What is the cause of single phase ground failure in generator stator windings?

Single-phase grounding protection of generator stator winding In order to be safe, the generator shell is grounded, therefore, the short circuit to the shell that occurs when the insulation of a certain phase winding of the stator of the generator is damaged is single-phase grounding. The hazard of single-phase grounding is mainly the arc burning stator core at the fault point, expanding the damage degree of winding insulation, and further developing it into an inter-turn short circuit or interphase short circuit, and even causing the generator to suffer more serious damage. The characteristics of single-phase grounding of stator winding are set when single-phase grounding occurs at the neutral point S of stator winding A, then the voltage to ground of the fault phase is UAD=0, and the voltage of the non-fault phase is the line voltage.

The zero sequence voltage of the fault point A is the phase voltage, and the voltage is proportional to S, that is, the farther the fault point is from the neutral point of B, the higher the zero sequence voltage. In the past, it was believed that the stator windings of the generator were fully insulated, and the operating voltage near the neutral point was low, making it difficult to generate a grounding point, and even if a grounding point occurred, the grounding current was small and would not cause damage to the core.

Therefore, it is allowed to have a certain dead zone near the neutral point of grounding protection. However, the actual operation experience shows that even if the grounding current is very small, if it is not found in time, so that the operation time continues, it will cause serious damage to the core, and at the same time, the water-cooled unit used by the factory should consider the leakage of the stator winding near the neutral point, which causes a great possibility of insulation damage, and should be installed without dead zone, that is, the grounding protection of the stator winding with 100% protection zone. Stator Grounding Protection:

Generally, the generator is set up with two kinds of grounding protection, one is UBx117 type, 95% stator connection protection, using a voltage relay that responds to voltage offset, and protects the stator winding from the end of the machine to the neutral point direction of 95%, in order to avoid the influence of the third and other higher harmonics, the relay is equipped with a low-pass filter to suppress the harmonic voltage, especially the influence of the third harmonic.

Single-phase grounding of stator windings is one of the frequent faults of generators. For the generator with no neutral point grounding, the size of the grounding current when the single-phase grounding occurs depends on the ground capacitance current of the generator voltage system (including the busbar, cable, overhead line, etc. connected to the generator), when the grounding capacitance current exceeds 5A, the arc suppression coil must be installed at the neutral point of the generator to compensate, otherwise it may burn out the stator core. The magnitude of the ground capacitance current can be estimated or calculated using the following methods: 1) For overhead lines:

l = UDR 3502) for cable routes: 7C=T div10 in the UD - line rated voltage, kv; l - line (cable) length, for hydro generators, generally can be read out of its capacitance current at the rated voltage during the withstand voltage test. The capacitance current of the generator and the line connected to it is added up to the capacitance current of the generator voltage system.

Due to the aging of the stator insulation, moisture or local defects, the stator insulation is broken down under the operating voltage or overvoltage. The insulation of the stator winding is burned due to the overheating of the stator joint or the local overheating of the iron core, resulting in insulation breakdown. Sudden short-circuited electrodynamic forces cause insulation damage.

Insulation damage caused by rotor parts flying out or end fixing parts falling off during operation. During the operation of the generator, due to external, internal and misoperation reasons, it may cause various faults or abnormal states of the generator, and the common faults are as follows: stator fault

Winding interphase short circuit, turn to turn short circuit, single-phase grounding, etc. Rotor winding failure: rotor two-point grounding, rotor loss of excitation function, etc.

Aging or damage of winding insulation: When the generator runs for a long time, the winding insulation material will gradually age, or the insulation will be damaged due to overheating, overvoltage and other conditions. This makes the windings susceptible to electric shock or other forms of damage, which can lead to single-phase ground faults.

Foreign body intrusion: During generator operation, some foreign objects (such as metal chips, fibers, etc.) may invade between the stator windings, resulting in insulation damage or short circuit.

Manufacturing or installation defects: During generator manufacturing or installation, if the windings are improperly manufactured or installed, it can lead to problems such as poor insulation or poor contact, which can cause single-phase ground faults.

Improper maintenance: If the generator is not properly maintained, such as incomplete cleaning, insufficient lubrication, etc., it can lead to corrosion or wear of the stator windings, which in turn can lead to single-phase ground failure.

External factors: External factors such as **, lightning strikes and other external factors may also cause single-phase ground faults in the stator winding.

Phenomenon:

1) The alarm bell rings, and the light sign of "generator stator grounding" is on.

2) The stator circuit insulation monitoring is indicated by the installed voltmeter.

3) The water leakage alarm may be installed.

4) When the voltage transformer for protection is blown at one time and the locking fails, the stator grounding protection will malfunction, pay attention to the distinction.

5) When the fundamental cast trips the action, the power and transformer group trips and stops.

6) The third harmonic fluctuation is used to send the signal.

Processing:

1) The unit trips, check the power switching of the plant, and deal with it according to the generator tripping. The transformer group is transferred to overhaul, and the maintenance is contacted to check the grounding reason.

2) If the generator is not tripped (grounding protection signal position), check the stator circuit insulation monitoring installation voltmeter, and judge whether the grounding phenomenon occurs in combination with the operation conditions.

3) Contact the secondary shift personnel to measure the secondary opening voltage of the generator TV, and check the protection installation. If the zero sequence voltage indication is higher than that of other units, it means that the unit is indeed grounded, and the value manager and the relevant plant leaders should be reported to apply for shutdown.

4) Wear insulating shoes to conduct a comprehensive inspection of the generator body and the lead wire, whether it is due to the loosening of the first and second plugs of the TV, and conduct a detailed inspection of the generator neutral point to the main transformer lead line and the high-voltage side casing area of the high factory transformer, and whether there is obvious grounding phenomenon (such as grounding caused by water leakage).

5) If it is a third harmonic grounding, attention should be paid to check and judge whether it is a protection malfunction.

Generator stator grounding reason:

Stator coil leakage or water seepage causes insulation to decline; ** generated during the operation of the lead wire, resulting in damage to the insulation; Condensation in the machine leads to grounding; Oil leakage from the bearings, resulting in a decrease in insulation; When the winding of the low-voltage side of the main transformer or the winding of the high-voltage side of the high-voltage plant transformer is single-phase grounded, etc.

Generator stator grounding phenomenon:

When the first phase of the stator is metallic ground, the relative ground voltage can be measured to be zero by switching the stator voltmeter.

The voltage of the non-ground relative to ground is the line voltage, and the voltage of each line is constant and balanced.

The stator insulation resistance measurement measures the "stator grounding" voltmeter indicating the zero sequence voltage value.

Handling of generator stator grounding:

If it is found that the grounding point is inside the generator (discharge sparks or arcs can be seen in the peephole), the load should be reduced immediately and shut down and reported to the superior dispatcher.

If the on-site inspection can not find obvious faults, but the "stator grounding" alarm does not disappear, it should be regarded as the internal grounding of the generator, and it must be stopped for inspection and treatment within 30min.

Answer]: B neutral point manuscript search bright non-grounding system, when phase A leakage world grounding, the neutral point voltage rises to the phase voltage, B and C rise to the line voltage relative to the width of the large bond, that is, <>

Phase voltage.

In the production process of generator stator wire rods, due to technological reasons, there may be air gaps or potato hopper impurities between the insulation layers or between the insulation layer and the strands; Under the combined action of electrical, thermal and mechanical forces during operation, KCB6743A will also directly or indirectly lead to insulation deterioration, resulting in new air gaps between insulation layers. Due to the different dielectric coefficients of air gap and solid insulation, the electric field distribution of this sandwich medium, which consists of air gaps (impurities) and insulation, is uneven. Under the action of the electric field, a partial discharge occurs when the operating voltage reaches the initial discharge voltage of the air gap.

The starting voltage of partial discharge is closely related to the dielectric constant of the insulating material and the thickness of the air gap.

The partial discharge of the gas in the air gap is a flow-type high-pressure glow discharge, in which a large number of high-energy charged particles (electrons and ions) collide with the main insulation at high speed, thereby destroying the molecular structure of the insulation. In the air gap where partial discharge occurs in the main insulation, the local temperature can reach 1000, which deteriorates the adhesive and strand insulation in the insulation, resulting in loose strands and short circuits between strands, so that the main insulation is partially overheated and pyrolyzed, and finally damages the main insulation. The further development of partial discharge is to produce dendritic discharge inside the insulation, which causes further deterioration of the main insulation, and finally forms a discharge channel and destroys the insulation.

The harm of partial discharge of the main insulation of the generator is considerable, because when the discharge of the electric stalk occurs, there will be a flow of high-voltage glow, a large number of high-energy charged particles, electrons and ions, high-speed collision of the main insulation, instantaneous stability up to 1000 degrees, so that the main insulation is heated and the coil is loose and damaged.

There are generally two reasons for the main insulation discharge of the generator nail set.

1. There is a gap or impurity between the insulation layer of the coil or between the insulation layer and the strand.

2. During the operation of the generator, the coil is affected by the heating and charge mechanical force, which leads to insulation cracking and creates a gap between the insulation layers.

Due to the difference in the dielectric coefficient of air gap and solid insulation, partial discharge occurs when the working voltage reaches the initial discharge voltage of the air gap under the action of an electric field.

The further development of the partial or denier discharge of the generator is to make the dendritic discharge inside the insulator, penetrate the insulator, and form a discharge channel, so that the insulation is damaged and the generator is damaged.

The winding is an integral part of the motor, aging, moisture, heat, erosion, foreign body intrusion, and the impact of external force will cause damage to the winding, and the motor is overloaded, undervoltage, overvoltage, and phase loss operation can also cause winding failure. Winding faults are generally divided into winding grounding, short circuit, open circuit, and wiring error. Now we will explain the fault phenomenon, the cause of the occurrence and the inspection method.

1. Winding grounding.

It refers to the grounding caused by the insulation of the winding and the core or the casing.

1. Fault phenomenon.

The chassis is electrified, the control line is out of control, and the winding is short-circuited and heated, resulting in the motor being unable to operate normally.

2. Causes.

The insulation resistance of the winding is reduced by moisture; Long-term overload operation of the motor; Harmful gas corrosion; Metal foreign matter intrudes into the winding and damages the insulation; When rewinding the stator winding, the insulation is damaged and touches the core; The end of the winding touches the end cover base; insulation burns caused by stator and rotor friction; The insulation of the outgoing wire is damaged and the shell is damaged; Overvoltages (e.g., lightning strikes) break down the insulation.

3.Check the method.

1) Observational method. Visually inspect the end of the winding and the insulation in the trunking to see if there are signs of damage and blackening, if any, the grounding point.

2) Multimeter inspection method. Check with a multimeter with a low resistance level, and if the reading is very small, it is grounded.

3) Megohmmeter method. According to different grades, different megohmmeters are selected to measure the insulation resistance of each group of resistors, if the reading is zero, it means that the winding is grounded, but the motor insulation is damp or broken down due to accidents, it needs to be judged based on experience, generally speaking, when the pointer swings at "0", it can be considered that it has a certain resistance value.

4) Test the light method. If the test light is on, it means that the winding is grounded, and if it is found that there is a spark or smoke somewhere, it is the winding grounding fault point. If the lamp is faintly bright, the insulation has a ground breakdown.

If the light does not come on, but the test rod also has a spark when it is grounded, it means that the winding has not broken down, but it is only heavily damp. You can also use hardwood to tap on the edge of the shell, and when you knock it to a certain place and wait for it to go off and on, it means that the current is on and off, and that place is the grounding point.

5) Current burn-through method. With a voltage regulating transformer, after connecting to the power supply, the grounding point will heat up quickly, and the place where the insulation smokes is the grounding point. Special attention should be paid to the fact that the small motor shall not exceed twice the rated current and the time shall not exceed half a minute; The large motor is 20%-50% of the rated current or gradually increases the current, and the power is cut off immediately when the smoke just rises at the grounding point.

6) Group elimination method. For the grounding point in the core of the core and the burning is more severe, the burned hole copper wire is fused with the core. The method used is to divide the grounded one-phase winding into two halves, and so on, and finally find the grounding point.

In addition, there are high-voltage test methods, magnetic needle exploration methods, power frequency vibration methods, etc., which will not be introduced here.

4.Solution:

1) The grounding caused by moisture in the winding should be dried first, and when it is cooled to about 60-70, it should be poured with insulating paint and then dried.

2) When the insulation at the end of the winding is damaged, re-insulate at the grounding place, paint it, and then dry it.

3) When the winding grounding point is in the slot, the winding should be rewound or part of the winding components should be replaced.

Finally, different megohmmeters should be used to measure and meet the technical requirements.

The reason for the leakage of the coil is very simple, and the outer insulation of the enameled wire is scratched or burned. The use of insulating paint to fill the repair, generally speaking, it is difficult to find ---

Due to the different potentials of each part of the core caused by multi-point grounding, a large current will be generated in the core, sometimes up to tens of amperes, if not found in time, it will cause the local heating of the core or the core burning at the arc grounding point. The overheating of the iron core caused a light tile action or a change in the gas composition of the oil, and the power was forced to be cut off for inspection. Changsha Lanhong Electric Technology Co., Ltd. has conducted in-depth research on the topic of multi-point grounding fault monitoring of iron cores, and has developed LHTX1306B-R series monitoring and protection devices.

The product can quickly and accurately monitor the core grounding current, and achieve the purpose of controlling the core grounding current of the transformer through the switching of the resistance network, so as to meet the needs of customers for this type of fault monitoring and maintenance. This product is also suitable for clamp grounding monitoring and protection (for details: 1*3*3*5*7*2*1*7*3*1*9).