Application and Analysis of Intelligent Current Transmitter in Thermal Power Plant
Reading guide： There is no change in the output, while the conventional current transmitter has only a single piece of information, so when the input changes, the output of the transmitter fluctuates greatly. After comparison, it is found that the output of the intelligent current transmitter is higher than that of the conventional current transmitter. It is more stable and reliable, has the functions of time synchronization, communication, and wave recording, and is more suitable for DCS and DEH control systems of power plants.

## Introduction

At present, the generator active current transmitters used in most thermal power plant generator sets are conventional transmitters, which convert the measured current parameters (such as current, voltage, current, frequency, current factor, etc.) into DC current, DC The voltage is isolated and the output analog signal or digital signal is sent to the DCS for the control and monitoring of the steam turbine and the generator by the DCS system. Faults and saturation of measuring current transformers will cause the output of the transmitter to be distorted and cannot correctly reflect the real state of the primary electrical system. On the one hand, it will cause the operator to misjudge the operating equipment.

The adjustment of the turbine unit may cause misadjustment of the steam turbine unit, resulting in protection action, the shutdown of the generator set, and serious losses. The active current of the machine adopts an intelligent current transmitter. In the case of abnormality in the secondary circuit of the current transformer, by comparing the output of the conventional current transmitter and the smart current transmitter, it is found that the output of the smart current transmitter is stable and has no fluctuations, which effectively solves the problem of current transformer or voltage The abnormality of the secondary circuit of the transformer leads to the problem of output distortion.

## Working principle of the ordinary active current transmitter

The current transmitter is a method that converts the measured current (AC voltage, current, active current, reactive current, active energy, reactive energy, frequency, phase, current factor, DC voltage, current, etc.) into linear proportional DC Measuring instruments for current or voltage output (energy pulse output), active current transmitters are three-phase AC input signals passed through a three-phase isolation sampling circuit to form a common ground tracking voltage signal of three-phase current and three-phase voltage signals. Under the control of the single-chip microcomputer, the A/D converter performs multi-point synchronous sampling on it, and the sampled data is calculated by the DSP device according to the electrical principle. The digital signal representing the three-phase current is converted into an analog quantity, which is amplified and calibrated by the calibration amplifier to form a DC voltage output VZ; the VZ output forms a 4-20mA DC current output after a dedicated thick-film integrated V/I converter.

## Working principle of the intelligent current transmitter

The intelligent transmission device is connected to the generator terminal voltage and current (protection level and measurement level) for sampling and uses the full-cycle Fourier algorithm to calculate the active current. Normally, the measurement level CT is automatically selected for normal operation. When the current of the protection level CT is greater than 1.1 times the rated current or when a CT disconnection occurs, the current calculation automatically switches to the protection level CT current, taking into account both normal operation and accurate measurement under abnormal conditions. , Accurate measurement of current, the device calculates the active current of the generator and outputs it with a current of 4~20mA, which is used by the generator current automatic adjustment system and DCS system. The generator's active current automatic regulation system and DCS system reliability are improved.

## Comparison and Analysis of Conventional Active current Transmitter and Intelligent Active current Transmitter

The operator of a power plant found that the active current of the generator received by the DCS system became smaller, causing the steam turbine valve to be opened larger, and a current imbalance occurred. However, the active current of the generator received by the DEH did not fluctuate. By calling the DCS The historical curve (as shown in Figure 1) found that the secondary current of phase B of the current transformer used for generator measurement suddenly and intermittently decreased, causing the active current of the generator to decrease intermittently. Further inspection of the drawings found that the active current sent to the DCS generator was used The conventional active current transmitter is used for DCS coordination operation; while the active current of the generator set to DEH uses the value of intelligent active current transmitter for DEH current load unbalance protection.

Figure 1 DCS historical trend

Since the conventional active current transmitter is only connected to a set of generators measuring current transformers when the B-phase secondary current fluctuates, the output value of the transmitter fluctuates accordingly; further, call the wave recording in the intelligent transmitter As shown in Figure 2, the purple curve is the current curve calculated using the measured current, and the green curve is the output current curve after the smart transmitter is switched to the protection current. It is found that the smart transmitter is connected to the measured current and Protection current, when the B-phase measurement current decreases, the intelligent transmitter switches the measurement current to the protection current instantaneously, which realizes no fluctuation in the active current of the generator and improves the reliability of the equipment.

Fig. 2 The oscillogram of the current switching of the intelligent transmitter

## Summary

The improvement of the intelligence of electronic products provides a solid foundation for the application of intelligent current transmitters. In some important application fields, especially in the control system of power plants, traditional current transmitters are no longer competent for special applications. For applications under working conditions, through the analysis of a field case, it is not difficult to find that the intelligent transmitter has obvious advantages over conventional active current transmitters, not only in the case of transient faults in the system and abnormalities in the secondary circuit of the current Guaranteed operation under certain conditions, and can record the accident data, which is convenient for post-accident analysis. However, due to the reason that the smart current transmitter is more expensive than the conventional current transmitter, it has not been well promoted and applied. As power plant Production personnel, in order to ensure the safe operation of the generator set, intelligent current transmitters should be installed and used in important control systems such as DCS and DEH to ensure that the control system can be controlled in the event of transient faults in the system and abnormalities in the secondary circuit of the current The system can work safely and stably.