The load switch has a simple interrupting device, the interrupting medium generally uses air, which can connect and break certain currents and overcurrents, but cannot break short-circuit currents and cannot be used to cut off short-circuit faults. Therefore, it is absolutely not allowed to simply use the load switch to replace the circuit breaker; if the load switch is to be used, it must be used in conjunction with the previously mentioned high-voltage fuse (in fact, fuses and load switches are often used in series, used as simple overload protection to reduce the cost of the project).
The load switch is similar to the disconnecting knife, which has an obvious disconnection gap and can easily distinguish whether the circuit is on or off.
Simply put, a transformer is a device that uses alternating electromagnetic fields to convert different voltage levels (actually, it is the conversion of electrical energy), and the voltage before and after the conversion does not change in frequency. According to its use can be divided into many kinds, such as power transformers, rectifier transformers, regulators, isolation transformers, as well as CT, PT, etc.. We often encounter power transformers in engineering sites.
Some of the main technical parameters related to transformers include
1, rated capacity : refers to the rated output capacity of the transformer under rated operating conditions (equal to U × I, the unit is kVA);
2, rated voltage: no-load, rated tap, the value of the terminal voltage (i.e., primary and secondary side voltage values);
3, no-load loss: no-load conditions, the transformer loss (also called iron consumption);
4, no-load current: no-load conditions, the value of the current flowing through the primary side coil;
5, short-circuit loss: the primary side through the rated current, the secondary short-circuit losses (mainly generated by the coil resistance);
6, the tap (tap) concept.
To suit the needs of grid operation, the general transformer high-voltage side have taps, the voltage value of these taps are expressed as a percentage of the rated voltage, the so-called tap voltage. For example, a transformer with a ±5% tap on the high voltage 10kV means that the transformer can be operated at three voltage levels: 10.5kV (+5%), 10kV (rated), 9.5kV (-5%). In general, on-load tap-changers have a high number of taps (tap points), such as 7 tap points (±3 x 2.5%) and 9 tap points (±4 x 2%). Since it is not possible to fully guarantee the synchronous switching of the tap changer, the on-load tap-changer is generally not able to operate in parallel.
7、Active load ：
The load that generates machine energy or thermal energy in the power system. But the pure resistive load in the load only consumes active power, such as electric heat, electric furnace, lighting and other electrical loads are completely active load. Asynchronous motor and synchronous motor loads consume both active power and reactive power, where the part of work to produce machine energy is active load. Active load to be supplied by the generator active power.
8、Reluctive power load
In the power load does not work part. Only in the inductive load to consume reactive power. Such as: transformers, motors, air conditioners, refrigerators, etc.. So in the generator output active power at the same time, but also need to provide reactive power.
Reactive power can not meet the grid, the system voltage will drop, in order to meet the needs of users, so in the substation to install reactive power compensator, to maintain the balance of reactive power, so as to maintain the voltage level.
9、Accidental backup :
One of the components of backup capacity in the power system. As the power generation equipment may occur temporary or permanent failure and affect the power supply, so the system must set a certain number of accidental backup power to ensure the safety of power facilities.
10、System unlisting ：
In order to prevent system failure and accident expansion, the complete power system is broken down into several independent systems that are no longer in synchronous operation, a measure. After unbundling, some local systems may experience power shortage, frequency and voltage drops, so it is necessary to remove part of the load to prevent the stability of the whole system from being damaged.
PT (TV)/CT (AV)
The transformer is actually a special transformer, mainly used to electrically isolate the primary circuit from the control circuit, thus expanding the use of secondary equipment (instrumentation, comprehensive security, etc.).
The use of PT/CT can avoid the high voltage / high current of the primary circuit directly into the secondary control equipment (such as: instrumentation, comprehensive security devices, etc.), but also to prevent the operation of the primary circuit due to the failure of the control equipment.
1, the current transformer (CT, AV) is characterized by
Primary side winding N1 thick and less, secondary side winding N2 thin and more, the secondary side of the rated current I2 is generally 5A (according to N1I1 = N2I2 can be approximated to calculate the primary side current I1, or according to the primary side of the current I1 selection of the corresponding ratio of current transformers). Since the primary winding and secondary winding are connected in series in the primary circuit and secondary control circuit when the CT is working, according to the characteristics of the transformer U1I1=U2I2 can be derived from the working voltage of the secondary side at work, the voltage is very large in the open circuit, so the CT is absolutely not allowed to open circuit. According to the use to divide, usually can be divided into protection and measurement with CT. measurement CT in a short circuit fault in the primary circuit, easy to saturate, in order to limit the secondary current (secondary winding side current I2) is too large, to achieve the purpose of protecting the comprehensive protection device; and protection CT in a short circuit fault in the primary circuit, should not appear to protect the phenomenon, in order to ensure that the comprehensive protection device reliable action.
2, ratio: transformer high-voltage side winding and low-voltage side winding turns ratio is called the ratio, an approximation can be used for high-voltage side and low-voltage side of the rated voltage ratio.
3, voltage transformer (PT, AV) is characterized by: the primary winding turns N1 more, the secondary winding turns N2 less, equivalent to a step-down transformer (secondary side voltage rating is generally 100V). Since the primary and secondary windings are connected in parallel to the primary and secondary control circuit voltage coils respectively when the PT is in operation, and since the impedance of the voltage coil is large, the current on the secondary side of the PT is very small and the secondary winding is approximately no-load; however, the impedance of the secondary winding itself is very small, so if the secondary winding is short-circuited, it will result in a very large secondary side current (N1I1= N2I2). Therefore, the secondary winding of PT should never be short-circuited.
The trolley and drawer are part of the high-voltage switchgear and low-voltage switchgear respectively, where components such as high-voltage circuit breakers and low-voltage circuit breakers and their relays are installed. This divides the trolley switchgear (high voltage) and drawer switchgear (low voltage), whose functions are basically the same as those of fixed switchgear, the main difference being the convenience of maintenance and overhaul (both trolleys and drawers can be pushed and pulled out by mechanical operating mechanism rockers). The trolley and drawer generally have three position states: working (normal operation), testing (trial operation and field test) and withdrawing (maintenance, overhaul).
Grounding knife (also called grounding switch) is mainly: one is used in line and equipment maintenance, in order to ensure the safety of personnel grounding with; two can be used to artificially cause the system ground short circuit, to control the purpose of protection.
The first role is well understood, not to be introduced. The second role is this: grounding knife is usually connected to the high-voltage side of the step-down transformer, when the receiving end of the fault or transformer internal fault, grounding knife switch should automatically close, resulting in a ground short circuit fault, forcing the sending end (upper end) circuit breaker to act quickly to cut off the fault, so that this is a personal ground short circuit fault, the purpose is to ensure that the sending end of the circuit breaker can act quickly.
Contactor is a kind of electrical appliances used for remote and frequent connection and opening of AC and DC main circuits and large-capacity control circuits, the main control objects are motors, lighting, capacitor banks, etc., divided into AC contactors and DC contactors. Compared with the circuit breaker, the difference is that: the action frequency is very high (and therefore requires a long enough electrical life and mechanical life); there is a high opening and connecting capacity, but generally used in 1 kV and below, can not be compared with the circuit breaker tens of kilovolts, hundreds of kilovolts.
Relays are used in the control circuit to control other electrical appliances (generally primary electrical equipment) action or in the main circuit as a protection and signal conversion appliances, only for long-distance breaking, connecting small capacity control circuit, such as: AC / DC current relays, voltage relays, time relays, intermediate relays, thermal relays, etc.
Common tests include.
1, type test: in accordance with a design requirement and one or more devices or equipment manufactured by the test, to test whether this design requirement is in line with certain specifications.
2, conventional test: also called factory test, for each device or equipment in the manufacture or completion of the test, to determine whether the device or equipment in line with a standard.
3, medium test: is a general term for various tests to test the electrical characteristics of the medium, including: insulation, static electricity, voltage resistance, etc.
Sampling test: a number of randomly selected samples of a batch of products for testing, is also used to determine whether the sample meets a certain standard.
4, life test: to determine the product in the specified conditions may reach the life of the test, or to evaluate the analysis of the product life characteristics of the test, is a destructive test.
5, endurance test: in a certain period of time, including a certain purpose taken for a specific operation and other specified conditions, the test of the product, such as repeated operation, short circuit, overvoltage, vibration, shock, etc., is a destructive test.
6、Commissioning test: The test conducted on the product at the site to prove that the installation is correct and the product operation is normal.