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Title:
APPARATUS AND CONTROL METHOD OF SELF ORGANIZED OPERATION OF DISTRIBUTION GRID SECTIONS WITHOUT NEW PHYSICAL COMMUNICATION INFRASTRUCTURE
Document Type and Number:
WIPO Patent Application WO/2018/020297
Kind Code:
A1
Abstract:
Apparatus (Frequency Decoupling Smart Transformer - FDST) and control method of self organized distribution grid segments/sections operation without new physical communication infrastructure, implemented by decoupling distribution grid segments operation frequency from the upstream and neighboring grid, allowing frequency variation, active power flow regulation and correlated control and energy management of generators, controllable loads and storage available capacity. Furthermore, voltage regulation is performed at the Frequency Decoupling Smart Transformer site and locally by the distributed active power electronic devices. Under new regulations and technical limitations plug and play functionality per user connection for new DER units and controllable loads at the grid segments (microgrids) can be established.

Inventors:
TSELEPIS EFSTATHIOS (GR)
Application Number:
PCT/IB2016/054496
Publication Date:
February 01, 2018
Filing Date:
July 28, 2016
Export Citation:
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Assignee:
TSELEPIS EFSTATHIOS (GR)
International Classes:
H02J3/38; H02M5/40
Foreign References:
CN104578046A2015-04-29
DE10205261A12003-08-21
US20140265607A12014-09-18
Other References:
DE CARNE GIOVANNI ET AL: "Variable frequency voltage control in a ST-fed grid by means of a Fractional-Order Repetitive Control", 2016 IEEE 25TH INTERNATIONAL SYMPOSIUM ON INDUSTRIAL ELECTRONICS (ISIE), IEEE, 8 June 2016 (2016-06-08), pages 1230 - 1235, XP033006430, DOI: 10.1109/ISIE.2016.7745070
T. STETZ; M. KRAICZY; K. DIWOLD; M. BRAUN; B. BLETTERIE; C. MAYR; R. BRIINDLINGER; B. NOONE; A. BRUCE; I. MACGILL: "High Penetration PV in Local Distribution Grids - Outcomes of the IEA PVPS Task 14 Subtask 2", 29TH EUROPEAN SOLAR PHOTOVOLTAIC CONFERENCE IN AMSTERDAM, 22 September 2014 (2014-09-22)
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Claims:
CLAIMS

1. Apparatus and control method of local self organized operation of grid segments/sections through a Frequency Decoupling Smart Transformer (FDST) without new physical communication infrastructure for a very large number of controllable devices (generators, controllable loads/devices, battery storage units, etc.) as represented in drawings 1,2,3,4,5. The invention presents a concept that uses and combines in a novel way, electric power devices, control methodology, a novel Frequency Decoupling Smart Transformer at the grid segment substation, grid frequency mediated communication and coordinated operation of all controllable components connected to electric grid segments without the need of a new communication infrastructure in the distribution grid segments.

2. The "Frequency Decoupling Smart Transformer" (FDST) and control method allows very large increase of the RES generator accepting capacity in the grid segment via voltage and frequency variation at FDST point of connection. Furthermore, it allows load management in the distribution networks with the associated environmental, economic and operational benefits. It also contributes to the transformation of the networks to frequency dependent self organized and regulated Smart Grids with optimized management of RES generator production, through storage management and power sharing between adjacent grid segments or even in coordinated exchange of power flow between several grid segments or "Microgrids" with the upstream distribution and transmission grid. The Distribution System Operator (DSO) may implement control strategies through the "Frequency Decoupling Smart Transformer" (FDST) such as: active power flow control, voltage control, temporary disconnection and re-connection of the grid segment from upstream network, in case of emergency or even adjacent grid segments in the same voltage level, in order to minimize flows through the "transformers", support adjacent LV grid segments and finally to optimize its flows with the upstream Medium and Higher Voltage grid in a coordinated way. This regulation approach could also be followed in the upstream grid nodes in Medium and High Voltage implementing a bottom up hierarchically coordinated electrical system (Drawing 3).

3. The apparatus (FDST) and control method decouples the LV grid segment (Microgrid) operation frequency from the frequency of the medium voltage upstream grid or even adjacent grid segments at the same voltage level, allowing grid frequency variation and thus power control, voltage control and energy management of generators, controllable loads, battery storage capacity and thus plug and play functionality, within certain predefined technical limits, for new DER units and controllable loads per connection point, within a defined grid segment of a "microgrid". Grid frequency variation in interconnected grid segments offers control simplification and increased reliability of the grid segment state communication and implementation scheme, reducing monitoring and control points.

The apparatus (FDST) and control method avoids investment in new communication infrastructure for direct communication systems and avoids security, latency, robustness and reliability issues with a very large number of grid connected controllable devices. It promotes the use of the grid frequency for power control and energy management of the generating and absorbing/consuming devices.

The various grid frequency windows for various grid segment operation states are presented in one potential implementation as proposed in Table 1. The frequency windows as presented in Table 1 (the frequency windows could be implemented with much tighter frequency ranges and be redefined as needed) may correspond to different tariff charges or service charges and credits according to the demand response and ancillary services received from the prosumers. When the grid segment is in normal frequency operation range, such as 49.8 to 50.2 Hz, then the usual electricity market procedures are followed. The excursion outside the normal operation range (such as 49.8 - 50.2 Hz), would energize automatic demand and generation response according to the urgent needs (Emergency condition levels - Table 1) of the grid segment implemented by the DSO. Therefore, that would require from the distributed interconnected electric devices to monitor and respond accordingly (Demand and Generation Response).

The apparatus (FDST) and control method applies a uniform methodology within grid segments per point of connection without the need for individual assessment. In practice, the local controllers of all the grid-connected units, such as energy generators, storage, and active loads in the grid segment will provide balancing and ancillary services such as voltage, frequency support as well as energy buffering, depending on their local voltage situation, the droop frequency logic, their own state/limitations within the grid segment.

7. One of the implementations of the frequency decoupling between electricity grid sections of different voltage level is the apparatus (FDST) in its generic form composed of two back to back 4-quadrant inverters with communication, monitoring, control and storage integration capabilities, implementing a "Frequency Decoupling Smart Transformer" (FDST), for frequency decoupling of the LV grid segment (Microgrid) at the MV/LV transformer level, or it could be extended to higher voltages in the approach, such as MV/HV FDST, as presented in Drawing 3. The "Frequency Decoupling Smart Transformer" has also the capacity to control power flow, to vary its voltage level to correct possible grid voltage excursions outside the accepted voltage operation window in the downstream network. The FDST can be instructed to vary power flow through the "transformer" by energizing storage reserves and/or shifting loads and generation and inducing temporary islanding of the grid segment or "Microgrid".

8. One realization of the FDST hardware and control at the MV/LV node substation, at the same voltage level, with many LV grid segments starting from the substation could be implemented as follows. The FDST is composed of power converters going from MV to LV as follows AC(MV) / DC - DC / AC(LV). On the LV AC side each grid segment departure from the substation will have a dedicated DC / AC(LV) converter, which are all connected to the same DC bus (see drawing 5), thus allowing each LV grid segment to be independently optimized in voltage profile and operate its own energy management (i.e. frequency variation profile) strategy according to the local segment conditions (load, generation, storage, neighboring grid segments, etc.). The DC bus of the FDST will be the electricity exchange point, when internal (same substation) electricity exchanges of the MV/LV node are taking place. The DC / AC(LV) converters may also parallel with adjacent converters on the LV side in order to increase efficiency and reliability in case of converter failures. The DC / AC(LV) converters could be expandable and modular in their design. The upstream side of the DC bus is connected to a single or several DC / AC(MV) converters appropriately sized for the substation needs for reliability and efficient operation. This way the FDST control can make use first of local grid segments resources, before resorting to the upstream MV grid. As more generation, storage and load shift flexibility resources are added to the LV grid segments the exchange with the upstream grid is reduced with benefits in terms of electricity use efficiency, reduction in new upstream infrastructure investment and increased grid reliability.

9. The frequency decoupling solution at the MV/LV "transformer", FDST, provides the opportunity to the Distribution System Operator to introduce storage media (battery bank, flywheel, capacitors, etc), sized appropriately to allow planned or emergency islanded and transitional grid segment or "Microgrid" operation between islanded and grid-connected modes and the associated benefits for the LV grid network and its stakeholders without the need to define any further the development of the grid segment in terms of resources, Drawing 2, but only introducing regulations and technical specifications of allowed interconnection capacity for power and storage devices per point of connection to the grid per "consumer" or "prosumer" (as consumers are usually called if they have their own RES generation). The DSO will operate the FDST node as the last actively controlled point of the downstream grid and through it, implement its monitoring and operational control.

10. The apparatus (FDST) and control method within the normal state of operation (one potential implementation is proposed in Table 1) is the tool that allows Demand and Generation Response implementation that may be incentivized through tariff variation to consumers that could be communicated through the grid frequency value variation instead of direct communication "signals". The FDST provides the tool to the DSO for wide application of tariff level response control, within the normal frequency operation window. Outside the normal operation window the FDST and control method is the emergency tool that enforces through the frequency variation (Table 1) the coordinated automated behavior of all active electric devices. The emergency level active power control scenarios as presented in Table 1, which could be compensated with an appropriate structure of fees depending on the case of emergency and level of grid support.

11. In case of high DER penetration conditions or lack of local energy resources at the considered grid segment level, the Frequency Decoupling Smart Transformer" (FDST) is monitoring the flow of power and the voltage variation and through its control logic (one potential implementation is proposed in Table 1) will vary the frequency appropriately in order to realize a pre-determined control and energy management strategy of all grid segment connected "active" devices that are monitoring frequency and voltage without any direct point to point communication.

12. The FDST is monitoring in real time Voltage and Current (power flow) of a number of consumer/prosumer smart meters along the length of the grid segment, communicated to it through the smart metering data communication infrastructure of the Distribution System Operator. This application is a current state estimation picture of the grid segment and in time the smart meter readings are used to validate the state of the grid segment by reiteration at appropriate time intervals. When excursions outside the normal state are occurring, the smart meter information assists in taking the right decision for the implemented operational control and it is enhanced and supported by regulations and technical limitations that are applied to generators, storage and loads per point of connection (consumer/prosumer). This function makes use of the available metering and communication infrastructure.

13. When within the grid segment, the local voltage compensation of distributed units and the voltage variation at the FDST point of connection are not sufficient to keep the voltage along the grid segment in the window of operation according to the grid operation code, then the FDST through its internal logic and collected smart meter measurements, possible load and RES generator forecasts (through local sensors and/or algorithms or through information relayed by the DSO or another provider) and communication with the DSO control, it will vary the grid segment frequency in order to "signal" active device control to all listening (monitoring) inverter connected devices and controllable loads to act in a coordinated and collective manner in order to stabilize the operation of the segment (Grid frequency dependent control in emergency conditions, as one potential implementation is proposed in Table 1).

14. The FDST due to its specifications and capabilities allows simple and smooth connection and disconnection of the LV grid segments (microgrids) to the upstream network and neighboring grid. Furthermore, it has the capability for voltage control, frequency variation, local monitoring, control logic according to provided data or set point by DSO and two-way communication capability (termed: Microgrid (or Grid Segment) Frequency Decoupling Controller - MFDC, see Drawings 1,2) with the Distribution System Operator.

15. The implementation of the Frequency Decoupling Smart Transformer may proceed initially by keeping the existing MV/ transformers and by adding a FDST system either on the LV side of the transformer, Drawing 4 a, or on the MV side of the transformer Drawing 4 b, with a DSO controllable bypass for technical and economic optimization of the FDST hardware and also to gain confidence in the first FDST applications. The same concept can be applied also to the higher voltage levels.

16. The grid segment (Microgrid) through the FDST will present itself to the Distribution System Operator as a single controllable entity in the network, simplifying the situation and providing to the DSO the capacity for coordinated and complemented grid segments (Microgrids) management that possess a very large number of "active" electric devices (generators, controllable loads, battery storage units, etc.) connected to the grids.

17. The proposed grid segment operation concept allows the collective organization and optimized operation, possibly through aggregation, within the normal frequency operation range, such as 49.8 to 50.2 Hz, where market procedures can be followed (Table 1). Therefore, aggregation and its benefits are encouraged within grid segments and "Microgrids", for consumers, producers, aggregators with their independent associated monitoring and control by Information and Telecommunication applications with aggregation at the level of buildings, houses, farms, small industry, etc.

18. The grid frequency variation works also in autonomous grid segments (microgrids, mini- grids, isolated power systems, multi-microgrids connected between them with power lines) through voltage control, frequency value variation control (one potential implementation is proposed in Table 1) and management of grid-connected generator inverters or/and storage, active loads, given the storage capacity of the grid segments (microgrids, mini-grids, isolated power systems, multi-microgrids connected between them with power lines). Each grid segment will have the FDST units with appropriate storage capacity, which will be the connection and exchange nodes between grid segments and at the same time they will be the grid forming inverters for each grid segment. The FDST units will also have sensors and algorithms to forecast load and RES generation for optimal grid segment resources management and if needed, multi grid segment operation optimization through higher level coordination. Fossil fuel thermal generators could be started when the frequency of the grid segment is, for example, under 49.8 Hz, to support the loads and charge storage systems until the grid frequency reaches, for example 50.2 Hz. The FDST therefore, could also be used for interconnecting adjacent remote mini-grids/microgrids in order to share resources (power and energy) for economic optimization as well as control of local resources.

19. The same concept of frequency decoupling through the FDST controllable transformer can also be applied, with the same logic and organization between the Medium Voltage and Higher Voltage grid, as it was presented between LV and MV. Between the DSO and TSO, there is exchange of information regarding the state of the MV grid segments, through the MV customer/prosumer Smart Meter information and other grid monitoring devices. The corresponding proposed control of the HV7MV FDST is implemented as needed by the TSO or the DSO for a coordinated and optimal operation of the electric system as presented in Drawing 3.

AMENDED CLAIMS

received by the International Bureau on 10 April 2017 (10.04.2017)

[Claim 1] Apparatus and control method of local self organized operation of grid segments/sections through a Frequency Decoupling Smart Transformer (FDST) without new physical communication infrastructure in the LV grid for a very large number of controllable devices (generators, controllable loads/devices, battery storage units, etc.) as represented in drawings 1,2,3,4,5. The invention presents a concept that uses and combines in a novel way, electric power devices, control methodology, a novel Frequency Decoupling Smart Transformer (AC/DC/ AC) at the grid segment substation, grid frequency mediated communication and coordinated operation of all controllable components connected to the LV electric grid segments without the need of a new communication infrastructure.

[Claim 2] The "Frequency Decoupling Smart Transformer" (FDST) and control method allows very large increase of the RES generator accepting capacity in the grid segment via voltage and frequency variation at FDST point of connection. Furthermore, it allows load management in the distribution networks with the associated environmental, economic and operational benefits. It also contributes to the transformation of the networks to frequency dependent self organized and regulated Smart Grids with optimized management of RES generator production, through storage management and power sharing between adjacent grid segments or even in coordinated exchange of power flow between several grid segments or "Microgrids" with the upstream distribution and transmission grid. The Distribution System Operator (DSO) may implement control strategies through the "Frequency Decoupling Smart Transformer" (FDST) such as: active power flow control, voltage control, temporary disconnection and re-connection of the grid segment from upstream network, in case of emergency or even adjacent grid segments in the same voltage level, in order to minimize flows through the "transformers", support adjacent LV grid segments and finally to optimize its flows with the upstream Medium and Higher Voltage grid in a coordinated way. This regulation approach could also be followed in the upstream grid nodes in Medium and High Voltage implementing a bottom up hierarchically coordinated electrical system (Drawing 3).

[Claim 3] The apparatus (FDST) and control method decouples the LV grid

segment (Microgrid) operation frequency from the frequency of the medium voltage upstream grid or even adjacent grid segments at the same voltage level, allowing grid frequency variation and thus power control, voltage control and energy management of generators, controllable loads, battery storage capacity and thus plug and play functionality, within certain predefined technical limits, for new DER units and controllable loads per connection point, within a defined grid segment of a "microgrid". LV grid frequency variation in interconnected grid segments offers control simplification and increased reliability of the grid segment state communication and implementation scheme, reducing monitoring and control points.

[Claim 4] The apparatus (FDST) and control method avoids investment in new communication infrastructure in the LV grid segments for direct communication systems between the FDST and the grid connected controllable devices and avoids security, latency, robustness and reliability issues with a very large number of grid connected controllable devices. The communication with all these devices will be through the frequency and the frequency monitoring in each of these devices, that will have a common algorithm of grid frequency versus active power response (as in Table 1 of the concept proposed) and therefore no new communication infrastructure is required for this action. It uses the grid frequency for power control, energy management and provides a price signal as a function of the grid frequency when the grid frequency remains within the window of the normal operation (as in Table 1 of description) for generating and absorbing/consuming devices.

[Claim 5] The various grid frequency windows for various grid segment operation states are presented in one potential implementation as proposed in Table 1. The frequency windows as presented in Table 1 (the frequency windows could be implemented with much tighter frequency ranges and be redefined as needed) may correspond to different tariff charges or service charges and credits according to the demand response and ancillary services received from the prosumers. When the grid segment is in normal frequency operation range, such as 49.8 to 50.2 Hz, then the usual electricity market procedures are followed. The excursion outside the normal operation range (such as 49.8 - 50.2 Hz), would energize automatic demand and generation response according to the urgent needs (Emergency condition levels - Table 1) of the grid segment implemented by the DSO. Therefore, that would require from the distributed interconnected electric devices to monitor and respond accordingly (Demand and Generation Response).

[Claim 6] The apparatus (FDST) and control method applies a uniform

methodology within grid segments per point of connection without the need for individual assessment. In practice, the local controllers of all the grid-connected units, such as energy generators, storage, and active loads in the grid segment will provide balancing and ancillary services such as voltage, frequency support as well as energy buffering, depending on their local voltage situation, the droop frequency logic, their own state/limitations within the grid segment.

[Claim 7] One of the implementations of the frequency decoupling between

electricity grid sections of different voltage level is the apparatus (FDST) in its generic form composed of two back to back 4-quadrant inverters with communication to the DSO, monitoring, control and storage integration capabilities, implementing a "Frequency Decoupling Smart Transformer" (FDST), for frequency decoupling of the LV grid segment (Microgrid) at the MV/LV transformer level, or it could be extended to higher voltages in the approach, such as MV/HV FDST, as presented in Drawing 3. The "Frequency Decoupling Smart Transformer" has also the capacity to control power flow, to vary its voltage level to correct possible grid voltage excursions outside the accepted voltage operation window in the downstream network. The FDST can be instructed to vary power flow through the "transformer" by energizing storage reserves and/or shifting loads and generation and inducing temporary islanding of the grid segment or "Microgrid".

[Claim 8] One realization of the FDST hardware and control at the MV/LV node substation, at the same voltage level, with many LV grid segments starting from the substation could be implemented as follows. The FDST is composed of power converters going from MV to LV as follows AC(MV) / DC - DC / AC(LV). On the LV AC side each grid segment departure from the substation will have a dedicated DC / AC(LV) converter, which are all connected to the same DC bus (see drawing 5), thus allowing each LV grid segment to be independently optimized in voltage profile and operate its own energy management (i.e. frequency variation profile) strategy according to the local segment conditions (load, generation, storage, neighboring grid segments, etc.). The DC bus of the FDST will be the electricity exchange point, when internal (same substation) electricity exchanges of the MV/LV node are taking place. The DC / AC(LV) converters may also parallel with adjacent converters on the LV side in order to increase efficiency and reliability in case of converter failures. The DC / AC(LV) converters could be expandable and modular in their design. The upstream side of the DC bus is connected to a single or several DC / AC(MV) converters appropriately sized for the substation needs for reliability and efficient operation. This way the FDST control can make use first of local grid segments resources, before resorting to the upstream MV grid. As more generation, storage and load shift flexibility resources are added to the LV grid segments the exchange with the upstream grid is reduced with benefits in terms of electricity use efficiency, reduction in new upstream infrastructure investment and increased grid reliability.

[Claim 9] The frequency decoupling solution at the MV/LV "transformer",

FDST, provides the opportunity to the Distribution System Operator to introduce storage media (battery bank, flywheel, capacitors, etc), sized appropriately to allow planned or emergency islanded and transitional grid segment or "Microgrid" operation between islanded and grid- connected modes and the associated benefits for the LV grid network and its stakeholders without the need to revisit the "Microgrid" structure every time new resources are added to it, but only introducing regulations and technical specifications of allowed interconnection of devices per point of connection to the grid by "consumers" or

"prosumers" (as consumers are usually called, if they also have their own RES generation) Drawing 2. The DSO will operate the FDST node as the last actively controlled point of the downstream grid and through it, implement its monitoring and operational control.

[Claim 10] The apparatus (FDST) and control method within the normal state of operation (one potential implementation is proposed in Table 1) is the tool that allows Demand and Generation Response implementation that may be incentivized through tariff variation to consumers that could be communicated through the grid frequency value variation instead of direct communication "signals". The FDST provides the tool to the DSO for wide application of tariff level response control, within the normal frequency operation window. Outside the normal operation window the FDST and control method is the emergency tool that enforces through the frequency variation (Table 1) the coordinated automated behavior of all active electric devices. The emergency level active power control scenarios as presented in Table 1, which could be compensated with an appropriate structure of fees depending on the case of emergency and level of grid support.

[Claim 11] In case of high DER penetration conditions or lack of local energy

resources at the considered grid segment level, the Frequency Decoupling Smart Transformer" (FDST) is monitoring the flow of power and the voltage variation and through its control logic (one potential implementation is proposed in Table 1) will vary the frequency appropriately in order to realize a pre-determined control and energy management strategy of all grid segment connected "active" devices that are monitoring frequency and voltage without any direct point to point communication.

[Claim 12] The FDST is monitoring in real time Voltage and Current (power flow) of a number of consumer/prosumer smart meters along the length of the grid segment, communicated to it through the existing smart metering data communication infrastructure of the Distribution System Operator. This application is a current state estimation picture of the grid segment and in time the smart meter readings are used to validate the state of the grid segment by reiteration at appropriate time intervals. When excursions outside the normal state are occurring, the smart meter information assists in taking the right decision for the implemented operational control and it is enhanced and supported by regulations and technical limitations that are applied to generators, storage and loads per point of connection (consumer/prosumer). This function makes use of the available metering and communication infrastructure.

[Claim 13] When within the grid segment, the local voltage compensation of distributed units and the voltage variation at the FDST point of connection are not sufficient to keep the voltage along the grid segment in the window of operation according to the grid operation code, then the FDST through its internal logic and collected smart meter measurements, possible load and RES generator forecasts (through local sensors and/or algorithms or through information relayed by the DSO or another provider) and communication with the DSO control, it will vary the grid segment frequency in order to "signal" active device control to all listening (monitoring) inverter connected devices and controllable loads to act in a coordinated and collective manner in order to stabilize the operation of the segment (Grid frequency dependent control in emergency conditions, as one potential implementation is proposed in Table 1).

[Claim 14] The FDST due to its specifications and capabilities allows simple and smooth connection and disconnection of the LV grid segments

(microgrids) to the upstream network and neighboring grid. Furthermore, it has the capability for voltage control, frequency variation, local monitoring, control logic according to provided data or set point by DSO and two-way communication capability (termed: Microgrid (or Grid Segment) Frequency Decoupling Controller - MFDC, see

Drawings 1,2) with the Distribution System Operator.

[Claim 15] The implementation of the Frequency Decoupling Smart Transformer may proceed initially by keeping the existing MV/ transformers and by adding a FDST system either on the LV side of the transformer, Drawing 4 a, or on the MV side of the transformer Drawing 4 b, with a DSO controllable bypass for technical and economic optimization of the FDST hardware and also to gain confidence in the first FDST applications. The same concept can be applied also to the higher voltage levels.

[Claim 16] The grid segment (Microgrid) through the FDST will present itself to the Distribution System Operator as a single controllable entity in the network, simplifying the situation and providing to the DSO the capacity for coordinated and complemented grid segments (Microgrids) management that possess a very large number of "active" electric devices (generators, controllable loads, battery storage units, etc.) connected to the grids.

[Claim 17] The proposed grid segment operation concept allows the collective organization and optimized operation, possibly through aggregation, within the normal frequency operation range, such as 49.8 to 50.2 Hz, where market procedures can be followed (Table 1). Therefore, aggregation and its benefits are encouraged within grid segments and "Microgrids", for consumers, producers, aggregators with their independent associated monitoring and control by Information and Telecommunication applications with aggregation at the level of buildings, houses, farms, small industry, etc.

[Claim 18] The grid frequency variation works also in autonomous grid segments

(microgrids, mini-grids, isolated power systems, multi-microgrids connected between them with power lines) through voltage control, frequency value variation control (one potential implementation is proposed in Table 1) and management of grid-connected generator inverters or/and storage, active loads, given the storage capacity of the grid segments (microgrids, mini-grids, isolated power systems, multi- microgrids connected between them with power lines). Each grid

segment will have the FDST units with appropriate storage capacity, which will be the connection and exchange nodes between grid segments and at the same time they will be the grid forming inverters for each grid segment. The FDST units will also have sensors and algorithms to forecast load and RES generation for optimal grid segment resources management and if needed, multi grid segment operation optimization through higher level coordination. Fossil fuel thermal generators could be started when the frequency of the grid segment is, for example, under 49.8 Hz, to support the loads and charge storage systems until the grid frequency reaches, for example 50.2 Hz. The FDST therefore, could also be used for interconnecting adjacent remote mini-grids/microgrids in order to share resources (power and energy) for economic optimization as well as control of local resources.

[Claim 19] The same concept of frequency decoupling through the FDST controllable transformer can also be applied, with the same logic and organization between the Medium Voltage and Higher Voltage grid, as it was presented between LV and MV. Between the DSO and TSO, there is exchange of information regarding the state of the MV grid segments, through the MV customer/prosumer Smart Meter information and other grid monitoring devices. The corresponding proposed control of the HV/MV FDST is implemented as needed by the TSO or the DSO for a coordinated and optimal operation of the electric system as presented in Drawing 3.

Description:
Apparatus and control method of self organized operation of distribution grid sections without new physical communication infrastructure

The present invention relates to a concept that uses and combines in a novel way all the following items as a whole that is: electronic devices, control methodology, communication and operation of all active and controllable components connected to electric distribution grid segments/sections without the need of a new communication infrastructure.

Introduction

Together with the integration of renewable energy generation in the distribution electricity grids the number of storage systems and controllable loads will increase as well. The distribution grids are changing and the implementation of a concept that would allow coordination and self regulation with the possibility of autonomous operation at critical times of the low voltage grid segments in relation to the upstream electricity grid, is attractive as it provides advantages due to higher reliability, efficiency, resilience and flexibility in the grid operation as well as optimized infrastructure usage [1]. As it is developing, the most active unit in the electricity grids due to the multiplicity of interactions of its components (generators, controllable loads, battery storage units, etc.) is considered to be the part of the network downstream from the Medium Voltage (MV) to the Low Voltage (LV) transformer. A term usually used for this grid concept, that could have the potential of self organization, is "Microgrid". One definition of the "Microgrid" is: a distribution network of a certain voltage level (for example Low Voltage grid) that may have one or more grid segments where consumers, electricity service providers and the distribution system operator participate in the energy market domain with small generation units, storage, loads and controllable loads in a coordinated manner through communication media for the benefit of all stakeholders in terms of energy cost, service, reliability, environmental benefits, etc. "Microgrids" in principle are usually interconnected to the main power network through a transformer and could be operated in autonomous mode, at times of faults or of service of the upstream network and seamlessly return back to the interconnected mode of operation. The collective operation strategy of a "Microgrid" is formulated by the grid code and the market operator and is implemented by the Distribution System Operator and energy market players. The "Microgrid" is a subsystem of the implementation of Smart Grids in the distribution power grid system. Background

The large integration of renewable energy sources (RES), active distributed energy resources (storage) and controllable loads in the electricity networks is imposing key operational, technical and economic challenges and barriers. These are voltage regulation issues, especially in rural areas but increasingly in urban grids and coordinated operation of a very large number of variable generation and controllable devices connected to the grids. Traditional grid reinforcement is the preferred solution to increase the hosting capacity for RES in the distribution grids, while control and management of controllable devices is attempted to be effected through point to point communication through mainly through internet communication solutions.

The aim of this invention is to increase RES generator accepting capacity and enhance load management at the distribution networks, without new physical communication infrastructure, enhancing the associated environmental, economic and operational benefits, while contributing to the transformation of the networks to self organized smart grids with minimized curtailment of RES generator production through storage management and power sharing between adjacent grid segments within a "Microgrid" or even in coordinated exchange of power flow through the upstream distribution and transmission system. The integration of Distributed Energy Resources (DER), such as RES generators, storage and controllable loads in the grids will increase dramatically in the near future, leading consequently to the dramatic increase of communication flows and data processing volumes, while data security issues will immerge. Therefore, it is proposed to avoid investing in new infrastructure for direct communication systems, in data processors and links for control and energy management in distribution grid segments (Microgrids). The key positive elements are avoiding the new communication infrastructure, the latency and vulnerability in distribution grid segments by controlling in a coordinated manner a very large number of grid connected controllable devices through the grid frequency variation. To allow grid frequency variation in distribution grid segments, the downstream operating frequency should be decoupled from the frequency of the medium voltage upstream grid, or other adjacent grid segments within the same "Microgrid". Each grid segment within a microgrid could have a separate Frequency Decoupling Smart Transformer (FDST) (Drawing 1). By allowing frequency variation, we will be able to realize control and energy management of generators, controllable loads and battery storage capacity. In this way, plug and play functionality, within certain predefined technical limits, can be applied for new DER units and controllable loads, within a defined grid segment. This will allow uniform application of a solution within a grid segment per point of user connection without the need for individual assessment. In practice, the local controllers of all the grid-connected units, such as energy generators, storage and active loads in the "Microgrid", Drawing 1, will have to be able to monitor and respond accordingly to local voltage variation, (already most recent Photovoltaic system inverters have this feature integrated), standardized local droop frequency logic for power control, energy management, black start, etc., allowing for self organized "Microgrid" operation. The distributed active DER units in the "Microgrid" will provide balancing and ancillary services such as voltage, frequency support as well as energy buffering depending on their local voltage situation, the grid frequency and their own operation state/limitations within a self-organized grid segment.

Drawing 1 : LV grid segments (Microgrid) example, in this case two grid segments with one Frequency Decoupling Smart Transformer (FDST), of the proposed concept: Frequency Decoupling Smart Transformer with storage between the Medium and Load Voltage grid (single line electric diagram).

The proposed communication and grid segment organization does not come for free, the frequency decoupling of the "Microgrid" means that an investment has to be made at the MV/LV transformer level. For this purpose, two back to back 4-quadrant inverters with communication, monitoring, control and storage integration capabilities, implementing a "Frequency Decoupling Smart Transformer" (FDST), are proposed, Drawing 2.

Drawing 2: Microgrid (or Grid Segment) Frequency Decoupler Controller generic concept- "Frequency Decoupling Smart Transformer" with storage (single line electric diagram).

The frequency decoupling solution will also provide the opportunity to the Distribution System Operator to introduce its own storage media (battery bank, flywheel, capacitors, etc), sized appropriately to allow islanded and transitional "Microgrid" operation between islanded and grid-connected modes and highlight the associated benefits for the LV grid network and its stakeholders without the need to define any further the development of the "Microgrid" in terms of resources, but only introducing regulations and technical specifications of allowed interconnection of devices per point of connection to the grid "consumer" or "prosumer" (as consumers are usually called, if they also have their own RES generation). To complete the innovative control solution, grid-connected inverters (generators and storage inverters) would have to monitor the local grid voltage in order to implement locally the reactive power control functions. Demand and Generation response may be incentivized through tariff variation to consumers that could be communicated through the value of the grid frequency variation instead of direct communication "signals".

This self organization approach could also be followed in the upstream grid implementing a bottom up hierarchically coordinated electrical system. The grid segment with the proposed FDST concept, while with low DER unit penetration, will be monitoring and correcting the voltage at the "transformer" connection point though the "Frequency Decoupling Smart Transformer" which also has the capacity to vary its voltage level to correct possible grid voltage excursions outside the accepted voltage operation window. As the DER unit penetration increases, the control devices of the distributed units may first respond to the local variation of the voltage, supporting the grid operation. In higher DER penetration or lack of local energy resources at the grid segment level, the "Frequency Decoupling Smart Transformer" (FDST) monitoring the flow of power and the voltage variation will adjust the frequency appropriately in order to realize a pre-determined control state and energy management strategy of all grid connected "active" devices. By active devices it is meant, those that monitor the voltage and frequency of the grid at their connection point and are able to perform some functions to contribute in the self-regulation of the grid segment. Depending on the state of the wider network, where the "Microgrid" is connected, various strategies may be implemented by the Distribution System Operator (DSO), who is in communication and control of all the "Frequency Decoupling Smart Transformers". The DSO may command the FDST to vary or minimize flow through the transformer by energizing storage reserves and/or shifting loads, inducing temporary islanding of the grid segment or the "Microgrid". The DSO will operate each FDST node as the last actively controlled point of the downstream grid and implement its monitoring and operational control as described.

Operational Control

In the not too distant future the low voltage grid segments, especially urban and semi-urban segments, are expected to be dominated by PV systems, small wind generators, μΟΙΤΡ, heat pumps, battery storage systems, electric vehicles, all connected to the grid through an inverter. In addition the "prosumers" and consumers will be connected to the grid through a smart meter and all buildings will have controllable loads, such that their operation could be deferred through a "signal" when it is considered appropriate and desirable in economic and system operation related terms. In low voltage grid segments like the ones described previously the operational control will be as follows: inverters will have to be able to monitor the voltage, current, frequency and to be equipped with local voltage support function depending on the local situation (Local Voltage Control). At the same time the grid is operating at its nominal frequency of 50 Hz and the Frequency Decoupling Smart Transformer (FDST) that interconnects the segment to the Medium Voltage is controlling, the power flow and the voltage at the LV side of the transformer in order to counterbalance any unwanted voltage excursions. The FDST is also monitoring in real time Voltage, Current of a number of smart meters along the length of the grid segment communicated through the smart metering data communication infrastructure of the Distribution System Operator (Drawing 3). The smart meter information is providing a snap shot of the state of the grid segment to the FDST through the DSO and a reiterated validation of the implemented operational control by the FDST.

Drawing 3 : Smart meter data communication flow between Distribution System Operator and Frequency Decoupling Smart Transformers (FDST) at LV and MV levels, as well as coordination between DSOs/TSOs and FDSTs control (single line electric diagrams).

The FDST controllable transformer may operate in voltage control mode which is particularly useful for long rural feeders, where reactive power control is less effective.

Load and RES forecasting and historic data will be also important tools for the DSO in anticipating the grid situation and having the resources ready to ride through the potential events.

Then, at a certain point in time, when the local voltage compensation of distributed units in the grid segment and the voltage variation at the point of connection of the FDST are not sufficient to keep the voltage in the window of operation according to the grid operation standard (see Table 1). Then the FDST through its internal logic and measurements collected will vary the grid segment frequency in order to "signal" active power control to all listening (monitoring) inverter devices and controllable loads to act in a coherent manner in order to stabilize the operation of the segment (Grid Frequency dependent control). Already in Germany all PV system inverters above a certain power level can be controlled with external signals and modulate their active and reactive power. The proposed control suggests the avoidance of direct communication, the extra investment cost, security, latency, robustness and reliability issues and promotes the use of the grid frequency variation for control of the power generating/absorbing devices. At the same time the frequency variation can be programmed to be the "signal" for tariff changes and/or service charges and credits that electric devices and consumers may monitor and respond accordingly (Demand and Generation Response) with an appropriate compensation incentive. In terms of frequency droop response, already for new inverters it is mandated in Germany to limit, for instance, their active power output according to the grid frequency increase.

Furthermore, the DSO may implement control and energy management strategies from the FDST transformer, such as to minimize flows through the transformer, support adjacent LV grid segments and finally to optimize its flows with the upstream Higher Voltage grid. Therefore, a bottom up organization of the grid is proposed (Drawings 1,3). Furthermore, the FDST due to its principal of operation and specifications will allow simple and smooth connection and disconnection of the LV segment (Microgrids) to the upstream network.

The same concept of frequency decoupling through the FDST controllable transformer can also be applied, with the same logic and organization between the Medium Voltage and High Voltage grid, as it was presented between LV and MV. Between the DSO and Transmission System Operator (TSO) exchange of information is suggested as presented in Drawing 3, regarding the state of the MV grid segments, through the MH/MV FDST, MV customer/prosumer Smart Meter information and other grid monitoring devices. The corresponding control of the HV7MV FDST is implemented as needed by the TSO or the DSO for a coordinated and optimal operation of the electric system as presented in Drawing 3.

Microgrid or Grid Segment Frequency Decoupler Apparatus

The power electronics and storage introduced to the MV/LV transformer site will have the capability for low voltage grid control, monitoring and two-way communication capability (Microgrid (or Grid Segment) Frequency Decoupling Controller - MFDC, see Drawings 1,2) with the Distribution System Operator, therefore implementing a Frequency Decoupling Smart Transformer. This way the grid segment or "Microgrid" will present itself to the Distribution System Operator as a single controllable entity in the network, simplifying the situation and allowing grid segment management for a very large number of connected electric devices. The above proposal does not exclude the collective organization and optimized operation (aggregation), within the grid segment or "Microgrid", for consumers, producers and the associated monitoring and control by Information and Telecommunication applications at the level of buildings, houses, farms, small industry, aggregation of consumers/producers, etc., within the frame of the safe operation (regulated by grid code and market operation) that the grid segment allows. In fact, it is suggested to couple the normal frequency variation controlled by the DSO, within the window of 49.8 - 50.2 Hz, with the electricity tariff level "signal", incentivizing the behavior of electricity market participants. Therefore, the FDST provides a tool to the DSO for wide application of tariff level response control, within the normal frequency operation window. Outside the normal operation window (Table 1), all active user electric devices would have to follow in an automated mode the emergency level active power control scenarios as presented in Table 1, with appropriate compensation and charges depending on the case.

Table 1 : Operation scenarios of FDST downstream (LV) grid according to frequency windows

This innovative concept opens a new field of hardware and control logic development which moves the last active point of monitoring and control for the Distribution System Operator at the MV/LV Smart Transformer for the entire grid segment (Microgrid) organized downstream. Grid frequency droop control in interconnected grid segments offers control simplification and increased reliability in the communication application, reducing monitoring, control points and investments. The addition of new DER units, within predefined capacities depending on the local grid segment or "Microgrid" infrastructure, local electricity consumption, etc., will allow plug and play operation, per customer connection point, due to the underlying control logic of the DER units that does not permit excursions outside preset windows of voltage and frequency.

Existing MV/LV transformers will not have to be replaced with more expensive on line tap changing transformers in order to be able to perform only voltage control in the LV grid segments at high renewable energy penetration levels. One other option to implement the Frequency Decoupling Smart Transformer is also to keep the existing MV/LV transformer and to add a FDST either on the LV side of the transformer, Drawing 4 a, or on the MV side of the transformer Drawing 4 b, with a DSO controllable bypass for technical and economic optimization of the FDST hardware and also to gain confidence in the first FDST applications. Appropriate business models would have to be developed and give value to the new features and compensate participating grid devices for services offered for frequency, voltage and other unusual conditions reinstatement.

Drawing 4 a, b: Microgrid (or Grid Segment) Frequency Decoupler - "Frequency Decoupling Smart Transformer - (FDST)" with storage, connected in series to the classical existing transformer with a DSO controllable bypass for reliability reasons with the FDST, a) in the LV side and b) in the MV side (single line electric diagrams).

One realization of the FDST hardware and control at the MV/LV node substation, at the same voltage level, with many LV grid segments starting from the substation could be implemented as follows. The FDST is composed of power converters going from MV to LV as follows AC(MV) / DC - DC / AC(LV). On the LV AC side each grid segment departure from the substation will have a dedicated DC / AC(LV) converter, which are all connected to the same DC bus (see drawing 5), thus allowing each LV grid segment to be independently optimized in voltage profile and operate its own energy management (i.e. frequency variation profile) strategy according to the local segment conditions (load, generation, storage, neighboring grid segments, etc.). The DC bus of the FDST will be the electricity exchange point, when internal (same substation) electricity exchanges of the MV/LV node are taking place. The DC / AC(LV) converters may also parallel with adjacent converters on the LV side in order to increase efficiency and reliability in case of converter failures. The DC / AC(LV) converters could be expandable and modular in their design.

The upstream side of the DC bus is connected to a single or several DC / AC(MV) converters appropriately sized for the substation needs for reliability and efficient operation. This way the FDST control can make use first of local grid segments resources, before resorting to the upstream MV grid. As more generation, storage and load shift flexibility resources are added to the LV grid segments the exchange with the upstream grid is reduced with benefits in terms of electricity use efficiency, reduction in new upstream infrastructure investment and increased grid reliability.

Drawing 5: The FDST is composed of power converters going from MV to LV as follows AC(MV) / DC - DC / AC(LV). Each LV grid segment may be independently optimized in voltage profile and operate its own energy management (i.e. frequency variation profile) strategy according to the local segment conditions (load, generation, storage, neighboring grid segments, etc..) but may also parallel with adjacent converters on the LV side in order to increase efficiency and reliability in case of converter failures (single line electric diagram).

Implementation example of control strategy, in terms of grid frequency variation, of active power control of generators, controllable loads and the operational behavior of battery charge/discharge cycles

Provided that the RES generators and storage devices do not exceed a nominal capacity, to be defined, at the connection point of the consumer and certain predefined technical limits, a plug and play operation concept of grid-connected devices may be implemented.

Plug and play operation of "active" (frequency and voltage monitoring) devices is implemented through active power variation according to a potential scenario, frequency window, as presented in Table 1 below and at the same time respecting voltage regulation at all points of the grid segment, through local voltage control logic of "active" distributed devices. The distributed active grid-connected devices measure the grid frequency variation, local voltage and respond accordingly (ancillary services) as in Table 1, regarding active power.

Furthermore, the FDST transformer measures and controls Voltage and Frequency and thus power flow at the MV/LV transformer point. Alternative grid frequency operation windows, to those of Table 1, may be needed and would have to be investigated, for controlling distributed electrical devices in order to comply with safety related issues. The controlled distributed grid-connected devices should also adopt a new local control logic according to operate coherently with the grid frequency variation. If there is concern for wider than desired frequency variation away from 50 Hz, it is technically possible to implement much tighter frequency windows compared to the ones currently presented in Table 1.

Reference:

[1] T. Stetz, M. Kraiczy, K. Diwold, M. Braun, B. Bletterie, C. Mayr, R. Brundlinger, B. Noone, A. Bruce, I. MacGill, B. Mather, K. Ogimoto, K. Washihara, A. Iaria, A. Gatti, D. Cirio, M. Rekinger, IT. Theologitis, K. de Brabandere, S. Tselepis, C. Bucher, Y. Wang, "High Penetration PV in Local Distribution Grids - Outcomes of the IEA PVPS Task 14 Subtask 2", 29 th European Solar Photovoltaic Conference in Amsterdam, Sept. 22-26 111 2014, Proceedings, 5DP.1.1.