The operating principle of a commonly used electromagnetic voltage transformer (VT) is identical to that of a standard transformer. Its structural design and connection to the system are also similar. However, the secondary voltage is low, and its capacity is quite small—typically ranging from tens to hundreds of volt-amperes (VA). The ratio of the rated voltage between the primary winding and the secondary winding is defined as the rated voltage ratio of the voltage transformer, denoted as
k_uk_u = U_{N1} / U_{N2} \approx N_2 / N_1 = k_NU_{N1}U_{N2}N_1N_2k_N
For most electrical professionals, voltage transformers are a familiar concept. However, for beginners in the field, they might only have a vague understanding. Voltage transformers are indispensable components in power generation plants, substations, and other transmission and distribution systems. Precision voltage transformers, in particular, are instruments used in electrical testing labs to extend measurement ranges for voltage, power, and energy. Improper selection of wiring methods or measurement accuracy can directly impact the precision of voltage, power, and energy measurements. Therefore, in the design of substations or distribution systems, correctly selecting a voltage transformer is critical to ensuring measurement and metering accuracy, as well as the reliability of relay protection and automatic control systems. So, what exactly is the principle and function of a voltage transformer? How is it wired? Let’s dive in and explore these topics together!
1. Working Principle and Characteristics of Voltage Transformers
Voltage transformers can be classified into two main types: electromagnetic and capacitive voltage-dividing. For voltage levels of 220 kV and below, electromagnetic types are more commonly used. For this discussion, we’ll focus on the electromagnetic type.
(1) Working Principle
Voltage transformers operate based on the principle of electromagnetic induction. They consist of a closed iron core with two mutually insulated windings of different turns counts. The primary winding () is connected to the power supply side, while the secondary winding () serves as the output side. When a voltage is applied to the primary winding, an alternating current flows through it, generating an alternating magnetic flux () in the iron core at the same frequency as the power source. Since both the primary and secondary windings are wound on the same core, an electromotive force (EMF), , is induced in the secondary winding per Faraday’s law of electromagnetic induction. This induced EMF has the same frequency as the primary side but differs in magnitude due to the difference in turns. The relationship between the voltages and turns is given by . According to national standards, the secondary output voltage of a voltage transformer is typically 100 V.
N_1N_2\Phi_1E_2N_1 / N_2 = U_1 / U_2
(2) Characteristics of Voltage Transformers
The primary voltage of a voltage transformer is unaffected by the secondary load. The impedance of the voltage coils in secondary-side instruments or relays is very high, resulting in minimal current flow. As a result, a voltage transformer operates near a no-load condition under normal circumstances. However, the secondary side must never be short-circuited. A short circuit would cause a large short-circuit current to flow, potentially damaging or burning out the transformer. To prevent this, fuses are typically installed on both the primary and secondary sides for short-circuit protection.
2. Wiring Methods of Voltage Transformers
Voltage transformers come in single-phase and three-phase configurations. Three-phase voltage transformers are generally used for voltage levels below 20 kV.
(1) Single-Phase Voltage Transformers
- V-V Connection: Two single-phase transformers are connected in a V-V configuration.
- Open-Delta Connection: Three single-phase transformers are wired in an open-delta arrangement.
(2) Three-Phase Voltage Transformers
- Y-Y0 Connection: A single three-phase, three-column transformer is connected in a Y-Y0 configuration to measure line voltages.
3. Summary
The operating principles of voltage transformers and current transformers are both rooted in electromagnetic induction. Through the processes of "electricity generating magnetism" and "magnetism generating electricity," they convert high voltages into low voltages and large currents into small currents. This enables the miniaturization of secondary-side equipment, such as measuring instruments and relays, while also isolating personnel from high voltages to ensure safety.
