Application of electronic current transformer in digital substation

    Digital substation is composed of intelligent primary equipment (electronic transformer, intelligent switch, etc.) and network The secondary equipment is constructed in layers (process layer, bay layer, station control layer) based on the IEC61850 communication specification, which enables modern substations to share information and interoperate between intelligent electrical equipment within the substation.IEC61850 divides digital substation into three layers: process layer, bay layer and station control layer.

 

High-speed network communication is adopted between the two substations. Compared with conventional substations, the equipment and network connection of the bay layer and the station control layer of the digital substation only change the interface and communication model, while the process layer needs to undergo a major change. From the traditional current and voltage transformers, primary equipment, and cable connections between primary equipment and secondary equipment, it gradually changes to electronic transformers, intelligent primary equipment, merging units (MU), optical fiber connections, etc. The data exchange between the devices in the process layer follows the requirements of IEC61850-9-1/2 or IEC60044-8 standards. The electronic transformer is a key device in the digital substation.

 

Disadvantages of traditional transformers

Current and voltage transformers (CT, PT for short) are important electrical equipment widely used in power systems. Traditional electromagnetic current and voltage transformers have been used in power systems for many years and are very mature products. With the development of power production and manufacturing technology, current and voltage transformers have formed a series of products with various types and parameters, which can fully meet the use needs of power systems. However, traditional CT and PT present some insurmountable problems due to their sensing mechanism, such as: with the increase of voltage level, insulation difficulties increase sharply; potential sudden failure, such as explosion and other dangers; susceptible to electromagnetic interference; CT is easy to saturate under fault conditions, and it is difficult to correctly reflect the non-periodic components in the transition process; if the CT output is open, high voltage will appear; PT is prone to ferromagnetic resonance; large size and heavy weight. The capacitor voltage divider type voltage transformer (CVT) still has the possibility of resonance due to the presence of nonlinear inductance elements such as intermediate transformers; the accuracy of capacitor voltage division is easily affected by external factors, and often needs to be recalibrated during on-site installation. Although many technical measures have been taken to improve it, the above problems cannot be fundamentally overcome.

 

As an ideal replacement product for traditional current and voltage transformers, electronic transformers do not have these problems. In addition, modern microcomputer integrated measurement and protection devices and instruments no longer need transformers to provide energy to work, but only require transformers to collect and transmit primary current and voltage information completely, timely and accurately. Only a few volts of voltage signal and very small power are needed to meet their interface requirements, and electronic current and voltage transformers can meet this requirement very well.

 

Electronic transformer

An electronic instrument transformer is a device consisting of one or more current or voltage sensors connected to a transmission system and a secondary converter to transmit a quantity proportional to the measured quantity to supply measuring instruments; meters and relay protection or control devices. In the case of a digital interface, this function is completed by a group of electronic instrument transformers sharing a merging unit.

 

Electronic transformers can be divided into two types. One type uses magneto-optical effect and electro-optical effect to directly convert current and voltage into optical signals, which is generally called passive type; the other type uses electromagnetic induction or voltage division principle to convert current and voltage signals into small voltage signals, and then converts the small voltage signals into optical signals for transmission to secondary equipment, which is generally called active type.

 

Active electronic transformer

Air-core current transformer

The air-core coil is used to sense the large current to be measured. The electronic module located on the high-voltage side converts the output signal of the coil into a digital optical signal and transmits it to the low-voltage side through the optical fiber according to the specified protocol. The active optical current transformer based on the air-core coil (Rogowski coil) is the most widely used electronic current transformer at present. The Rogowski coil is a coil that is evenly wound on a non-magnetic ring frame. The measured current passes through the center of the coil. When the measured current passes through the center of the coil, an induced voltage will be generated at both ends of the coil. The induced signal of the air-core coil is proportional to the differential of the measured current.

 

The magnitude of the measured current can be obtained through signal processing such as integral transformation. There is an electronic module composed of electronic circuits on the high-voltage side of the active optical current transformer. The electronic module collects the output signal of the coil, and converts it into a digital signal after filtering, integral transformation and A/D conversion. The digital signal is converted into an optical signal through an electro-optical conversion (LED) circuit, and then the digital optical signal is sent to the secondary side through optical fiber for relay protection and electric energy metering equipment.

 

Low power current transformer

LPCT is still a CT based on the principle of electromagnetic induction. The schematic diagram is shown in the figure below. It consists of a primary winding, a very small iron core and a secondary winding with minimum loss connected to the sampling resistor Rsh. Rsh is a component of the secondary winding and plays the role of converting current output into voltage output.

Compared with traditional CT, the special feature of LPCT is that the core material used is microcrystalline alloy core, not silicon steel sheet. This core is made of iron-nickel alloy steel sheet and adopts a special annealing process. The core of LPCT has high magnetic permeability in weak magnetic circuit, which can make the transformer measuring winding meet the accuracy requirements with a smaller cross-section. Therefore, the size of LPCT is greatly reduced compared with traditional.

 

Even compared with Permalloy, the density and lamination coefficient are lower than Permalloy core. Under the same technical conditions, the manufacturing cost can be reduced by about 1/3, the weight is lighter by more than 1/4, but it has a wider linear range than Permalloy core. Due to the small loss of LPCT, it has high accuracy and will not saturate when measuring large currents (even short-circuit currents). Therefore, LPCT has a wide measurement range. In certain application fields (for example, the primary current ranges from tens of amperes to thousands of amperes), one core can meet the requirements of 0.2 level measurement and 5P20 protection at the same time. The size of LPCT is greatly reduced compared with traditional CT. At the same time, since the secondary winding integrates the sampling resistor, there is no danger of open circuit.

 

Active Electronic Voltage Transformer

Active optical voltage transformers use the principles of capacitive voltage division, resistive voltage division or capacitive-resistive voltage division, use electronic modules similar to active current transformers to process signals, and use optical fiber to transmit signals.

 

Passive electronic current transformer

Optical Current Transformer

Passive optical transformers use the Faraday magneto-optical effect. The principle of the Faraday magneto-optical effect is that due to the magnetic field generated by the current to be measured, when a beam of linearly polarized light passes through a Faraday material (such as magneto-optical glass) placed in a magnetic field, if the direction of the magnetic field is the same as the propagation direction of the light, the polarization plane of the light will rotate, and the rotation angle is proportional to the magnetic field intensity and the line integral along the path of the polarized light through the material. The angle is proportional to the current to be measured, and the change in angle is converted into a change in output light intensity using an analyzer. The current to be measured can be obtained through photoelectric conversion and corresponding signal processing.

 

Optical Voltage Transformer

The passive optical transformer uses the Pockels electro-optic effect. The light emitted by the light emitting diode is a linearly polarized light after passing through the polarizer. Under the action of the external voltage, the linearly polarized light undergoes birefringence after passing through the electro-optic crystal. The phase difference between the two birefringent light beams has a definite relationship with the external voltage. The phase difference is proportional to the external voltage. The polarizer is used to convert the change of the phase difference into the change of the output light intensity. The measured voltage can be obtained through photoelectric conversion and corresponding signal processing.

 

Comparison of active and passive electronic transformers

The key technologies of active electronic transformers are power supply technology, reliability of remote electronic modules, and maintainability of acquisition units. Based on the operating experience of traditional transformers, the maintenance of Rogowski coil and voltage divider (resistance, capacitance or inductance) faults can be ignored. GIS electronic transformers are directly connected to the DC power supply of the substation, and no additional power supply is required. The acquisition unit is installed on a grounding shell that is closely connected to the earth. This method has strong anti-interference ability. It is convenient to replace and maintain, and the abnormal processing of the acquisition unit does not require a system power outage. For independent electronic transformers, the power supply and remote module on the high-voltage platform work in a harsh environment with frequent alternation of high and low temperatures for a long time. Their service life is far less than that of the protection and measurement and control devices installed in the main control room or protection room, and actual engineering experience needs to be accumulated; in addition, when the power supply or remote module is abnormal and needs maintenance or replacement, a system power outage is required. The key technologies of passive electronic transformers are the stability of optical sensing materials, assembly technology of sensor heads, modulation and demodulation of weak signals, the influence of temperature on accuracy, the influence of vibration on accuracy, and the stability of long term operation.

 

However, since the electronic circuit parts of passive electronic transformers are installed in the main control room or protection room, the operating conditions are excellent and the 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, which is the development direction of transformers. Passive electronic transformers are highly reliable and easy to maintain,and are an ideal solution for independently installed transformers.

 

Advantages of electronic transformer

No iron core, eliminating magnetic saturation.Electronic transformers generally do not have an iron core and do not have saturation problems, so their transient performance is better than that of traditional transformers.This greatly improves the accuracy of various protection fault measurements, thereby increasing the correct action rate of protection devices and ensuring the safe operation of the power grid.

 

Rapid response to power system failures

Existing protection devices (including microcomputer protection) are limited by the performance of traditional transformers. They are basically based on power frequency quantities for protection judgment. They are easily affected by system oscillations, magnetic saturation and other factors. The protection performance is difficult to meet the requirements of ultra-high voltage, large capacity and long-distance development of today’s power system. Using the transient signal quantity at the time of fault as the protection judgment parameter is the development direction of microcomputer protection. It has very high requirements for the linearity and dynamic characteristics of the transformer. Electronic transformers can meet this requirement, but traditional transformers cannot.

 

Eliminates ferromagnetic resonance and has strong anti-interference ability

Electronic transformers do not have the conditions for ferromagnetic resonance and have strong resistance to electromagnetic interference. It changes linearly with the primary side voltage. It does not produce ferromagnetic resonance when operating in the power system, thus preventing equipment damage and ensuring the safety of system operation.

 

Excellent insulation performance

As the voltage level increases, the insulation difficulty of CT, PT, CVT increases sharply. The use of insulating materials such as oil is explosive, and the volume is large and the weight is heavy. The insulation of electronic transformers is relatively simple. The signal transmission between the high voltage side and the ground potential side uses glass fiber made of insulating materials. It is small in size, light in weight, and has good insulation performance. Due to the structural characteristics of the electronic transformer, the electronic current transformer will not generate high voltage that endangers the safety of equipment and personnel when the secondary circuit is open, and the electronic voltage transformer will not generate large current when the secondary side is short-circuited, ensuring the safety of personnel and equipment. Adapted to the development of digitalization, computerization and automation of power measurement and protection.

Electronic transformers can directly provide digital signals to metering and protection devices, which helps the system integration of secondary equipment, accelerates the digitalization and informatization process of the entire substation, and triggers major changes in power system automation devices and protection.

 

Large dynamic range and high measurement accuracy

The output of the secondary side of the electronic current transformer is completely linear with the current of the primary side, without ferromagnetic saturation, no distortion, and a wide dynamic range. Photoelectric and air-core coil current transformers can range from a few amperes to hundreds of kiloamperes. The iron core of low-power electronic current transformers also begins to saturate at double the rated current. Due to the large measurement range, only two or three current specifications are needed within the effective range to cover the entire range of CT from 20A to 5000A.

 

Wide frequency response range

The frequency response of the sensor head of the electronic current transformer depends on the transit time of the light beam on the sensor head, which can reach 1MHz at present. The electronic current transformer has been proven to be able to measure the harmonics on the high-voltage power line, and can also measure transient current, high-frequency large current and DC current. The sensor head of the electromagnetic current transformer is an iron core, and its frequency response is very narrow.

 

Good economic efficiency

In terms of economy, as the voltage level of the transmission line increases, the cost of traditional transformers (mainly insulation cost) increases exponentially, while the cost of electronic transformers only increases slightly when the voltage level increases. The low cost of electronic transformers is the key to attracting people. In addition, due to the small size and light weight of electronic transformers, they can be combined with circuit breakers or other high-voltage equipment, share supporting insulators, reduce the area occupied by substations and their foundation projects, and reduce the installation projects of separate equipment. Electronic transformers do not use copper core cables as output connection lines, and the accuracy level is not affected by the secondary connection circuit. Therefore, the consumption of copper core cables is reduced, which reduces the cost of engineering construction.

 

Engineering Practice

Take a 220kV substation as an example:

1) The current transformer is configured as follows:

220kV bays, 110kV bays, and three sides of the main transformer use full-fiber current transformers; 10kV bays (except the main transformer) and the center point of the main transformer use conventional current transformers. 220kV, full-fiber current transformers on each side of the main transformer, each transformer contains 2 independent current sensing/collection optical paths, with an accuracy of 0.2S (5TPE), which can be used for protection, measurement, and metering. 110kV full-fiber current transformers, each transformer contains 1 independent current sensing/collection optical path, with an accuracy of 0.2S (5TPE), which can be used for protection, measurement, and metering.

 

2) The voltage transformer is configured as follows:

220kV and 110kV voltage transformers use capacitor voltage-dividing electronic voltage transformers. The data are converted into digital signals for protection, measurement and control. The 10kV busbar uses conventional voltage transformers and uses a merging unit with analog plug-in for digital conversion.

 

Each 220 and 110kV three-phase electronic voltage transformer contains two independent output circuits with an accuracy of 0.2 (3P) and can be used for protection, measurement and metering. 110kV active line A phase electronic voltage transformer contains 1 independent output circuit, accuracy 0.2, for synchronization.

 

5 Conclusion

Compared with traditional transformers, electronic transformers have greater advantages in insulation, dynamic range, saturation performance, and economy. Electronic transformers have been applied in 220kV and 110kV systems. For example, electronic transformers have been successfully put into operation in the 110kV Yuanshi substation of Wuxi Power Supply Company and have been running well so far. As the development direction of substations, electronic transformers are one of the key products for the intelligentization of power systems. Its wide application will bring great changes to the power system and will comprehensively improve the level of intelligence.

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