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Posted on 01 June 2019

Harmonic and Flicker Testing

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Meeting the requirements of the EN62019-3-2/3

All designers of electrical or electronic equipment which is intended to be connected to the mains supply need to be familiar with the performance requirements of EN62019-3-2 (harmonics) and EN62019-3-3 (flicker and voltage variations) and the test methods required to ensure compliance.

By Malcolm Robinson, Senior Design Engineer, Thurlby Thandar Instruments Ltd.

 

EN62019-3-2 is the standard that defines permitted harmonic current limits. It categorises equipments into four classes (A, B, C and D) and imposes different harmonic current limits on each class. It also defines cases where no limits apply, but places prohibitions on some control techniques that apply to all equipment within its scope, even if they are not subject to limits. The original standard was released in 1995, but significant revisions were made in 2000, particularly to the treatment of fluctuating harmonics and in the definition of class D.

The standard calls for harmonic levels to be checked using both the average and maximum values over the whole test interval. Any harmonic is allowed to fluctuate up to a maximum of 150% of its limit, provided that its average value is below 100% of the limit. In addition, some trade-off between harmonics is allowed for odd harmonics of orders 21 to 39, based on a value called the partial odd harmonic current (POHC), which is the RMS sum of all the odd harmonics between 21 and 39.

EN62019-4-7 is the standard that defines the requirements for the test equipment for measuring harmonic currents. This standard has also undergone substantial revision. The 1993 version covered both digital and analogue implementations (such as tuned level meters), while the present 2002 version anticipates an implementation based on digital signal processing techniques and extends the measurement complexity.

At the heart of the measurement technique is the discrete Fourier transform, which takes a sequence of samples of a waveform in the time domain and computes the frequency spectrum of the waveform on the assumption that the signal repeats indefinitely. Much of the complexity of the standard lies in the fact that it also covers situations, known as fluctuating harmonics, where this assumption is untrue. The transform is taken across a number of cycles of the mains waveform over a period known as the window length. The present standard calls for a 200 ms window: 10 cycles at 50 Hz or 12 cycles at 60 Hz. The digital system is required to be synchronous, generally involving a phase-locked loop linking the sampling clock to an exact multiple of the mains frequency. Results are required up to the 40th harmonic, and applying the wellknown Nyquist criterion means that the sampling rate must be at least 80 points per cycle. The HA1600A instrument shown in Figure 1 uses transforms based on the prime factor algorithm at 150 samples per cycle.

The HA1600A harmonics flicker and power analyser and the AC1000A low distortion power source from TTi

The standard demands that the instrument must sample continuously and perform transforms on all the data without any gaps. This imposes a burden of realtime availability on the processor, which is virtually impossible to achieve on a general-purpose computer running a desktop operating system. All compliancegrade instruments will have digital signal processing built in. Incidentally, if a PC is used to carry out the signal processing, then that PC becomes part of the instrument and must be included in the annual calibration cycle.

A major addition to the EN62019-4-7 standard in the 2002 amendment is the need to include spectral components at frequencies which fall between two harmonics (known as inter-harmonics in the standard). If a discrete Fourier transform is conducted over a window of ten mains cycles, it gives spectral amplitudes at ten points between the harmonic ‘bins’. Previously, these components were ignored; now they must be added (by an RMS sum of the magnitudes) into the amplitude of the nearest harmonic (with half the amplitude of the mid-point bin added to the harmonics on either side) before comparing that result to the limit.

Systems whose harmonic levels change over a time frame comparable to the window width cause sidebands on the harmonic signals, and these are now included. Other systems may directly generate non-harmonic components, commutator noise from asynchronous motors being one example. A washing machine with an unbalanced spin load may produce such a waveform (it might also produce flicker, although generally the motor current is too small for this). Any such equipment will give different results when tested using the new methods.

Figure 2 shows a typical mains voltage waveform as available at a normal socket outlet. The flat-top distortion is clearly evident, showing the effect on the supply network of harmonic currents caused by large numbers of electronic products taking a sharply peaked current waveform such as that illustrated.

Typical mains voltage waveform showing the effects of harmonic distortion

EN62019-3-3: flicker and voltage fluctuations

EN62019-3-3 is the standard that defines limits for two related effects: flicker and voltage fluctuations. Both are intended to ensure that equipments do not cause annoyance to neighbouring consumers connected to the same supply network by causing voltage changes across the shared source impedance of the network. The magnitude of these voltage changes depends on the both the network source impedance and on the changes in the current consumed by the unit under test.

Flicker is the impression of changes in the brightness of a lamp as perceived by a human observer. Over the years, surveys have been conducted to establish the threshold of annoyance to the population caused by flicker, leading to the definition of the flicker perception unit. EN61000-4-15 (which has replaced IEC868) describes the complex processing sequence of AGC, demodulation, filtering, squaring and smoothing, and statistical classification by which the real-time voltage waveform is reduced to two simple numbers for the short and long-term flicker severity indicators Pst and Plt. As with harmonics, testing to this standard presents a continuous realtime processing burden and requires a DSP in the instrument.

Assessing voltage fluctuations requires an examination of the history of the RMS values of each half cycle over a period of time, categorising an interval as either a steady state or a voltage change, and comparing the magnitude and duration of the changes against the limits.

Supply source and reference impedance

The standard environment for testing requires a perfect sine-wave mains source with a defined source impedance called the reference impedance. This is intended to be representative of the actual characteristics of the supply mains at a domestic socket outlet, and the reference value is derived from survey work carried out many years ago. The voltage change produced by any given current change is directly proportional to the impedance value, so that any error in the magnitude of this impedance translates directly into an error in the measured result. As the permitted overall error is 8%, the impedance must be quite precise. It is also necessary to allow for the resistance of any connecting leads, plugs and sockets.

Some laboratory standard mains generators have provision for simulating the reference impedance using their internal feedback loop, but such methods are subject to the limited bandwidth of that loop, and it is possible for unforeseen interactions to occur.

Many laboratories do not have access to an expensive mains source but, because of waveform distortion and indeterminate source impedance, it is not possible to conduct these measurements by using the public mains supply in conjunction with the normal voltage sensing method. Therefore the HA1600A also implements an alternative current sensing technique that can be used without a laboratory-grade mains supply.

Inrush current

The 2001 amendment to EN62019-3-3 also introduced an explicit requirement to test the voltage fluctuation caused by manual switching events. This imposes a limit on the maximum peak half-cycle RMS inrush current that a unit may take whenever it is switched on. This requirement arises from concerns among supply network operators about the difficulty of reconnecting a network after a supply failure.

These tests have to be conducted by physically actuating the actual switch on the product under test; the use of electronic switches is not permitted. Because of the random phasing of the user’s switch action with the mains waveform, a series of tests must be performed to obtain a statistically valid average result. As equipment containing surge-protection measures may take several minutes to cool down between tests, this is a time-consuming requirement.

Because of the nature of modern rectifier circuits, this test needs to be conducted on almost all electronic products.

Complete solution

The HA1600A harmonics flicker and power analyser, combined with the AC1000A low distortion power source (Figure 1), provides a complete solution for the compliance quality measurements described above.

However, in addition to its EMC measurement capabilities, it provides basic power meter measurements, such as RMS and peak current, power, VA and power factor, as well as a visual display of the voltage and current waveforms on-screen, without any of the safety issues involved in using a standard oscilloscope for live mains measurements.

In production testing, a quick check of the waveform can easily show up manufacturing faults such as partially faulty bridge rectifiers or transformers with high magnetising currents in a quick and safe way that allows manufacturers to dispatch their product confident that it is safe.

Another measurement provided is of instantaneous peak inrush current. There are no EMC standards limiting this parameter, but it is a critical aspect determining the life expectancy of switch and relay contacts. Using this measurement, product designers can check that the ratings of the selected switch are not being exceeded.

 

 

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