No load (transformer) refers to the working state in which the primary winding of the transformer is connected to the power supply and the secondary winding is open circuit.At this time, the current in the primary winding is called the no-load current of the transformer.No load current generates no load magnetic field.Under the action of the main magnetic field (that is, the magnetic field that crosslinks the primary and secondary windings at the same time), the electromotive force is induced in the primary and secondary windings.[1]
transformerNo load operationAlthough there is no power output at the secondary side, the primary side still draws part of the active power from the grid to compensate for the magnetic flux saturation caused in the iron coreHysteresis lossandEddy current lossabbreviationIron consumption。The magnitude of hysteresis loss depends on the frequency of power supply and core materialHysteresis loopArea;Eddy current lossAnd MaxMagnetic flux densityIt is proportional to the square of the frequency.In addition, there is no load currentCopper consumption。For transformers with different capacities, the no-load current and no-load loss are different.No load of motor means that the motor operates without load equipment.
No load means no connectionload。The meaning of no-load is neither open circuit norshort circuit。A circuit has both input and output. Generally, no load means that there is input but no load connected to the output. Then the output of this circuit is equivalent to“Open circuit”, but it also has input and current (no-load current). There is no open circuit or short circuit for this circuit.If an amplifier is powered on but the horn is not pressed, the amplifier is called no-load.One motor belt pulley drives one water pump. When no belt is installed, the motor is powered on. This motor is called no-load.
No load test
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definition
No load test, refers to inspectionTurbine generator unitStart procedures for operation stability and reliability under no-load idling conditions.This belongs to the performance test of hydraulic unit under no-load condition.No load test isSynchronous generatorOne of the basic tests of.The no-load test can not only check the working condition of the excitation system,Armature windingWhether the connection is correct can also know the saturation degree of the motor magnetic circuit.[2]
content
No load test is to check the accuracy of automatic operation control, and determine the no-load characteristics related to settingrelay protectionValue.Before starting, it is necessary to conduct a comprehensive inspection of the unit, mainly including: ① the clearance around the rotating and fixed parts is qualified, and there is no foreign matter inside. ②The safe distance between the rotor pole joint and the nearby conductor shall not be less than 10 mm. ⑧The oil level of each bearing is within the specified range. ④thrust bearingThe insulation resistance of the upper guide bearing to the ground meets the requirements of the regulations, and the high-pressure oil jacking oil pump of the thrust bearing works normally. ⑤The temperature measuring device and signal device of the water turbine generator unit have been put into use. ⑥The opening and closing positions of all oil, gas and water system valves are normal. ⑦The cooling water pressure of each part shall be adjusted to the specified range and the water can be supplied normally. ⑧operationHydro generatorThe brake action is correct. ⑨The inside of the hydro generator is clean and free of metal and other leftovers. ⑩The carbon brush of excitation slip ring needs to be pulled out. ⑩The circuit breaker and disconnector at the outlet of the outgoing line of the hydro generator are cut off.After the above inspection meets the requirements, start the machine according to the operation program command.
The first startup of the units in the new hydropower station is manual operation, and it stops at 60% and 80% of the rated speed for a while to have time to check and find problems.When there is no abnormality, it will rise to the rated speed,No load operation。During the first startup and operation, the following contents shall be monitored: ① Monitor the turbine chamber,Hydro generatorWhether there is abnormal sound at the stator wind tunnel and the upper and lower wind tunnel covers. If there is abnormal sound, quickly identify the location and cause, and if necessary, emergency stop for inspection and handling. ②Check whether the working seal leakage and head cover drainage of the hydraulic turbine are reliable, and if necessary, emergency stop to check and handle. ③Monitor the temperature rise of each bearing bush. If the temperature rises too fast or the temperature is close to the allowable limit, stop the machine immediately. ④Monitor the swing of the water guide, lower guide, upper guide and slip ring, and the vibration of the top cover and upper and lower frames, check the oil level change of each bearing, and adjust accordingly if necessary. ⑤Check the leakage of oil, gas and water pipe joints and valves, and dispose accordingly. ⑥Check whether the lubrication friction of the oil receiver is good.
When the above operating conditions are normal and the temperature of each part is stable within the limit, the test shall be carried out under no-load condition. The main items are: ① measure the residual voltage of the hydro generator and check the phase sequence. ②Turbine governorAutomatic regulation test, measure various characteristics and parameter setting, and finally conduct disturbance test to check sensitivity and stability. ③Reassemble and grind carbon brush of excitation slip ring, test and adjust excitation system, and record no-load characteristics of exciter. ④If the swing and vibration of water turbine generator unit are too largeDynamic balance test of hydro generator set, and add counterweight. ⑤Hydro generatorShort circuit test, check the secondary power circuit, set the current protection of hydro generator, record the short circuit characteristics and short circuit degaussing characteristics of hydro generatormain transformerCurrent rise test, short circuit drying if necessary. ⑥Turbine generator unitOver speed test, set the speed relay. ⑦Stop the machine for inspection, and conduct ACDC withstand voltage testEtc.[3]
Circuit working state
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The working state of the circuit includes: loaded state, no-load state and short circuit state.Their respective characteristics are described as follows.
On load status
For electrical installation works, the circuit is in normal working condition with load.At this time, the voltage supplied by the power supply meets the rated value, the percentage of its deviation meets the regulations of the local power supply department, and the current flowing through the power supply and its switchgear is within the rated range.In a word, all electrical parameters (such as current, voltage, power, etc.) and non electrical parameters (such as noise level, heating, electric stress, etc.) in the power circuit under the load state are in the expected normal state.The most obvious feature is that there is both current and voltage in the circuit, and normal conversion of electric energy and other energy occurs.
No load status
For electrical installation engineering, the circuit is in standby state when it is unloaded, and the standby state can be divided into hot standby and cold standby.No matter what part of the circuit is composed of, its power supply side has voltage, but no current flow is hot standby;If there is neither voltage nor current flow on the power supply side, it is cold standby.The most obvious feature is that there may be voltage in the circuit, but there is no current flow, and there is no conversion between electric energy and other energy.
Short circuit status
For electrical installation engineering, the circuit short circuit is in a fault state. The fault may occur at any part of the circuit, but the current flowing through the load is usually indicated as a short circuit.At this time, various electrical parameters and non electrical parameters in the circuit are abnormal, the power supply voltage drops sharply, the current increases sharply, the switching equipment and line temperature through which the short circuit current flows rise sharply, the electric stress increases, and the local noise increases.If switchgear, etcRelay protection deviceIf the function is normal, the power supply of the circuit in the short circuit state will be cut off quickly to avoid greater losses caused by the expansion of the fault state.If the function is abnormal, the fault expansion causes the action of the relay protection device of the upper level switch equipment of the circuit to cut off the power supply of the short circuit circuit, which is called skip tripping. This is usually an undesirable phenomenon that the coverage area of the power failure expands.The most obvious feature is that the voltage of the power supply drops, the current in the circuit sharply increases, and unexpected energy conversion occurs.
significance
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The loss of the transformer is an important performance parameter of the transformer. On the one hand, it indicates the efficiency of the transformer during operation; on the other hand, it indicates whether the performance of the transformer in design and manufacture meets the requirements.No load loss of transformerAnd no-load current measurement, load loss and short-circuit impedance measurement are routine tests of transformers.
TransformerNo load testIt is to measure the no-load loss and no-load current of the transformer when the rated voltage is applied to any group of coils of the transformer and other coils are open circuited.The no-load current is expressed as a percentage of the rated current.
1. No load test is to measure the no-load loss and no-load current under the rated voltage. During the test, the high-voltage side is open and the low-voltage side is pressurized. The test voltage is the rated voltage of the low-voltage side. The test voltage is low, and the test current is a few percent or a few thousandths of the rated current.
2. Selection of power capacity for transformer no-load test: ensure that the power waveform distortion does not exceed 5%, and the no-load capacity of the test object should be below 50% of the power capacity;The no-load capacity shall be less than 50% of the capacity of the voltage regulator;When the generator set is used for test, the no-load capacity shall be less than 25% of the generator capacity.
No load testThe test voltage of the transformer is the rated voltage at the low-voltage side. The no-load test of the transformer mainly measures the no-load loss.No load loss is mainly iron loss.The size of iron loss can be considered independent of the load size, that is, the loss at no-load is equal to the iron loss at load, but this refers to the situation at rated voltage.If the voltage deviates from the rated value, due toTransformer coreInMagnetic inductionAt the saturation section of the magnetization curve, the no-load loss and no-load current will change sharply. Therefore, the no-load test should be conducted at the rated voltage.
Note: When measuring no-load or load losses of large transformers, cos φ can be less than or equal to 0.1 because of the low power factor.Therefore, a wattmeter with low power factor must be used.
3. ViaNo load testThe following defects of the transformer can be found: poor insulation between silicon steel sheets.The local short circuit between poles and pieces of iron cores is burned out, the insulation parts of core bolts or binding steel strips, pressure plates, upper yoke iron, etc. are damaged and short circuited, the silicon steel sheets in the magnetic circuit are loose, misplaced, the air gap is too large, the iron core is grounded at multiple points, the coils have inter turn and inter layer short circuits, or the parallel branch turns are unequal, and the ampere turns are unbalanced, etc., the high consumption and low-quality silicon steel sheets are misused or the design calculation is wrong.
