Building electrical basic knowledge of DC system


1、 DC system provides reliable DC power supply for control, signal, protection, automatic device and emergency lighting in substation

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It also provides reliable power supply for operation

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The reliability of DC system plays an important role in the safe operation of substation, which is the guarantee of safe operation of substation

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2

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At the critical moment of production equipment failure, DC system failure, especially the loss of the whole station control DC, will cause serious damage to the main equipment or fire, explosion, power system blackout and other extremely serious consequences and huge economic losses

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2、 The main components of DC system are as follows: 1

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Battery, charger, DC bus, insulation monitoring device and feed out load

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2

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DC system schematic diagram: 3

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Form an integral whole

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If the battery is compared to the heart of the body, the DC circuit is the blood vessel in the body, the DC load is the muscle of the body, and the charging device is the spleen of the body, which bears the functions of blood generation and hematopoiesis

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4

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Relevant technical measures of DC system I

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The AC and DC circuit of substation can not share a common cable: the reason is that the AC and DC systems are isolated from each other The AC system is a grounding system, both of them share a common cable

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If there is a short circuit between the two systems, it will cause DC grounding and affect the two systems at the same time

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The consequences of AC into DC are unimaginable, and similar accidents occurred in North China power grid

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Causing a large area of power failure

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At the same time, the public cable will interfere

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II

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Special DC fuse shall be used for DC system of substation

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The DC arc extinguishing performance of AC fuse is poor III

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DC power supply cannot be used for lighting and heater in high voltage switch cabinet and switch mechanism box IV

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the capacity and service time of emergency lighting should be limited when battery is used for emergency lighting

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3、 Battery 1

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The characteristics of VRLA battery (this handout mainly explains this kind of battery, hereinafter referred to as battery) 1) the lean solution design is often adopted (there are colloidal type, but it is not widely used)

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A gas channel is reserved between the positive and negative plates

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During the charging process of the battery, the oxygen generated on the positive electrode smoothly reaches the negative electrode through the partition plate, reacts with the negative active material and recovers At the same time, the design of the grid can inhibit the precipitation of hydrogen, so as to achieve the goal of no water loss

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During the whole service life of the battery, there is no need to add acid and water

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2) the battery is airtight and liquid tight, and there is no acid mist overflow during use

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3) the normal floating charge service life is 10 years

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4) the self discharge is small

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5) the structure is compact

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6) safety valve: when the internal air pressure exceeds the predetermined value, the safety valve will automatically open and release gas

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When the internal air pressure decreases, the safety valve will automatically close, and the external air pressure will be prevented The external characteristics of the battery are relatively flat, and the internal resistance of the battery is smaller than that of the charging device

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When the battery passes through the large current, most of the current is borne by the battery

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When the charging device suddenly needs to output tens or even hundreds of amperes of large current from the usually small output current, it is equivalent to a short circuit During the charging process, the general charging device is equipped with limited current protection device

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In addition, the storage battery can still provide power for the most important DC loads such as protection device and monitoring system after the loss of AC power and the stop of charger, so as to ensure the normal operation of these devices

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3

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Battery floating charging operation charging device is connected to the battery with DC load

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During normal operation, the charging device will charge the battery while bearing the regular load, so as to supplement the self discharge of the battery, and make the battery in full capacity state for standby

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4

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Battery model: 5

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Several concepts of battery 1) rated capacity of battery: the minimum capacity that the battery can guarantee under the conditions specified by the manufacturer

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It generally refers to the capacity of the battery to discharge at a 10 h discharge rate when it is fully charged

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It is represented by C10

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Common, such as 400A

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H2) battery capacity: discharge current and discharge hour product, capacity CN = it, unit A.H, abbreviated as ampere hour

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3) Terminal voltage of storage battery: when the storage battery is over discharged, it will not cause damage to the electrode plate

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Generally 1.8V (single 2V battery, 10 hours discharge rate) (mobile phone battery termination voltage) 4) battery self discharge: battery internal plate short circuit, external positive and negative pole insulation bad, resulting in capacity loss

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(how to protect notebook battery) 6

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Operation requirements of valve controlled battery 1) voltage deviation value and discharge end voltage value of valve controlled battery in operation shall comply with the provisions of (table)

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2) fireworks shall be strictly prohibited in battery room

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Good ventilation and heating facilities shall be provided to avoid sunlight

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Explosion proof lamps shall be used

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3) voltage value of single battery shall be tested regularly, and four qualified ones shall be used Semi digital voltmeter

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It is allowed to use the qualified automatic monitoring device to measure the battery cell voltage, which can be used as the basis of detection

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4) The battery shell and pole should be checked regularly to see if there is any trace of leakage

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5) Clean the shell of the battery pack regularly

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6) The operating temperature of the battery should be kept at 5 ~ 30 ℃, and the recommended operating temperature is 25 ℃

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Too high temperature will reduce the capacity of the battery and shorten its service life

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7) During operation, the terminal voltage value, floating charge current value, voltage value of each battery, ground resistance value and insulation state of battery and DC bus are mainly monitored

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8) The main purpose of choosing the appropriate floating charge voltage is to make the battery achieve the ideal service life and rated capacity

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If the floating charge voltage is too high, the floating charge current of the battery will increase, which will accelerate the grid corrosion rate

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In addition, overcharge will generate gas inside the battery, increase the internal pressure, and the gas will be discharged from the safety valve, resulting in the reduction or drying of electrolyte and excessive water loss The results show that the service life of the battery is advanced; the floating charge voltage is too low, the battery can not be maintained in a fully charged state, which easily leads to irreversible sulfation, capacity reduction, and shorten the service life of the battery

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9) The valve regulated battery with large capacity is generally installed in the special battery room

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10) Valve regulated batteries of 200ah and below can be installed in the battery cabinet

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A thermometer shall be installed in the battery cabinet

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The battery cabinet structure should have good ventilation and heat dissipation

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11) The nominal voltage of DC control bus in substation is 110V and 220V

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During operation, the voltage variation range is 95% ~ 110% of the nominal voltage

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In normal floating charging operation, the DC control bus voltage should be adjusted according to the upper limit

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7

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Selection of the number of batteries: it is calculated according to the DC bus voltage of 105% of the rated voltage of DC system during normal floating charge operation

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Valve regulated battery rated voltage selection: 1) there are two common monomer 2V and 12V

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2) The advantages of 2V battery are long design life and high reliability

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If one or two batteries are damaged, they can be divided into two types: 2V and 12V

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2) The advantages of 2V battery are long design life and high reliability

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If one or two batteries are damaged, they can be short circuited and will not have a great impact on the system voltage

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The disadvantages are high cost, large maintenance and large floor area

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3) The advantage of 12V battery is that each group has only 18 pieces (220 V system), maintenance and replacement are more convenient, the cost is lower than that of 2V battery with the same capacity, the structure is compact, and the floor area is small

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The disadvantage is that the damage of 1-2 batteries has a greater impact on the system voltage, and can not be short circuited, so it generally needs to be replaced

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8

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Balanced charging 1) definition of balanced charging: to compensate for the unbalanced voltage generated by the battery in the process of use Phenomenon, so that it returns to the specified range of charging.

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