Keywords: digital, measuring relay, protection

Design and Application of Digital Measurement Relay Based STM32F103R8T6

Jing Shenfeng1 Yao bo 2 Tang Jianjun2
(XianEngineeringDesignCo. Ltd, Chinacoal, Xian Shanxi 710054)
(Shanghai Acrel Co., Ltd., Jiading Shanghai 201801)

Abstract: The digital measurement relay which has many merit, such as exactitude measurement setting simple, many protect function, speed quick and so on, is the traditional electromagnetism, the static relay ideal substitution product. This article introduced in detail based on the STM32F103R8T6 digital measurement relay's overall project design. this Relay have many protect functions for protecting the objects which using in the errors, to make the objects not damaged in the state of errors, increase the reliability, decrease the loss.
Keyword: Digital, Measurement Relay, Protect

0 Introduction In electric power and industrial automation control systems, various types of relays are commonly used in situations where status monitoring is required, as a blocking action element or starting element for protection. At present, most of the relays used for such occasions are static relays, which are completely designed by analog circuits. The parameter setting and setting value settings are all adjusted by knobs. In this way, the setting accuracy is low, the error is large, and the protection function is single. There is a display device, the user can not know the current various electrical parameters from the relay, and can not network to achieve intelligent networking.
There is another type of multi-functional comprehensive protection relays, such as Nanjing Intelli's PA100 series integrated digital relays and ABB's 615 series relays. The name of this type of relay is relay, which is actually a multi-function relay protection device. The relay has powerful functions and integrates multiple functions such as protection, measurement, control, monitoring, and communication. It is a high-end power system automation hardware device. However, the use of such relays in many applications requiring simple applications has high use costs and waste of functions. And other disadvantages.
This article will introduce the design and application of a digital measuring relay (ASJ series digital measuring relay). In addition to the features of traditional static relays, this relay also has the characteristics of convenience, flexibility and intelligence. There are many protection functions, high sensitivity, flexible operation time setting, over-under mode and same-machine setting, real-time measurement and display of the current electrical parameter value, which is equivalent to adding the function of protection relay on the basis of intelligent electrical measuring instruments.

1 Hardware design of the relay The hardware circuit of the digital measuring relay includes the main CPU chip, power supply, signal acquisition circuit, human-computer interaction unit, RS485 communication interface and relay output interface (Figure 1).

Figure 1 hardware circuit block diagram

1.1 Main control CPU
The CPU of this measuring relay adopts ST's 32-bit processor STM32F103R8T6 based on ARM's latest Cortex-M3 architecture core. The clock frequency can be up to 72MHz. It has built-in 64K Flash, 20K RAM, 12-bit AD, and four 16-bit timers. , 3-way USART communication port and other resources, with extremely high cost performance.
1.2 Power
The power supply is a key part of whether a device can work normally, stably and reliably. The relay uses the common switching power supply module commonly used by the company. The input voltage of the power module is AC85V ~ 265V, the input frequency is 45Hz ~ 60Hz, and it has multi-channel isolated voltage output to meet the requirements of different functions for different power supply voltages. The output voltage is stable, the failure rate is small, and the output ripple is <1%. With overvoltage and overcurrent protection. The module has high stability, reliability and anti-interference ability after being used in actual field.
1.3 Signal acquisition circuit The signal acquisition circuit uses a transformer to isolate the input to isolate current, voltage and other power signals to improve the safety and reliability of the system. The sampled signal is amplified by the amplifier circuit for A / D conversion. figure 2.

Figure 2 Signal acquisition circuit

1.4 Human-computer interaction unit The human-computer interaction unit uses LED display and key input. The system uses a single row of four LED digital tubes to display various information. Users can set according to actual needs. The menu and parameters are displayed in the programming state. The digital tube display adopts dynamic scanning mode, and its driving circuit is composed of a 74HC595 and a triode.
1.5 RS485 communication interface The communication interface module adopts the common RS-485, Modbus RTU communication protocol, which can realize the functions of telemetry, remote control and remote communication, see Figure 3.

Figure 3 Schematic diagram of the communication circuit

1.6 Relay output interface The relay output interface (Figure 4) is the actuator of action. When a fault occurs, the relay will generate an action and send an alarm or trip signal.

Figure 4 Schematic diagram of relay output

2 Software design Because the measuring relay uses digital circuits, the core component uses a 32-bit microcontroller, the calculation speed is fast (clock frequency 72MHz), the protection algorithm is implemented by software, so the protection function derived from the same electrical parameter can be integrated In one (such as measuring three-phase current can achieve multiple protections such as overload, underload, unbalance, phase failure, phase sequence, etc.), unlike static relays, different protection functions require different analog circuits to achieve, resulting in a single static Relays often have only one protection function.
2.1 Programming Design The software design of this relay mainly includes various function subroutines such as calculation, protection, display, button, communication and so on. Among them, the calculation subroutine is mainly used for signal collection and calculation, real-time measurement of the electrical parameters of the protection object; the protection subroutine is mainly integrated with various protection algorithms, and compares various measured parameters with preset values, To determine whether there is a fault and protect it in time; the display and button subprograms are used for human-computer interaction; the communication subprogram is used to remotely transmit various parameters to the background control system through the communication interface.
Due to the large amount of program content, the main program flow (Figure 5) and the protection subroutine flow chart (Figure 6) are now given.

Figure 5 main program flow chart

Figure 6 Flow chart of protection subroutine

2.2 Oversampling is designed to be used as both a protective relay and a low-voltage electrical measuring instrument during the design of this product, so the measurement accuracy of this product is required to be high, but for cost considerations, the main control chip is used STM32F103R8T6 internal 12-bit AD, in order to achieve high-precision measurement, you need to use oversampling technology in sampling. According to Nyquist's theorem, for every additional bit of resolution, the signal must be oversampled at a rate of 4 times: fos = 4w × fs. Among them, w is the number of resolution bits you want to increase, fs is the initial sampling frequency requirement, and fos is the oversampling frequency. According to this formula, the sampling frequency is increased by 256 times during sampling, that is, the number of resolution bits is increased by 4 bits, and the resolution of 16 bits is reached. After adopting this method, the measurement accuracy of this product reaches 0.5 level, which can fully meet the requirements as a measuring instrument. Because the CPU speed is very fast, it will not cause the protection speed to be insufficient due to the increase in sampling points and the amount of calculation after oversampling.

3 Performance test
3.1 Measurement accuracy test (current type)

Picture 7

The test device is shown in Figure 7, and the signal generation source adopts the RTU verification device CL301V2-R of Shenzhen Kelu Company, with an accuracy level of 0.05. CL301V2-R outputs three-phase AC current to test the measurement accuracy of ASJ. The test results are shown in Table 1.

Table 1

From GB / T22264.1-200 "Installed Digital Display Electrical Measuring Instruments" Part 1: Definitions and General Requirements 5.2 Basic Error Calculation Method, combined with the measurement data in Table 1, the accuracy level can meet 0.5.
Calculated as follows:
The basic error of the meter should not exceed the absolute error â–³ of the measured value expressed by formula (1).

In the formula: Ux－The measured reading value;
Um-measured full-scale value;
a－The error coefficient related to the reading value;
b－The error coefficient related to the full scale value;

The following relationship should be satisfied in formula (1): a≥4b

3.2 Action error test (current type)
The action error of the relay is an important indicator. Therefore, a dedicated relay tester is required to test the action time error of the ASJ.
Here, "ZS-740 microcomputer relay protection calibrator" is used to test the operation error time of ASJ. The test results are shown in Table 2.

Table 2

4 Product features
The structure of ASJ series digital measuring relay adopts DIN35mm guide rail installation type and 96mm × 48mm appearance embedded installation type. This is a digital relay, real-time sampling of three-phase AC power, and the calculation method of true effective value, which achieves higher accuracy measurement in a wider frequency range. It integrates a variety of protection functions, and it can realize the input or exit of protection, alarm or trip by simple selection; high precision, low power consumption, fast action, high return coefficient and no jitter. This product integrates the function of low-voltage electrical measuring instruments and the function of measuring relays, and is an ideal alternative to traditional static relays. The product conforms to the relevant standards of GB / T 14598 measuring relays and the relevant standards of GB / T 22264 mounted digital display electrical measuring instruments.
According to the function, it can be divided into various types such as current, voltage, frequency, etc. The main technical indicators are shown in Table 3.

table 3

5 Application Case
ASJ series measuring relays have been widely used in various occasions because of their wide frequency measurement range (accurate measurement within 20Hz ～ 70Hz), high measurement accuracy and multiple protection functions.
Figure 8 is a diagram of the product applied to a diesel generator to protect the undervoltage and overvoltage of the diesel generator. Since the output voltage of the diesel generator is related to its speed, it is required that the device for measuring the output voltage of the diesel generator has a wide frequency measurement range. In this application case, the undervoltage point of the diesel generator is 230VAC (corresponding to 23Hz), rated 460VAC (corresponding to 50Hz), overvoltage point 530VAC (corresponding to 55Hz), and the measurement frequency range of the ASJ10-AV3 voltage type measuring relay used is 20Hz ~ 70Hz, within this frequency range, the voltage value can be accurately measured, which can fully meet Use requirements for this occasion. The working process is as follows: ASJ10-AV3 measures the output voltage of the diesel generator set, and compares this voltage with the preset undervoltage and overvoltage values. Once the measured value exceeds the limit, it will enter the fault delay, delay When the time is up but the fault state is not lifted, the alarm or trip relay will act, and the alarm signal will be issued or the diesel generator will be stopped directly, so as to protect the stable operation of the diesel generator.

Figure 8 Voltage type application diagram

Figure 9 is a diagram of the product applied to the motor to protect various faults that occur during the operation of the motor. In the operation of the motor, the common faults are overload and phase failure, and the protection of overload is divided into timing limit, inverse time limit, quick break and so on. In this application case, ASJ10-AI3 can provide multiple protection functions such as underload, overload (time limit, inverse time limit, quick break), unbalance, and break. The working process is as follows: ASJ10-AI3 measures the current value in the motor power supply circuit, and compares this current value with the preset value. Once the measured value exceeds the limit, it will enter the fault delay. If the fault state is not lifted, the alarm or trip relay will act. The alarm signal will be issued or the contactor KM will be disconnected to stop the motor, thus protecting the motor.

Figure 9 Current type application diagram

6 Conclusion
ASJ series digital measuring relay adopts advanced design scheme, which can provide a variety of protection methods for different use occasions. The product is stable and reliable, and is an ideal substitute for static analog relays.

The article comes from: "Modern Building Electric" 2010-11.

references
[1] Shanghai Ankerui Electric Co., Ltd., ASJ series measuring relay selection manual, 2009.
[2] China Electric Power Press, Electrical and Electronic Technology, 1999.
[3] Ren Zhicheng, Zhou Zhong, Principles and Application Guide of Electric Power Measurement Digital Instruments, China Electric Power Press, 2007

About the Author:

Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system

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