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Prospects for Implementation of Non-conventional Instrument Transformers

The Brainstorm block is a collection of industry experts’ opinions on actual problems. It allows specialists to look at different problems from different perspectives.

Energy professionals from around the world answered several questions about non-conventional instrument transformers:

  1. How would you evaluate the prospects for implementation of non-conventional current and voltage transformers? What implementation strategy of non-conventional instrument transformers should be chosen? What types of instrument transformers should be implemented in different types of bays and equipment?
  2. What factors prevent non-conventional current and voltage transformers from being widespread? If some of the factors are technical, what are they?
  3. How can you evaluate the prospects of the implementation of merging units? Will they be implemented in the future in anticipation of mass adoption of non-conventional current and voltage transformers, and if yes, for what purposes?
  4. What types of current and voltage transformers for what voltage levels can be used in the most reasonable way — and why?
  5. In what ways will mass adoption of non-conventional current and voltage transformers influence secondary systems of substations?
  6. How long will it take non-conventional current and voltage transformers to be massively adopted? What is their mass adoption determined by (e.g. in terms of volume or market share)?

* * *

Maxim Maltsev
RusHydro, Russia

1. It is inappropriate to talk about the prospects of all types of non-conventional instrument transformers (NCITs). In power electronics, such devices have been used for decades and have shown only their best side. At our stations, dozens of excitation systems are equipped with so-called LEM sensors, and these are just electronic instrument transformers. The application of such measuring devices in high-voltage circuits of power plants and networks is more of a confirmation of the competitive advantages of technologies used in NCIT design, compared with conventional instrument transformers, which becomes more apparent with the advent of optical instrument transformers.

However, we must not forget about the ’infantile sicknesses’ of new devices, which do not yet allow one to state that the optical electrical technologies (OET), according to their performance characteristics, have reached the level of conventional transformers.

Nevertheless, currently, I believe that optical transformers are almost ready to be used in industrial facilities of an average responsibility level. As for power plants, in the next three years, RusHydro plans to install such devices in one of the facilities currently under construction.

2. Mass introduction of optical instrument transformers is primarily hampered by the following factors:

  • A lack of a single-industry, standard documentation, establishing the requirements for NCITs. During the design, testing and certification of NCITs, manufacturers currently apply regulatory documents that establish requirements for traditional current transformers/voltage transformers (CT/VT), which eliminates several potential advantages of NCITs.
  • Designers lack an understanding of the qualities, characteristics and properties of new equipment; inertness in the development of new, specific knowledge, computer networks, for example, a shortage of computer-aided design (CAD) diagrams for designing digital substations.
  • An absence of reference materials and methods of technical maintenance of new equipment that would define the requirements of personnel qualifications, tooling and work deadlines.
  • A lack of accumulated statistics characterizing the operational indicators, in particular, life-time metrics. It should be noted that these obstacles are relevant for virtually any new solution and technology. The mechanism for overcoming these issues is traditionally based on a gradual transition from research and development (R&D) to single implementations at low-priority facilities and then extend to wider applications in the industry. Meanwhile, each next step should be followed by a period sufficient for the collection and analysis of the obtained results, namely design modification, service, personnel training and subsequent organization of the production environment at equipment suppliers. I believe that at present we are in the stage of transition to single implementations of such equipment.

3. It should be noted, that even now, without mass implementation of NCITs, it cannot be said that MUs are widely used at power facilities.

In the current understanding, an MU is a device designed to convert analogue measurements into digital form with subsequent transmission of the results via the SV IEC 61850 protocol. In this form, obviously, the need for such devices will be further reduced with the mass introduction of NCITs.

There are good prospects of such devices as bay controllers (connection controllers).

However, in addition to measuring circuits, there are also signalling and monitoring circuits, the transfer of which into digital lines of communication will remain a vital task. To solve this problem, perhaps, there are good prospects of such devices as bay controllers (connection controllers). At the same time, I believe that development of such devices is directed towards their combination with circuit breaker automatic monitoring and soon we will see the mass application of devices such as a circuit breaker automatic-monitoring control processor, which will play the role of grass-roots devices in automation systems of both stations and substations. Furthermore, the wide introduction of NCITs will only contribute to this. Specifically, in our company, there are already three realized projects of gas insulated switch-gear (GIS) automation with voltage from 110 to 330 kV, complete with application of a circuit breaker automatic-monitoring control processor.

4. Optical current transformers — for all voltage classes from 110 kV and higher, as well as for applications at generator voltages. Regarding the voltage instrument transformers, the answer is not so obvious yet, and I believe that it will take another one or two years to test the devices already proposed, to draw some conclusions.

There are many technical, organizational and regulatory issues to be resolved, but there are no fundamental obstacles here.

5. Obviously, the impact will be quite strong. As a result, we will get one device (NCIT), which will be able to provide any required number of secondary devices with measuring information, and the concept of instrument transformers (IT) system separation on current transformer (CT) cores will be history, and will remain only as a measure for ensuring reliability due to hardware redundancy. There are many technical, organizational and regulatory issues to be resolved, but there are no fundamental obstacles here.

6. It will take 5–7 years before the full-edged competition of NCITs with traditional instrument CTs becomes apparent.

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Mikhail Khaikin
Gomelenergo, Belarus

1. The introduction of non-conventional and, in particular, the optical transformers, is not only promising, but also unavoidable for many reasons (production cost, metrological characteristics, ease of installation and maintenance, explosion and fire safety). It is obvious that it is possible to apply such NCITs only on those sites where secondary circuits are ready for it (relay protection and monitoring devices, metering devices, etc.), and a corresponding local area network (LAN) is built. Therefore, the strategy for implementing NCITs should be comprehensive: everything should be changed at the facility, including psychology of personnel, and not only of service personnel.

2. The feeling that everyone is waiting for something: manufacturers — for the emergence of demand, service — for those who will take the risk first. In addition, there are the technical factors: there are too few proposals on the market, and the technical (metrological) characteristics of NCITs are not ideal.

3. In conditions of mass introduction of NCITs, analogue MUs will not be needed, but it will take a long time to reach that point.

4. Optical transformers are the most promising for all voltage classes, in my opinion.

5. In the most revolutionary way — there will already be LAN circuits.

6. I think the process will be explosive, i.e. during new construction and partially during reconstruction only NCITs will be applied.

* * *

Oleg Bolshakov
FGC UES, Russia

When discussing the introduction of new instrument transformers (IT), two different aspects are often mixed. The first is the expansion of measurement capabilities (expansion of the measurement frequency range, absence of memory or magnetization effects, low
sensitivity to external EM interference, etc.). However, these measurements are not possible without an external power supply and electronic modules/converters/amplifiers. Well, if the presence of such electronic modules is still mandatory, why not equip IT with a digital output? Hence, the second aspect arises — the transition to the digital technology field with a standardized exchange on the digital bus and to the concept of a complete digital substation. However, for further consideration, I suggest that it is possible to switch to new measurement methods while remaining in the field of analogue signals, as well as to realize the entire structure of the digital substation using conventional CTs and VTs.

The greatest effect from implementing NCITs can be expected at sites with a high level of relay protection and automation responsibility (where there is a high risk of magnetization) and a high level of harmonic components (correct measurement with harmonics considered).

1. Based on the specifics of the new (or perhaps not really) physical principles of measurement, the greatest effect from implementing NCITs can be expected at sites with a high-level of relay protection and automation responsibility (where there is a high risk of magnetization) and a high-level of harmonic components (correct measurement with harmonics considered). It is in these cases that the difference in the measurement results of NCIT and traditional IT will have the greatest importance and one can expect the maximum effect from the introduction of new technologies.

The desire to expand the scope of IT requires the development of numerous technical documents.

2. Of course, technical problems when implementing new technologies always require a solution, and that is, in my opinion, the main issue generally, and is also in this case. Part of the solution is hampered by underestimating the importance of new measurement capabilities. After application of NCITs, measurements will be made, whose results will differ from traditional ones, and documents regulating the coexistence of new dimensions with the old ones are not yet available. Even a method for measuring the amount of electric power does not exist! But everyone understands that the energy measured at the fundamental frequency and in a wide frequency range (with harmonics considered) can vary significantly and it will be determined by the presence of these harmonics. With the tuning relay protection and automation settings, the speed (inertia) of the core magnetization is now considered, while in new technologies, there is no magnetization at all. Furthermore, how, in this case, will one coordinate the work of relay protection and automation bordering facilities built on different principles? Moreover, we have not yet touched upon the structure of the digital substation (SS). It is clear that the desire to expand the scope of IT requires the development of numerous technical documents from the fields of relay protection and automation, metrology, accounting, etc. Though, we must also recognize that the existing structures of the service divisions are optimized for the existing hierarchy of secondary devices.

3. I think that merging units are an atavism of the transition period. The secondary SS system must be either traditional or digital. Transforming from analogue to digital and vice versa reduces both the accuracy of the measurements and the reliability (due to increased equipment required); therefore, it has no independent technical and functional sense. At the stage of absence of many necessary regulations (see above), we will compare them with each other to confirm the correctness of the measurements. As soon as all the necessary regulatory documents that legalize the ’digital’ measurements appear, such transformations will not be necessary.

4. I think that these are magneto-optical CTs, equipped with electronic modules and combined (capacitive/resistive) dividers as a VT. At the same time, the regulations should indicate that they are not just scale converters/dividers, but full instruments of physical quantities (voltage or current). In addition, the service documents should consider the limitations imposed on the electronic components of these measuring instruments. If to speak on the prospects of implementation — I will repeat myself: the greatest efficiency will be obtained at sites with a high-level of relay protection and automation responsibility (where there is a demand for no magnetization) and a high-level of harmonic components (correct measurement, considering the harmonics). Of course, for this purpose, the NCIT assortment itself must cover all levels of voltage.

Deep specialized knowledge should be defined, formalized, approved and implemented in the form of standard algorithms.

5. It appears to me that, like everywhere else, there will be a struggle between the two trends: first — the structuring and development of each individual task (accounting, relay protection and automation, automatic management system of technological processes, wide area measurement system (WAMS), power quality monitoring and control system, diagnostic tasks, etc.) in terms of improving the algorithms, considering the additional factors, increasing the accuracy of the approximation; the second trend is the unification of all the ’secondary stuff’ on one hardware platform. Clearly, this can save a lot of money compared with the fully-divided traditional structure. But then the main issue will be the possible conflicts related to tasks and the servers. After all, all the tasks of the secondary system services require deep specialized knowledge, which IT specialists do not possess now, and will not in the future. Hence, all secondary system services will crowd around these SS monitoring servers to control their tasks. But this is the way of integration, reducing the ’hardware’ and increasing reliability, so it definitely will be in demand. Deep specialized knowledge should be defined, formalized, approved and implemented in the form of standard algorithms.

6. Unfortunately, making predictions is a thankless task. Especially considering our time — the time of multiple unordered local initiatives. As can be seen from the arguments given above, to solve the problem of mass introduction, the following is necessary: to correct the legislation, to develop new regulatory documents for legalizing digital (other) measurements, to develop and standardize the hardware, to make a metrological base available, to conduct successful testing on test sites for 3–5 years, as well as to make rules for the coexistence of new and old traditional systems. It is clear that this requires coordinated work of more than one organization and can be implemented in 5–7 years with a common program (project) with a clear role for distribution. Otherwise, the efforts of all interested parties will not be combined, and the situation will remain in the form of unordered local initiatives. It is almost impossible to predict this option.

* * *

Artyom Perepelitsyn
Transneft, Russia

1. In general, I see the application of NCITs to be promising. At the same time, I want to make a reservation, that not all technical solutions offered by the manufacturers are currently suitable for mass application. Before introduction of such IT’s, it is necessary to understand, what tasks we will solve, and whether it is expedient or not. There is no sense to consider the introduction of NCIT’s as a separate task, as it is not interesting either from a technical or economic point of view. This issue should be considered only in the context of a comprehensive digitalization of the power industry and the strategy for their implementation should be similar to any serious innovation in the strategic industry:

  • Complex preliminary analysis of the effect of NCIT applications with de ned implementation objectives.
  • Pilot projects with a duration of testing of at least 5 years and with a detailed analysis of the operation of the equipment.
  • Decision on the need for mass implementation.

2. There are many factors hampering the introduction of NCITs. Here are the ones I see as the crucial ones:

  • The main factor now is the cost. Currently, almost no one considers the application of NCITs as the only source of measurement without the use of conventional transformers, the cost of pilot projects, which should be added to the cost of installation of a traditional solution, deters most companies from testing this equipment.
  • Technical personnel cannot see advantages of NCITs clearly, but any shortcomings are obvious. All these ‘greater safety’, ‘hypothetical accuracy’, etc. are empty words for a protection engineer at a substation. In response to this list of advantages, most of them will say: ’It’s already fine as it is. Accuracy’s fine, and I already know how to work in main circuits with safety.’ But the protection engineer is 100% convinced that there will be more problems with the new equipment. Given the speed of decision to cut bonuses when dealing with malfunctions, the protection engineer starts thinking he or she does not need these problems. Therefore, the implementation is initiated usually from those above, while being resisted by technical specialists.
  • Technical imperfection of the proposed solutions. I will not take any time to describe these problems, but there are issues with all manufacturers, both Russian and foreign ones.
  • The conservatism of the power industry. Since reliability is one of the key parameters of any equipment, all specialists are very cautious about any innovations, and introducing something fundamentally new is a whole problem in itself.
  • Priority of the problem. The implementation of NCITs solves problems that are not of top priority. Most substations in our country have equipment that is more worn-out than IT, and one can spend their finances more efficiently than implementing NCITs.

3. The use of MUs in combination with traditional CT/VT is, in my opinion, an exceptionally temporary solution for testing a part of the digital substation technology. In the future, either NCITs will be applied with the appropriate justification, or one will continue to use traditional solutions.

4. I am not yet ready to answer the question of prospects of specific solutions, because my preferences are at the level of ’like/dislike’ and are not supported by the necessary experience with specific samples.

5. Cardinally. There is a lot that can be written on the subject, but with little point to it. I propose to wait for completion of the pilot projects to speak about this based on decisions taken.

6. More than 5 years. The specific time will be clear from the results of testing at actual sites and through financing developments in this area.

* * *

Chen Dehui
State Grid Electric Power Research Insitute, China

1. From the standpoint of technology development related to the power industry, conventional transformers will be replaced by non-conventional instrument transformers due to their good characteristics. In addition, the deployment of NCITs is also a key feature of a digital substation.

A better way of adopting NCITs is a pilot model.

2. From the experience gained on site during these years, I may say that the most important factors which prevent NCITs from being used worldwide in electrical substations are vibration, instability, etc. Most products could perform well in the laboratory, but their characteristics did not meet the requirements from some applications such as relay protection. Owing to this, NCITs are unsuitable for projects. A better way of adopting them is a pilot model.

3. A merging unit is a new concept or device from IEC 60044-7/8, and its wide use on site depends largely on the usage of non-conventional transformers. Along with it, a breaker IED is also emerged and adopted in some substations. In the future the interface associated to primary equipment will be specified and may be integrated partly with primary equipment in terms of construction.

* * *

Fred Steinhauser
Omicron electronics, Austria

1. There is a desire to get away from classical CTs and PTs with copper windings, iron cores and oil insulation. The reasons may vary from case to case. In some places, it is even the risk of exploding oil-filled CTs. There is also a potential for cost savings with new sensor technologies, which can be built cheaper than classical transformers. By now, the prices for the first specimen of non-conventional sensors are still high and oriented at the prices of classical CTs and PTs. But when the market will be established, with numbers growing and competition increasing, the prices will drop.

The most reliable, performant and cost-effective principle or product shall win.

There is no reason to favour a specific technology in the first place. The most reliable, performant and cost-effective principle or product shall win. The vendors need to make this stuff easy to use. Just specify the electrical performance and the protocol and leave us alone with the implementation details. The products must be made for the users, not for the scientists.

2. The communication networking issues are also involved, which adds another challenge when designing a system. Until recently, we had no serious solution for the redundancy problem in place. Several different options for designing the communication networks make it even more confusing for protection engineers.

3. Of course, when applying non-conventional sensors and when making actual use of a real process bus, merging units need to be used. They make sense not only with non-conventional sensors: merging units with inputs for classical CTs and PTs offer advantages when upgrading a secondary system to digital technology and still using already present CTs and PTs.

4. As said, do not let us discuss how this is implemented ’inside the box’. We should not need to know. The industry must come up with the best package.

5. A process bus with possibilities to distribute data from/to the process easily over the whole protection and control system provides options for the application of new concepts, which appeared to not be feasible until now because of the unreasonable efforts for the required wiring.

6. Now it is time for the concept to take off. Successful pilot projects need to prove that the concept works and delivers suitable benefits. This can substantially speed up the process.

* * *

Ivan Višiс́
GridClone, The Netherlands

Implementation of NCITs is one of the key changes in the process of full digitalizing of substation PAC systems.

Some of the technologies used with NCITs are quite old and some are newer, but for NCITs to be massively adopted, they need to prove their value in a few key aspects: they need to be cost-effective, to be cheaper than conventional transformers (or similarly priced but with added value), to provide better quality of measured values in terms of precision and/or range, if possible, through various atmospheric conditions and their measurement outputs need to be universally understood, as is the case with conventional analogue measurement transformers.

Implementation of NCITs is one of the key changes in the process of full digitalizing of substation PAC systems. To achieve the before mentioned interoperability, NCIT output measurements need to be standardized. A standardized form of such measurements definitely depends on the type of PAC systems it will be used in, but a huge success toward standardization of such measurements has been achieved through the development of the IEC 61850 9-2, UCA Guideline, also known as 9-2 LE and IEC 61869-9.

To understand how big an impact digitalization would have on PAC, one needs to understand the key impacts of digitalized (sampled) measurements. Analogue values are transmitted over one dedicated hard-wired channel and are available only at the time that they occur on the primary side. Sampled values are available in the communication network that can be proprietary or shared with other network traffic and can be delayed, mixed, corrupted or lost. From the delivery point of view, the most important aspects to address are communication network and a transportation mechanism. The task is to provide quick and stable delivery of the measurements, avoid network congestion and overload of subscriber’s communication controller capacity. All of this greatly impacts the communication topology as well as the types of redundancy methods, physical as well as functional (redundancy protocols). Delivered digital or sampled values should be used only if they are in real-time and in the correct sequence. To achieve that, all the equipment needs to be very precisely synchronized, somewhere in the range of hundreds to tenths of nanoseconds.

For NCITs to be widely accepted, the whole system should be based on digital measurements and needs to be designed around them.

For NCITs to be widely accepted, the whole system should be based on digital measurements and designed around them. Such a system can be based on conventional measurement transformers as well. In both cases, we will need merging units (MUs), the devices that incorporate logical MU devices consisting of logical nodes for current (TCTR) and/or voltage transformers (TVTR). An MU is an IED that delivers sampled values to a communication network and can have additional features such as the ability to digitalize binary signals. So, an MU becomes a basic element of any digital PAC system. It can be a standalone device or it can be implemented in the primary device itself. It will probably be more often used as the only measurement delivery device when used in larger facilities where the rest of the bay equipment is far apart from each other, such as the air-insulated high-voltage switch-yards. While in more compact facilities, an MU will also be used for controlling and monitoring of most or all bay equipment.

With digitalized measurements, distribution of the functions per devices becomes much more flexible. Functions can be more easily distributed over more IEDs, they can be more adaptive with a dynamic subscription to measurement streams as well as binary signals. On the other hand, with all signals digitalized close to the primary equipment, many functions can be centralized in a single IED that can cover more than one bay. At the end, more PAC schemas can be used at the same time as they will be independent of the primary equipment, enabling different schemes of functionality, redundancy and easy migration between the generations of IEDs.

Cybersecurity schemes will have to be implemented to prevent a whole range of unauthorized intrusions in the system.

Communication networks will become a backbone for fully digital PAC systems and their topology, network traffic routing and redundancy will have to support functional requirements of protection and control schemes. We should not forget about cybersecurity schemes which will have to be implemented to prevent a whole range of unauthorized intrusions in the system.

Massive acceptance of NCIT is directly connected with massive acceptance of fully digital substations, and it will progress as the industry proves that such systems can cost-effectively achieve the same level of availability and reliability that power systems require.

* * *

Priyanka Mohapatra
ScottishPower, Scotland

1. The financial benefits from future installations of non-conventional instrument transformers (NCITs) (optical CTs, VTs, sensors) hugely contributes to the business case of digital substations. The financial and operational benefits are listed as follows:

  • Saving footprint and civil costs: NCITs have a smaller footprint and are much lighter than conventional CTs/VTs. Thus, they require a smaller, or no foundation, and can be directly mounted on the primary equipment. This allows reuse of the existing structures and saves on civil costs for future foundations. The fibers from the NCITs carrying the measurements also have a smaller footprint than the copper cables, thus allowing reuse of the trenches in the substations in case of future expansion. This a huge financial and environmental benefit.
  • Operational flexibility: NCITs are easy to maintain and the primary equipment can remain energised while the secondary equipment, such as protection relays, are being replaced if they are receiving their measurements over a digital process bus. Thus, NCITs provide operational flexibility by increasing the availability of digital substations.
  • Increase in substation safety: CTs exploding is not uncommon in many parts of the world. But NCITs do not explode through moisture ingress or overheating as they contain no oil or gas. This hugely enhances the substation’s safety for its engineers working on site and also for the primary plant.
  • They are a good enabler for future wide-area applications requiring much better data quality as they are not prone to noise and have a higher bandwidth of measurement.

2. Lack of an appropriate technical standard that could standardise the way measurements are performed from a NCIT to the secondary equipment was a major barrier in the use of NCITs in the past. However, with the evolution of IEC 61850, especially IEC 61850-9-2, defining a standard for SVs and the GOOSE over process bus has revolutionized the use of NCITs in substations for various applications such as protection and monitoring.

There are still challenges around time synchronization, as unreliable time synchronization can negatively affect the reliability of digital measurements; thus, leading to critical function failure. However, the international standards using PTP and IEEE 1588 have significantly addressed this issue and provide much more reliable time synchronization. Other issues such as communications failure and loss of packets are currently being tested in simulation and live environments and do not pose a large challenge to the use of NCITs in substations.

3. MUs, especially standalone MUs, provide a way to reuse existing CTs/VTs in digital substations. They form a crucial part of the digital substation architecture as they implement the analogue-to-digital conversion algorithm and are ultimately responsible for the transfer of the standardised SVs to the process bus. The MUs need to be interoperable with multiple vendors’ secondary equipment and the standard needs to be developed to ensure this, as MUs have a longer lifetime than secondary equipment and utilities will prefer to be vendor-agnostic while choosing their next set of equipment.

With mass adoption of the digital substation standards as the prices of the NCITs will ultimately drop, the benefits will be realised at all voltage levels.

4. NCITs currently have larger financial benefits in HV/MV applications due to the size of the substations, and bigger SS footprint. There are bigger savings to be made in civil costs at HV/MV substations. However, with mass adoption of the digital substation standards as the prices of the NCITs will ultimately drop, the benefits will be realised at all voltage levels. Aside from actual capital expenditure (CAPEX) costs, there is no barrier in my opinion to the use of this technology at any voltage level.

5. It will greatly increase the adoption of applications relying on better data quality, such as power quality monitoring, phasor measurements and fault locations. However, the current limitations on sampling rates from various MUs commercially available are constraining the use of this technology for a few specific applications. I am positive with the wider use of this technology those challenges can also be addressed.

6. It depends! Markets catering to higher volumes of intermittent renewable generation will see a quicker adoption of the digital technology as it will increase operational flexibility and serve as an enabler to applications enhancing substation intelligence. In my opinion, the next 5 years will see a rise in pilot and business applications of NCITs. Furthermore, I hope that utilities, vendors and internal standard committees working together can make the decision easier in the future where every new SS will be a digital substation without any concerns regarding reliability and dependability.

* * *

Thomas Rudolph
Schneider Electric, Germany

1. ENCITs provide many advantages, namely cost, size, weight and dimensions; therefore, they will be used more extensively. Presently, I see application areas only in transmission and sub-transmission above 90 kV and mainly in AIS on which large VTs and CTs are used today. In MV, LPITs will provide same advantages and the interface will be kept as analogue, or SV will be derived from the relays itself for voltage distribution as an example.

2. There are 5 points which have limited the use:

  • Missing product standards regarding the functions, performances, communication (should be overcome by IEC 61869-9, IEC 61869-13).
  • Anticipated lower-lifetime compared to conventional CT/VTs.
  • Test procedures for each part (sensors, electronic).
  • Additional power supply.
  • Missing standardisation on sensors and their interface to the electronics.

3. I see SAMU as important for refurbishment of the existing substations moving to process bus architectures as well as for new substations which should retain conventional CT/VT but should go for the process bus as well. Nevertheless, on new HV substations, there will be a trend to use full-packaged sensors/MU functions.

4. As stated above: LPIT on MV; fiber-optic based CTs/VTs above 90 kV.

5. It will influence the architectures concerning the communication between the process level and the bay level as well as the functional distribution. A major effect will be on the replacement of ring-wiring with consequences in engineering, commissioning, testing and documentation.

6. 5 years for wide use (identical to that experienced with the introduction of digital relays).

* * *

Igor Mets
Elering, Estonia

The price/quality ratio of a new technology for measuring electrical ratings should be competitive with traditional instrument transformers.

1. The application of a non-conventional instrument transformer (NCIT) technology should not be an end in itself. Although it is an opportunity to introduce the latest technology at the process level, the use of NCITs should be primarily due to effective planning for materials and costs — for example, in the construction framework of new high-voltage substations. In other words, the price/quality ratio of a new technology for measuring electrical ratings (values) should be competitive with traditional instrument transformers. Nevertheless, the purchase of NCITs is affected by a variety of aspects: the length of the life cycle, operating time to failure, operating time between failures.

However, all the preceding arguments are irrelevant if there is an insufficient — or non-existent — legislative framework for testing, verification, and application of NCITs. For example, the lack of metrological principles for verification of the NCIT (e.g., a standardised test), specified by legislative acts, will not allow the use of NCITs for the commercial accounting of electricity consumption.

The strategy for implementing NCITs directly depends on the technological process in which NCITs are intended to be applied. In the case of high-voltage substations, measurements of frequency, current, voltage, active and reactive power, and voltage on buses and in cubicles of power transmission lines are all important. But in metal-smelting plants, attention is primarily on current and frequency measurements. The use of NCITs as indication devices alone is not advisable. NCITs should be functionally reliable in relay-protection systems. Of course, the application of NCITs as commercial electric meters would further strengthen the position of innovative instrument technology.

2. The main factors that prevent mass introduction of NCIT are the following:

  • The lack of a legislative framework for large-scale application.
  • The lack of suitable voltage sensors on the market.
  • The relative monopoly on the production of current sensors.
  • The sensitive temperature dependence of the current measurement error at small values (≤300 A);
  • The paucity of specialists for implementing NCIT.
  • The inconsistency in the dynamic behavior of analogue-to-digital converter units in NCIT as pertains to implementing differential protection on high-voltage power transmission lines with a traditional solution present at the substation on the opposite side.
  • A market shortage of relay-protection devices that possess a discrete measurement signal interface (process bus).
  • The need for a standard when testing NCIT at a facility.

3. The prospects of using solely classical analogue signal converters when contemplating a transition to discrete signal measurement is complicated by the fact that relay protection devices remain classical. A solution will need to be found for partial renovation of traditional high-voltage substations. In the future, it is unlikely that there are any more possibilities for the application of traditional systems if the interface of classical analogue signals is not integrated into the converters along with the bus interface of the process.

4. The principal difference among NCITs depends primarily on the type of substation under construction (i.e., air-insulated switch-gear (AIS) or gas-insulated switch-gear (GIS)). Here, Rogowski coils (GIS) or current sensors (based on the Faraday effect) can be used. However, there is a problem when using a voltage sensor that is based on the Pockels effect, because a complete product does not exist yet.

Recently, a capacitive voltage divider (RC-divider) has been widely used as a voltage sensor. The main advantages of this solution are the linearity of the rating from zero to several kHz, high measurement accuracy not only of the voltage, but also of the harmonics (less than 1%), a small deviation of the phase angle over the whole frequency range, and the lack of a saturation effect (owing to the absence of a core).

Voltage classes in which NCITs can be applied may be identified using the quality and price ratings of the current sensors. Usually, this means a 110 kV voltage class or higher. Current sensors for lower voltage classes are more expensive owing to manufacturing technology. Thus, classes of medium and low high-voltage will incur greater cost when implementing NCITs.

5. A mass demand for NCITs should result in the following effects on the secondary systems of energy facilities:

  • Reduction in cost of all relay-protection devices.
  • Increase in the cost of the implementation of local computer networks and their hardware.
  • Cheaper implementation of primary measuring equipment.
  • Increase in the cost of secondary hardware of the NCIT intended to provide redundancy.
  • Increased attention to the implementation of the process bus and the corresponding increase in the cost of work and the amount of man-hours (commissioning, testing).
  • Control over the hardware compatibility of the process bus devices will increase the cost of the introduction of NCIT for the entire project implementation period.
  • Reduction of the cost of the project because of a reduction of time needed to procure and install electric circuits.

6. Mass introduction of NCITs will be tempered by market opportunities for meeting the demand of buyers for relay-protection devices that have inputs for digital flows of measurement values coming from the NCIT. Some of the criteria for mass introduction of NCITs are listed below:

  • Readiness of the legislative framework for the application of NCITs.
  • Compliance of NCIT systems with international standards.
  • Readiness of a NCIT technology to provide discrete signals measurements (process bus) for relay protection as well as for power quality analysis. In the latter case, the recognition of the required number of harmonics by the NCIT is a prerequisite.
  • Readiness of the NCIT to provide analogue signal measurements (classical analogue signal: 1 A or 5 A, 200 mV or 4 V) for relay protection, as well as for power quality analysis (this is important at the time of introduction of NCITs in the presence of traditional relay protection).
  • Presence of relay-protection devices that accept digital inputs (process bus).
  • Suitable measurement accuracy of the NCIT (for example, digitalisation of an analogue signal at a high discrete frequency).
  • Resilience to electromagnetic interference.
  • Dynamic stability.
  • Maintain ability and standardisation of components of NCIT systems (interchangeability of standard components).
  • Favourable cost of NCITs in comparison to traditional ITs.
  • Comparable life cycle of NCITs in comparison to traditional ITs.
  • Infrequent maintenance of NCITs.
  • Presence of qualified personnel to setup and repair the NCIT.

The editorial team is deeply grateful to all the speakers for their answers.

Digital Substation