DA realization. Thedissertation recommends the resolvent of bad measurement data and structure mistake on the ground of describing state estimate way. It also advances a practical test and judging way on topology mistake in state estimate about bad data test and abnormity in state estimate as well as the problem and effect on bad data from statemeasure to state estimate .As for real time monitor and control problem, the dissertation introduces a new way to solve them by electricity break and exceptional analysis, and the way has been tested in Weifang DA.

Fourthly, about the difficulty for building the model of load forecasting, big parameter scatter limit and something concerned, the dissertation introduces some parameters, eg. weather factor, date type and social environment effect based on analysis of routine load forecasting and means. It presents the way for electricity load forecasting founded on neural network(ANN),which has been tested it’s validity by example and made to be good practical effect.

Fifthly, concerning the lack of concordant wave on preve nting concordant wave and non-power compensation and non-continuity on compensation, there is a topology structure of PWM main circuit and nonpower theory on active filter the waves technique and builds flat proof on the ground of Saber Designer and proves to be practical. Meanwhile, it analyzes and designs the way of non-power need of electric network tre- nds and decreasing line loss combined with DA, which have been tested its objective economic benefit throu- gh counting example.

Sixthly, not only do the dissertation design a way founded on the magrginal electricprice fitted to our present national electric power market with regards to future trends ofelectric power market in China and fair trade under the government surveillance, that isgroup competitio n in short-term trade under the way of grouped price and quantity harmony, but also puts forward combination arithmetic, math model of trading plan and safty economical restriction. It can solve the original contradiction between medium and long term contract price and short term competitive price with improvement on competitive percentage and cut down the unfair income difference of electric factory, at the same time, it can optimize the

electric limit for all electric factories and reduce the total purchase charge of electric power from burthen curve of whole electric market network.

2. Power system voltage level and substation type

The voltage level of the power system is 220/380v (0.4kv), 3 kV, 6 kV, 10 kV, 20 kV, 35 kV, 66 kV, 110 kV, 220 kV, 330 kV, 500 kV.With the improvement of the manufacturing process of the motor, the 10 kV motor has been mass-produced, so 3 kV and 6 kV are less used, and 20 kV and 66 kV are also rarely used.The power supply system is based on 10 kV and 35 kV.The transmission and distribution system is based on 110 kV.The power plant generator has 6 kV and 10 kV, and now it is based on 10 kV and the user is 220/380v (0.4kv) low pressure system.

In accordance with the urban power network design rules "regulation: the grid for 500 kV, 330 kV, 220 kV, 110 kV, the high voltage power distribution lines of 110 kV, 66 kV, medium voltage distribution network of 20 kV, 10 kV and 6 kV, low voltage power distribution network is 0.4 kV (220 v / 380 v).

Issue 6 kV and 10 kV electric power plant, in addition to power plant oneself use (apc), 10 kV voltage can also be used to power plant near the users, including 10 kV power supply range of 35 kV, 10 km, is 20 ~ 50 km, 66 kV is 30 ~ 100 km, 110 kV is 50 ~ 150 km, 150 kV to 100 kV ~ 300 km, 300 to 330 ~ 600 km, 500 kV to 600 ~ 850 km.

All kinds of power system voltage level transformation through power transformer, voltage rise for the step-up transformer (transformer substation for booster station), reduced to step-down voltage transformer (transformer substation of step-down station).A double coil transformer with two coils (winding) of a voltage to another voltage, and a three-coil transformer of three coils (windings) of two voltages.

In addition to lifting and lowering of the substation, the substation is divided into hub stations, regional stations and terminal stations.The voltage level of the hub station is generally three (three-ring transformer), 550kV /220kV / 110kV.The regional stations generally have three voltage levels (three ring transformers), 220 kV /110kV /35kV or 110kV /35kV / 10kV.The terminal station is usually directly received by the user, most of which are two voltage grade (two ring transformer) 110kV /10 kV or 35 kV /10 kV.The user's own substation generally has only two voltage grades (double coil transformer) 110 kV / 10kV, 35kV / 0.4kv, 10kV / 0.4kv, of which 10kV / 0.4kv is the most.

3 preventive measures of the distribution network system

1. Leakage of fire hazards and causes of leakage

After electrical wiring or equipment insulation damage, in certain circumstances, to close to the material (threading tube metal shell, electrical device, wet wood, etc.) will happen leakage, make the local material charged, cause serious and even fatal electric shock to people, a spark, arc, such as high temperature overheating and cause a fire. Leakage occurs when the electrical equipment which touch the shell when short circuit, current to connect equipment shell, protecting the zero line (protective earthing line), zero line (earth) to form a closed loop, the leakage current will be very

big, often can make fuse action and cut off power supply.But because of many reasons, such as fuse specifications may be artificially increase several times or replaced by copper wire, grounding device is not in conformity with the requirements of grounding resistance is bigger, grounding line terminal connection, protection device failure or set unreasonable, etc.) can make the over current protection device don't over-current protection function, so that once the leakage occurs, will continue to exist, result in electric shock or electric fire accident, many leakage fire cases also proves this point.

There are many factors that cause leakage, which can be summarized as follows: (1) the installation of low voltage power distribution system is the electrical professionals, quality is uneven, difficult to guarantee quality, displays in: wet or have acid and alkali corrosive environment, Ming fu, wire equipment did not protect the direct installation, wiring, knife, pliers, hammer damage such as insulating layer;The non-standard phenomenon of quality of wire joint connection and quality of insulation and rolling quality is not in conformity with the requirements.

(2) the electrical wiring or equipment is neglected for inspection, and the reason of the insulation deterioration due to overloads or longer service life;

(3) choosing fake and shoddy electrical products;

(4) external factors: water immersion, extrusion, rat bite, etc.

2. Leakage causes fire

(1) leakage current causes fire.The leakage point of leakage is usually not real, and the contact resistance is very high, which causes the contact resistance to be large, which makes it difficult to move the current protection device, and also produces an electric arc at the point of failure.According to the survey, only 0.5 A current of arc temperature can exceed 2000 ℃ above, all enough to ignite fuel.

(2) the connection between protection of the zero line or the connecting terminal of the protection ground line is not real, causing the fire.Phase line and zero line terminal connections are not real, the operation of the equipment can be found in a timely manner, protect the zero or ground terminal connection untrue, resistance is too big, the equipment work as usual, fault point is difficult to be found.Once leakage, due to the fault point joint is too loose or corrosion, etc., the presence of high resistance, cause local overheating, terminals at high temperature or electric arc, can spark around flammable substances, or burn out electrical socket and switch, ignition wooden base, this is the more common forms of residual current fire.In July 2000, a fire in a hotel in hangzhou was a case of fire. Before the fire, the restaurant closed and the load was in a low ebb.At the time of the fire, lighting was still normal.The total air switch in the distribution box is seriously carbonized, and the protection of the zero wire terminal has a metal concave shape, which is consistent with the leakage.

(3) fire caused by leakage voltage.After leakage continues, because the current cannot diaspora, and guide to find the other one road leads to resistance, zero line (ground) along the protection take conduction that all connected with the metal shell of the electric devices with the voltage, then can to nearby low potential of warm water pipe, gas pipe and other metal components flash-over become fire origin, 20 v voltage alone can make the arc occurs continuously, can also be ignited fuel around.If an arc

is to the gas, it may break through the wall, causing a gas leak to cause a fire.It is necessary to note that the leakage point is not necessarily consistent with the fire point due to the conduction of the voltage.

(4) to protect the line diameter of the zero line or to protect the ground wire, if the selection is too small, the line will rise faster when passing the larger leakage current, which can also cause the fire.

3. Prevention measures of leakage fire

3.1 strengthen the management of electrical workers

To establish and improve electrical operation procedures, all electrical practitioners must learn to master these operating procedures, and non-electrical professionals are not allowed to work.In order to strengthen the training of electrical workers, organize training courses regularly, improve the technical and safety awareness of electrical workers, and eliminate the human factors causing the fire.

Take necessary technical protection measures

(1) install the leakage protector.Current low-voltage distribution system in the protection of the zero and overcurrent protection devices and other measures can't effectively prevent the occurrence of leakage fire, therefore, in the current building the into the line should be set dedicated to fire prevention of the leakage protector.To prevent blackouts in power distribution box and switch box customers should set up the leakage protector, rated its rated current action and action time should be reasonable to cooperate, make it has the function of graded protection.

(2) reasonable selection of protection of zero (protective grounding line) and the design of grounding resistance.The selection of the cross-sectional area of the protection of the zero and the protection of the ground wire must be calculated and verified with the short circuit current of the shell.Its terminals must be securely connected, not allowed to be loose, and check its connection quality frequently.Electrical equipment protective earthing resistance value should not be more than 4 Ω, such as the capacity of electrical equipment is larger, the melt fuse current is bigger also, should increase the grounding line cross section or parallel body to fully decrease grounding resistance, increase the leakage short-circuit current, thus is advantageous to the protective device.

(3) carry out equipotential connections.Leakage protector for single-phase 220 v circuit provides only indirect contact protection, at the same time there are for parts wear short life, poor contact, quality instability caused by factors such as action failure of pitfalls, couldn't become a reliable protection, so is the equipotential connection should be implemented, can effectively eliminate the leakage of electric circuit or device with low potential metal components between the generation of electric arc and spark, which eliminate the leakage voltage may cause fire.Equipotential connection refers to the protection by zero bus and the structure of the water main, total gas pipe, hvac duct device such as a metal pipe or use wire connection measures, to achieve the purpose of equilibrium potential within a building, especially for flammable and explosive place more has its irreplaceable role.

3.3 strengthen the audit of electrical design.For building power distribution system, the relevant departments should be on the electrical design review on strictly

according to the related technology, for building interior decoration engineering must be strictly implemented the code for fire protection design of building interior decoration, use less as far as possible need not or flammable combustible material, especially in the electrical wiring through fuel, flame retardant should wear metal or hard plastic tube protection, because the plastic insulation performance is good, so it can well prevent leakage, using metal wiring, must prevent the insulating layer is broken.Power distribution devices (switches, sockets, distribution boxes, etc.) and electric equipment and combustible materials should be kept safe and distance, which are indeed inseparable, should be protected from heat protection measures.After the completion of the construction project, its electrical installation shall be put into use after being tested by a special testing institution, and the occurrence of leakage fire can be eliminated from the source as far as possible.

4 Development of power distribution system

1. Development status of voltage distribution system

With the rapid development of modern industrial technology, the low-voltage distribution system in people for operational reliability and the requirements of the intelligent management is becoming more and more high, with the improvement of the computer system reliability, and the application of a slightly processor technology, made in the electric power system, a lot of intelligent low-voltage electric elements obtained rapid development, which appeared in the intelligent low-voltage distribution system, at present in our country there are a lot of low voltage intelligent monitoring system is based on traditional SCADA system transformation, although it can satisfy the basic monitoring function, but not the electrical system fully reflected its own characteristics and requirements, the development and launch of meet the demand of industrial control and have high reliability in the low-voltage distribution system gradually become the primary task of low voltage electrical equipment manufacturing sector.

Although this kind of intelligent low-voltage distribution system has been widely applied at present, but because its price is quite high, the main application in the following aspects: first, the hair distribution system such as substation and power plant;Second, some important industrial fields, such as mining, automobile manufacturing, petrochemical and steel, etc.Third, some infrastructure such as subway, terminal or airport;Fourth, some large commercial buildings, such as high-rise buildings, smart buildings and superstores.At present the system has been widely used in these projects, the amount is relatively large, especially in recent years, along with our country economy and the rapid growth of demand in the low-voltage distribution system, the products are all imported products, at the same time, the dosage of the will be more and more big, in the future in power system, intelligent low-voltage distribution system will become the main development trend of power system.

In addition, in low voltage complete sets of equipment in our country, its intelligent power distribution system also started gradually, such as xiamen electric control equipment factory has successfully developed to implement data acquisition, motor control center in tianjin xiali car project, the application of the power cabinet and low

voltage power distribution cabinet have realized computer control, centralized monitoring system and the running status of equipment and all its faults, but in terms of technology, although our country has already mastered the microprocessor in the electronic products of all kinds of application technology, there are also some new type of digital products, but can reflect the level of intelligent controller and intelligent low-voltage distribution equipment is less, the need to strengthen the research and exploration.

Ii. Future development trend of medium and low voltage distribution system

In the intelligent low-voltage distribution system can collect real-time data, remote operation and program control, all kinds of statements and equipment maintenance information management, fault analysis, protection, digital communication and the event log and alarm and other functions.Because of the low voltage power distribution system directly to the controlling terminal, the required equipment is more and more widely distributed, at the same time in the field condition by more complex and so on, in the future development, can intelligent low-voltage distribution system should be implemented in the object-oriented operating mode, must have strong anti-interference ability.

In this system, its main control functions are completed by intelligent elements in the equipment layer, gradually formed to a network integrated distribution control system, all of them, and can satisfy the system running reliability, real-time and fast requirements, in the low-voltage distribution system in intelligent low-voltage intelligent component in terms of its function, which can be roughly divided into the following kinds: switch the protection and control, motor control, and power quality monitoring, etc.

(I) the necessity of realizing intelligence and automation of low-voltage power distribution system

1. Meet the requirements of national economic development and improve the quality of life of people.In modern society, the quality of power supply not only reflects a country or region's economic development and people's living quality and level, but also with an important factor affecting the development of its economy.Which the intellectualization of low-voltage distribution system and automation level directly determines the size and direction of the development of the industry, only to realize low-voltage distribution system in intelligent and automation, can only be in the maximum extent, improve the quality of the power supply, to adapt to the demand of economic development and people's life and production requirements.

2. Realizing the intelligent and automation of low-voltage power distribution system is one of the important ways for the development of electric enterprises.Implementation of low-voltage distribution system in intelligent and automation, can effectively reduce the number of failure occurs, narrowing the scope of the accident and to shorten the time of the accident, for the power supply recovery and rapid analysis and report the cause of the accident to provide corresponding effective basis.At the same time also can improve the whole economic benefit of power system, improve the security and reliability of the equipment operation, effectively reduce the work intensity of maintenance personnel, to a great extent, can

effectively prolong its service life.In addition, it is more convenient to monitor the power supply situation and the situation of the users, achieve overall control and provide more detailed and rich information for its management.

3. The effective management to improve the low-voltage distribution system, demand for the load input to respond quickly, at the same time in the system can optimize the distribution of energy consumption, parking balanced load so as to reduce the potential of various events.According to the actual operation of each equipment in the system, the corresponding maintenance plan should be made according to the plan, so as to effectively reduce the cost of maintenance.In addition, but also potential accident forecasting warning, and take corresponding measures to prevent accidents, failure to reduce the loss, understand the relevant fault information at any time, effectively guide the maintenance work.Can through the way of report or graphics to the operation of various power distribution circuit in the display or a device in the system operation, and can be stored in the disk or print it out, to a great extent, improve the efficiency of its work.

(ii) development trend of low-voltage distribution system

From the above content, we can know that the future low-voltage distribution system should be developed in the following aspects;First, the function is stronger, the system is more perfect and the reliability is higher, etc.Secondly, it is more flexible, easier to program and easier to control.Third, the openness of communications protocols is stronger.

Implementation of automatic control and intelligent control, low voltage switchgear can be done through the following two ways: first, set in the surrounding of the switchgear several PLC remote terminal station, the various motor control circuit and the power distribution circuit state and direct electricity parameters collected in the PLC control machine for processing, through the PLC I/O points to realize the signal control.Second, in the traditional low voltage switchgear equipped with corresponding power data acquisition module, make it become a kind of intelligent switch cabinet, then pushes the data acquisition module unit reuse communication network in the PC for processing, but also accept computer control command, this method can effectively improve the reliability of switchgear and power distribution system, and its performance, etc.In switchgear intellectualization and automation control, because the many manufacturers products in our sample is relatively simple, the author suggested that may follow the same type products abroad samples to its WanZhengHua, if conditions permit, should adopt automatic switch cascade protection measures.

5 conclusion

To sum up, with the progress of science and technology and the continuous development of social economy, at present our country adopted in low voltage power distribution system has yet to be further research and innovation, to improve the reliability of electricity at the same time, improve the quality of the power supply and the level of the enterprise itself, realize the low-voltage distribution system in the intelligent and automation, in the low-voltage distribution system more simple and reliable, convenient for people to its application, to enhance the reliability of system

operation, improve the compatibility of low voltage products in network, make it meet the needs of businesses and individuals.

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