Power Quality Monitoring Part 2: Design Considerations for Standards-Compliant Power Quality Instruments

Summary

This article describes how to use a ready-to-use platform to accelerate development and efficiently design standards-compliant power quality (PQ) measurement instruments. The article explores in detail different solutions for designing Class A and Class S energy meters, including a new Class S power quality measurement integrated solution that can significantly shorten the development time and cost of power quality monitoring products. The article "Power Quality Monitoring Part 1: The Importance of Standard-Compliant Power Quality Measurements" explains in detail the power quality IEC standards and their parameters.

Challenges in implementing power quality solutions

Figure 1 shows the basic components of a meter used to measure power quality. First, the current and voltage sensors must support the operating range of the instrument, and the input signal should be able to adjust to the dynamic range of the analog-to-digital converter (ADC) input. Traditional sensors are the number one source of inaccurate measurements; therefore, correct sensor selection is critical. The signal is then transmitted to the ADC; its various characteristics such as offset, gain, and nonlinearity errors become a second source of inaccurate measurements. Proper selection of the ADC to perform this function is a major difficulty when designing power quality instruments. Finally, a series of signal processing algorithms must be developed to obtain electrical and power quality measurements from the input signal.

Figure 1. Main components of a power quality measurement instrument.

Voltage and current sensors

Depending on the location and application of the power quality instrument, the nominal supply voltage (UNOM), nominal current (INOM) and frequency will also be different. In addition to the nominal value measured by the instrument, the IEC 61000-4-7 standard requires the power quality measuring instrument to achieve the accuracy shown in Table 1; therefore, when selecting a sensor, you must ensure that the instrument can achieve the required measurement after using the sensor Accuracy.

Table 1. Current, voltage, and energy measurement accuracy requirements specified by the IEC 61000-4-7 standard

The IEC61000-4-71 standard recommends using these nominal voltages (UNOM) and nominal currents (INOM) when designing input circuits:

►For 50 Hz systems: 66 V, 115 V, 230 V, 400 V, 690 V

►For 60 Hz systems: 69 V, 120 V, 240 V, 277 V, 347 V, 480 V, 600 V

►0.1A, 0.2 A, 0.5 A, 1 A, 2 A, 5 A, 10 A, 20 A, 50 A, 100 A

Furthermore, the characteristics and accuracy of the sensors used to measure voltage and current must remain unchanged when 1.2× UNOM and INOM are continuously applied. Applying four times the nominal voltage signal or 1 kV rms (whichever is lower) to the instrument for 1 second shall not cause any damage. Likewise, applying 10× INOM to the instrument for 1 second should not cause any damage.

Analog to Digital Converter

Although the IEC 61000-4-30 standard does not explicitly specify a minimum sampling rate requirement, the ADC's sampling rate must be sufficient to measure some oscillations and fast power quality phenomena. Insufficient sampling rates can result in incorrect classification of power quality events or failure to detect events. The IEC 61000-4-30 standard states that a meter’s voltage and current sensors should be able to support up to 9 kHz. Therefore, the sampling frequency of the ADC must be selected according to the signal analysis rules to measure energy spectrum components up to and including 9 kHz. Figure 2 shows the consequences of insufficient sampling rate. The waveform in the upper left contains 64 samples every 10 cycles (200 ms), and the waveform in the upper right contains 1024 samples every 10 cycles. As shown in Figure 2, the upper left image shows a voltage sag event, while the upper right image shows that this sag is caused by a transient.

The IEC standard applies to single-phase and three-phase systems; therefore, the ADC selected must be able to sample the specified number of voltage and current channels simultaneously. The ability to perform measurements on all voltage and current channels on the meter simultaneously allows all parameters to be checked and triggered immediately when a power quality event occurs.

Digital Signal Processing (DSP)

Although selecting sensors and ADCs for power quality measurement applications requires a comprehensive engineering effort, there is no doubt that developing algorithms to process the ADC raw measurement data is the most time- and resource-consuming task in the power quality measurement process. To build a standard-compliant meter, the correct DSP hardware must be selected, and algorithms that calculate power quality parameters based on waveform samples must be developed and properly tested. This standard not only requires calculations, but also requires integration based on different times. The time accuracy of category A is less than ±1 second/24 hours, and the time accuracy of category S is less than ±5 seconds/24 hours. These algorithms must perform harmonic analysis. Additionally, power quality parameters rely on Fast Fourier Transform (FFT) analysis (harmonics, interharmonics, supply signal voltages, imbalances), which is difficult to implement. FFT analysis requires that the waveform be sampled at a minimum rate of 1024 samples every 200 ms (10 cycles). To resample the ADC's raw waveform at a specified rate, great care must be taken to avoid harmonic distortion and aliasing.

After the algorithm was developed, IEC standards required the instrument to pass more than 400 tests in order to be fully certified.

The block diagram shown in Figure 3 shows the most relevant functions required by a DSP system for power quality measurements.

Figure 2. ADC sampling rate affects power quality measurements.

Figure 3. Block diagram: Related functions of the DSP power quality system.

Power Quality Measurement Solutions from Analog Devices

Multi-channel simultaneous sampling ADC, compliant with IEC 61000-4-30 Class A standard

When developing Class A PQ instruments, we need to consider accuracy, channel number, and sampling rate requirements, so we recommend using the AD777x and AD7606x series products for ADC conversion of the signal chain/system. Note that these solutions only provide raw digitized data from the input signal. A DSP system must be developed to obtain certified PQ measurement results.

AD777x Series Σ-Δ ADC

The AD777x is an 8-channel, 24-bit simultaneous sampling ADC family of devices. Eight complete Σ-Δ ADCs are integrated on-chip, providing 16 kSPS/32 kSPS/128 kSPS sampling rates. The AD777x offers low input current, allowing direct connection of sensors. Each input channel has a programmable gain stage with gains of 1, 2, 4, and 8 that maps low-amplitude sensor outputs to the full-scale ADC input range, maximizing the dynamic range of the signal chain. The AD777x supports VREF voltages from 1 V to 3.6 V and has an analog input range of 0 V to 2.5 V or ±1.25 V. The analog inputs can be configured to accept true differential, pseudo-differential, or single-ended signals to match different sensor output configurations. The sampling rate converter can be used to provide fine resolution control of the AD7770 and can also be used in applications where ODR resolution is required to keep the sampling frequency following and maintain coherence when the line frequency changes as much as 0.01 Hz. The AD777x also provides a 5 kHz large signal input bandwidth (10 kHz for the AD7771). The data output and SPI communication interface provided through SPI can also be configured to output Σ-Δ ADC conversion data. Its temperature range is –40°C to +105°C, up to +125°C when operating from a 3.3 V or ±1.65 V supply.

Figure 4 shows a typical 3-phase application system block diagram of the AD777x series ADC used by PQ Instruments, using a current transformer as the current sensor and a resistor divider as the voltage sensor.

AD7606x series 16/18-bit ADC data acquisition system

The AD7606x is a family of 8-channel 16/18-bit simultaneous sampling analog-to-digital data acquisition systems (DAS). Each channel includes analog input clamp protection, programmable gain amplifier (PGA), low-pass filter, 16/18-bit successive approximation (SAR) ADC. The AD7606x also includes built-in flexible digital filters, a low-drift 2.5 V precision reference, and a reference buffer to drive the ADC and flexible parallel and serial interfaces.

The AD7606B operates from a single 5 V supply, supports ±10 V, ±5 V, and ±2.5 V true bipolar input ranges, and can sample at a throughput rate of 800 kSPS (AD7606B)/1 MSPS (AD7606C) on all channels. Input clamp protection tolerates different voltage inputs and they are user-selectable analog input ranges (±20 V, ±12.5 V, ±10 V, ±5 V, and ±2.5 V). The AD7606x operates from a single 5 V analog supply. It operates from a single supply and features on-chip filtering and high input impedance, eliminating the need for external drive op amps that require bipolar supplies.

In software mode, the following advanced features are available:

►Additional oversampling (OS) option, up to OS × 256

►System gain, system offset and system phase calibration per channel

►Analog input open circuit detector

►Multiplexer for diagnostics

►Monitoring functions: SPI invalid read/write, cyclic redundancy check (CRC), overvoltage and undervoltage events, busy card monitoring and reset detection

Figure 4 shows a system block diagram of a typical 3-phase application of the AD7606x family of ADCs for power quality instrumentation, using a current transformer as the current sensor and a resistor divider as the voltage sensor.

Figure 4. System block diagram of a power quality 3-phase application for the AD777X and AD7606x series ADCs.

Analog Devices’ pre-certified IEC Class S power quality solutions

The ADE9430 is a high-precision, fully integrated multi-phase energy metering IC that, combined with the ADSW-PQ-CLS software library running on the host microcontroller, forms a complete solution compliant with the IEC 61000-4-30 Class S standard. Through integration, the development time of PQ monitoring products is greatly shortened and the cost is reduced. The ADE9430 + ADSW-PQ-CLS solution tightly integrates the acquisition engine and calculation engine, simplifying the implementation and certification of power and PQ monitoring systems. Figure 5 shows the system block diagram of a 3-phase application of the ADE9430 + ADSW-PQ-CLS solution for power quality instrumentation, using a current transformer as the current sensor and a resistor divider as the voltage sensor.