​The relay coil is represented in the circuit by a long box symbol, and if the relay has two coils, two long boxes are drawn side by side. At the same time, mark the relay word symbol "J" in or beside the long box. There are two ways to represent the contacts of relays: one is to draw them directly on one side of the long box, which is more intuitive. The other is to draw each contact into its own control circuit according to the needs of the circuit connection. Usually, the contact of the same relay and the coil side are marked with the same text symbol, and the contact group is numbered to show the difference.

Self-locking and interlocking are achieved by auxiliary contacts of relays. First of all, you need to understand what is called self-locking and what is called interlocking. Self-locking is to short off the button switch of the contactor coil loop with your own contacts, and make the coil loop open continuously after the button switch is released, which is self-locking.

Electromagnetic Voltage Transformer (PT) and Capacitive Voltage Transformer (CVT) are widely used in power system. Although capacitive voltage transformers and electromagnetic current transformers commonly used in power grids have mature technology and long-term operation and maintenance experience, their measurement linearity is poor, the transient response speed is slow, and the transient error characteristics of electromagnetic current transformers are not ideal.

The rise and development of intelligent power grid technology has put forward new requirements for voltage and current sensors in power system. Miniaturized non-contact sensors will be the mainstream and need. This paper summarizes the application status of traditional electromagnetic voltage transformers and capacitive voltage transformers, and points out their limitations. On the basis of introducing the D-dot measurement principle in high voltage pulse electrical measurement, an electronic voltage transformer based on self-integrating D-dot sensor is proposed, and it is pointed out that it will be the mainstream trend of voltage signal measurement in power system.

A power transformer is a static machine used for transforming power from one circuit to another without changing the frequency. As there is no rotating or moving parts, a transformer is classified as a static device. Transformer operates on an AC supply. Transformers operate based on the principle of mutual induction.

Potential transformer or PT can have the same construction as any normal transformer. It has primary & secondary winding. The number of turns in primary windings is greater than the number of turns in the secondary winding because it is a step-down transformer.

A transformer ​is a device that converts voltage or current proportionally. The function of the transformer is to convert high voltage or high current into standard low voltage (100V) or standard low current (5A or 1A, both referring to the rated value), so as to achieve the standardization and miniaturization of measuring instruments, protection equipment and automatic control equipment.

In the power system, the role of the current transformer is to change the large current into the small current, isolate the secondary loop connected to the relay and measuring instrument from the high voltage system of the primary current, and convert the primary current to the secondary current value of 5A or 1A standards.

In order to ensure the safe and economical operation of the power system, the operation of the power equipment must be monitored and measured.

Wound Current Transformer – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer.