Design of a low voltage protection measurement and control device based on OMAP5910

The hardware and software structure and characteristics of the multi-core processor OMAP5910 are introduced, and a design scheme for a low-voltage protection measurement and control device with OMAP5910 as the core processor is proposed. The hardware and software design schemes of the protection measurement and control device are briefly described, and the design schematics of the A/D conversion circuit, digital input circuit and digital output circuit are given. The characteristics of the relay protection function are introduced. Due to the use of a high-performance hardware platform and an embedded real-time operating system, the device has the characteristics of complete functions, flexible protection configuration, reliable operation, convenient maintenance, and good expandability, which better meets the performance requirements of the low-voltage protection measurement and control device.
With the continuous improvement of the automation level of the power system, the trend of digitalization and intelligentization of relay protection measurement and control devices is becoming increasingly obvious, and it has the characteristics of diversified functions, rich communication interfaces, high reliability and high performance indicators. At present, the hardware platform of traditional low-voltage protection and control devices mostly uses a multi-CPU structure of ARM+DSP+FPGA. This structure can ensure the real-time data exchange and the reliability of the protection function, but there are problems such as data sharing, equipment interval expansion, clock signal synchronization, and high power consumption. In order to solve these problems, this paper proposes a design scheme for low-voltage protection and control devices with a multi-core processor OMAP5910 (with DSP and ARM cores integrated inside) as the control core, and achieved good results.

Overall structure of protection and control device

The multi-core processor OMAP5910 is an open multimedia application platform launched by TI. It integrates DSP processor and ARM processor on the chip. The DSP processor is based on the TMS320C55X core and provides 2 multiplication and accumulation (MAC) units, 1 40-bit arithmetic logic unit and 1 16-bit arithmetic logic unit. Since the DSP adopts a dual ALU structure, most instructions can run in parallel, its operating frequency reaches 150MHz, and the power consumption is lower. The ARM processor is the TI925T processor based on the ARM9 core, including a 16KByte instruction cache and an 8KByte data cache, a coprocessor, and the instruction length can be 16 bits or 32 bits. OMAP5910 has the advantages of high integration, strong hardware reliability and stability, strong data processing capability, and low power consumption.

 

Figure 1 Structural diagram of protection and control device

In order to facilitate maintenance, the protection measurement and control device adopts a modular structure in design, mainly including AC analog input module, digital input module, digital output module, button and display module, communication module, microcontroller module, etc. Its structural block diagram is shown in Figure 1. The AC analog input module includes voltage transformer, current transformer, signal conditioning circuit and A/D conversion circuit, which is used to convert AC analog signal into digital signal that can be processed by OMAP5910; the digital input module is used to collect common switch quantity signals such as load switch position signal, low voltage circuit breaker position signal, fuse blown signal, etc., and can also collect non-electric quantity signals such as heavy gas action tripping and light gas action alarm; the digital output module is mainly used for various protection devices. The export tripping, signal alarm and other functions; the button and display module is mainly used for human-computer interaction; the communication module is used to communicate with other intelligent devices and monitoring centers. The

DSP processor in OMAP5910 is the core of the protection measurement and control function, which is mainly responsible for the acquisition of AC analog and digital input signals, digital filtering, electrical quantity calculation, protection logic judgment, fault information processing, protection action export and other real-time tasks. The ARM processor in OMAP5910 is mainly responsible for processing non-real-time tasks such as human-computer interaction, GPS timing, and network communication. Since the DSP processor and the ARM processor are integrated in one chip, their power consumption is much lower than that of the hardware platform with a multi-CPU structure, and there is no problem of clock signal synchronization; the DSP processor and the ARM processor can realize data sharing through the 192K bytes of internal SRAM, and there is no data sharing and problem, and the entire hardware platform has flexible scalability, which better solves the problems existing in the multi-CPU hardware platform.

Main hardware circuit design

A/D conversion circuit The

AC analog signals collected by the low-voltage protection measurement and control device include three-phase measurement current, three-phase protection current, zero-sequence current and three-phase voltage [4]. The three-phase measurement current is sampled by a 5A/3.53V linear current transformer, the three-phase protection current signal is sampled by a 100A/7.07V current transformer, the zero-sequence current is sampled by a 20A/7.07V current transformer, and the three-phase voltage is sampled by a 220V/7.07V voltage transformer. A total of 10 A/D conversion channels are required.

 

Figure 2 TLC3578 interface circuit

The A/D conversion chip uses TLC3578 produced by TI, which is an 8-channel 14-bit serial analog-to-digital converter. It is powered by a single 5V analog power supply and a 3V~5V digital power supply. The analog input range is -10V~+10V, which can fully meet the design requirements of connecting multiple transformers at the same time. The interface circuit of TLC3578 is shown in Figure 2. The serial interface between TLC3578 and OMAP5910 mainly consists of four pins: chip select signal, clock signal SCLK, serial data input SDI and three-state serial data output SDO. The /EOC terminal is connected to the interrupt input terminal of OMAP5910. When the internal FIFO storage area of ​​TLC3578 is full, a corresponding external interrupt is generated, triggering the corresponding interrupt program to read the data.

Digital input circuit

The digital input circuit can not only collect common switch signals such as load switch position signal, fuse blown signal, low voltage circuit breaker position signal in the low voltage power supply system, but also collect non-electrical signals such as heavy gas tripping, light gas alarm, ultra-high temperature tripping, high temperature alarm, etc. of low voltage transformer. The device is equipped with 20 strong electric digital input interfaces and 4 programmable spare non-electrical input interfaces to facilitate the expansion of non-electrical functions. The digital input interface circuit is shown in Figure 3. DIIN is the digital input terminal, DICOM is the common end of the digital input circuit, DIOUT is the output end of the digital input, and the input AC signal at the DIIN end is converted into a digital signal output after rectification, filtering, and optocoupler.

 

Figure 3 Digital input circuit

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The digital output circuit adopts the form of start relay locking. The control signal of the start relay is controlled by the ARM core of OMAP5910, and the control signal of the output relay is controlled by the DSP core of OMAP5910. Only after the start relay is activated can the positive power supply of the output relay be opened, thereby realizing partial decoupling of the digital output control and avoiding misoperation of protection due to device damage. The digital output interface circuit is shown in Figure 4. When output is required, first set the DOENH of the start relay to a high level, DOENL to a low level, the photocoupler EL852 is turned on, the start relay is activated, and its normally open contact is closed, so that +24VE is connected to +24V; then set DOOUT to a low level, the photocoupler EL852 is turned on, the output relay is activated, and its normally open contact is closed, so that the tripping or alarm circuit is turned on.

 

Figure 4 Digital output circuit

The ARM core of the communication interface circuit

OMAP5910 is the main hub for information exchange. The device is equipped with two 10/100Mbit/s fiber optic Ethernet ports and two RS485 interfaces. The two Ethernet ports form a dual GOOSE network, which is responsible for receiving and parsing digital information such as tripping and switching from the process layer intelligent operation unit, and transmitting digital output information to the intelligent operation unit in real time, and realizing information sharing with the process layer equipment. The RS485 interface is mainly used for the debugging and maintenance of the device, and can also be used for data communication with other intelligent devices. The

Ethernet interface circuit is implemented using the Ethernet control chip RTL8019AS. RTL8019AS is a 10Mbit/s Ethernet controller produced by REALTEK, which supports 8-bit or 16-bit data bus, realizes all functions of the Ethernet media access layer (MAC) and physical layer (PHY), and is connected to the Ethernet through the RJ45 interface. The RS485 interface circuit is implemented using the RSM485D chip. The RSM485D is a dual-channel isolation transceiver module that integrates dual-channel power supply isolation, electrical isolation, RS485 interface chip and bus protector. It has excellent isolation characteristics and an isolation voltage of up to 2500VDC.

Flexible configuration of protection functions The protection

and measurement control device is equipped with a wealth of protection functions, including three-stage time-limited overcurrent protection with re-pressure locking, three-stage overload protection, inverse time-limited overcurrent protection, zero-sequence current protection, negative-sequence current protection, low voltage protection, overvoltage protection and PT disconnection alarm. According to the modular design concept, the protection and measurement control device divides different protection functions into independent modules. Each module has independent entry conditions and exit states, and each module is equipped with a control soft pressure plate. The protection function of the device can be configured by controlling the soft pressure plate to be put into or out. The setting value and exit mode (tripping or alarm) of each protection function can be configured through buttons or communication networks. This modular design makes the protection function highly readable and portable, and the collaborative relationship between modules is clear, which is conducive to improving the reliability of protection.

According to the configured protection function, protection setting value and export mode, the DSP core of OMAP5910 digitally processes the collected AC analog quantity for protection and compares it with the protection setting value. When the protection action conditions are met, it acts according to the configured export mode and passes the export information to the ARM core for LCD display, status indication and data communication. When the device is equipped with one or more protection functions, the relevant setting values ​​and event information of other unconfigured protection functions become invisible, and the relevant protection functions are not executed in the system program. Therefore, only the setting values ​​of the required protection functions need to be configured, which can minimize the number of setting values, simplify the user's setting value management, and reduce the possibility of errors.

Software Design

OMAP5910 is a highly integrated hardware and software application platform that supports multiple operating systems such as WinCE, EPOC, Nucleus, VxWorks and Linux. Since the VxWorks operating system has efficient task management functions, supports multi-task and multi-priority, supports priority preemption and round-robin scheduling mechanisms, and has extremely high real-time and reliability, it is very suitable for use in protection and measurement and control devices, which can improve the real-time performance of the device, the reliability of the protection software, and the efficiency of software development and maintenance.

Since the VxWorks operating system adopts multi-task and priority preemption mechanisms, the focus is placed on the division of tasks and interrupts and the implementation of task scheduling in programming. The system mainly includes three interrupts, one task scheduling and multiple tasks. The three interrupts include A/D sampling interrupts, timer interrupts and key input interrupts. The tasks include analog quantity calculation tasks, protection logic judgment tasks, protection function tasks, digital quantity control tasks, fault recording tasks, communication processing tasks, key management tasks, alarm functions (LCD display and indicator light indication) tasks and GPS time synchronization tasks. Real-time multi-task scheduling is the core of the entire system and the key to ensuring the rational and orderly execution of multiple tasks. During design, task scheduling is performed in the data sampling interrupt processing. The task scheduling block diagram is shown in Figure 5.

 

Figure 5 Task scheduling flow chart

Conclusion

This paper proposes a design scheme for low-voltage protection measurement and control device with OMAP5910 as the core processor. With the help of OMAP's powerful hardware platform and the software environment of the VxWorks operating system, the hardware structure of the entire device is more concise and optimized, effectively reducing the overall power consumption of the device, improving the efficiency of internal data exchange and the flexibility of software development, and improving the reliability and scalability of the device. At the same time, the device has flexible protection function configuration and protection export configuration functions, which simplifies the management and use of protection setting values ​​and facilitates use and maintenance.