Source: Electrotechnical Electrical Forum
Anecdotes about the management of transformers
◾ Distribution transformer and station area:
As mentioned earlier, the transformer looks like the backbone node of the power grid (transmission professionals raise their hands against it).
One is that he is a dot on the diagram, and the other is that the voltage level is divided through the transformer and the management requirements are distinguished.
In the hottest distribution connection of the past decade, the transformer acted as a major connection point. In the power distribution major, the transformer is the distribution transformer, and in the marketing major, the distribution transformer and its power supply range are called the station area, and then the distribution transformer is given a station area code in the information system.
Many people who are engaged in informatization often get confused in the distribution and transformation and station areas. This matter can be explained in a special chapter later.
◾ Public transformer and special transformer:
The distribution transformer used by residents is directly managed by the power grid company, which is called public transformer (public transformer, does it feel like being the master?).
The power company should provide low-cost and high-quality operation and maintenance guarantee services, and the loss above the meter is counted as the power grid company, which is a loss-making business.
The transformers of enterprises, governments, institutions, industrial and commercial units are called special transformers, and they are specially used transformers. By the enterprise operation and maintenance, there are also many enterprises subcontracted to the power grid company operation and maintenance, from the demarcation point down the loss is counted as the enterprise, is the power grid company and the electricity sales company to make money.
◾ Calculated metrics:
As the backbone node of the power grid, the transformer is the focus of various assessment indicators, and the point is always better than the line compared to the line. The transformer is equipped with a complete meter, which is much better than the intermediate node of the line.
The relevant calculation indicators are: power quality series (voltage fluctuation, current balance, etc.), power failure series (power outage duration, power outage times), load metering (electricity, power, etc.), economic operation (load, overload, light load) and so on.
◾ Economical Operation:
Other indicators are too professional, or money is important.
The economic operation of the transformer consists of two items, one is the utilization rate and the other is the loss.
Taking the residential distribution transformer as an example, the new investment community is not used, the load is low, the high and low peak difference is large, it is called light load, uneconomical, generally 95% of the time the average load rate is below 20%, if the load is more than 80%, it is heavy load, may develop to overload (more than 100%), to consider expansion.
20%~80% is considered economic operation, which is what the power grid company wants to see the most, in fact, the community is too different because of the peak and valley level, and the good situation is 30% of the full-time average load.
The loss is born with the transmission medium, there is no-load loss (iron loss) and load loss (copper loss), both of which are not good for the power company, the iron loss is improved by the manufacturer, and the copper loss is improved by improving the utilization rate mentioned above.
Operation of the transformer
Now that we've talked about transformer management, let's take a look at the operational aspects. There is a standard for the operation of the transformer, and it is necessary to pay attention to its temperature, load and voltage at all times.
1 permissible temperature
Taking dry-type transformers as an example, the allowable temperature should generally not exceed 110 degrees, and the maximum temperature should not exceed 120 degrees.
Other types of transformers can be specified on request.
2. Allowable load
The transformer should generally not exceed the rated capacity operation, but normal overload operation is allowed, and the overload operation cannot exceed 10% of the transformer capacity, and the temperature rise should be paid attention to.
3. Transformer three-phase unbalanced load
When the three-phase is unbalanced, the current of the maximum phase should be monitored, and if the three-phase current is found to be seriously unbalanced, the average distribution of the load of the single-phase electrical equipment should be adjusted in time.
4. Fluctuating voltage is allowed
The power supply voltage of the transformer shall generally not exceed 5% of the rated value, that is, the line voltage of 10KV shall not exceed 10.5KV and shall not be less than 9.5KV.
Transformer overload requirements
Transformer overload is a very normal thing, after all, if there is an accident, ensuring uninterrupted power is the most important thing to consider, and insulation aging must be put aside.
However, it is not possible to overload the transformer, and there are still requirements:
Transformer overload requirements
1. Overload is allowed
The allowable overload of the transformer means that the degree of overload is not large, and the load before the overload is light, the oil temperature of the transformer is not high, and the hot spot temperature of the winding will not reach the degree of harm.
2. Limit overload
The degree of overload of the transformer is large, which increases the oil temperature, and the hot spot temperature of the winding may reach a hazardous level, but it has not yet reached a dangerous level. At this time, although the transformer can continue to operate, it will reduce the dielectric strength, threaten the safety of the transformer, affect the life of the transformer, and this overload must be limited.
3. It is forbidden to overload
The degree of overload of the transformer is large and the time is long, so that the oil temperature has exceeded the allowable value, and the hot spot temperature of the winding has reached a dangerous level. If the operation continues, the insulating oil around the hot spot will decompose and form bubbles, and the dielectric strength will be severely reduced, and this overload must be prohibited.
Operation and maintenance of transformers
Transformer operation and maintenance
In addition to paying close attention to the overload of the transformer, what aspects need to be paid attention to in the daily operation and maintenance of the transformer?
For example, the quality of insulating oil, the prevention of overtemperature, etc., are all points that need to be paid attention to in the daily operation and maintenance of transformers.
Operation and maintenance of transformers
Prevent transformer overload operation:
If it is overloaded for a long time, it will cause the coil to heat up, make the insulation gradually age, and cause short circuit between boxes, short circuit between phases or short circuit to ground and oil decomposition.
Guarantee the quality of insulating oil:
In the storage, transportation or operation and maintenance of transformer insulating oil, if the oil quality is poor or there are too many impurities and moisture, the dielectric strength will be reduced. When the dielectric strength is reduced to a certain value, the transformer will be short-circuited and cause sparks, arcing or dangerous temperatures. Therefore, the oil quality of the transformer should be tested regularly during operation, and the unqualified oil should be replaced in time.
Prevent aging and damage to transformer core insulation:
The insulation aging of the iron core or the damage of the clamp bolt sleeve will cause a large eddy current in the iron core, causing the core to heat up for a long time and cause the insulation aging.
Prevent inadvertent damage to insulation during maintenance:
When the transformer overhauls the lifting core, attention should be paid to the protection coil or insulating sleeve, and if there is abrasion damage, it should be dealt with in time.
Make sure the wires are in good contact
Poor contact between the internal joints of the coil, the connection points between the coils, the contacts leading to the bushing on the high and low voltage sides, and the poor contact of each fulcrum on the tap-changer will cause local overheating, damage insulation, and short circuit or open circuit. The high-temperature arc generated at this time will cause the insulating oil to decompose, producing a large amount of gas, and the pressure in the transformer will increase. An explosion occurs when the pressure exceeds the gas cut-off protection value without tripping.
Protection against electric shock
The power supply of power transformers generally comes through overhead lines, and overhead lines are easily struck by lightning, and the transformer will burn out due to breaking down insulation.
Short-circuit protection should be reliable
In the event of a short circuit in the transformer coil or load, the transformer will be subjected to a considerable short-circuit current, and if the protection system fails or the protection value is too large, the transformer may be burned. For this purpose, reliable short-circuit protection must be installed.
Maintain good grounding
For low-voltage systems with protective zeroing, the neutral point on the low-voltage side of the transformer should be directly grounded When the three-phase load is unbalanced, a current will appear on the neutral line. When this current is too large and the contact resistance is large, a high temperature will appear at the grounding point, igniting the surrounding combustible materials.
Prevent overheating
Changes in temperature should be monitored during the operation of the transformer. If the transformer coil wire is Class A insulation, the insulator is mainly paper and cotton yarn, the temperature has a great impact on the insulation and service life, and the insulation life should be reduced by about 50% for every 8 °C increase in temperature.
Therefore, when the transformer is running, it is necessary to maintain good ventilation and cooling, and forced ventilation can be taken if necessary to achieve the purpose of reducing the temperature rise of the transformer.
Loss calculations for transformers
As mentioned earlier, the loss is inherent in the transmission medium, and there are no-load losses (iron loss) and load losses (copper loss). How do you calculate how much the transformer loses?
In fact, the economic operating point with the least loss of the transformer is when the load factor is 0.5-0.6, that is, when the iron loss of the transformer is equal to the copper loss. Depending on the economic operating point of the transformer, it can be decided whether to run the transformers in parallel.
The power loss of a general transformer can be calculated as follows:
△P=△P0+(Scp/Sn)*△Psc
where △P0 = no-load loss of transformer;
△Psc = short-circuit loss of transformer;
△P = total loss of transformer;
SCP = average capacity of the transformer;
Sn = rated capacity of the transformer
To improve efficiency, it is possible to avoid the transformer operating at lower loads.