Design Guide for relay protection system of the ho

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High performance power line monitoring and relay protection system design guide

with the continuous development of global electricity, power line monitoring and relay protection products are constantly upgrading and changing the design mode. As the world's leading supplier of high-performance signal processing solutions, the series of high-performance ADC launched by Adi company has always led the technical development route in this field: the first generation of power relay protection products adopt analog switches and are designed with single channel 16 bit ADC (such as ad976, AD574); Later, the second generation of relay protection products using 16 bit AD7656 and 14 bit AD7865 with analog switches appeared. AD7656 and AD7865 still have very successful application cases in many current power relay protection products; With the update of technology and the improvement of product technology, especially its technical characteristics such as ± 10V bipolar multi-channel synchronous input, AD7656 has become the mainstream choice of the previous generation of power relay protection. At present, this product still plays an important role in a large number of power monitoring and protection equipment

with the development trend of electric intelligent management, the design of power line monitoring and protection products is facing more and more challenges. Designers of multi-channel current and voltage monitoring systems need to deal with a series of complex design challenges, such as dual power supplies, limited analog input range, low analog input impedance, and high cost caused by the use of expensive discrete devices. As a key solution provider for power secondary equipment manufacturers, ADI company has a deep understanding of the technical needs of global power equipment enterprises. Based on the successful application experience of AD7656, it has successfully launched the 16 bit 8-channel synchronous sampling ad7606 series again to help customers better cope with the technical challenges faced by the development of secondary equipment in the era of smart electricity

Figure 1: ADI's high-performance ADC product development roadmap

ad7606 simplifies the design of power line monitoring system

ad7606 series devices use a single 5V power supply, and support true ± 10V and ± 5V bipolar signal input. The sampling rate of each channel can reach 200ksps. Multiple channels integrated in a single chip can support the measurement of three-phase current, voltage and zero line in substation automation equipment. The synchronous sampling function allows the phase information to be retained, and the bipolar voltage and current can be sampled in a wide dynamic range

all 8 channels of the ad7606 series can achieve a maximum sampling rate of 200 ksps. It has a built-in low-noise, high impedance input and signal adjustment amplifier, which can handle input frequencies up to 22 kHz. The signal-to-noise ratio (SNR) of the ad7606 is as high as 90dB. Choosing an on-chip digital filter can further improve SNR performance, reduce bit error, spread spectrum and improve anti aliasing suppression. The conversion process and data acquisition are controlled by convst signal and internal oscillator. Through two convst pins, all eight analog inputs or two groups of analog input channels (four analog inputs form a group) can be sampled at the same time to take into account the phase difference between transformers

figure 2:ad7606 further simplifies the circuit design (the upper and lower figures are the circuit diagrams using AD7656 and ad7606 respectively)

ad7606's internal signal conditioning circuit has included a signal conditioning circuit with low noise and high input impedance. Its equivalent input impedance is completely independent of the sampling rate and is fixed at 1m Ω. At the same time, a filter with 40 dB anti aliasing and suppression characteristics is integrated at the input, which simplifies the front-end design and eliminates the need for external drive and filter circuits. Therefore, the signal output from the secondary transformer can be directly connected to the ad7606 without being buffered by the operational amplifier. The ad7606 internally integrates a 2.5V bandgap voltage reference and a reference buffer circuit. In the design and application, the built-in reference or external reference can be selected according to the system requirements. In the design of multiple ADC, if high absolute accuracy is required, the external reference with high initial accuracy and low temperature coefficient should be used to eliminate the error caused by the difference between the built-in references of different devices. Adr421b with initial accuracy of 0.04% and temperature coefficient of 3ppm/℃ is recommended. If you need the value matching between multiple ADC channels, you can set the first ad7606 to work in the built-in reference mode, and the rest of the ad7606 to the external reference mode, and then supply the rest of the ad7606 through the built-in reference output of the first longmang Baili. Thus, the data matching between multiple ad7606 channels can be guaranteed without external benchmarks

Figure 3: the outstanding performance advantages of the ad7606 make your system design simpler

and the operating power of the ad7606 as low as 100MW maintains ADI's advantages in ADC low-power technology by calculating the maximum zigzag stress consumption at the cross section of the load and the standby power consumption of only 25MW, especially when there are several multi-channel ADCs on a circuit board (some system power losses plus up to hundreds of ADC channels on a circuit board), Power consumption is an important consideration. This low-power characteristic is one of the key factors to simplify the system thermal design and improve the system reliability

ad7606 series includes 8-Channel, 6-channel and 4-channel synchronous sampling ADC devices. For applications requiring multi-channel systems, designers can adopt flexible design combinations of 8+4 and 8+6 channels respectively to ensure low cost of the scheme while taking advantage of the high performance of ADI's new ADC series products (the cost of four channel and six channel AD and ad is 19% and 34% lower than that of dual 8-channel ad7606, respectively). Moreover, the pins of these devices are compatible, and the same circuit design can be applied to a variety of system configurations with different channel numbers

circuit design suggestions

most power line monitoring systems will use multiple ad7606 devices to achieve multi-channel synchronous sampling. In order to ensure a good performance match between devices, these devices must adopt a symmetrical layout. The avcc voltage plane is arranged along the right side of the two devices, and the vdrive power line is arranged along the left side of the two ad7606 devices. The reference voltage chip adr421 is located between two ad7606 devices. The reference voltage is routed up to pin 42 of U2 and down to pin 42 of U1. Use solid ground plane. These symmetrical layout principles apply to systems with more than two ad7606 devices. The ad7606 device can be placed along the north-south direction, the reference voltage is located in the middle of the device, and the reference voltage is routed along the north-south direction

good decoupling is very important for the overall performance of the power line monitoring system. Proper decoupling configuration can reduce the power impedance and power peak amplitude of the ad7606. The decoupling capacitance of the refin/refout pin and the refcapa, refcapa pins is an important capacitance related to performance, which should be as close to the corresponding ad7606 pin as possible. If possible, these capacitors should be placed on the same side of the circuit board as the ad7606 device. The decoupling design of the ad7606 is very simple, requiring only 9 low value ceramic capacitors, including 2 10uF, 2 1uF, and 5 0.1uF. In high channel system, good performance matching between channels and devices can greatly simplify the calibration process. The symmetrical layout of ad7606 devices, analog input channels and decoupling capacitors helps to match the performance of multiple devices

Figure 4: recommended circuit layout in multi-channel applications

(for more detailed design suggestions, see "layout considerations of scalable multi-channel synchronous sampling data acquisition system based on 16 bit 8-channel Das ad7606")

ad7606+bf518 complete solution

to adapt to the rapid development of the global power industry, the complexity of power systems and the improvement of protection requirements, In addition to the basic testing functions of traditional relay protection equipment, current relay protection products also have strong data processing and communication capabilities, and have high requirements for reliability, selection of Mr. Gu Yongtao, President of China Hengrui Co., Ltd. (left) and Mr. Gu Zhiqiang, director of the Investment Promotion Bureau of Changshu National High tech Industrial Development Zone (right), who jointly signed the settlement agreement in the presence of Jiabin. Moreover, with the gradual implementation of iec61580 specification, higher requirements will be put forward for the processing capacity of the processor, including higher communication capacity, storage capacity, intelligent characteristics and so on. ADI and the third party Beijing Yiqi jointly developed bf518 based on ad7606 and Blackfin Processor (complete power line monitoring solution realized.

Figure 5: the 16 channel power line monitoring system demo implemented by bf518 and dual ad7606

Blackfin Processor is the core flagship product of ADI company. Its powerful processing capacity and converging architecture make it unnecessary to add a separate communication processor in the design of relay protection products, which can help customers realize the scheme that the bus does not leave the chip, greatly improve the anti-interference ability of the system, and make it easy to be full Meet the requirements of international IEC standards. Bf518 is a converged processor that has been widely used in new products of power equipment all over the world. The processor has built-in Ethernet 10/100 Mac with media independent interface and simplified media independent interface, and has introduced built-in PTP into Blackfin Processor for the first time_ Tsync engine realizes the hardware supported IEEE1588 (precision clock synchronization protocol for network measurement and control system) clock synchronization, which is fully compatible with IEEE1588 version2 standard. The networking and intellectualization of the power industry have expedited the networking of relay protection devices, and the introduction of the international standard IEC61850 protocol for substation communication networks and systems reflects this development trend. The bf518 processor supporting IEEE1588 protocol will help power equipment enterprises occupy a more active position in this development trend

this scheme has shown excellent performance in the test. Because ad7606 provides functional characteristics such as oversampling, anti aliasing filter and digital filter, the sampling accuracy of this scheme can reach 19 bits at most in practical application. In the application of high-voltage protection, ad7606 has more obvious performance advantages and higher single channel cost performance


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