A power transformer is one of the most critical elements of typical station equipment, the stability of which depends on the stability of the power supply to the consumer network. A general view of such a conversion device is shown in the photo below.
Due to the special importance of equipment of this class, special attention is paid to its protection from possible damage and operational disturbances. The types and procedure for its organization are stipulated in the relevant regulatory documents (PUE 7 p.p.3.2.51-71 and PTEEP Ch.2.1).
Types of protection
According to the above normative documents, the existing means for the protection of station transformers (ST) are divided into main and backup. The first ones are triggered immediately after a violation of the normal operating mode and are divided into the following types:
- Differential longitudinal.
- Protection against short circuit of working windings on the case and overload.
- Gas or jet.
Overcurrent (overcurrent) protection is usually used as a backup. It is sometimes also called relay.
Note:The back-up protection has a time delay and comes into effect when the main ones do not work at interphase faults on one of the sides for some reason.
Overloads in one or all phases, which led to a sharp surge of current in the windings, are considered violations of the normal operation of a power transformer. This often happens for the following reasons:
- short circuit of input or output transformer terminals to ground or to neutral elements;
- interphase short-circuits in the CT windings themselves or in its load buses (the latter are clearly distinguishable in the photo below);
- inside winding closures.
For all types of protection, a sharp drop in the supply voltage and a surge in current in this section of the circuit serve as a signal about a violation of the normal mode.
Features of the main and backup protection
The first two types of protection (differential and short-circuit protection) are triggered when the transformer windings are damaged, as well as when the busbar and input circuits of the CT are faulty. The third on the list of protective functions is provided in case of damage to the interior of the tank with a coolant (transformer oil), as well as its level falls below a critical value.
Relating to the category of reserve overcurrent protection or relay protection (see GOST 9680-77) is triggered in case the current exceeds the permissible value for this winding. Examples for a single-phase and 3-phase transformer device are shown in the photo below.
Relays A, A1, A2 and A 3 are triggered if the current exceeds the value permissible for the windings with a sharp decrease in the load resistance or short circuit. For taking readings, special transformers with the required current indicators are used.
The time delay provided by relay T is necessary to improve the selectivity of the protective circuit. After it is triggered, the control unit L is turned on, disconnecting the product from the power line until the causes of the malfunction are clarified. Due to the presence of the shutter speed, the considered scheme is used only in situations where the main types of protection for some reason do not work.