Electrical Power Quality: The rationale
What is PQ and what is QoS?
Voltage characterise the “ability” of the utility to sell a product (energy in the electrical form). The supply system (utility) delivers energy when the user energise the electrical circuit so that the applied voltage will cause the flow of current.
The quality of the current being withdrawn to a client with only a perfectly resistive load, is set by the quality of the voltage, hence the reason why PQ documents mostly deals with the concept of QoS.
Ohm’s law do cause a complex interaction when the user withdraws current in an irregular, non-linear and/or unbalanced fashion. Voltage drops over supply impedances can then degrade the quality of the voltage waveform at the Point of Connection (Poc). The utility has the obligation to maintain QoS within minimum standards at all points where customers connect and for that reason has manage both the QoS and the manner by which customers use the supplied energy.
Is PQ important to governments?
Yes, it is. We do not have competition in the Electricity Supply Industry (ESI). With competition and customers having the choice from whom to buy electrical energy, some degree of self-regulation can be possible, as suppliers have to compete for customers. Without this competition, government has to be involved to protect the economy and why a government tasked an Energy Regulator to look after the interests of both users and suppliers of electrical energy.
Electrical Power Quality (PQ) is known as a major contributor to operational risk at both the suppliers and users of electrical energy. Energy Regulators in Southern Africa recognised that the Quality of Supply (QoS) to customers requires compliance to minimum technical standards, as this will contain the operational risk.
It could be achieved by a licence condition set to an electrical utility. A well-known example is that of the National Energy Regulator of South Africa (NERSA) who has attempted by means of a Power Quality Directive and legislative action, to operationalize the governmental mandate given to NERSA to protect the interest of users and suppliers of electrical energy.
Is QoS important to utilities in Southern Africa?
It should be. For one, it could be a license condition set to utilities. But, in Southern Africa, the experience during the past 20 years has shown a significant diversity in success with regulatory efforts to the extent that many case studies exist (specifically in South Africa) of users not able to remain competitive under the existing economic challenges. That challenges were brought about not by the price of electricity as much as by the quality of the electrical energy sold to them.
What is QoS?
QoS or PQ and it describe both voltage waveform events and the quality of the steady state voltage waveform as depicted.
A simple example on the impact of voltage waveform events is where the dip performance at a manufacture PoC of wood products caused computerised process controls to regularly interrupt production. Not only production, product and man-hours are lost; the quality of the product can also be affected. With many competitors in such a market, poor PQ can be the reason why profit targets are not met and investors then decide to terminate business. The consequential damages to the local economy are evident.
The quality of the voltage waveform is also important. Although generators at power stations produce perfectly sinusoidal voltage waveforms perfectly balanced between phases and perfectly repetitive at 50 Hz (or 60 Hz), that is not the case where most users withdraw electrical energy. Many reasons to the degradation of the voltage quality exist. Most are due to the manner in which users withdraw current, but it can also be due to the manner in which some “users” inject current.
For example, Independent Power Producers (IPP’s) “inject” current into the distribution network. This current is not perfectly sinusoidal as a grid-connected inverter interface the energy source (such as PV) to the grid. With the distribution grid being impedance, the harmonics being injected can contribute to the degradation of the voltage waveform at the PCC. This component of voltage degradation is normally referred to as the harmonic distortion.
Similarly, the unbalance between phase currents can cause unbalanced voltage drops over the system impedance, resulting in unbalanced voltages at the PCC. Loading that continuously changes such as at an arc furnace can modulate the amplitude of the voltage, resulting in voltage flicker.
Why is PQ not perfect?
Because of Ohm’s law. Also known as the law of constant misery, when impedance is present and current withdrawal is not perfectly sinusoidal, and/or not by a purely resistive load, and/or not perfectly balanced between phases, Ohm’s law dictate the QoS.
A relevant example of how the change to environmentally friendly energy sources can impact the quality of the voltage waveform can be found where sources of renewable energy being connected to distribution networks all over Southern Africa. Sources of renewable energy inject energy by means of a solid-state interface. The operation of these power electronics is non-linear in principle. Increased levels in voltage waveform distortion due to current harmonics being injected, an increase in voltage flicker and even in voltage waveform asymmetry can result in the network to which these sources connect.
This impact on the supply network is collectively referred to as emission. The owner of the distribution network requires QoS to be contained within acceptable levels. “Acceptable” means that QoS has to remain well within the minimum standards set by the NRS 048 part 2-2007. From a network operation perspective, a “planning” level is the operational concept of QoS to be maintained lower than the minimum NRS 048 levels.
Voltage dips are mostly caused by short-circuit conditions. Many reasons exist to short-circuits. Lightning is a well-known cause. When a lightning stroke (direct or in-direct) cause a flash-over condition over an insulator, then some over-current protection upstream of that fault has to clear the fault. Reclosing of the breaker should restore the supply, as the arcing over the insulator would have been extinguished. During the time that the short-circuit current is flowing, voltage at one or more busses in the power system will be reduced due to the voltage drops towards the fault. It can be to the extent that it is classified as being a “voltage dip”.
Distance to the fault sets the depth of the voltage dip. The impedance near or at the fault will be small, resulting in a small residual voltage. The further away, the depth of the voltage dip will be less. Duration of the dip is set by the time the protection takes to clear the fault.
Protection schemes can be designed to minimise both the depth of a dip and the duration of the dip. Proper discrimination and coordination of protection settings is important to manage dip performance.
PQ has to be managed.
The Southern African power system is an interconnected system with sources of generation and consumers all over. A complex supply chain between areas and countries exist. A quality assurance process in the acquisition (buying of generation) of electrical energy makes sense.
With customers served all of the time with electrical energy within the minimum requirements of a technical standard (NRS048 part 2), the expectation is that these customers will bring about sustainable income for the utility or even additional income if the customers are profitable enough to need even more electrical energy.
A QoS assessment of each PoC is ideally needed to understand network performance. With this information available, management strategies and intervention schemes can be devised to maintain QoS within targets.
The management of PQ in South Africa is done mostly at Eskom but only at a few municipalities in South Africa. Some of the utilities in neighbouring countries do manage QoS.
As a whole, the regulatory approach to gain acceptance for the importance of QoS was not successful in Southern Africa. Legislation to enforce and to force utilities to comply with the regulatory requirements is not evident and possibly contributing to the lack of participation by utilities. The risk that investors have to deal with in Southern Africa when buying electrical energy is a contributor to why no recent significant investment in sophisticated manufacturing and processing industries are seen on Southern Africa.
QoS management at electrical utilities should therefore be a strategic priority and not only due to a license condition. Electrical energy is a major income stream for any utility and for the economy at large and it is in the interest of all parties to have a QoS management program operational.
PQ is about compatibility
The aim of PQ is mostly to maintain compatibility between supply and use condition. Any PQ parameter has to remain below the level at which user equipment will either fail or not operate as designed, known as the immunity level. Practically, the utility will attempt to maintain PQ disturbances well below a compatibility level to cater for levels in a PQ parameter that can experience a daily, weekly or seasonal change for example.
A typical statistical normal distribution can be expected in the behaviour of the disturbance level of a specific PQ parameter. For example, the total harmonic distortion of voltage will change as a function of time and this change over a period of time being possible to be depicted as a normal distribution as shown above
The idea is to have equipment as shown on the graph to be as immune as possible, in other words being able to be able to operate as designed even at high levels of disturbance in the supply voltage. That is not practical or cost-effective. A compromise between performance and cost of equipment therefore result in the importance of considering the immunity level of the equipment in use.
This concept is referred to as compatibility between supply and use conditions. Operationally, it results in both the utility and the user having a responsibility to consider this when using electrical equipment or supplying the electrical energy to the equipment. If utilities agree to manage the supply network to remain within certain levels of disturbance, it is helpful for the users as they can specify and use equipment with immunity levels well above that compatibility level.
Users have a right to good PQ
Electrical energy is a strategic commodity to many users, not only in the industrial and manufacturing sectors, but also to commercial institutions such as data centres, financial institutions, the pharmaceutical industry and many others who do not use as much energy, but that requires the energy used, to be of good quality and reliable.
The economic losses to all sectors are huge. It was measured scientifically in the European Union some years ago (2010) and the results indicated that even in such first world conditions, 150 Billion Euro’s are lost every year! Relatively speaking, it will be much worse in Southern Africa.
Even residential users have started to note the importance of proper QoS. The sophistication in modern electronic consumer products can only be realised when connected to a suitable voltage supply. Flat screen TV’s are known to be sensitive to the regulation of the voltage magnitude. A high failure rate of these TV’s exist in South Africa where the push for electricity for all has resulted in poorly regulated voltages at consumers due to the constraints on the supply fault levels.
Quality of Supply in electrical energy is a right to any South African and protected by the Consumer Protection Act, No. 68 of 2008. Every user has a legal right to a minimum quality in electrical energy, as South African courts have recognized electrical energy as consumer product. Regardless of the inability of the utility to properly serve customers with electricity of good quality, users can therefore consider legal recourses. As understanding for the importance in quality of electrical energy develops, it is expected that users will more and more claim their legal rights.