In this method, the transformer is disconnected from the main power supply when the valve setting needs to be changed. Adjusting the faucet is usually done manually. The shutdown transformer is shown in the figure below The stage change can be made when the transformer is in an unloading or non-load state. In a dry-type transformer, the cooling phenomenon occurs mainly with natural air. Unlike load stage change, in which arc quenching is limited by oil during transformer load operation, tapping with a load changer is performed only when the transformer is in the OFF switch state. If only one valve changer is required, manually operated valves are usually attached to the transformer`s high-voltage (primary) or lower-current winding to minimize the current load capacity of the contacts. However, a transformer may include a valve changer charging on each winding if it is beneficial. For example, in electricity distribution systems, a large buck transformer may have a pick-up changer on the primary winding and an automatic pick-up changer on the winding or secondary windings. The high-voltage valve is adjusted to match the long-term profile of the system in the high-voltage network (usually supply voltage means) and is rarely modified. The low-voltage valve can be instructed to change position several times a day without interrupting the power supply to follow the load conditions in the low-voltage network (secondary winding).
This continuous power requirement does not allow you to disconnect the transformer from the system to change the load valve. Therefore, support changers are preferred in most power transformers. Cut-off mechanisms require the transformer to be isolated before its tapping parameters can be adjusted, which is typically the case with smaller distribution transformers. Charging valve changers typically use many plug selectors that are not allowed to be switched under load, are divided into even and odd banks, and switch between banks with a high-performance bypass switch that can switch between them under load. The result works like a dual-clutch transmission, with step-by-step selectors taking the place of the gearbox and deviation switches replacing the clutch. Load valve changers are of two main types,[1] valve changers (NLTC), which must be powered off before adjusting the rotational ratio, and load valve changers (OLTCs), which can adjust their rotational ratio during operation. The selection of a charging valve changer can be done via an automatic system, as is often the case with ALTC, or via a manual valve changer, which is more common with NLTC. Automatic plug changers can be placed on a lower or higher voltage winding, but for high-power generation and transmission applications, automatic plug changers are often placed on the higher voltage transformer winding (lower current) for easy access and minimize current load during operation. [2] Tap changers and load changers (OLTCs) can be installed on hermetic transformers and conservator transformers.
In general, all major MVA electrical transformers use charging valve changers. For single-phase electrical transformers, each transformer has its own manually operated vacuum switch. The idle switchgear is located with the core and windings in the main tank, the control handle is usually located outside on the side of the transformer tank. So that the power supply is not interrupted, the Powershift transformers are continued. Such a transformer is called a valve-to-load change subload transformer. When tapping, two essential conditions must be met. The following figure shows the offload switching transformer (1 to 5) on the secondary winding. The position of the movable arm on the first bolt gives the minimum secondary tension and on the fifth bolt the maximum tension on the secondary tension. Transformer support changers (DETCs) are used to change the transmission ratio to adapt the transformer to network conditions. Therefore, transformer valve changers have multiple pins to connect to the coil windings.
We produce a large selection of valve changers for distribution transformers of different sizes for a current up to 1250 A. The diagram can be generated as a delta, star, serial parallel, or common output per phase application. Some types of faucet changers can also be used in parallel. This is a relatively recent development where thyristors are used both to switch the transformer winding valves and to relay the steady-state charging current. The disadvantage is that all non-conductive thyristors connected to the unselected valves still dissipate current due to their leakage currents and have limited short-circuit tolerance. This energy consumption can be added to a few kilowatts, which appears in the form of heat and leads to a reduction in the overall efficiency of the transformer; However, this results in a more compact design that reduces the size and weight of the faucet changer. Solid-state valve changers are typically only used for smaller power transformers. In this way, the continuity of the power supply is maintained and a higher rotation ratio of the pitch change can possibly be compared to the change in transformer idle. Care must be taken to avoid short circuits with windings during the tapping process. In transmission and distribution systems, voltage fluctuations (i.e., increasing or decreasing voltage levels) can occur when system load changes.
These fluctuations can also be caused by a voltage drop in the distribution network. Sometimes, these fluctuations in voltage levels can result in completely unsatisfactory performance. In order to minimize the number of winding valves and thus reduce the physical size of a tapping changer, an “inverted” tapping changer winding can be used, in which part of the main winding can be connected in the opposite direction (buck) and is therefore opposite to the voltage. The Load Changer (OLTC), also known as the Circuit Valve Changer (OCTC), is a load valve changer in applications where a power interruption during a load valve changer change is unacceptable, the transformer is often equipped with a more expensive and complex load valve changer mechanism. Load valve changers can usually be classified mechanically, electronically or fully electronically. A mechanical valve changer physically establishes the new connection before releasing the old one with multiple step selectors, but avoids generating high traffic currents by using a switch to temporarily place a large switch impedance in series with the short-circuited turns. This technique overcomes problems with open or short-circuit valves. In the case of a resistance valve changer, the change must be made quickly to avoid overheating of the bypass. A reactance valve changer uses a dedicated preventive autotransformer winding to act as a switch impedance, and a reactance valve changer is typically designed to withstand tapping stress indefinitely.
It should be noted that the deflection switch operates under load and no current flows through the selector switches during the stage change. When changing sockets, only half of the current limit in the circuit is connected. Support changers are also used in high-voltage distribution transformers where the system includes an open-circuit changer on the primary winding to compensate for fluctuations in the transmission system in a narrow band around the rated power. In such systems, the valve changer is often adjusted only once at the time of installation, although it may be changed later to accommodate a long-term change in the system`s voltage profile. You should install an open load changer on a transformer if the required voltage change is rare. Valves can be changed after a transformer has been completely disconnected from the circuit. This type of change is usually installed on a distribution transformer. The disadvantage of changing the charge level can be overcome by a special arrangement of coil connections to the transformer, called the transformer charge level change. The transformer connection for the load pitch change is shown below. A support changer is a mechanism in transformers that allows variable rotation ratios to be selected in different stages. This is done by connecting to a series of access points called taps along the primary or secondary winding. The valve change using a central capture R reactor is shown in the figure above.
Here, S is the deviation switch and 1, 2, 3 are selection switches. The transformer works with closed switches 1 and S. To switch to key 2, switch S is opened and switch 2 is closed. Switch 1 is then opened and S is closed to complete the tip switch. It should be noted that the deflection switch operates under load and no current flows through the selector switches during the stage change. When changing the valve, only half of the reactance is limited, which limits the main function of the switch. Its main feature is that during operation, the main circuit of the switch should not be opened. This means that no part of the switch should be shorted. Due to the expansion and interconnection of the power supply system, it is crucial to change the conversion valves several times a day to reach the required voltage according to the load demand. A possible design (flag type) of a loaded mechanical valve changer is shown on the right. It starts operation at socket position 2, with charging delivered directly through the right connection. Soft resistor A is short-circuited; Deflector B is not used.
When you switch to Tap 3, the order is as follows: It is imperative that you choose the right type of faucet changer according to your needs. In a typical switch switch, powerful springs are clamped by a low-power motor drive unit (MDU) and then released quickly to perform the valve change. To reduce arcing at contact, the valve changer operates in a chamber filled with insulating transformer oil or in a container filled with SF6 compressed gas.