With the trend of the transformation of traditional substations to digital substations becoming more and more obvious, electronic transformers occupying an important position in the primary equipment of digital substations are getting more and more attention. This article describes the advantages, classification, comparison and current problems in the application of electronic transformers.
With the rapid development of optical fiber sensing technology and optical fiber communication technology, optoelectronic technology has become more and more widely used in power systems. Electronic transformers are one of them. Electronic transformers have many advantages such as small size, light weight, wide band response, no saturation, good anti-electromagnetic interference performance, no oil structure, reliable insulation, easy to digital, microcomputer development, etc., and will be widely used in digital substations. application.
The birth of electronic transformers is the inevitable result of transformer sensing accuracy, transmission fiber optics, and output digitalization. Electronic transformers are one of the key equipment for digital substations. Sensing methods have a great influence on the structural system of electronic transformers. The optical principle of the electronic transformer is simple and it is a passive electronic transformer. Electromagnetic measuring principle electronic transformer is an active electronic transformer.
1, the advantages of electronic transformers
1.1 High and low voltage isolation, high safety, excellent insulation performance, no iron core, eliminating magnetic saturation and ferromagnetic resonance
The signal of the electromagnetic transformer is coupled with the secondary coil through the iron core, and the insulation structure is complex. The cost of the electromagnetic transformer rises exponentially with the voltage level. Unconventional transformers transmit high-side signals to secondary devices through optical fibers with good insulation properties, which greatly simplifies the insulation structure. The higher the voltage level, the more obvious the cost-effectiveness advantage. Unconventional transformers use fiber optic cables instead of cables as signal transmission tools to achieve complete isolation of high and low voltages. There are no hazards to the equipment and personnel caused by the short circuit of the secondary circuit of the voltage transformer or the open circuit of the secondary circuit of the current transformer. Safety and Reliability is greatly improved.
Due to the use of iron cores, electromagnetic transformers inevitably suffer from problems such as magnetic saturation and ferromagnetic resonance. Unconventional transformers are fundamentally different from traditional transformers in principle, and generally do not use iron cores for magnetic coupling. Therefore, magnetic saturation and ferromagnetic resonance phenomena are eliminated, so that the transient operation of the transformer is good and the stability is good. The high reliability of the system operation.
1.2 Good anti-electromagnetic interference performance, there is no open circuit on the low pressure side
The secondary circuit of the electromagnetic current transformer cannot be opened, and there is danger of open circuit on the low voltage side. There is only an optical fiber connection between the high-voltage side and the low-voltage side of the unconventional transformer. The signal is transmitted through the optical fiber, and the high-voltage circuit and the secondary circuit are completely isolated electrically. The transformer has good anti-electromagnetic interference capability and there is no open circuit on the low-voltage side. High voltage danger.
1.3 Large dynamic range, high measurement accuracy, wide frequency response range
The current flowing through the current transformer during the normal operation of the power grid is not large, but the short-circuit current is generally large, and the short-circuit current increases as the capacity of the power grid increases. Due to the magnetic saturation problem of electromagnetic current transformers, it is difficult to achieve a wide range of measurements. It is difficult for the same transformer to meet the needs of measurement and relay protection at the same time. Unconventional transformers have a wide dynamic range and can meet both measurement and relay protection needs.
The frequency range of the non-conventional transformer depends mainly on the relevant electronic circuit part, and the frequency response range is wider. Unconventional transformers can measure the harmonics on the high-voltage power line, and can also measure the grid current transients, high-frequency high-currents and DCs, and electromagnetic transformers are difficult to work in this area.
1.4 Data transmission has strong anti-interference ability
Electromagnetic transformers transmit analog signals. Measurement, control, and relay protection in power stations traditionally transmit electrical signals measured by electrical sensors to the control room via coaxial cables. When multiple different devices require signals from the same transformer, complex secondary wiring is required, and this conventional structure is inevitably hampered by electromagnetic fields. The digital signal output by the photoelectric transformer can be conveniently used for data communication, and the photoelectric transformer and the device that needs to take the transformer signal can constitute a field bus network. Data sharing is achieved to save a large amount of secondary cables. At the same time, the inherent anti-electromagnetic interference performance of fiber-optic sensors and optical fiber communication networks shows an unparalleled superiority in the harsh environment of power plants. The replacement of traditional electrical systems with fiber optic systems is The inevitable trend of future power station construction and renovation
1.5 There is no potential flammable, explosive or dangerous letter due to oil filling
The insulation structure of non-conventional transformers is relatively simple. Generally, oil is not used as an insulating medium and it will not cause fire or explosion.
1.6 Small size and light weight
Unconventional transformers have no iron core and their weight is much lower than electromagnetic transformers of the same voltage class.
In summary, non-conventional transformers, with their superior performance, have been adapted to the needs of digital, intelligent and networked development of power systems, and have obvious economic and social benefits, and are used to ensure the increasingly large and complex power system security. It is of far-reaching significance to run reliably and increase its degree of automation.
2, electronic transformer classification
2.1 Active Electronic Transformer
Active electronic transformers use electromagnetic induction and other principles to sense the measured signal. Rogowski coils are used for current transformers, and resistance, capacitance, or inductance is used for voltage transformers. The high-voltage platform sensor head of an active electronic transformer has an electronic circuit that requires power supply to perform analog digital sampling on a primary platform (ie, a remote module) and uses optical fiber transmission to transmit digital signals to secondary protection. , measurement and control and metering systems.
Active electronic transformers can be divided into closed-type gas-insulated combinational appliances (GIS) and stand-alone, GIS-type electronic transformers are generally combined current and voltage type, and their acquisition modules are installed in the grounded enclosure of GIS. The insulation is solved by the GIS. The remote acquisition module can directly use the substation 220 V/110 V DC power supply at the ground potential. The collection unit of the independent electronic transformer is installed on the insulating porcelain column. Due to the insulation requirements, the power supply of the collection unit includes laser, low-current transformer, voltage divider, and photovoltaic cell power supply. The actual engineering application generally adopts laser. Power supply, or laser and small current transformer cooperate with power supply, that is, the current is supplied by the small current transformer when there is current in the line, and it is powered by laser when there is no flow. For stand-alone electronic transformers, in order to reduce costs and reduce the floor space, generally combined type is used. That is, current transformers and voltage transformers are installed on the same composite insulator. The remote module can simultaneously collect current and voltage signals, which can be used together. Power supply circuit
2.2 Passive Electronic Transformer
Passive electronic transformers are also known as optical transformers. The passive electronic current transformer uses the Faraday magneto-optical effect to sense the measured signal. The sensor head is divided into two methods: block glass and full fiber. Passive electronic voltage transformers use the Pockels electro-optic effect or inducing signals based on inverse piezoelectric effects or electrical pinching effects. The optical voltage transformers currently studied are mostly based on the Pockels effect. Passive electronic transformer sensor head part does not require a complex power supply device, the linearity of the entire system is better. Passive electronic transformers use optical fiber to transmit primary current and voltage sensing signals, and modulate and demodulate them to the main control room or protection room, and output digital signals to MU for protection, measurement, and measurement. The sensor head of the passive electronic transformer is a relatively complicated optical system and is easily affected by various environmental factors such as temperature and vibration, which affects its practical application. Information, please visit: Transmission and Distribution Equipment Network
3. Comparison of active and passive transformers
The key technologies of active electronic transformers are the power supply technology, the reliability of remote electronic modules, and the maintainability of the acquisition unit. Based on the operating experience of traditional transformers, maintenance of faults of Rogowski coils and voltage dividers (resistive, capacitive or inductive) may not be considered. The GIS type electronic transformer directly connects to the substation DC power supply and does not require additional power supply. The acquisition unit is mounted on a grounding shell that is closely connected to the earth. This method has strong anti-interference ability, convenient replacement and maintenance, and abnormal processing of the acquisition unit does not require a system blackout. For stand-alone electronic transformers, the power supply and remote modules on the high-voltage platform are operated in the harsh environments with high and low temperature and frequent alternation. Their service life is far less than that of the protection and control device installed in the main control room or the protection room. It is necessary to accumulate practical engineering experience; in addition, when the power supply or the remote module is abnormal, needs maintenance or replacement, it needs a system power outage processing. Source: http://tede.cn
The key technologies of passive electronic transformers are the stability of optical sensing materials, the assembly technology of sensing heads, weak signal modulation and demodulation, the influence of temperature on precision, the impact of vibration on accuracy, and the stability of long-term operation. However, since the electronic circuit part of the passive electronic transformer is installed in the main control room or the protection room, the operating conditions are superior, and replacement and maintenance are convenient. The application of active or passive electronic transformers greatly reduces the floor space and reduces the secondary cable connection of traditional transformers. It is the development direction of transformers. Passive electronic transformers are highly reliable and easy to maintain, making them an ideal solution for independently installed transformers.
       The main problem in the application of electronic transformers is: Because of the need to power sensors, long-term high-power laser energy will affect the service life of optical devices, Rohs. coil output signal has a strong correlation with its structure, temperature Changes will lead to structural changes that affect the accuracy of electronic circuit measurements.
       The main problems in the application of photoelectric transformers are: temperature changes will cause changes in the optical system caused by crystals in addition to the electro-optical effect of the optical effect, thermal effects and other interference effects, resulting in the work of the optical voltage sensor in the insulator Stability weakened. The influence of temperature on the measurement error of photoelectric transformer has always been the focus of people's discussion. In practical applications, the influence of measurement error caused by temperature change should be improved the anti-interference ability of the optical system (such as photodiode). For example, the use of a light source with good temperature stability, a small wavelength drift, and pure electro-optic crystals that have been repeatedly pulled, has attracted attention from domestic and foreign researchers in recent years in the study of improving temperature stability. Double optical path temperature compensation method, double crystal temperature compensation, hardware circuit compensation and software compensation methods. In addition, the birefringence effect of the magneto-optical material affects the measurement accuracy of the photoelectric current transformer. Due to the birefringence effect of the magneto-optical material, the linearly polarized light of the injecting magneto-optical medium becomes an elliptically polarized light. The result is: The change of the light intensity output from the polarizer is not proportional to the measured current, which makes the sensitivity of the photoelectric current transformer unstable, thereby reducing the measurement accuracy of the photoelectric current transformer.
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