CIGRE Reference Paper : Power quality trends in the transition to carbon-free electrical energy systems

03 February 2020

By C.F. FLYTKJAER (DK) and Z. EMIN, SC C4 Chair (GB)

Due to the accelerated shift towards a carbon-free electrical energy system, the power system is changing in terms of both planning and operation with an increasing integration of converter-interfaced renewable generation at all voltage levels. One area strongly affected by these changes is power quality where, if not managed correctly, it can result in equipment mis-operation, accelerated aging, tripping of plant, loss of production process, etc. Power quality is ultimately a customer-driven issue but failure to provide the adequate supply can also have negative impact on system operators, including customer complaints, reputational damage and financial liability.

Power systems globally are experiencing a transition towards decarbonisation of electricity production through large-scale deployment of central and distributed renewable energy sources (RES), which are gradually replacing conventional thermal plant. The connection of RES to the power system is mostly achieved using power electronic (PE) converters. Equipment interfaced through PE-converters can have both a positive or negative effect on power quality, depending on the type of disturbance evaluated and the applied control strategy of the PE-converter.

Presently, the understanding of the impact of PE-converters and some related phenomena is not fully developed. However, it is widely accepted that the consequences of degraded power quality can have severe financial implications and most studies in the US and Europe point to an excessively high level of cost if serious problems arise. This reference paper provides a high-level summary of the main and topical power quality issues in this changing environment with the aim of raising awareness of the main facets of power quality.

Harmonics

Harmonics can be present in voltage and current waveforms and are defined as “a sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency”. Harmonic distortion is caused by non-linear devices connected to the power system. Unlike linear devices, a non-linear device reacts to a perfect sinusoidal voltage waveform with a distorted current

It is generally expected that power systems around the world will experience an increase in harmonic distortion as the green transition progresses. This is partly due to the sheer number of converter-interfaced equipment being connected and partly due to possible modification of existing distortion levels. However, emphasis on limitation of harmonic emissions is gaining more attention and driving a trend in the opposite direction, that harmonic emission of new plants, as a whole, is reduced at equipment level due to more advanced switching and control technologies being implemented and the stricter enforcement of grid code requirements. Moreover, with local resonance introduced by the use of cables, it will be difficult to generalise. A trend towards a more profound focus to undertake detailed analysis at the planning stages, to ensure adherence to statutory limits and hence secure power system operation is however manifesting.

Voltage variations

Voltage variations refer to the changes of the voltage waveform. This could take place throughout the day as slow variation due to gradual customer load variation and/or variable RES output or it could take the form of rapid voltage changes and dips caused by various switching operations. Larger and more frequent voltage variations can be expected due to increasing penetration of intermittent generation resources such as wind and solar. Such voltage variations can lead to both under voltage and over voltage where both situations can have an impact on network operation and on customer equipment; the effect will be strongest in areas of the power system having low system strength. In distribution networks, over voltage can lead to excess energy consumption, transformer core saturation and stressing of insulation leading to their premature failure. Under voltage can lead to reduced energy consumption, malfunctioning of high-intensity discharge lamps and reduction of torque developed by mains connected motors.

Reduced system strength at transmission levels means that voltage dips, typically caused by system faults, transformer energization or large motor starting, can become more frequent and severe and will also propagate to downstream distribution networks. Intermittent power output, combined with a reduction in system strength will result in a higher volatility in the system voltage at transmission level making fast voltage variations a possible issue at high voltage levels.

Voltage unbalance

Voltage unbalance is defined as a “condition in a poly-phase system in which the magnitudes of the phase voltages and/or the phase angles between consecutive phase voltages, are not all equal Proliferation of technologies such as photovoltaic systems at the low voltage level is taking place as single-phase connections. single-phase PV connections can be altered between the phases but in places where only single-phase laterals are available, all connections end up on the same phase which can lead to significant voltage unbalance levels on three-phase low voltage systems. Other technologies that will have an impact include electric vehicle charging points and heat pumps at the low voltage level. These will have an increased level power capacity and are likely to introduce more voltage unbalance and hence their capacities may be limited depending on the fault level at the point of connection. Large scale wind and solar farms are often connected at remote locations supplied by relatively long un-transposed lines, and hence voltage unbalance can arise due to the lines although the wind or the solar farms inject balanced currents.

Concluding remarks
The power quality in the future power system is expected to be significantly affected by the shift towards a carbon-free electrical energy system. Several trends point to degraded power quality indices of future power systems due to the integration of many power electronics devices, use of power cables at all voltage levels, increasing amount of fluctuating production and generally reduced system strength. However, with the ability to control power electronics new possibilities are emerging and if used correct many of the challenges introduced can be mitigated by the same components that create them. Doing so successfully requires high focus on power quality studies both at individual connection and system wide level, focus on grid code requirements and their implementation and robust system monitoring with a strategic approach.

CIGRE Reference Paper : The need for enhanced power system modelling techniques and simulation tools

03 February 2020

B. BADRZADEH, C4.56 Convenor (AU) and Z. EMIN, SC C4 Chair (GB)

The transition to a clean energy future requires thorough understanding of increasingly complex interactions between conventional generation, network equipment, variable renewable generation technologies (centralised and distributed), and demand response. Secure and reliable operation under such complex interactions requires the use of more advanced power system modelling and simulation tools and techniques. Conventional tools and techniques are reaching their limits to support such paradigm shifts.

Emerging types of power system simulation models

Power system simulation models can be broadly divided into static and dynamic models. Root mean square (RMS) dynamic models have been the most widely used type of dynamic models for assessing most power system technical performance issues of classical power systems, from a planning and operations perspective. These models cannot represent the sub-cycle phenomena and control systems associated with the controls of inverter-based resources.

These limitations often manifest themselves when the phenomenon of interest has a dominant frequency deviating by more than ± 5 Hz with respect to the network fundamental frequency, or when the system strength available to inverter-based resources approaches close to or drops below their withstand capability. The latter situation applies even if the dominant frequency of interest is at or near the network fundamental frequency.

Electromagnetic transient (EMT) dynamic models can fully address this limitation, however, inclusion of significantly higher level of details generally results in a higher computational burden relative to the RMS dynamic models limiting their applications Despite this, EMT models are being increasingly used in some countries for large-scale stability studies for operating scenarios with a high penetration of inverter-based resources.

EMT models can be divided into offline and real-time models. Offline EMT models are being progressively used by network owners and system operators for large-scale power system studies and are generally available from most major Original Equipment Manufacturers (OEMs). The use of real-time EMT modelling addresses some concerns regarding the speed of simulation for offline EMT studies. However, it is understood that the required software/hardware are not supported by many OEMs. The use of hybrid dynamic simulation aiming to combine the advantages of each of the RMS and EMT simulations has been recently implemented in some commercial power system analysis tools. These include hybrid RMS and offline EMT, or RMS and real-time EMT simulation.

Modelling tools for planning and operational studies

Power system simulation models are used in a wide range of applications including long-term planning, connection studies, operational planning and real-time operations. Despite increasing applications of EMT dynamic models, both the static types and RMS dynamic models have strong roles in some applications, and this is unlikely to change in the near future. This stems from the need for faster speed of simulation for real-time and near real-time applications, availability of accurate and adequate data when looking at several years or decades ahead, or simply the need to conduct simulation studies in other tools, for example for most power quality studies.

Model validation

Fundamental to all types of power system modelling approaches and plant models is model validation. Gaining confidence in the accuracy of power system models is paramount as these models are heavily relied upon for the development and operation of the actual power system. Model validation provides a measure of how accurate a given model is for the intended purpose(s). Different approaches are applied depending on the stage of connection project, country specific requirements, and whether the testing is initiated by the respective OEM or transmission/independent system operators. These include staged testing on a complete generating system, e.g. full wind or solar farm, hardware-in-the-loop simulation of individual plant such as a wind turbine or solar inverter, or leveraging on data captured during system disturbances for aspects of the dynamic model where congruence between plant and model dynamic responses may be difficult to demonstrate until a network disturbance occurs, e.g. fault ride-through function of inverter-based resources.

Distributed energy resources (DER) and load modelling

As DER levels grow, their behaviour becomes increasingly significant. The control systems in inverter-based DER may have settings that cause large numbers to act in unison, and the possibility of the mass mis-operation of large numbers of devices during power system disturbances poses a serious risk to system security. State-of-the-art RMS dynamic models of the DER has been recently developed. Activities are ongoing to validate these aggregate models against measured system disturbances. Operation under low system strength conditions with reduced levels of synchronous generators and increased uptake of the inverter-based resources, would increase the need for EMT dynamics models of the DER as well as the large-scale inverter-based resources.

There has been an increasing trend in deploying inverter-based loads in the distribution system as well as the inverter-based DER, both of which share somewhat similar characteristics and pose similar challenges to power system planning and operation. The validation and potential modification of the RMS dynamic load models, and their adaptation into EMT dynamic models is therefore expected to become equally important.

Concluding remarks

The transition to a clean energy future will require the use of more advanced and detailed power system models and simulation tools in order to ensure the power system can be planned and operated securely and reliably. There is increasing evidence that EMT modelling will be required when studying the impact of inverter-based resources under low system strength conditions, including scenarios with high share of inverter-based DER and loads. RMS models of inverter-based resources may be unable to reliably predict control instability. Furthermore, control interaction studies are increasingly becoming a requirement for connection of inverter-based resources in parts of the network with low levels of available system strength.

The ability to conduct EMT simulation studies for large-scale power systems is also becoming a necessity in some jurisdictions who are implementing ambitious renewable energy targets. To date, EMT analysis has not been integrated in any control room environment for real-time assessments. These types of simulations are usually significantly more computationally expensive than RMS models. To address simulation speed issues associated with EMT models, state-of-the-art solution techniques are being progressively developed by software and hardware developers.