Design Guide for High-Performance Power Line Monitoring and Relay Protection Systems Based on AD7606-EEWORLD

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With the continuous development of the global power grid, power line monitoring and relay protection products are constantly being updated and changing their design patterns. As the world's leading supplier of high-performance signal processing solutions, the series of high-performance ADCs launched by ADI have been leading the technical development route in this field: the first generation of power relay protection products all use analog switches and are designed with single-channel 16-bit ADCs (such as AD976 and 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 processes, especially its technical features 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 intelligent power grid management, the design of power line monitoring and protection products faces 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 costs caused by expensive discrete devices. As a key solution provider for power secondary equipment manufacturers, ADI has a deep understanding of the technical needs of global power equipment companies. 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 in developing secondary equipment in the era of smart grids.

Figure 1: ADI's high-performance ADC product roadmap.
AD7606 simplifies power line monitoring system design
The AD7606 family of devices uses a single 5V supply and supports true ±10V and ±5V bipolar signal inputs, with a sampling rate of up to 200ksps per channel. Multiple channels integrated into a single chip support the measurement of three-phase current, voltage, and neutral line in substation automation equipment. The synchronous sampling function allows the phase information to be retained while sampling bipolar voltage and current over a wide dynamic range.

All 8 channels of the AD7606 family can achieve a sampling rate of up to 200 kSPS. It has built-in low-noise, high-impedance inputs and signal conditioning amplifiers that can handle input frequencies up to 22 kHz. The AD7606 has a signal-to-noise ratio (SNR) of up to 90dB, and the optional on-chip digital filter can further improve the SNR performance, reduce bit errors, spread the spectrum, and improve anti-aliasing suppression. The conversion process and data acquisition are controlled using the CONVST signal and internal oscillator. Through the two CONVST pins, all eight analog inputs or two groups of four analog input channels can be sampled simultaneously to take into account phase differences between transformers.

Figure 2: AD7606 further simplifies circuit design (the upper and lower figures are circuit schematics using AD7656 and AD7606, respectively).

The signal conditioning circuit inside the AD7606 already includes a low-noise, high-input impedance signal conditioning circuit. Its equivalent input impedance is completely independent of the sampling rate and is fixed at 1MΩ. At the same time, the input end integrates a filter with 40 dB anti-aliasing suppression characteristics, which simplifies the front-end design and no longer requires external drive and filtering circuits. Therefore, the signal output by the secondary transformer can be directly connected to the AD7606 without being buffered by an op amp. The AD7606 integrates a 2.5V bandgap voltage reference and a reference buffer circuit. In the design application, the built-in reference or external reference can be selected according to the system requirements. In the design of multiple ADCs, if high absolute accuracy is required, an 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. It is recommended to use the ADR421B with an initial accuracy of 0.04% and a temperature coefficient of 3ppm/℃. If the values between multiple ADC channels need to match, the first AD7606 can be set to work in the built-in reference mode, and the other AD7606 can be set to the external reference mode, and then the built-in reference output of the first AD7606 can be supplied to the other AD7606. In this way, the matching of the data between multiple AD7606 channels can be guaranteed without adding an external reference.

Figure 3: The outstanding performance advantages of AD7606 make your system design simpler.

The AD7606's low operating power consumption of 100mW and standby power consumption of only 25mW maintains ADI's advantage in ADC low-power technology. Especially when there are several multi-channel ADCs on a circuit board (some systems require up to hundreds of ADC channels on a circuit board), power consumption is an important consideration. This low-power feature is one of the key factors in simplifying system thermal design and improving system reliability.

The AD7606 series includes three synchronous sampling ADC devices: 8-channel, 6-channel, and 4-channel. For multi-channel system applications, designers can use flexible design combinations of 8+4 and 8+6 channels, respectively, to take advantage of the high performance of ADI's new ADC series products while ensuring low cost (the four-channel and six-channel AD7606-4 and AD7606-6 are 19% and 34% lower than the dual 8-channel AD7606 respectively). Moreover, these devices are pin-compatible, and the same circuit design can be applied to a variety of system configurations with different channel counts.
Circuit Design Recommendations
Most power line monitoring systems use multiple AD7606 devices to achieve multi-channel synchronous sampling. To ensure good performance matching between devices, these devices must be arranged symmetrically. The AVCC voltage plane is arranged along the right side of the two devices, and the VDRIVE power supply trace is arranged along the left side of the two AD7606 devices. The reference voltage chip ADR421 is located between the two AD7606 devices, and the reference voltage trace is arranged upward to pin 42 of U2 and downward to pin 42 of U1. Use a solid ground plane. These symmetrical layout principles apply to systems that contain more than two AD7606 devices. The AD7606 devices can be placed in a north-south orientation with the reference voltage in the middle of the device and the reference voltage traces in a north-south orientation.

Good decoupling is very important for the overall performance of the power line monitoring system. Proper decoupling configuration can reduce the power supply impedance of the AD7606 and the amplitude of its power supply spikes. The decoupling capacitors of the REFIN/REFOUT pins and the REFCAPA and REFCAPB pins are important capacitors that are critical to performance and should be as close to the corresponding AD7606 pins 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 and only requires 9 low-value ceramic capacitors, including 2 10uF, 2 1uF, and 5 0.1uF. In high-channel systems, good performance matching between channels and devices can greatly simplify the calibration process. The symmetrical layout of the AD7606 device, analog input channel, and decoupling capacitors helps to match the performance between multiple devices.

Figure 4: Recommended circuit layout for multichannel applications.

(For more detailed design suggestions, please refer to "Layout Considerations for Scalable Multi-channel Synchronous Sampling Data Acquisition System Based on 16-bit 8-channel DAS AD7606")
Complete Solution of AD7606+BF518
In order to adapt to the rapid development of the global power industry, the complexity of power systems and the improvement of protection requirements, current relay protection products have powerful data processing and communication capabilities in addition to the basic test functions of traditional relay protection equipment, and have high requirements for reliability, selectivity, sensitivity and speed. Moreover, with the gradual implementation of the IEC61580 specification, higher requirements will be placed on the processing power of the processor, including higher communication capabilities, storage capabilities, intelligent features, etc. ADI and the third party Beijing Yiqi jointly developed a complete power line monitoring solution based on AD7606 and Blackfin processor BF518.

Figure 5: Demo of a 16-channel power line monitoring system implemented by BF518 and dual AD7606.

Blackfin processor is the core flagship product of ADI. Its powerful processing capability and convergent architecture make it unnecessary to add a separate communication processor in the design of relay protection products. It can help customers realize the solution of bus not leaving the chip, greatly improve the anti-interference ability of the system, and easily meet the requirements of the international IEC-6100-4 standard. BF518 is a convergent processor that has been widely used in new products of power equipment around the world. The processor has built-in Ethernet 10/100 MAC with media independent interface and simplified media independent interface. It also introduced the built-in PTP_TSYNC engine in the Blackfin processor for the first time, realizing the hardware-supported IEEE1588 (Precision Clock Synchronization Protocol for Network Measurement and Control Systems) clock synchronization, which is fully compatible with the IEEE1588 version2 standard. The networking and intelligentization of the power industry has given rise to the networking of relay protection devices, and the introduction of the IEC61850 protocol, an international standard for substation communication networks and systems, reflects this development trend. The BF518 processor that supports the IEEE1588 protocol will definitely help power equipment companies take a more active position in this development trend.

The solution showed excellent performance in the test. Since AD7606 provides oversampling, anti-aliasing filter and digital filtering, the sampling accuracy of this solution can reach up to 19 bits in practical applications. In high voltage protection applications, AD7606 has more obvious performance advantages and has a higher single-channel cost performance.