Step down transformer

A step-down transformer  is to refer to the higher voltage at the input, converted to a relatively low output of the ideal voltage, so as to achieve the purpose of step-down transformer. Step-down transformer is a very important equipment in the transmission and substation system, its normal operation is not only related to its own safety, reliable power supply for users, but also directly affects the stability of the power system.

The configuration of the step-down transformer protection should meet in any case, can not burn the transformer, so that the accident expands, affecting the stability of the power system. Details of its working principle, relay protection principle, operating conditions, operation and requirements, as well as abnormal operation and handling methods are introduced.

Working principle.

1. Basic principle

Is the principle of electromagnetic induction, single-phase double-winding transformer as an example of its basic working principle : when the primary side of the winding plus voltage, the flow of current in the core will produce alternating flux these fluxes are called the main flux, the main flux will pass through, primary and secondary windings, the winding will produce induction electromotive force, then if the secondary side access to the load, there will be current flow, generating electricity.

2. Principle description

E – the effective value of the induced potential f – frequency N – the number of turns of the main flux maximum due to the secondary winding and primary winding turns are different, the induced potential E1 and E2 size is also different, when omitted after the internal impedance voltage drop, the voltage size is also different.

When the secondary side of the transformer is unloaded, the primary side only flows through the current of the main flux, which is called the excitation current. When the secondary side is loaded with the load current, also in the core to produce flux, trying to change the main flux, but the primary voltage remains the same, the main flux is unchanged, the primary side will flow through two parts of the current, part of the excitation current for the balance, so this part of the current changes with the change. When the current is multiplied by the number of turns, this is the magnetic potential.

3. Frequency characteristics.

Frequency characteristics: low frequency Power supply phase number: single-phase Core shape: E type Cooling form: dry Core structure: heart type Number of windings: double winding Moisture-proof way: open cooling way: natural cooling Form structure: vertical Voltage ratio: 220/110 (V) Efficiency (η): 95 Rated power: 50-10000 (KVA) as shown in the figure “Transformer Schematic”.

Second, common step-down voltage.

Main step-down transformer input end voltage: 460V, 380V, 220V, 400V ,

Output side voltage: 380V, 220V, 110V, 36V, 24V

Electromagnetic device: transformer can transform a voltage AC energy into another voltage AC energy of the same frequency voltage device is the main part of a core and two windings set on the core. The coil connected to the power supply, receiving AC energy, called the primary winding and the coil connected to the load, sending AC energy, called the secondary winding primary winding of the secondary winding voltage phase amount U1 voltage phase amount U2 current phase amount I1 current phase amount I2 electric potential phase amount E1 electric potential phase amount E2 turns N1 turns N2 while cross-chaining once, the phase amount of the magnetic flux of the secondary winding is φm ,the flux is called the main magnetic flux.

Third, the principle of relay protection.

In the study of the operation of the power system and faults, as well as to explore countermeasures to deal with abnormal reactions, in these processes mainly with contact relays to protect the power system and step-down transformers, generators, transmission lines and other components to avoid damage, which is also known as relay protection, which is mainly when the power system failure can be issued to the operating staff in a timely manner warning signal, or to the The main purpose of relay protection is to send an early warning signal to the operating staff when a fault occurs in the power system, or to send a jump ICJ command to the control circuit breaker to control the closest jump ICJ device to the faulty component, so that the faulty component can be disconnected from the power system in time, thus reducing the damage to the power component to a greater extent and reducing the risk of safe power supply in the power system. Figure .

When there are abnormalities in the step-down transformers in the power system, in addition to strengthening supervision, it is also necessary to contact the relay protection personnel to deal with those protections that may cause misoperation to withdraw their exit pressure plates, and the following abnormalities are common when withdrawing

(1) When the signals of “AC shorting of busbar” and “DC voltage disappearance of busbar” are issued, and when the unbalanced current of busbar is not zero, and when the busbar switch without special bypass busbar is operated on behalf of the line, more attention should be paid to these cases ;

(2) when the DC current disappears and the regular channel test parameters do not meet the requirements, device failure or channel abnormal signal issued when it cannot be restored, the relay protection personnel need to make timely exit response measures; and in the transformer operation when refueling, oil filtering or silica gel change; need to open the breathing system to release the oil plug, bleeder door and so on, all need to carry out strict monitoring, a party relay protection problems , of course, for these problems will have some corresponding protection measures.

Basic operating conditions.

220kV step-down transformer takes the responsibility of transforming from 220kV high-voltage input to low-voltage, its cooling conditions under the specified cooling conditions, can be operated according to the nameplate, regardless of the transformer tap in any position, as long as the added primary voltage does not exceed 5% of the corresponding rated value, the temperature to meet the temperature rise requirements, the secondary side of the transformer can carry rated current.

For special use, it is allowed to operate at a rated voltage not exceeding 110%. For the mutual relationship between current and voltage, if there is no special requirement, when the load current is K(K≤1)times of the rated current, the voltage U is limited according to the following formula U(%)=110-5K2.

220kV step-down transformer cooler operation requirements

The transformer is not allowed to operate under the condition of no cooler, the cooler should be equipped with double power supply, and both sections of power supply are put into use during normal use, and standby for each other. The main transformer cooler in normal operation, in addition to the main use, must also be equipped with a group of auxiliary, a group of standby, and according to the provisions of the regular rotation of use [6] .

Transformer temperature limit of the operating requirements

Oil-immersed transformer in the operation of the upper layer of oil and the temperature rise of each component shall not exceed a certain specified value.

Transformer insulation resistance requirements

According to the national standard requirements, the insulation resistance is measured with a 2500V shaking table for coils with rated voltage above 1000V, and with a 500 or 1000V shaking table for coils with rated voltage below 1000V. Transformer insulation absorption ratio R60/R 15 is not less than 1.3) Transformer insulation resistance shall not be lower than the previous measured value of 700Ic at the same temperature, and the minimum resistance value shall not be less than 1 MΩ/kV.

Fourth, the operation and requirements.

General operation and requirements

The operation of the transformer must sound a perfect operating system or protocol, and strictly in accordance with the system or protocol implementation [6].

1. Before transformer power supply operation, all preparatory work must be done. Such as transformer insulation resistance qualified, cooling device test run normal, residual

gas is eliminated and oil level, oil color, bushings and other normal work, etc., until all the preparation and inspection work is completed, before the next step can enter the

Work.

2. Before closing the gate and sending power, put in the protection device and check whether it works normally and whether the cooler and transformer fire extinguishing system are normal.

3. Close the switch (first close the power side switch), close the circuit breaker power outage operation on the contrary.

In addition to special circumstances, is strictly prohibited from the transformer low-voltage side charging, when the transformer from the high-voltage side charging should be put into the neutral grounding knife gate, charging is completed and then pulled open or not according to the requirements.

Normal operation and standby in the transformer, should be heavy gas protection device into the tripping position.

Transformer impact test operation

The transformer’s quick-break protection device is set to convert to the low-voltage side of the maximum three-phase short-circuit current of about 1.25 times, heavy gas into the proposed full-voltage impact closure 5 times; each impact, according to the reverse operation and the relevant provisions of the national standard to record the relevant data. After the shock experiment is completed, the no-load operation is performed for 24 hours, and the no-load current and oil surface temperature rise are recorded once an hour.

On-load regulator switch operation

Start the on-load device before starting the transformer, the on-load device should be powered on first and then put into operation, the adjustment of the on-load regulating tap should be regulated step by step, while monitoring the tap position and the change of voltage and current.

V. Operation and maintenance

Operation and maintenance is the most fundamental guarantee of equipment operation, must develop a corresponding scientific operation and maintenance system to match, such as the operation and standby in the main transformer, should be regular and mobile inspection, each mouth evening shift in the peak load, a light out check, check the joints everywhere there is no discharge sparks, corona and overheating burning red phenomenon; special circumstances to have a special inspection system and so on.

Abnormal operation processing: transformer abnormalities are inevitable, such as timely detection and take reasonable countermeasures, can greatly reduce the occurrence of accidents, transformer common abnormalities are mainly [6] :

Increased sound or tone abnormal;

The temperature rises significantly or rapidly;

Transformer or casing oil level is lower or higher than the allowable value, oil color changes, and fails the test;

Cracks, discharge traces or discharge sounds appear in the casing;

Casing screw or terminal block is overheated and red;

The following is an introduction to several common causes of abnormal conditions and handling methods:

When the transformer temperature has abnormal rise

1. Check the load of the transformer and whether the three-phase load is balanced, when overload occurs, the output should be reduced to limit the load.

2. Check whether the radiator is related to the wood body temperature: whether the cooler is operating normally; whether the unit cooler ventilation device is normal; if the fan motor is out of service, check whether the various power supply insurance corresponding to the out-of-service fan is fused, whether the thermal relay is operating, whether the switch contact is good, whether the secondary circuit is disconnected, etc.

Check the thermometer of the transformer;

3. check whether there is oil leakage or other reasons to make the oil level too low and cause the temperature to rise; after checking the cooling device and temperature measuring device are good, and after reducing the negative load and the oil temperature still rises.

After checking the cooling device and temperature measuring device are good, and after reducing the load and the oil temperature still rises, should immediately stop the transformer.

Transformer oil level drops

1. If the oil level drops slowly, should be a comprehensive check whether the oil leakage or low temperature so that the oil level drops; if the oil level drops rapidly, the gas protection will be deactivated, should be immediately set up to system oil leakage, and notify the maintenance personnel to deal with or oil injection, such as can not system should contact the power outage processing.

2. light gas protection action processing

Have a spare transformer should be put into; no spare transformer, should pay close attention to the operation of the transformer, at this time is strictly prohibited to exit the heavy gas protection.

Take gas and oil samples for analysis; check whether there is gas in the transformer gas relay, to determine whether the protection action. Before checking, the protection pressure plate should be put into signal and put into tripping after sampling. If the transformer internal fault must be shut down, if the air, should open the gas relay bleed valve to release the air. If the gas protection wood body failure, abnormal or secondary circuit insulation is poor, should be tripped out II relay disconnected.

Transformer overload operation

Before overload operation, should check whether the transformer has more serious defects (such as cooling system is not normal, serious oil leakage, there is local overheating phenomenon, dissolved gas in the oil analysis results are abnormal, etc.) or insulation has a weak point, if so, should not overload operation. Transformer overload operation, should immediately put into the standby cooler, observe the transformer upper temperature, ambient temperature and time. And record the overload time and overload multiplier, overload multiplier in normal overload operation.