Analysis of current transformer structural design principles
The concept of digital modulation current transformer has been proposed for nearly half a century, but it has not been developed due to the constraints of components. In recent years, ABB Company and German RITZ Company have reported on some different types of electronic current transformer and electronic voltage transformer products. Developed active electronic current transformer/voltage transformer for 220kV/1250A equipment. The overall structure of the prototype is shown in Figure 2.
Current transformer equipment consists of a current detection unit and a voltage detection unit. The former uses a Rogowski coil as the current sensor, and the active electronic device performs digital modulation of the signal; the latter uses a capacitive voltage divider as the voltage sensor. The high-voltage side electronic circuit power is taken directly from the busbar by an auxiliary power induction coil. Rogowski coils and power induction coils are installed on the busbar. The high-voltage end signal modulation and power dispatch circuit board is placed in a ferromagnetic shielding box. The current signal is modulated into an optical signal after AD conversion and E/O conversion at the high-voltage end, and is transmitted to the low-potential end through optical fiber. Depending on the needs, the signal can be continuously transmitted to a remote control room using optical fiber, or it can be converted locally through O/E and D/A and amplified into a simulated current signal. After the bus voltage is converted into a low-voltage signal through a capacitor divider, it can be modulated. The optical signal is transmitted to the control room through optical fiber. It can also be calibrated and temperature compensated locally to give a simulated voltage signal. A ceramic sleeve is used as an insulating support for high-voltage components and at the same time it is an optical fiber for transmitting current signals. and capacitive divider channels.
Current and voltage signal measurement of current transformer
In active electronic current transformers. There are many kinds of components in the primary current sampling sensing head, including traditional electromagnetic current transformers, specially designed small signal current transformers, shunt resistors, Rogowski coils, etc. Among them, Rogowski coil has become the first choice due to its excellent frequency response, high detection accuracy, simple structure, low cost and other characteristics. Therefore, the active ETA based on the Rogowski coil has become the photoelectric transformer product with the most development potential. It can be used as a current detection device in closed electrical GIS and plug-in combined electrical appliances PASS, and can also be used in open Type independent active ETA. Rogowski coil is an air-core coil made by winding wires evenly around a skeleton of non-magnetic material, as shown in Figure 3.A current-carrying wire passes through the center of the coil. When there is current passing through the wire, an induced potential e will be generated at both ends of the coil, with a magnitude of
It can be seen from equation (2) that in order to obtain the measured current signal, it is necessary to integrate the output voltage signal of the coil, which can be accomplished in two ways: using an analog integrator or using digital integration. In short, through subsequent circuits and related signal processing, we can obtain the measured current signal.
In the field of medium and low voltage power distribution, precision resistive voltage dividers and capacitive voltage dividers are now widely used. The use of this voltage sensor technology greatly simplifies the design of the high-voltage sensing part. At the same time, optical fiber is used to transmit signals, retaining the excellent electrical isolation effect of optical fiber. Therefore, a capacitive voltage divider is selected as the voltage sampling element in the design of this article.
Signal processing equipment for current transformers
The signal processing process is as follows: after the current signal is sampled at the high voltage side. It is converted into a digital signal, and through an appropriate power unit, it is driven by the photodiode to become an optical signal, which is sent to the low voltage end of the transformer under the optical fiber. After the voltage signal is sampled by the voltage divider, it becomes a digital signal. After appropriate power amplification, the signal drives the light-emitting diode and becomes an optical signal. In the low potential state of the transformer body, the current and voltage signals of the actuated light 1 are transmitted to the substation 2 to 3 through the optical fiber. After O/E conversion in the control room, after appropriate scheduling, the signals are then sent to the industrial computer. Carry out signal demodulation and processing, and the demodulated simulated signal can be used for planning and maintenance.In the signal processing unit, transient signal detection is used to maintain the detection of current and voltage signals. It is necessary
Consider the response speed and frequency bandwidth of the signal processing unit.For example, the highest natural frequency of the transient recovery voltage of 220kV equipment is
10kHz, if sampled according to the 1/10 interval, the main frequency bandwidth of the E/O conversion must be greater than 200kHz. In the O/E conversion, a fast photodiode needs to be used.
Problems with power supply of primary electronic circuit of current transformer
The primary side electronic circuit power supply problem is a key technology in active ETA. The primary side power supply must provide stable power supply to the signal processing part of the sensing element. There are two ways to deal with this problem. One is to transfer energy from the secondary side on the ground to the primary side to supply power, and the other is to take power directly from the busbar on the secondary side. Because there must be complete isolation between the primary and secondary sides, the best way to send the energy from the secondary side to the primary side is through photoelectric conversion and transmission via optical fiber: Photoelectric energy conversion generally uses high-power semiconductors Laser diodes come to an end. Laser diodes serve as light sources to supply optical power for driving photovoltaic cells. Diodes with appropriate optical power and output power are selected based on the total power requirements of the equipment. The energy transmitted from the optical fiber is directly coupled to a photoelectric converter on the primary side, usually in a photovoltaic cell, to convert the light energy into electrical energy. Commercial diode arrays are now available for photoelectric conversion. The advantage of using the ground energy supply method is that the power supply is stable, reliable and not affected by the bus current. However, the power that this energy supply solution can generally supply is relatively small, usually at the mW level or even the pW level. Sometimes this energy supply solution cannot supply satisfactory energy to the primary side. In addition, high-power, high-power laser diodes and photoelectric conversion devices are relatively expensive. There are no strict assessment reports on service life.
Because the bus current changes greatly in the design, taking an ETA with a rated current of 1250A as an example, the bus steady-state current can be changed within 5%-120%1n (rated current), that is, within the design of 62.5-1500A; short circuit fault situation Downstream and bus transient currents can reach 201n or even higher. In these cases, it is required to provide stable power supply required by the primary electronic circuit. In view of the above requirements, a self-contained power supply solution that takes power directly from the bus bar is proposed. The design work mainly focuses on taking out a voltage source with a certain power and stable output from the current source that changes within a large design. The schematic diagram of a self-contained power supply is shown in Figure 4. Here we use an annular induction coil with an iron core to extract electric energy from the bus.
The required stable voltage Ul can be output. Therefore, the problem can be transformed into designing a load impedance controllable circuit for the power induction coil. When the bus current is small, the equivalent impedance is large; when the bus current is large, the equivalent impedance is small. Reasonable design of the controllable impedance circuit can achieve the purpose of providing stable feed power under large design bus current.
Review Editor: Huang Fei
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