Design Conception of Electronic Transformer
1 Research on the energy supply of the high-voltage electronic circuit of the sensor
The output signal of the electronic transformer is digitized on the high voltage side in order to make the measured information in the process of information transmission, no new error is generated, and it is not affected by the load. Therefore, the power supply to the electronic circuit of the high-voltage end signal processing part is a key factor to ensure reliable and stable operation of the sensor, and is also a technical problem common to various hybrid electronic transformers. Moreover, the high-voltage side power supply must be suspended to ensure complete isolation of the high and low voltage side electrical signals. According to the current situation, many units at home and abroad are studying the power supply problem of the high-potential side of the hybrid photoelectric transformer. The feasible technical solutions are as follows: (1) The power supply mode of the coil from the busbar. The power supply mode utilizes the principle of electromagnetic induction, and the alternating current electric energy is induced from the high voltage bus through an ordinary ferromagnetic transformer, and then the power is supplied to the high voltage side circuit after rectification, filtering, and voltage regulation. (2) Power supply mode of high voltage capacitor divider. A high-voltage capacitor divider is connected between the high-voltage bus and the ground, and energy is directly obtained from the high-voltage bus. After rectification, filtering, and voltage regulation, the power is supplied to the high-voltage side circuit. (3) Battery energy supply mode. This is a way of using a battery to power an electronic circuit on the high potential side.
2 Electromagnetic compatibility design of electronic voltage transformer
Electronic transformers are generally installed on outdoor lines, and their working environment is harsh. The electronic circuits will be affected by various electromagnetic interferences from the external environment and the electronic transformer itself. These surge voltages or electrostatic discharge interferences will harm the electronic type. The safety of the transformer's equipment, so improving the electromagnetic compatibility (EMC) capability of the electronic transformer is an important step to ensure its safe and reliable operation in the power system site. The design of the anti-interference ability of the electronic transformer can only be based on the existing experience, minimize the adverse effects caused by electromagnetic interference, and reduce the harm to the safe operation of the power system.
Since the electronic circuit of the sensing element is at the high voltage end, the electromagnetic environment is complex, the external electromagnetic interference signal is relatively strong, and the interference source is relatively large. Therefore, in the anti-interference design adopted, the most commonly used means is to use the shielding technology to block or Reduce electromagnetic radiation to interfere with energy transfer. Shielding is to use electromagnetic or magnetically-conducting closed surfaces (such as metal boxes of iron or aluminum) to electromagnetically isolate the space on the inside and outside sides, and the electromagnetic energy transmitted from one side space to the other side is suppressed to a minimum. Amount to achieve the effect of attenuating external interference signals. Grounding is another important way to improve the electromagnetic compatibility of electronic devices. In the design of the electronic transformer, the floating power technology is adopted, and the anti-interference wiring of the signal processing is connected to a common shielding layer, and the distributed capacitance between the power supply line and the casing is minimized, so that the working power source can be operated in the electromagnetic interference effect. Synchronous floating with the potential of the casing greatly reduces the inrush current flowing through the power supply caused by interference, thereby increasing the ability to resist common mode interference. If the double-shielded cable is used for signal transmission, it can be grounded with a shielded cable on both sides of the cable; the outer shield is grounded on both sides, and the inner shield is grounded; the outer shield is grounded and the other side is passed through a capacitor. Three methods, such as grounding and grounding of the inner shield side, solve the EMC requirements of the substation cable. The suppression of interference to the working power supply is mainly achieved by the method of the power supply filter. At the same time, the power supply part is shielded to eliminate its radiation interference; in addition, the separation of the digital power supply from the analog power supply is also beneficial to the operation of the signal processing circuit.
3 Protection measures for electronic transformers
In the increasingly large and complex power system, the impact of direct lightning and inductive lightning, changes in the operation mode of the power system, frequent switching operations, sudden load changes, and short-circuit faults in the system occur frequently, resulting in transformers and secondary energy metering in the power system. The probability of overvoltage in the system is greatly increased. Since voltage-sensitive microelectronic chips, semiconductor components, and the like are used in the electronic transformer, the probability of damage is drastically increased under the impact voltage. As an indispensable part of the power system, the energy metering system will be inoperable due to overvoltage. Therefore, the secondary metering system overvoltage protection device must be used to prevent the PT secondary disconnection caused by the loss of the meter. The device is switched off to avoid the huge loss caused by the flaw of the metering system.
The overvoltage acting on the PT secondary metering system has induced lightning overvoltage, operating overvoltage, system short circuit fault overvoltage, resonant overvoltage and fault overvoltage. Among them, the induced overvoltage and the operating overvoltage are the most harmful to the overvoltage generated by the PT secondary system. This type of overvoltage has a high amplitude and short impact time, which can easily damage the PT secondary system metering equipment. Overvoltage protectors can be divided into switch type and voltage limit type.
The main components of the switch-type protector are the main components of the discharge tube voltage-limiting protector, which are transient suppression (TVS) diodes and zinc oxide varistor (MOV). The characteristic of the gas discharge tube is that the discharge is large, the work is normally in the off state, and there is no leakage current, but the discharge reaction is slow and the discharge time is long. The TVS diode has the fastest response, up to 10.125, but it has a weak surge capability and a long turn-off time. MOV has low residual pressure, no freewheeling, small action delay, good response of steep wave, large circulation capacity, strong ability to absorb overvoltage, and can be used as the main component of the protector voltage limit. According to the accuracy, stability, continuity and safe and reliable operation characteristics of the energy metering system, reasonable and economical protection equipment is adopted.
4 Resistor-capacitor voltage-divided electronic voltage transformer system design
It can be seen from the above that the RC capacitor is an excellent solution for voltage measurement in high voltage and ultra high voltage power networks. At the same time, as a new type of electronic transformer, its effective transmission of signals must be considered.
If the measuring instrument on the secondary side adopts the analog interface, the small power PT is connected to the sensing element to achieve electrical isolation, and then the integral link and the analog-to-digital conversion link are added on the secondary side of the PT. The main advantage of adopting this method is that the high-voltage side-resistance and voltage-dividing transformer acts as a passive component to transmit voltage, which simplifies the circuit of the sensing head portion, and the signal processing circuit is conveniently implemented on the low-voltage side. The disadvantage is that the front end is used as an analog signal output, the signal transmission circuit uses a copper wire, and the anti-interference performance in a strong electromagnetic environment is poor.
The rapid development of grid automation has made the development and application of digital primary equipment more and more important. Therefore, when considering the digital interface of the resistor-capacitor voltage divider type transformer, the signal processing should be performed on the high voltage side, and then the signal transmission is performed by the optical cable to the merging unit. The signal processing on the high voltage side includes filtering, integration, A/D conversion, etc. Under the existing conditions, an analog integrator should be used to realize the reduction of the differential signal. Direct A/D conversion is more mature and perfect than VFC conversion. The conversion accuracy is not affected by system harmonic component changes and frequency fluctuations. The sampling method is relatively reliable and is a general purpose suitable for measurement and protection control. Signal processing scheme.