New power quality issues in distribution networks

September 11th, 2015, Published in Articles: Energize

 

The increasing application of energy savings devices, the move towards the smart grid and the increasing appearance of distributed generation as well as own generation using inverters has all impacted energy usage, and at the same time has brought about new challenges in the field of power quality.

The primary concern is the increase in network harmonics, as well as the appearance of higher order harmonics, as a result of the widespread and large scale introduction of smaller energy savings devices, and the impact of distributed power factor (PF) devices on harmonic resonance.

In addition to low power devices such as compact fluorescent lamps (CFLs), computers, cell phone chargers and energy efficient appliances, a new higher power source of harmonics is appearing in distribution networks, in the form of rooftop PV inverters, and domestic electric vehicle chargers. Renewable energy systems, primarily rooftop solar, play a major role in generating and exacerbating harmonics.

Because energy efficient appliances must be cost effective, i.e. the cost of the energy savings technique must not exceed the value of the energy saved, this often means that the cheapest solution is adopted, with minimum attention to the impact on power quality.

Harmonics can affect not only the power quality on the distribution network, but where the result is voltage harmonics, can also affect the operation of devices and appliances connected to the network.

Of prime concern is the effect on smart meters and security systems. More and more devices are being connected, with the future possibility of electric vehicle chargers becoming common, and steps need to be taken now to prevent further deterioration of power quality in distribution networks.

Network harmonics

The focus on harmonics in networks in the past has been in the low frequency band. Current studies, however, show that there is a significant effect at higher frequencies which are not multiples of the fundamental.

Fig. 1: Total fifth harmonic current as a function of the time of day measured at the busbar of  a public LV grid with installed PV power of 332 kW.

Fig. 1: Total fifth harmonic current as a function of the time of day measured at the busbar of
a public LV grid with installed PV power of 332 kW.

Harmonics

In network engineering the evaluated frequencies are often limited to the 40th harmonic (2 kHz). This is defined as frequency component multiples of the fundamental frequency present in the grid in addition to the fundamental 50 Hz component ranging up to 2 kHz.

Supraharmonics

Recent studies indicate that future problems to distribution grids may include supraharmonics (SH).  These are multiples of the fundamental frequency which fall in the range between 2 kHz and 150 kHz. These frequency components are partly the effect of the normal operation of equipment due to power-electronic converters and the switching techniques used.

Supraharmonics are not well researched, and there are no international or industry standards in place covering the allowable limits for these, although it is understood that preparation of relevant standards is in progress. One of the results of this is the proliferation of devices producing SH at levels which may be considered dangerous.

Interharmonics

These are frequencies that fall in the ranges of harmonics and supraharmonics, but which are not multiples of the fundamental frequency. As such they are not strictly harmonics, but have the same effect as harmonics on power quality and are still a source of interference and current and voltage distortion on the network.

Primary and secondary harmonics

Primary harmonics are a direct result of the operation of a device. Secondary harmonics are the result of the interaction between harmonic producing devices, which may result in a level of harmonics which are greater than that of the two devices operating separately. Some devices are sensitive to background voltage harmonics, and the harmonic level increases as the background harmonic level increases [2].

Impact of supraharmonics

One of the main concerns about SH is the impact on power line communication (PLC) which operates between 9 and 95 kHz, and on ripple control systems. Even though from an equipment viewpoint there is no difference between a signal used for communication and a signal which is a residue from a switching circuit, PLC is a useful signal for operation of the grid and for communication with smart meters, and the treatment of harmonics in this range needs to take this into account.

The emerging smart grid is characterised by intercommunication between elements and devices, and as PLC is one of the options being considered for this purpose, it is susceptible to higher order harmonics. In addition the smart grid contains a large number of devices where power consumption is controlled or regulated by electronic devices, which are themselves a source of harmonics, and can also be affected by SH.

Voltage and current harmonics

Although current harmonics can result in increased losses and the maloperation of network protection systems, voltage harmonics have the greatest potential to disturb the operation of equipment and appliances connected to the network, and from a customer’s point of view are the prime factor related to power quality. Voltage harmonics can also be amplified by various devices such as intelligent voltage following inverters. High order harmonics at odd multiples of the third harmonic are troublesome as they are zero sequence harmonics and add together in the neutral conductor. Where the neutral is sized for zero current this can result in high levels of harmonics developing. This is particular concerning at the nineth and 15th harmonics.

Current harmonics

These are caused by the operation of devices connected to the network, and flow into the network at the point of common coupling (PCC). With a modern consumer installation several devices may be operative at any one time and the current harmonics at the PCC will be the sum of the individual device harmonics. Different current harmonic profiles of devices (amplitude and phase angle) make summation difficult, and the mix of devices in operation changes with the time of day making a profile even more complex. Current harmonics result in increased network losses, overheating of network components, false tripping of protection devices and other affects.

Fig. 2: Phase angle of fifth harmonic current of CFLs from  different suppliers are similar [6].

Fig. 2: Phase angle of fifth harmonic current of CFLs from
different suppliers are similar [6].

Voltage harmonics

These are the result of interaction between the current harmonics and the network impedance. The network impedance will be different for different harmonic frequencies and harmonic resonance can result, which leads to amplified harmonic voltage levels. Voltage harmonics are the cause of interference with equipment connected to the network, and the maloperation of such equipment.

Network impedance

Calculating network impedance in a modern distribution network is complicated by the varying number of appliances and devices connected to the network. A common practice used to improve the power factor of appliances is to connect capacitors across the input. This results in significant distributed capacitance in the network, which affects its impedance and resonant frequency. The capacitance will also change as appliances are switched in and out of service, changing the impedance and resonant frequency. This makes the analysis of the network difficult, and often the only solution is direct measurement and monitoring.

Standards governing harmonic emissions

Formal standards are confined to the harmonic band below 2 kHz and few standards exist for the supraharmonic range, although standardisation work is ongoing. The lack of any limits in this range has led to the development of unregulated and uncontrolled emissions. Standards in existence tend to cover the emission of current harmonics by individual items of plant connected to the network, and the overall voltage harmonic levels on the network. Standards do not cover combinations of harmonic generation items, although network harmonic limits are specified. An installation may consist of a number of items, all complying with the standard but which in combination produce an unacceptably high level of harmonics.

Current harmonic standards are applied to consumer installations and voltage harmonic standards to the utility distributor. Thus, knowing the network impedance, the utility should be able to calculate the limit of current harmonics allowable in the network, and by agreement or by allocation, set the current harmonic emission limits for consumers connected to the network. An industry standard,
IEEE 519, has set up a method of allocating current harmonic emission levels based on the load drawn by the consumer [1]. At present, this is mainly applied to MV networks and larger consumers, but might well find application in the LV residential and small business networks in future.

There are two sets of standards governing harmonic emissions:

Harmonic current standards

Standards specify the maximum limits for harmonic current emitted by individual items of equipment at the PCC. Existing standards do not cover summed harmonics emitted by combinations of equipment in a consumer’s installation.

In South Africa the relevant standard is SANS 61000-2-2:2002/IEC 61000-2-2:2002, Electromagnetic compatibility (EMC) – Part 2-2: Environment – Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems. This standard gives guidance for the limits to be set for disturbance emission into public power supply systems, but deals mainly with compatibility limits for equipment connected to the network.

SANS 61000-3-2:2006+A1+A2 is applicable for electrical equipment that is supplied from mains network with voltage not less than 220 V and current up to and including 16 A. This standard sets the limit for harmonic current limits for appliances connected to the network.

The standard which covers the MV network is IEC 61000-2-12: Electromagnetic compatibility (EMC) – Part 2-12: Environment – Compatibility levels for low-frequency conducted disturbances and signalling in public medium-voltage power supply systems.

IEEE 519 is a standard or guideline which limits the harmonic current which an installation can inject into the network. This current limit is based on the load drawn by the installation, relative to the total capacity of the network (or short circuit capability at the PCC). This is mainly intended for industrial and commercial applications and would not be applied to residential consumers, but gives an interesting insight as to how the problem could be tackled. The philosophy adopted to develop the limits was to restrict harmonic current injection from individual customers so that they would not cause unacceptable voltage distortion levels when applied to normal power systems [1].

Voltage harmonic standards

Voltage standards are applied to the utility network and limit the network voltage harmonics which may be present at the PCC. The onus is on the utility to ensure that these limits are met. One can appreciate the problem faced by a utility where there are no current harmonic limits on consumers connected to the network. The change in harmonic profiles in residential networks can also produce unanticipated results for the utility. In South Africa, voltage harmonic limits are specified in NRS 048-2 and the NERSA power quality directive of March 2002.

Fig. 3: Differing phase angles of harmonics from different  devices can result in cancellation.

Fig. 3: Differing phase angles of harmonics from different
devices can result in cancellation.

Network standards

IEC 61000-3-6 sets the standards for MV and HV distribution networks, as well as methods for calculating the allocation of harmonic levels on each of the feeders connected to the network. The document also recommends planning levels for total harmonic distortion on the MV network for each harmonic up to the 40th (2 kHz). MV loads comprise high load users as well as low voltage distribution networks. With this system it is possible to determine the harmonic allocation for a low voltage distribution network. Allocations are not fixed but must be calculated based on the configuration of the network, and different LV networks may have different allocations.

Standards are focussed on planning levels for MV networks, but with the increasing use of harmonic generating equipment in LV networks there would seem to be a case for establishing planning limits for LV networks as well.

Sources of harmonics

Compact fluorescent lamps (CFLs)

The low power factor of CFLs is a well-known phenomenon which has received attention in studies on lighting systems, but little attention has been paid to the harmonic generation from these devices. Essentially, in a mass-produced energy savings device more attention is paid to cost than to power quality. A possible cause could be that lighting constitutes a small portion of the total load. Studies have shown that lighting constitutes only 6% of the network load, but can contribute more than 50% of the allowable harmonics in a network [2]. Most of the studies done so far have concentrated on the effect of harmonics from the large scale use of CFLs and have shown that the effect of harmonics extends upwards into the MV network as well as the LV network.

LED lamps

LED lamps require direct current (DC) for operation and are fitted with AC/DC converters, which are the source of harmonic generation (HG). Some LEDs are equipped with simple converters and show a very high harmonic generation level. Although high prices limit application, LEDs allow a wide variety of designs and applications and it is anticipated that this could change in future as prices fall and LEDs replace CFLs as the primary illumination source. LEDs consume less current for the same output which could result in a drop in harmonic levels from lighting. LEDs are also being considered for street lighting which may have a significant impact on network harmonics.

Computers

Computers are DC driven devices and their power supplies have been developed over the years into highly efficiency units. Most computers will use switched-mode power supplies, which in a basic version generates significant harmonics. Price again plays a factor here and cheaper units used in domestic applications may be problematic. From a residential point of view the number of computers is far less than the number of lights or other appliances, although the power consumed by a computer may exceed the power consumed by a high-efficiency lighting system.

UPS systems

In the past these were used to provide back-up power for computers, and in distribution networks the units are generally small. However there is an increasing trend, especially in South Africa, to use larger units to provide power for a mixture of loads during load shedding, and again, price being an overruling factor, the harmonic generation from such units could be quite high.

Domestic appliances

This includes microwave ovens, induction cookers and a host of other appliances which convert AC to DC to power energy-efficient devices. All generate harmonics at some level. Most appliances will comply with harmonic standards, but a combination of units, all operating at the limit, may cause problems.

EV chargers

A new entrant in the market, the electric vehicle (EV) charger is unusual in that it places a fairly high load on the network connection and can therefore be expected to contribute significantly to total harmonic distortion (THD).

Distributed energy resource (DER) inverters

DER inverters are usually associated with rooftop PV systems in distribution networks. Harmonics can be generated by the inverter or amplified as secondary emissions by the inverter. Some inverters operate on a voltage-following principle and replicate existing background voltage harmonics in the output waveform. The level of the harmonics can also change depending on the level of generation, and are dependent on the manufacturer [4]. Inverters generally switch off when there is no solar generation, and the harmonic content is zero during this period. This gives a fairly consistent pattern of HG as shown in Fig. 1 [3].

This can be a problem where there is a high penetration of PV systems, although diversity can result in cancellation. Studies have shown that a high concentration of similar DER systems can result in series resonance at harmonic frequencies which can increase background voltage harmonics, while small current harmonics from a large population of DER inverters will excite resonance in distribution networks [4].

Active power factor correction and lower harmonic filters

Use of active devices to suppress lower order harmonics and for active power factor correction often has the undesired effect of increasing the higher order harmonics [5].

Harmonic resonance

The phenomenon of harmonic resonance in industrial installations with large power factor (PF) correction capacitors is well known, and the design of the PF system will take this factor into account. HR in industrial applications usually occurs at the lower order harmonics. In distribution networks, we are faced with the problem of numerous distributed small capacitors, often installed in appliances and other devices to improve the power factor. This usually results in resonance at higher order harmonics which can cause interference with other equipment connected to the network. This is problematic as the utility has no means of controlling the capacitance connected to the grid, and this can vary during the day as appliances are switched in and out of service. So the resonant frequency can change with time of day, and over the longer period as new appliances are added to the network.

Summation and cancellation of harmonics from different devices

The effect on the total harmonic content of a distribution network will depend on both the amplitude and phase of harmonics generated. The phase angles between individual harmonics generated by different devices may result in addition or cancellation of harmonics.
Studies have shown the phase angles of individual harmonics from different types of device can differ considerably [6], and the combination of harmonic currents can result in mutual cancellation and reduction of the network harmonic currents. Even similar devices, such as CFLs, can show a variation in phase angle from different suppliers.

This makes it very difficult to assign a characteristic harmonic profile to any class of device, such as CFLs for instance. Although a study of CFLs from several suppliers in Germany has shown very similar phase angles there is a fairly wide variation in phase angle amongst the same products from the same supplier
(Fig. 2) [6]. In the case where mass replacement of incandescent lamps with CFLs from a single supplier has taken place, such as the Eskom programme, this may result in very close group of phase angles on harmonics in the network.

This can be an important factor when analysing and designing networks. The wide number of devices in network and the variance between the harmonic profiles makes estimation of network harmonic profile very difficult.

Cancellation occurs between harmonic waveforms of the same harmonic number when there is a phase difference between the two waveforms (Fig. 3). With a large number of similar devices the phase angle will not be so well defined. The cancellation of harmonics will vary with time unless dominant devices are involved which are operational simultaneously most of the time, and the cancellation effect and potential has to be determined by actual measurements and monitoring of the network as well as individual consumer harmonic profiles.

References

[1]    T Blooming: “Application of IEEE 519”, https://ewh.ieee.org/r3/atlanta/ias/IEEE_519.pdf
[2]    AM Blanco, et al: “Impact of supply voltage distortion on the harmonic emission of electronic household equipment”, SICEL, Columbia, November 2103.
[3]    Dr. M Basu: “Coping smartly with harmonic penetration, propagation and interaction in the distribution network”, Dublin University seminar, 2105, http://dit.ie/dublinenergylab/media/ditdublinenergylab/seminars/may2015/Malabika%20Basu.pdf
[4]    S Rönnberg, M Bollen and A Larsson: “Emission from small scale PV-installations on the low voltage grid”, International conference on renewable energies and power quality (ICREPQ14), Cordoba (Spain), 8 to 10 April, 2014.
[5]    JH Enslin: “Harmonic interaction in high populations of distributed power resources”, IEEE Energy conversion congress and expo, Pittsburgh, USA, September 2014.
[6]    J Meyer, et al: “Harmonic summation effects of modern lamp technologies and small electronic household equipment”, 21st International Conference on Electricity Distribution (CIRED), Frankfurt, June 2011.

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