Measuring the reality of the grid: the impact of energy transition on current measurement
01 September 2022
There is a proverb in the Netherlands, ‘Meten is weten’, Roland Bürger, Business Development Engineer at next gen power grid measurement technology expert, Senseleq tells us here. A good translation would be ‘to measure
is to know’, or more freely, ‘the numbers tell the whole story’.
This article was originally featured in the September 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
The expression seems highly appropriate as we face the challenges of transitioning our energy generation from traditional sources to renewables. Without accurate measurement, it’s impossible to know what exactly is happening in the grid. Only when we have the numbers will the picture become clear – and a solution can be proposed to face these challenges.
Following the setting of general decarbonation targets in most countries, this article describes energy transition challenges in power grids, and how this will impact substation equipment, including the conventional current transformer. This is followed by a discussion outlining how a future-proof current transformer is expected to perform, before concluding by suggesting the use of wide-range, universal, highly accurate current transformers based on zero-flux technology.
An introduction to energy transition challenges on power grids
Today, there is little debate on the urgency to reduce CO2 emissions in all aspects of our lives. Typically, in Europe, there is a clear ongoing dynamic, symbolised by the European Commission’s proposal to cut greenhouse gas emissions by at least 55% by 2030, which sets Europe on a responsible path to becoming climate neutral by 2050. One of the biggest elements is related to electricity power generation, with a challenging goal expressed in the Clean Energy for All Europeans Package to achieve 32% for renewable energy sources in the EU’s energy mix by 2030. Europe’s energy sector is shifting from a fossil fuel dominated and supply-centric model to a clean, digitalised and electrified consumer-centric system with many distributed resources.
This looks great on paper, but what does it mean concretely for Transmission System Operators (TSOs) and Distribution System Operators (DSOs) in their daily power grid exploitation? In line with the ‘European Green Deal’, ENTSO-E (the European association for the cooperation of electricity TSOs) has established their view in the report, Vision on Market Design and System Operation towards 2030. One of the main conclusions is the need for transition of power generation from a dominated synchronous system to a hybrid synchronous and invertors one (see figure 1). In addition, it will increase the electricity production volatility coming from the renewable energy intermittency.
The logical result will be a significant increase of power electronic converters connected to the HV (high voltage) and MV (medium voltage) grids (see Figure 2), with a strong impact on power quality.
So what? How is this trend impacting the existing power grid and substation equipment? This is precisely the topic discussed in the next section.
Power quality matters!
Figure 1. The logical result will be a significant increase of power electronic converters connected to HV & MV grids (see Figure 2), with a strong impact on power quality
The consequences of the multiplication of power electronic devices on power quality is already well known in the low voltage arena. Indeed, the presence of switch mode power supplies, variable speed drives, UPS (uninterruptible power supplies) and diverse power converters used at the electricity consumption level have increased power quality challenges. Therefore, we can expect to see the same consequences in HV and MV grids, as the following phenomena are becoming more and more common:
• Harmonic pollution in AC grids: sub-harmonics, low harmonics, higher harmonics up to kHz can all impact loadability, useful lifetime, operational efficiency and output accuracy of how transformers perform in the grid.
• DC pollution in AC grids: this is changing the load point and saturation status of power transformers, distribution transformers and instrument transformers. This also impacts loadability, useful lifetime, operational efficiency and output accuracy of how transformers perform in the grid.
• Reduced useful lifetime of HV & MV equipment: this is due to operational conditions which are not aligned with design specifications. It has also resulted in further development of condition monitoring systems and asset management platforms, as well as proactive maintenance scheduling programmes.
• Increased importance of power quality due to the increased cost of damage/loss of load to customers.
At the heart of power systems are measurement systems which facilitate the delivery of a true picture of what is happening in the grid. Hence, the upgrade of current transformer technology is already a given. The following section explains this more fully.
In the past, grids were either AC (clean AC, without much harmonic distortion or DC pollution), or DC (clean DC, without AC harmonics). This is not the case anymore, due to shifts in the nature of power generation, transmission, distribution and application. Given this scenario, the question facing a highly polluted current flow is: ‘what should the nature of new measurements be?’
Is it still enough to record a single value of peak or rms (root mean square) current level, or must current transformers of the future deliver more information than just a number?
Current transformers of future
The previously mentioned ENTSO-E organisation explained in its Innovation Roadmap 2020-2030 report the emerging needs and challenges to “enable secure operation of widespread hybrid AC/DC grids”; among which, the following apply to current transformers (CTs) of future.
A future-proof CT is required that can measure a wide range of occurrences from extremely fast, high frequency phenomena to extremely low frequency DC phenomena. Only then can we truly know the ‘reality of the grid’. When we can measure everything, we can understand it, prepare improvement plans and take corrective actions.
Figure 2. How is this trend impacting the existing power grid & substation equipment?
The new CT technology is expected to find more applications with different use cases supported by one CT which is able to feed different IEDs (intelligent electronic devices) with different functionalities – such as protection relays, accounting and metering, power quality, including DC pollution and low/high kHz harmonics, digitalisation over a wide range of currents 0A to kA, condition monitoring, and so on.
The new CT must be able to deliver the following specifications:
• Highly accurate AC & DC current measurement (exceeding existing accuracy classes) across a wide range of currents – from milliamperes to multiple kiloamperes.
• DC measurements in AC grids/DC pollution
• Power quality, harmonics (sub-harmonics, inter-harmonics, supra harmonics and so on)
• Extreme accuracy needed for a wide range of digitalisation – which enables the same CT to be used for different applications
• Accounting/billing standards and certifications
• Able to be combined in a hybrid solution with existing inductive CT technology through a transition phase and for retrofit applications
• Robust and flexible
• Simplified and safe – to deliver more functionality without introducing any new safety risks
Is all of this possible – or is it just a utopian dream?
Universality comes from hybridization
In addition to the conventional current transformer principle, there are also a large diversity of current measurement technologies, from the simple shunt to complex electronic sensors, such as optical, Hall effect or Rogowski. However, none of them can cover all the emerging measurement needs described above, except one: zero-flux. Indeed, this technology allows measurement from DC to several hundreds’ kHz with a very high accuracy (ppm range) and robustness equal to CT.
Thanks to its zero-flux universal measuring technology, Senseleq can provide a stable and reliable solution to solve the new measuring challenges.
The Senseleq solution includes three main components as follows:
• Ring core CT which provides the basic measurement of the primary current. The ring core can be delivered in cast resin set-up, in sub-assembly set-up for installation inside power transformers, or on a four-wheel solution for laboratories and the like.
• An analogue electronic system reads the secondary current and injects back current into separate windings on the ring core. This approach is based on zero-flux technology and makes sure that the core remains linear and accurate. This system includes a 19-inch rack solution and needs to be installed in an IP-rated environment.
• Secondary cable between the ring core CT and the electronic system, which provides the connection between the ring core windings – secondary winding, feedback winding, compensation winding, and so on.
This CT can also be delivered in a hybrid configuration, including traditional existing inductive cores, because the active part of the CT is the same technology as a ring core CT; the new addition is the analogue electronic system, which manages the linearity of the core to provide high accuracy measurements across a wide range.
In a hybrid CT solution, the existing metering and protection cores are also delivered as with the existing ring core CT around the transformer bushing. Therefore, in addition to the existing inductive ring cores – in the same cast resin unit – Senseleq delivers the new sensor with its output analogue current, which can be used by the customer for special purpose DC measurements. By implementing the new technologies in a controlled manner, the continued safe operation of CTs is ensured. The output signal of the analogue electronic system will provide the output signal which will be fed into the IEDs. This signal includes the entire spectrum of components – including AC 50Hz, DC, harmonics up to 50kHz – all at a high accuracy level.
Within the context of a major revolution for power grids for self-adaption in order to achieve carbon neutral objectives by 2050, there is a clear need for significant improvement of existing infrastructure. It is important, therefore, to acknowledge that in addition to on-going investment in software applications, there is also a definite requirement to upgrade the technology used in high- and medium-voltage equipment as well. The impact of power electronics, by increasing power quality monitoring importance, will reveal the urgency to change the existing approach by exploring new current measurement technology. At Senseleq, thanks to our long-term experience in zero-flux and conventional CT, we are well-positioned to help TSOs and DSOs to address the challenge.
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