For electrical systems to function in their intended manner without significant loss of performance or life, they require a supply of electricity that is of good quality. Good quality electrical power has the following characteristics:
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The term 'clean power' is used to describe electricity that is considered to be of good quality (see above) with particular reference to a very low harmonic content. Therefore, the term 'dirty power' is used to describe electricity that is considered to be of low quality (opposite to the above) with particular reference to a very high harmonic content.
In Australia, our electricity is supplied in alternating current at a frequency of 50Hz. In alternating current (AC) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction. 50Hz means that the AC has a frequency of 50 cycles per second. This is also known as the 'fundamental' frequency. AC is the form in which electricity is delivered to businesses and residences. Most electrical devices (such as motors) need clean electricity to function properly. This means that the electrical supply needs to be a clear sinusoidal wave.
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Yes! This is the best way to eliminate harmonic related problems.
Harmonics mitigation is most cost-effective when 'designed in'. Mitigation in existing installations is possible but more than likely to cost significantly more without the performance of an 'ab initio' approach.
Good reasons for designing-in harmonics mitigation relate to economy, performance and reliability. All three of the following reasons have dollar implications.
Harmonic mitigation solutions should be tailored to the type of application. In many ways each installation is unique but broad guidelines apply.
The three-phase loads are generally HVAC, moving walkways and escalators, and elevators. HVAC plant generates significant harmonics.
Single-phase loads usually provide the highest aggregate harmonic load. Primary sources are IT equipment, and lighting ballasts. Load balancing for single-phase circuits is an important design consideration when there is high harmonic content. Two-phase loads are less frequent but might be reserved for some commercial kitchens.
Changes in tenancies, and expansion of facilities (more offices, shops, etc) can overload the reticulation because of increases in harmonic content. The capacity of a sub-station transformer can easily be surpassed unless harmonic mitigation has been installed.
Many loads are three-phase, three-wire. Sub-station transformers are likely for large consumers connected to medium voltage distribution and plant expansion can challenge the sub-station beyond capacity.
Single-phase loads in aggregate are likely to be less than three-phase loads. Careful consideration of circuit and load mitigation will provide high power quality (eg: mitigation of negative sequence harmonics, commutation notching from converters) and allow control gear (PLCs, micro-controllers, industrial bus systems) to operate from clean supply.
Power factor is often low or variable due to loading and unloading of motors. Consideration of power factor control in combination with harmonic mitigation is important and best done at the design phase.
Harmonics mitigation adds cost to electrical installations and for that reason is often ignored. Sometimes a ‘suck it and see’ policy is adopted.
Harmonics are not just an internal problem. They are exported into the supply system in spite of interposing transformers—even the ubiquitous third harmonic, since installations are never phase-balanced.
If the supply authority complains about the degree of voltage distortion at the point of common coupling (pcc) harmonic filtering might be specified. Note that the relevant standard for harmonic voltage distortion in Australia is AS/NZS .3.6. Supply authorities can insist on their own maximum voltage distortion levels.
As energy regulators look more closely at 'gold-plating' of poles and wires, often defended on the basis of having to cater for high demand intervals, consumers can expect considerable additional charges for poor power factor and/or high apparent power demand. Flattening demand peaks is the single most important way of achieving cost-effective distribution systems.
Demand is measured in various ways by:
There can also be combination of the above demand parameters and power factor, this normally being the displacement power factor. Harmonics severely lower power factor and therefore increase the apparent power.
Consumers who buy electrical energy at medium rather than low voltage will have a sub-station transformer which may or may not owned by them but the problems caused by harmonics will be owned by them. Harmonics can severely de-rate the transformer and it is not an exaggeration that in Australia not much attention has been paid to 'harmonics-hardened' transformers. Eddy current losses, proportional to the square of both the amplitude and harmonic frequency will shorten transformer life span.
Proximity and skin effects in conductors due to harmonics will often increase energy losses well in excess of the I2R losses at the fundamental frequency (50 Hz). These additional losses can wreck switchboards, overload conductors including neutrals and cause voltage distortion in the customer’s installation. Reliability of protection is affected. Magnetically tripped breakers can be tripped through severe current distortion (very high peak current) and thermally tripped breakers through the heating effects of harmonic current components. Harmonics can therefore upset coordination of protective gear. Residual current devices (RCDs) can also malfunction.
Energy waste in the reticulation system is best avoided by harmonics mitigation as close to the harmonics generating sources as possible. Economies can be achieved by aggregating some harmonics generating loads for example variable speed drives. Large rating loads are often best connected on separate circuits. Single-phase loads are generally most economically mitigated at sub-distribution board level using three-phase, 4-wire harmonic filters (active filters are recommended for high variability loads).
Neutral current compensation can be a problem in the case of unbalanced phases, and zero sequence (3rd, 9th, etc) harmonic compensation best achieved on a per phase basis. Transformer selection for sub-stations should be on the basis of having a line diagram of the installation with predicted harmonic content available. For installations with stand-by generators, mitigation filters should avoid creating leading power factors. For installations using passive capacitor-bank displacement power factor correction, detuning chokes have to be employed to avoid resonance problems.
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This article explains how to efficiently design a standards compliant power quality (PQ) measurement instrument using a ready to use platform that accelerates development. It will discuss different solutions for designing a Class A and Class S meter, including a new Class S power quality measurement integrated solution that significantly reduces the development time and costs for power quality monitoring products. The article “Power Quality Monitoring Part 1: The Importance of Standards Compliant Power Quality Measurements” provides an understanding of the IEC standard of power quality and its parameters.
The basic components of an instrument designed for power quality measurement are shown in Figure 1. First, the current and voltage transducers must account for the operational range of the instrument and adapt the input signal to the dynamics of the analog-to-digital converter (ADC) input. Traditional transducers are the first source of uncertainty in the measurement; therefore, correct selection is of great importance. Next, the signal goes to an ADC; its individual characteristics such as offset, gain, and nonlinearity errors create a second source of uncertainty. Selecting the correct ADC for this function is a demanding effort for designing a power quality instrument. Lastly, a series of signal processing algorithms must be produced to get electrical and power quality measurements from the input signals.
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Depending on the location and application of the power quality instrument, the nominal supply voltage (UNOM), nominal current (INOM), and frequency varies. Independently of the nominal values that the instrument measures, the IEC -4-7 standard requires power quality measurement instruments to reach the accuracies presented in Table 1; therefore, the transducers must be selected such that the instrument fulfills the accuracy requirements.
Table 1. Accuracy Requirements for Current, Voltage, and Power Measurements Specified by IEC -4-7 Standard Class Measurement Conditions Maximum Error A Voltage UM ≥ 1% UNOMINOM: Nominal current range of the measurement instrument
UNOM: Nominal voltage range of the measurement instrument
UM, IM, and PM: Measured values
The IEC-4-71 standard recommends designing the input circuitry following these nominal voltages (UNOM) and nominal currents (INOM):
Additionally, the transducers selected for measuring voltage and current must keep their characteristics and accuracy unchanged when a 1.2× UNOM and INOM applied continuously. A signal four times the nominal voltage or 1 kV rms, whichever is less, applied for 1 second to the instrument must not lead to any damage. Likewise, a 10× INOM current for 1 second shall not produce any damage.
Even though the IEC -4-30 standard does not specify a minimum requirement for sampling rate, the ADC must have enough sampling rate to measure some oscillatory and fast power quality phenomena. An insufficient sampling rate could result in the misclassification of a power quality event or the failure to detect one. The IEC -4-30 standard states that the instrument voltage and current sensors should be appropriate for up to 9 kHz. Thus, the sampling frequency of the ADC must be selected following the rules of signal analysis to perform a measurement of frequency components up to 9 kHz included. Figure 2 illustrates the consequences of when the sampling rate is not sufficient. The top left waveform contains 64 samples per 10 cycles (200 ms) and the top right waveform has samples per 10 cycles. As shown in Figure 2, the top left graph shows a voltage dip event while the top right graph shows that the dip is transient induced.
The IEC standard applies to single-phase and three-phase systems; therefore, the selected ADC must be able to sample the required number of voltage and current channels simultaneously. Having measurements for all the voltage and current channels on the instrument at the same time allows all parameters to be examined and immediately triggered when a power quality event occurs.
Even though selecting the transducers and ADC for power quality measurements requires a comprehensive engineering effort, developing the algorithms for processing the raw ADC measurements is undoubtedly the task that demands most of the time and resources to make a power quality instrument. To implement a standard compliant instrument, the right DSP hardware must be chosen and the algorithms to calculate the power quality parameters from the waveform samples have to be developed and properly tested. The standard not only requires calculations but also different time dependent aggregations with time accuracies less than ±1 seconds per 24-hour period for Class A and ±5 seconds per 24-hour period for Class S. These algorithms must perform harmonic analysis. Additionally, power quality parameters rely on fast Fourier transform (FFT) analysis (harmonics, interharmonics, mains signaling voltage, unbalance), which are challenging to implement. The FFT analysis requires the waveforms to be sampled at samples per 200 ms (10 cycles) minimum. Performing resampling of the raw waveforms from the ADC to the required rate requires care to avoid harmonic distortion and aliasing.
After the algorithms are developed, the IEC standard requires a comprehensive list of more than 400 tests that the instrument must pass to be fully certified.
Figure 3 shows a block diagram with the most relevant functions a DSP system needs for producing power quality measurements.
Considering the accuracy, number of channels, and sampling rate requirements to develop a Class A PQ instrument, the AD777x and ADx family of products are recommended for the ADC conversion of the signal chain/system. Note that these solutions provide just the raw digitized data from the input signals. A DSP system must be developed to get certified PQ measurements.
The AD777x is an 8-channel, 24-bit simultaneous sampling ADC family of devices. Eight full sigma-delta (∑-Δ) ADCs are on-chip providing sampling rates of 16 kSPS/32 kSPS/128 kSPS. The AD777x provides a low input current to allow direct sensor connection. Each input channel has a programmable gain stage allowing gains of 1, 2, 4, and 8 to map lower amplitude sensor outputs into the full-scale ADC input range, maximizing the dynamic range of the signal chain. The AD777x accepts a VREF voltage from 1 V up to 3.6 V and analog input range: 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. A sample rate converter is provided to allow fine resolution control over the AD and it can be used in applications where the ODR resolution is required to maintain coherency with 0.01 Hz changes in the line frequency. The AD777x also provides large signal input bandwidth 5 kHz (AD 10 kHz). A data output and SPI communications interfaces are provided although the SPI can also be configured to output the sigma-delta conversion data. The temperature range is from –40°C to +105°C, functional up to +125°C with a power supply of 3.3 V or ±1.65 V.
Figure 4 shows a 3-phase typical applications system diagram for the AD777x family of ADCs for a PQ instrument using current transformers as current transducers and resistor dividers for voltage.
The ADx provides a 16-/18-bit, simultaneous sampling, analog-to-digital data acquisition system (DAS) with eight channels. Each channel contains analog input clamp protection, a programmable gain amplifier (PGA), a low-pass filter, and a 16-/18-bit successive approximation register (SAR) ADC. The ADx also contains a flexible digital filter, low drift, 2.5 V precision reference and reference buffer to drive the ADC, and flexible parallel and serial interfaces.
The ADB operates from a single 5 V supply and accommodates ±10 V, ±5 V, and ±2.5 V true bipolar input ranges when sampling at throughput rates of 800 kSPS (ADB)/1 MSPS (ADC) for all channels. The input clamp protection tolerates different voltages with user selectable analog input ranges (±20 V, ±12.5 V, ±10 V, ±5 V, and ±2.5 V). The ADx requires a single 5 V analog supply. The single-supply operation, on-chip filtering, and high input impedance eliminate the need for external driver op amps, which require bipolar supplies.
In software mode, the following advanced features are available:
Figure 4 shows a 3-phase typical applications system diagram for the ADx family of ADCs for a power quality instrument using current transformers as current transducers and resistor dividers for voltage.
The ADE, a highly accurate, fully integrated, polyphase energy metering IC combined with the ADSW-PQ-CLS software library running on a host microcontroller, is a complete solution that is IEC -4-30 Class S standard compliant. This integration significantly reduces the development time and costs for PQ monitoring products. The ADE + ADSW-PQ-CLS solution simplifies the implementation and certification of energy and PQ monitoring systems by providing a tight integration of acquisition and calculation engines. Figure 5 shows a 3-phase applications system diagram for the ADE + ADSW-PQ-CLS solution for a power quality instrument using current transformers as current transducers and resistor dividers for voltage.
With seven input channels, the ADE can be used on a 3-phase system or up to three single-phase systems. It supports current transformers (CTs) or Rogowski coils with an external analog integrator for current measurements. It provides an integrated analog front end for power quality monitoring and energy measurement. The ADE is pin-compatible with the ADE and ADE with equivalent analog and metrology performance. Its features include:
The ADSW-PQ-CLS software library is designed specifically to be integrated with the ADE to generate standard compliant IEC -4-30 Class S PQ measurements. It implements all parameters defined in IEC -4-30 for Class S instruments. Users can decide which PQ parameters to use. This library needs low CPU/RAM resources and is core/OS agnostic (Arm® Cortex®-M minimum). Supported MCU architectures include Arm Cortex-M0, Cortex-MO+, Cortex-M1, Cortex-M3, and Cortex-M4. For distribution to end users, the library is provided as a CMSIS-PACK file (.pack) compatible with Keil Microvision, IAR Embedded Workbench version 8.x, or Analog Devices CrossCore® Embedded Studio. The license for software library is included with the purchase of the ADE. A PC serial command line interface (CLI) example is provided to evaluate the library and its features. Figure 6 shows how PQ parameters are displayed by this CLI.
The EVAL-ADEARDZ enables quick evaluation and prototyping of energy and Class S power quality measurement systems with the ADE and the ADSW-PQ-CLS Power Quality Library. The power quality library and application example are provided to simplify implementation of larger systems. This kit provides a plug and play type of experience that is easy to use to test the power quality parameters of a 3-phase electrical system.
The kit has the following hardware features:
Figure 7 shows the connections required to use the EVAL-ADEARDZ with a PC.
The EVAL-ADEARDZ consists of a PCB with four current and three voltages + neutral input connectors and on-board ADE, isolators, a real-time clock, a Cortex-M4 STM NUCLEO-413ZH development board with an example application of the ADSW-PQ-CLS library, and three current sensors.
The ADE + ADSW-PQ-CLS solution has been certified to accurately measure power quality parameters following the requirements of the IEC -4-30 Class S standard.
Designing a standards compliant power quality meter is a challenging task. To reduce the time and engineering resources needed to produce an IEC -4-30 Class S standard compliant PQ measurement instrument, the ADE + ADSW-PQ-CLS is a complete go-to solution that enables designers with a ready to use platform to accelerate development and solve for many critical design challenges.
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