(3.2.1)--15电流互感器及运行.ppt

Electrical Equipment and Operation1ContentsThe Working Principle of Electromagnetic Current TransformerThe Influence of the Open Circuit on the Secondary Side of the Current TransformerPolarity and Wiring Method of Current TransformerONThe current transformer turns a large current into a small current(5A or 1A),which is used as the AC power supply and current signal of measuring instruments and relays.Electromagnetic current transformers are widely used in power systems,represented by TA(the old symbol is CT or LH).Overview of Current Transformers3The Working Principle and Characteristics of Electromagnetic Current TransformerThe current transformer also works according to the principle of electromagnetic induction.Its structure is similar to that of an ordinary transformer,mainly composed of iron core,primary winding and secondary winding.Fig.3-1 Principle structure diagram and wiring diagram of current transformer(a)Principle structure diagram (b)Wiring diagramThe ratio of the primary and secondary rated currents of the current transformer is called the rated transformation(current)ratio Ki of the current transformer,which can be expressed as:The secondary rated current of the current transformer has two types:5A and 1A.Generally,5A is used for high-current system,and 1A for weak-current system.1A can also be considered when the power distribution device is far from the control room.At present,the primary rated current levels of current transformers are:5,10,15,20,30,40,50,75,100,150,200,300,400,600,800,1000,1200,1500,2000,3000,4000,5000,6000,8000,10000,15000,20000,25000A.5The Working Principle and Characteristics of Electromagnetic Current Transformer(1)The primary winding is connected in series in the tested circuit with few turns.The current in the primary winding completely depends on the current of the tested circuit,and has nothing to do with the secondary current.Fig.3-1 Principle structure diagram and wiring diagram of current transformer(a)Principle structure diagram (b)Wiring diagram(2)The secondary winding has many turns,and the resistance of the current coil of the instrument or relay connected in series is very small,so during normal operation,the secondary side of the current transformer is close to working in a short-circuit condition.6The Influence of the Open Circuit on the Secondary Side of the Current TransformerFor current transformers in operation,the secondary circuit must be connected to a load or be directly short-circuited,and no open circuit is allowed.nIn normal state:nAnalysis:F2 plays the role of demagnetizing F1,and the role of F0 is very small;nIn an open circuit:F20,nWhen F1 is unchanged,F0 will be very large,the core is saturated,and becomes a flat-top wave.To voltage transformer7 7The Influence of the Open Circuit on the Secondary Side of the Current TransformerFor current transformers in operation,the secondary circuit must be connected to a load or be directly short-circuited,and no open circuit is allowed.Consequences:The secondary side voltage is as high as a thousand volts,which endangers the equipment insulation;If the magnetic flux is large,the core loss will increase and become severely heat,and even the transformer will be burnt;The error increases.8 8 8The Influence of the Open Circuit on the Secondary Side of the Current TransformerFor current transformers in operation,the secondary circuit must be connected to a load or be directly short-circuited,and no open circuit is allowed.uIn order to prevent an open circuit on the secondary side of the current transformer,it is not allowed to install a fuse or low-voltage circuit breaker on the secondary side,and the secondary connecting wire should use a copper core material with a cross-sectional area not less than 2.5mm2.uDuring operation,if it is necessary to replace the current transformer secondary circuit instrument or relay,you must first short-circuit the current transformer secondary circuit,and then disconnect the instrument or relay that needs to be replaced.Polarity and Wiring Method of Current TransformerThe polarity of the current transformer is marked according to the principle of minus polarity,as shown in the figure.When the primary side current I1 flows from L1 to L2,the secondary side current I2 flows from K2 to K1 in the secondary winding,and flows from K1 to K2 in the secondary load,it is stipulated that L1 and K1 are the same polarity terminals(L2 and K2 are also the same polarity end).For power meters and relay protection devices,the polarity of the current transformer is very important.Wrong polarity can cause count error in the power meter or malfunction to the relay protection device.10Polarity and Wiring Method of Current TransformerSingle-phase wiring:The current passed through the ammeter is one-phase current,which is usually used to measure one-phase current of a symmetrical three-phase load.Three-phase star wiring:The three current transformers reflect the three-phase current and various types of short-circuit fault currents respectively.It is widely used in three-phase three-wire circuits and low-voltage three-phase four-wire circuits with balanced or unbalanced loads for measurement and protection.1111Polarity and Wiring Method of Current TransformerIncomplete star wiring:The current flowing in the common line is the sum of the two-phase current,so this kind of wiring is also called two-phase current and wiring.It can be seen from that the current in the secondary-side common line is exactly the secondary current of the v-phase that is not connected to the transformer.Therefore,this kind of wiring can be connected to three ammeters to measure the three-phase current separately,so,no matter whether the load is balanced or not,it is widely used in three-phase three-wire neutral point ungrounded systems for measurement or protection.uIn all wiring of current transformers,the secondary winding should have a grounding point to prevent insulation breakdown between the primary and secondary windings from causing high voltage on the secondary side,which will endanger the safety of people and equipment.Thank you for watching!12