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Some basic protection knowledge of transformers

Word:[Big][Middle][Small] QR Code 2020/5/20     Viewed:    

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  1.Failure of the transformer

  1.Internal failure

  Transformer internal faults mainly include winding phase-to-phase short-circuit,winding-to-turn short-circuit and neutral-point grounding system winding ground-to-ground short circuit.These faults are very harmful,because the high temperature arc generated by the short-circuit current will not only burn the core of the winding insulation box,but also decompose the insulation material and transformer oil to generate a large amount of gas,which may locally deform and rupture the transformer oil tank,or even explode the oil tank.accident.Therefore,when an internal fault occurs in the transformer,the transformer must be quickly removed.This must be remembered.

  2.External fault

  Transformer external faults are mainly phase-to-phase short circuit and ground short circuit occurring in the transformer bushing and the lead-out line.When this type of fault occurs,the transformer should also be quickly removed to minimize the impact of the short-circuit current on the transformer.

  Second,the transformer does not work normally

  The abnormal working state of the transformer mainly turns into:

  (1)Current caused by external short circuit.

  (2)Overload.

  (3)The oil level of the tank is reduced due to oil leakage.

  (4)Over-excitation caused by transformer neutral point voltage increase or external voltage too high or frequency decrease.

  Third,the protection device that the transformer should be installed

  (1)Gas protection that reflects internal faults and reduced oil levels in the transformer tank.

  (2)Reflect the longitudinal difference protection or current quick-break protection of the single-phase grounding short circuit between the transformer winding and the lead-out short-circuit,the neutral point direct grounding system winding and the lead-out.

  (3)Reflects the external phase short-circuit of the transformer and serves as backup overcurrent protection(or overcurrent protection or negative-sequence overcurrent protection for composite voltage starting)of the gas protection box differential protection(or current quick-break protection).

  (4)reflects the zero-sequence current protection of the external and internal grounding short circuit of the transformer in the neutral point direct grounding system.

  (5)Overload protection reflecting the symmetrical overload of the transformer.

  (6)reflects the protection of transformer over-excitation.

  Fourth,the main protection of the transformer

  (1)Gas protection

  (1)Basic working principle of gas protection

  Reflecting the gas quantity and oil flow speed at the time of fault protection is called gas protection.When the internal fault of the transformer,the local high temperature of the fault point increases the temperature of the transformer oil,expands the volume,and the air in the oil is discharged to form a rising gas.If an arc occurs at the fault point,the transformer oil and insulation material will decompose a large amount of gas,and these gases flow from the oil tank to the oil storage cabinet.

  The more serious the fault,the more gas is generated,and the faster the oil flow to the oil tank.Because the amount of gas and the oil flow rate can directly reflect the nature and severity of the transformer fault,when a small amount of gas is produced and the gas flow rate is small,the light gas acts on the signal;when the fault is serious and the oil flow rate is high,the heavy gas protection acts on the trip instantaneously.

  Gas relay is the main component that constitutes gas protection.It is installed in the middle of the joint pipe between the oil tank and the oil storage tank,so that the gas inside the tank must flow through the gas relay to the oil storage tank.In order to make the gas flow to the oil storage tank smoothly,the old-fashioned transformer requires a certain inclination of the oil tank and the joint pipe,of which the oil tank requires 1%-1.5%,and the joint pipe requires a slope of 2%-4%.

  The new transformer is equipped with gas collecting manifolds in places where gas is easily collected(such as casing riser seat).Each gas collecting manifold is connected to the gas collecting main pipe,and then the gas collecting main pipe is connected to the connecting pipe at the front end of the gas relay.In this way,as long as there is a certain inclination of the gas collecting pipe and the connecting pipe,the gas can flow into the oil storage tank,so the oil tank has no inclination requirements.

  At present,the domestic use of open cup baffle gas relay,its working principle is as follows:

  1)During normal operation,the open cup is filled with oil.Since the torque generated by the open cup's own gravity is less than the torque generated by the counterweight,the open cup is pushed up and the reed contact is opened.

  2)When there is a slight failure inside the transformer oil tank,a small amount of gas will gather on the top of the relay,causing the oil level in the relay to drop,and the opening cup is exposed to the oil surface.The torque due to the weight of the opening cup plus the weight of the oil in the cup It is greater than the torque generated by the counterweight,so the open cup rotates downward.When the magnet fixed on the open cup descends with the open cup close to the reed contact,the contact closes to send a light gas action signal.

  3)When a serious fault occurs in the oil tank,a large amount of gas will be generated and the oil flow will hit the baffle.When the oil flow speed reaches the setting value of the relay,the baffle is washed to a certain position and fixed on the baffle The magnet is close to the reed contact,which closes the contact,and the contact closing action causes the circuit breaker to trip.

  (2)Setting of gas protection

  1)Setting of light gas protection

  Light gas operation value is expressed by gas capacity.Generally,the gas volume of light gas protection is in the range of 20-300cm 3;the adjustment of the gas volume can be achieved by changing the length of the arm of the weight.

  2)Setting of heavy gas protection

  The value of heavy gas protection operation value is expressed by the magnitude of oil flow velocity.General requirements for oil flow:0.8-1.0m/s for self-cooled transformers,1.0-1.2m/s for strong oil circulation transformers,and 1.2-1.3m/s for transformers above 120MVA.

  (3)Advantages and disadvantages of gas protection

  The main advantage of gas protection is its simple structure,which can comprehensively reflect various faults inside the transformer tank.Especially when there is a short-to-turn short circuit and the number of short-circuited turns is small,although the current in the fault loop is large,which may cause serious local overheating,the current change reflected in the external circuit is very small,even the sensitivity is relatively high.High differential protection may also not operate.Therefore,gas protection is of special importance to reflect such failures.In addition,gas protection is the only protection against iron core burnout.Gas protection is widely used because of its simplicity,sensitivity,and economy.Gas protection should be installed in oil-immersed transformers of 800KVA and above and indoor oil-immersed transformers of 400KVA and above.

  The main disadvantage of gas protection is that it cannot reflect the failure of the transformer bushing and the lead wire.Therefore,gas protection can not be used as the only main protection of transformer ground,and it can be used as the main protection of transformer ground together with differential protection.

  (2)Differential protection

  1.The basic principle of differential protection

  Transformer differential protection is constructed according to the principle of circulating current,which can correctly distinguish the internal and external faults of the transformer and can instantly remove the faults in the protection area.The current transformers TA1 and TA2 are installed on both sides of the transformer,the secondary coils are connected in series according to the circulating current principle,and the differential relay is connected to the differential current loop.

  During normal operation or external faults,current flows on both sides of the transformer.If the ratio of the two current transformers is selected appropriately,the secondary currents I12 and I22 have the same size and the same direction,while in the differential circuit I12 and I22 The direction of is opposite,so the current in the differential relay KD is equal to the difference between the secondary currents of the current transformers on both sides,and the current is zero,so the relay will not operate during normal operation or external faults.

  When a fault occurs inside the transformer,the secondary currents Id12 and Id22 of the current transformers on both sides have the same direction in the differential circuit,and the current flowing through the differential relay is the sum of the two currents,so that the differential relay operates.

  In fact,due to factors such as transformer excitation inrush current,wiring method,and current transformer error,an unbalanced current will flow through the differential relay.The greater the unbalanced current,the greater the relay operating current,resulting in differential protection.Sensitivity is reduced.Therefore,one of the main problems to be solved by differential protection is to adopt various measures to avoid the impact of unbalanced current.Under the condition of ensuring selectivity,it is necessary to ensure sufficient sensitivity and quickness during internal faults.

  2.Special problems of differential protection

  1)Influence of excitation current

  The excitation current of the transformer during normal operation only flows through the power supply side,and is reflected to the differential circuit through the current transformer to cause an unbalanced current.However,under normal circumstances,the excitation current of the transformer is very small,generally not exceeding 1%of the rated current;when the external fault occurs,the voltage is lowered and the excitation current is also reduced,so its impact is smaller,so it is not in actual adjustment.Be considered.

  2)Influence of excitation inrush current

  When the transformer is closed with no load,a large excitation inrush current may occur,and its value can reach 6-8 times the rated current of the transformer.It is transmitted to the secondary side through the current transformer on the power supply side of the transformer.If it flows into the differential circuit,it will often lead to the malfunction of the differential protection.

  Measures to prevent differential protection from malfunction due to excitation inrush current:

  (1)Use differential quick-break protection.Because the differential quick-break protection has an inherent operating time,the operating current does not need to avoid the maximum current.This scheme has low sensitivity and is only suitable for small transformers.

  (2)A differential relay with an intermediate speed saturation converter is used.The intermediate-speed saturation converter can suppress the transmission of excitation inrush current,thereby preventing the protection from malfunction.But because the transient current also contains non-periodic components during internal short circuit,the protection should be delayed.In addition,because there is often a non-periodic component in one phase of the three-phase inrush current,the phase velocity saturation converter does not work,which in turn must increase the protection action value,so the sensitivity of the protection decreases.Due to the slow action and poor sensitivity,this method is only suitable for medium and small transformers.

  (3)Use second harmonic braking.In the excitation inrush current,except for the fundamental wave and non-periodic component current,the second harmonic current is the largest,which is the most obvious characteristic of the excitation inrush current,because in other working conditions,the second harmonic is rarely generated.This is the main measure to prevent excitation inrush current in large transformer differential protection.

  (4)Use the characteristics of excitation inrush current waveform with obvious discontinuous angle to avoid excitation inrush current.There are currently two options,one is to directly identify the size of the discontinuity angle to determine whether it is an inrush current or an internal short circuit.The other is to compare the rate of change of excitation inrush current and secondary short-circuit current.

  (5)The differential protection is separately installed on the windings of each voltage side of the transformer,so the excitation inrush current no longer enters the differential circuit.

  3)The influence of different current phases on each side of the transformer

  The transformer length is Y,d11 wiring,so the phase of the current on both sides of the transformer is not consistent.Under normal circumstances,the line current on the triangular side of the transformer leads the current on the star side by 30°.If the current transformers on both sides use the same wiring method,the secondary currents also differ by 30°.Therefore,it is necessary to compensate for the unbalanced current caused by the different phases of the current on both sides.

  4)The effect of different current transformer errors on each side

  Due to the different excitation characteristics of the current transformers and the different secondary loads,a large unbalanced current will be caused in the differential circuit.

  5)The effect of the difference between the calculated transformation ratio of the current transformer and the selected standard transformation ratio

  Different transformation ratios will cause unbalanced current.When this unbalanced current is greater than 5%of rated load current,compensation measures should be taken.The commonly used compensation method is to use auxiliary auto-transformer,or use the balance coil of the differential relay to balance.

  6)Influence of transformer voltage regulation

  Transformers need to be adjusted according to the requirements of the system voltage during operation.In fact,the transformer's transformation ratio is changed,so unbalanced current will be generated.The size of the unbalanced current is related to the range of voltage regulation.Since it is impossible to readjust the relay with the change of the tap of the transformer during operation,the unbalanced current caused by the voltage regulation of the transformer should be avoided when setting the protection action value.

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