Industrial power quality standards

August 12th, 2014, Published in Articles: EE Publishers, Articles: Energize

 

Power quality becomes more and more important as energy efficiency equipment is brought into use in industrial networks, and as embedded generation is installed in distribution networks and own generation equipment is brought into use by industrial users. This article examines the standards for both utility networks and industrial users.

All networks, however well designed and well maintained, suffer from disturbances of some form or another. These may take the form of voltage dips, spikes, noise, harmonics, power factor variations, and EMC interference. These disturbances can be caused by faults, load switching, power factor correction, induction and many other sources during the normal operation of a network. The aims of any utility and the user are to limit the range of disturbances to those which will not cause harm or damage to either utility or user equipment. To this end national standards for power quality are compiled.

The purpose of power quality standards are:

  • To ensure safety for both the consumer and the provider of electricity
  • To prevent or limit damage to equipment and plant connected to the network
  • To ensure continuous and efficient operation of plant connected to the network, by limiting disturbances on the network to levels acceptable to the plant.

Quality standards define the limits of variations of the electrical parameters of a network, the main one being voltage. Agreed limits are a compromise between the design of the network and the design of the plant. Improving quality (or imposing tighter limits) on the network increases the cost of the network, while improving the tolerance of the equipment to variations on the network increases the cost of the equipment. Agreed quality standards are thus a balance between these two extremes.

South African situation

Power quality is regulated by the national electricity regulator NER directive on power quality. The directive covers power quality in all sectors of the network but this article will cover only those portions pertaining to the distribution network and consumers. The regulator is responsible for issuing licences to suppliers of electricity, and one of the conditions of the granting of a licence is performance according to the power quality directive.

Standards and managed contracts

The directive makes provision for regulation of power quality both by a standard and a managed contract. The variety of distribution feeders and the variety of different industrial users, each with their own combination of equipment and plant make it difficult to set a universal standard, and quite often a managed contract between the utility and the user is required. In instances, such as domestic feeds, a general standard can be used, but often a application specific agreement may be required.  This could involve additional cost to the utility to reach the customers required level of performance, and this is the subject of regulator scrutiny.

Power quality management systems

Part of the requirements of the directive cover power quality management systems. Requirements of a licensee to set up and maintain a power quality management system. Measurement and recording of power quality takes place in accordance with a defined time based regime.

Steady-state power quality.

It is important for there to be an alignment between the power quality requirements of the utility and the disturbance withstand ability of industrial networks and equipment. Although values and limits are designed into a network, actual values will exhibit a distribution over time due to variable factors such as load, climatic conditions, and values will vary between limits. Limits can be defined for individual items of equipment that make up a network, but the combination of items will result in a variation of values with time which will exhibit a statistical pattern.
In a similar fashion the withstand ability or immunity of equipment will vary from the design value depending on component tolerances, aging, climatic and other conditions depending on the installation, and will also exhibit a statistical distribution. The aim of quality standards is to ensure co-ordination of the disturbance and vulnerability regions. Co-ordination is done by referring to statistical models that characterise the distribution of disturbance and immunity levels.

Fig. 1 shows typical distribution curves of disturbance and immunity levels. The following parameters are used when specifying power quality by this method.

Fig. 1: Distribution curves for disturbances and equipment immunity.

Fig. 1: Distribution curves for disturbances and equipment immunity.

Compatability level

The disturbances produced by individual factors combine to create a level of disturbance in the whole network. The level of disturbance will be higher for some parts than for others, depending on their impedance and loading, and will vary according to the time of day, day of the week and time of year. The compatibility level is commonly defined as the disturbance level that must not be exceeded for 95% of the measurements in the entire network. Note that compatibility level is a statistical value that characterises the state of the whole network – it cannot be used to describe the situation on a particular section. Using statistics to characterise the supply’s performance makes it possible to compare the results with the statistical characteristics of equipment immunity to power quality variations.

Planning level (PL)

Planning levels are used in MV and HV networks and represent internal objectives of the electrical utilities.
They are used in network design, for example in deciding how to connect new loads. In many regulatory regimes, planning levels are applied to industrial and commercial consumers to limit the harmonic currents that can be imposed on the network by a consumer. Planning levels are lower than compatibility levels, partly because there are many unknown loads on the system (e.g. domestic loads) that can only be estimated and partly because the problem is a statistical one.

Equipment immunity level

Equipment performance may be negatively affected if this level is exceeded. In practice, the immunity of the equipment to disturbance is also affected by other factors. Component tolerances  and installation conditions, such as cable lengths and earthing arrangements, are also likely to introduce variations. As a result, the immunity level of equipment is also distributed statistically in the same way as the disturbance level.

Each piece of equipment is designed and manufactured to a standard that requires it to be immune to disturbances below a certain level. The immunity level  is the maximum value of a disturbance, present in the network, that does not degrade the behaviour of a particular item of equipment under test conditions   There should be some margin between the compatibility level for the supply and the equipment immunity level.

Utility planning level

The electric utility will establish this level as its design objective. Usually, the utility sets the planning level somewhere below the compatibility level to help ensure it won’t exceed the actual compatibility level. For example, the compatibility level for harmonic voltage distortion might be 5% but the planning level might be 4%, ensuring the 5% level isn’t exceeded.

Assessed or measured level

This is the level that exists on the system, based on measurements and used for assessing performance and licence requirements. The evaluation of performance requires measurements over a time period and could record the number of deviations from the limits over the period.

Supply network standards

These cover the standards that the supplier is expected to maintain at the point of connection to the consumer. Values, or limits, may be specified in a standard, such as NRS048, or may be specified in an agreement between the supplier and customer. The following section describes the main quality parameters specified at the point of connection,  and their potential impact on the operation of the network.

Supplied voltage

This is normally specified as the national standard voltage. Variations in voltage occur as the load on the network changes, but the voltage must be maintained within limits specified.

Under-voltage events, voltage dips and sags

These events seem to be the most frequent cause of power quality problems.as they receive the maximum amount of attention in quality standards. Many of the most costly and most common phenomena (dips, swells and interruptions) do not have compliance limits but only indicative values. Voltage dips and interruptions disturb many  types of devices connected to the network. A voltage dip or interruption of a few hundred ms may have damaging consequences for several hours. The paragraphs below cover the main consequences of voltage dips and interruptions on equipment used in the industrial, service and domestic sectors.

When a voltage dip occurs, the torque of an asynchronous motor drops suddenly which slows down the motor, and may result in motor stalling. Following an interruption, the motor tends to re-accelerate and absorb current whose value is nearly its starting current, the duration of which depends on the duration of the interruption.  This may result in unwanted operation of contactors or protective devices, incorrect operation, or shutdown of a machine. Rapidly reconnecting (~150 ms) the power to an asynchronous motor which is slowing down without precautionary measures may lead to reclosing in opposition to the phase between the source and the residual voltage. In this case the first current peak may reach three times the startup current. The overcurrents and consequent voltage drops have consequences for the motor  (excessive overheating and electro-dynamic force in the coils, which may cause insulation failures and torque shocks with abnormal mechanical stress on the couplings and reducers, leading to premature wear or even breakage) as well as other equipment such as contactors (wear or even fusion of the contacts). Overcurrents may cause tripping of the main general protective devices of the installation causing the process to shutdown.

The effects on synchronous motors are almost identical to those for asynchronous motors. Synchronous motors can however withstand deeper voltage dips (around 50%) without stalling, owing to their generally greater inertia, the possibilities of over-excitation and the fact that their torque is proportional to the voltage. In the event of stalling, the motor stops and the entire complex start-up process must be repeated.

Control devices (contactors, circuit breakers with voltage loss coils) powered directly from the network are sensitive to voltage dips. For a standard contactor, there is a minimum voltage value which must be observed (known as the drop-out voltage), otherwise the poles will separate and transform a voltage dip (lasting a few tens of milliseconds) or a short interruption into a long interruption until the contactor is reenergised.

Computer equipment is sensitive to voltage dips and operation outside limits leads to loss of data, incorrect commands, and shutdown or malfunction of equipment. Variable speed drives are affected by voltage dips and may trip out when a deep voltage dip occurs. Voltage dips can cause premature ageing of incandescent lamps and fluorescent tubes

Voltage swells

These are generally due to events such as switching out large loads or sections of network, or problems with voltage control devices such as regulators, tap changes, or capacitors.

Frequency deviations

Frequency deviations, which are generally beyond the control of the distribution utility, can cause damage to synchronous motors and other equipement.

Voltage unbalance

This is difference between phase voltages in a three phase system and can be caused by unequal loading of phases. The main effect is the overheating of 3-phase asynchronous machine as the current unbalance factor will be several times that of the supply voltage. Phase currents can differ considerably. This increases the overheating of the phase(s) which the highest current flows through and reduces the operating life of the machine.

Voltage harmonics and interharmonics

The consequences of harmonics are an increase in  peak and RMS values of voltages and currents. This results in energy loss, overloading and malfunctioning of switchgear. Harmonics acan also be responsible for power factor correction equipment resonance.

Voltage flicker

Flicker is a continuous series of short voltage sags in rapid succession, causing a noticeable disturbing variation in light levels.  Flicker can also result in damage to sensitive equipment.

Forced interruptions

Forced interruptions are caused by operation of protection devices or failure of one or more phases of a supply. An unplanned event that results in the disconnection of one or more phases of the network that supplies the customer for a period of more than 3 s, is categorised as a forced interruption. Such cases usually result in under-voltage events on the phase(s) that remain connected at the point of supply.

Planned interruptions

Planned interruptions, necessary for maintenance or repair work on the network, are considered as quality of service interruptions, for the purposes of measurement of quality of service.

Momentary interruption events (MV/LV):

These occur where an interrupting device on an MV or LV circuit has a sequence of operations, for example if a recloser or breaker operates two, three or four times and then holds.

Consumer power quality standards

These standards cover the power quality issues which the industrial consumer may cause on the grid, i.e. feed back into the grid or affect the power quality on the grid. Standards applicable to the supplier at the point of connection are also applicable within the customers installation, and may be especially so where a large installation is involved. Disturbances may occur within the customer installation which do not propagate into the grid but which nonetheless may affect the performance of customers plant. In addition to the disturbances propagated into the grid customers need to pay attention to internal disturbances and ensure that they are within the required limits of plant withstand. Consumer power quality standards at the point of connection cover the following:

Supplier network power quality standards

The  customer is required to take into account the power quality standards applicable to the supplier at the point of connection. These may be defined in a standard or contracted. The most important issue here is immunity of customer equipment to disturbances within the limits specified for the supply network. The process of dip immunity evaluation is described in NRS 048-7[6]

Emissions

This covers harmonic currents and EMC interference. Harmonic currents or non-sinusoidal distorted current waveforms are due to non linear loads such as variable frequency drives (VFDs), rectifiers, fluorescent lighting and a variety of other appliances. Any item of plant which converts the incoming AC to a DC supply and then to a variable AC voltage or frequency is a potential source of harmonic currents. The limits of harmonic current at the point of connection will be specified by the supplier.

Harmonic currents can be more of a concern to the customer than to the supplier, as distorted current waveforms can cause high I2R losses and overloaded switch gear. Most modern equipment . High harmonic currents can cause can cause distorted voltage waveforms on the supply which could interact with power factor correction equipment used by other customers. Harmonics can also result in EMC interference which can propagate into the network and cause faulty operation of electronic equipment. Broadband noise stretching over a wide spectrum can result from poor condition of contacts, commutation, arc furnaces, welding  and many other sources.

Power factor (PF)

Often overlooked in power quality standards but still of great significance, PF has more of an effect on the customer kVA supply than on the network. Compliance with supplier limits for PF is usually achieved by PF correction equipment at the point of connection, but this may cause losses within the customers network [8].

Voltage dips and sags

Voltage dips and sags caused by heavy load switching, such as motor starting, within the customer network are more of a problem to the customer than the supplier, but still have the potential to propagate into the supply network and affect other customers, depending on the configuration of the network. The supplier will specify voltage dip and sag limits. Mitigated by soft start or other methods.

Voltage levels

Equipment is designed to work at its optimum at a specified voltage. A new development in energy efficiency controls the voltage applied to equipment within close limits, and saving are claimed for this practice. It is in the interest of customers to ensure that voltages in their installations are kept within limits specified for the equipment.

Exclusions

Standards do not apply in cases where the supplier plant is subject to force majeure conditions such as violent actions  pilfering, theft, sabotage, attack and malicious damage or damage caused by accidental and unavoidable occurrences attributable to third parties.

Important to note as the increasing occurrence of cable and busbar theft in low voltage networks is a source of risk to  users. Particularly prevalent and damaging is the theft of neutral conductors. Removal of the neutral is particularly risky for single phase customers as this places two customers in series between phases with 400 V across them. The voltage across each customer will depend on the load that each draws and could exceed the limits across one of the users. Current will flow through both customers. Three phase customers will experience a similar situation, but balanced phase loads may reduce the voltage across each load.

Future possible impacts on power quality in industrial networks.

Changes to a smart grid concept and the possible introduction of distributed generation, particularly variable sources of renewable energy, will have an impact both on the quality of power delivered to consumers and power used by consumers. Prime here is the growing trend for own generation, particularly rooftop solar, primarily systems below  1 MW and many below 100 kW in size . Within the distribution network, the tendency will be to place the generating plant as close as possible to load centres. If a high percentage of a customers demand is supplied by own generation, sudden drop in the output of the generation plant will cause voltage dips on the supply. Although some large users have installed wind turbines at their plants, this is a rare occurrence and not usually applicable to industrial networks. Studies have shown that wind turbines using induction generators could cause number of problems, including flicker, voltage sags etc., when connected as embedded generators.

References:

[1]    NRS 048-2 2003: Electricity supply- quality of supply: Voltage characteristics, compatibility levels, limits and assessment methods.
[2]    NRS 048-7: Electricity supply-quality of supply: Application practices for end customers.
[3]    P Ferraci: “Power Quality” Schneider Electric, ECT 199 October 2001.
[4]    RASensi: “Power quality application guide: understanding compatibility levels”, Copper development association, March 2005.
[5]    M McGranaghan: “What power quality can you expect from your utility”, Electrical construction and maintenance, June 2003,  http://ecmweb.com/archive/inside-pq-1
[6]    M Muhlwitz, J Meyer and G Winkler: “Advanced power quality rating under the conditions of deregulated markets”, www.cired.net/publications/cired2001/2_51.pdf
[7]    B Kingham: “Quality of Supply Standards: Is EN 50160 the answer?”, White paper from Schneider, www.oasis-open.org/committees/download.php/37248/Power%20Quality%20White%20Paper%20from%20Schneider.pdf
[8]    JS Shakya, RK Saket and G Singh: “Power quality and reliability issues of induction generators for wind plants”, IJRRAS 11 (3), June 2012.

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