Connecting and operating storage units in low voltage networks

Transcription

1 Connecting and operating storage units in low voltage networks June 2013

2 Publication Information Forum Network Technology / Network Operation in the VDE (FNN) Bismarckstr. 33, Berlin, Germany Phone: + 49 (0) Fax: + 49 (0) fnn@vde.com Internet:

3 Connecting and operating storage units in low voltage networks VDE FNN Connecting and operating storage units in low voltage networks Page II

4 Contents 1 Scope Normative references Terms and definitions Technical requirements Technical requirements Supplementary observations with regard to storage units Connection criteria Symmetry and monitoring of infeed power Impacts on network loads Reactive power Active power limitation Active power reduction in the event of overfrequency Storage system design / NC protection Technical balance requirement Validation of compliance with technical requirements Connection and operating concepts General Storage units without generator plant and consumption system, directly connected to the grid Generator with storage unit without consumer Storage system in the generation path Storage system in the consumption path Isolated network operating mode (in the low voltage network) Page III Connecting and operating storage units in low voltage networks VDE FNN

5 List of figures and abbreviations Figures Fig. 1: Generator with storage unit without consumer Fig. 2: Storage system in the generation path Fig. 3: Storage system in the generation path with CHP system Fig. 4: Storage system in the consumption path Fig. 5: Storage system in the consumption path with CHP system Abbreviations FNA BMU EZA EZE FNN HöS HS MV LV Federal Network Agency German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety Generator plant Generator unit Forum Network Technology / Network Operation in the VDE Ultra-high voltage High voltage Medium voltage Low voltage VDE FNN Connecting and operating storage units in low voltage networks Page IV

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7 Foreword In light of the challenges surrounding the energy turnaround, we can assume that a large number of energy storage units are going to have to be integrated into the existing system over the long term. As such, the current general conditions and requirements need to be reviewed and, if necessary, adapted and expanded. The use of energy storage units is currently focused mainly on the low and medium voltage networks. Initial studies by VDE FNN have shown that the VDE-AR-N 4105 technical regulations, BDEW-MV guideline, technical connection requirements for low and medium voltage networks, VDN guideline for emergency generators and the ENTSO-E-NetworkCodes should serve as the basis and orientation for defining the requirements for connecting storage units to the grid and operating them. This Technical Information aims to technically implement the connection and operation of storage systems and, in particular, to implement the Guidelines for promoting stationary and decentralized battery storage systems for use in connection with photovoltaic systems, which was published on Friday, April 19, 2013 /1/. It is primarily aimed at network operators, system developers, system operators and manufacturers of generators or storage units and/or storage systems in connection with generators VDE FNN Connecting and operating storage units in low voltage networks Page 1

8 Introduction Generally, the widely established technical connection requirements (e.g., TAB 2007, VDE-AR-N 4105, technical guideline for emergency generators, etc.) must be observed when connecting and operating storage units and/or storage systems. In light of the complexity of the issue and the anticipated increase in the use of storage units and/or storage systems, explanations and practical examples are needed. The definition of the requirements for storage systems in terms of network engineering and network operation, in particular, must ensure the continued safe and reliable operation of the networks and the exclusion of any additional risks for people and property. The aim is to better integrate storage systems into the energy supply system based on existing schematics. Generally, storage units can be used to optimize the load within a customer's system, to even out load flows in the low voltage network (load management), and to support the network (e.g., by sustaining voltage levels). The motivation for integrating storage units comes from the broad spectrum of possible applications that can, in certain conditions, positively impact the public grid. These effects include: Reduction of additional network loads by means of load management in the customer's system (e.g., by increasing own consumption accordingly); Improved integration of generated power into the public grid (reduction of limitation measures) Load flow homogenization in the low voltage network Grid support, e.g., sustaining voltage levels by feeding in reactive power accordingly As a result, possibly avoiding the need to expand the network Provision of system services (frequency stability, network dynamics, etc.) The use of suitable information and communications technology (ICT) with standardized interfaces can further enhance the positive effects of storage units. These storage units can be electrical, thermal, chemical or mechanical. Page 2 Connecting and operating storage units in low voltage networks VDE FNN

9 1 Scope This Technical Information aims to technically implement the connection and operation of storage systems and, in particular, to implement the Guidelines for promoting stationary and decentralized battery storage systems for use in connection with photovoltaic systems, which was published on Friday, April 19, 2013 /1/. The information applies when planning, developing, operating and modifying storage units or storage systems that are connected to the low voltage network and operated in parallel with a network operator's grid (low voltage network connection point). Particular attention must be paid in this respect to modifications of storage units and/or storage systems that materially impact electrical behavior at the network connection point. The observation and reference point for technically evaluating a storage unit or system is always the boundary separating responsibility for the customer's system and for the public distribution network (e.g., house junction box). In order to ensure that the needs of all areas of application of storage systems are considered both now and in the future, these systems must be analyzed in their entirety at the network connection point (generation, consumption and storage) VDE FNN Connecting and operating storage units in low voltage networks Page 3

10 2 Normative references The documents cited below are necessary to use this information. In the case of dated references, only the cited version applies. In the case of undated references, the most recent version of the cited document applies (including all revisions). 1 D-A-CH-CZ DIN V VDE V VDE FNN TAB 2007 Technical guideline for emergency generators VDE-AR-N 4101 VDE-AR-N 4105 VDE-AR-N 4400 Technical rules for the assessment of network disturbances Grid integration of generator plants Information on technical / operational implementation of infeed management Technical conditions for connection to the low voltage network Guidelines for planning, constructing and operating emergency generating facilities Requirements for meter panels in low voltage electrical systems Power generation systems connected to the low voltage distribution network - Technical minimum requirements for the connection to and parallel operation with low voltage distribution networks Electricity metrology Legal specifications, such as derived from the Low Voltage Connection Regulation (Niederspannungsanschlussverordnung, NAV), apply irrespective of these requirements /2/. Compliance with product-specific requirements and norms is taken for granted. 1 Further information on all documents can be found at: Page 4 Connecting and operating storage units in low voltage networks VDE FNN

11 3 Terms and definitions System operator: Entrepreneurs, or natural or legal entities commissioned by them, who assume responsibility for safely operating and ensuring the proper condition of the customer's system. System developer: Individuals or enterprises who build, expand, modify or maintain electrical systems; but also individuals or enterprises who did not build, expand, modify or maintain the systems but acted as experts in inspecting the completed works and assumed responsibility for the proper completion of the same. Battery storage: A rechargeable electricity storage unit based on the conversion of chemical energy. Battery storage system: A system comprising the battery storage unit, battery management system and all of the components needed from a system engineering perspective to enable the intended operation in connection with a power generator that are not equally necessary when investing in, and operating, a generator. Storage system operating mode: Describes the operating function of the storage unit Mode: Energy consumption The storage system is charged from the public or customer's own AC grid Mode: Energy supply The storage system discharges into the public or customer's own AC grid Mode: Isolated network (max. time allowed to operate in parallel in isolated network mode 100 ms) The storage system is disconnected from the public grid. The storage unit is charged from, or discharged into, the customer's own AC grid. Final circuit: Circuit designed to supply power straight to electrical consumers or sockets. Energy flow direction sensor (EFDS): Technical equipment for determining the net energy flow direction; communicates with the storage system (e.g., by current directional relay). Note 1: If electricity is metered on a phase-selective basis, the EFDS must also be designed with phase selectivity. Note 2: Metering systems that comply with EnWG 21, are available in the marketplace, and can satisfy the requirements listed in this information, can also be used to monitor energy flow direction. Generator plant (EZA): the entirety of units from one source of energy that are used to generate power and are connected to a mains/house connection (e.g., all PV units). Generator unit (EZE): Single unit for generating electrical power Customer's system: the electrical system as defined in 13 and 14 NAV, i.e., the entirety of all electrical equipment downstream of the transfer point (with exception of the metering equipment) that is used to supply power to the users. Power: Unless explicitly described otherwise, the term is used to mean active power. Network operator: Operator of a network that supplies electricity to the general public VDE FNN Connecting and operating storage units in low voltage networks Page 5

12 Storage unit: A system that can draw electricity from, or feed electricity into, the customer's own network and/or the public grid, depending on the operating mode, irrespective of the nature of technical implementation. Storage system: A system comprising the storage unit, storage management system and all of the components needed from a system engineering perspective to enable the intended operation. Page 6 Connecting and operating storage units in low voltage networks VDE FNN

13 4 Technical requirements 4.1 Technical requirements Storage units and/or storage systems must satisfy the following requirements: In "Energy consumption" mode (from the public grid and/or the customer's own power generator), the storage unit acts like a consumption system from the grid's perspective. In "Energy supply" mode (from the public grid and/or the customer's own power generator), the storage unit acts like a generator plant from the grid's perspective. Accordingly, the relevant existing technical regulations governing consumption and generation systems must be observed when connecting storage units to the network and operating them. Connection and operation in LV networks: Compliance with TAB 2007 for "Energy consumption" mode, and with VDE-AR-N 4105 for "Energy supply" mode, and with the technical conditions (TAB) for connection to the low voltage network. Observance of the technical guideline for emergency generators in "Isolated network" mode. 4.2 Supplementary observations with regard to storage units When using storage units and/or storage systems, the documents listed above (Section 2) must be supplemented by the terms and definitions in Section 3. The description of the possible operating modes of storage units and/or storage systems constitutes a further supplement. As storage units can be equipped to perform multiple functions, different parameters must be set for the relevant operating modes and their functions. Operating modes: Energy consumption: Parameters as specified in the application procedures of TAB 2007 and/or by the operators of the relevant distribution networks Energy supply to the customer's own system and/or the public grid: Supplementary data sheets as for generators (VDE-AR-N 4105) Isolated network (max. time allowed to operate in parallel in isolated network mode 100 ms) Data sheet as per the technical guideline for emergency generators Information describing how the storage unit works is also necessary: Load optimization in the customer's system Load flow homogenization in the low voltage network (load management) Grid support, e.g., sustaining voltage levels, system services, isolated network operation as a standby system Provision of electricity for electric road vehicles Maximum and usable storage capacity in kwh are further parameters required just for the storage unit. These supplements are implemented by the following specific additions to the technical regulations: VDE FNN Connecting and operating storage units in low voltage networks Page 7

14 "Scope of application" section: "This VDE application rule/guideline/... applies for the planning, construction, operation and modification of consumption and generation systems and storage units that are connected to the low voltage distribution network of a network operator and that are operated parallel to the network. Particular attention must be paid in this respect to modifications of generators and storage units that materially impact electrical behavior at the network connection point." Note: "Storage units can support load management if they are flexibly operated as specified by the network operator (e.g., remote connection/disconnection of the storage units). Appropriate separate contractual agreements between the operators of the system and the network may be necessary in this respect." The technical requirements and conditions for connection as described in this information must be satisfied by an energy storage unit that is used in combination as a variable load, generator or standby system. The following properties and combinations of the same are possible: Storage unit that does not draw power from the public grid; Storage unit that does not feed into the pubic grid; Storage unit that draws power from the public grid; Storage unit that feeds into the pubic grid; All application scenarios must be derived from these properties. Section 5 describes possible concepts for connection and operation. It is therefore particularly helpful when planning, building and operating such systems. 4.3 Connection criteria The technical conditions for connection as stipulated by the network operator must be observed when designing the connection of the storage unit and/or customer system with storage facility. Within the customer's system, the cable connecting the storage unit must be attached to the meter panel for full infeed into the operator's network; the meter panel must then be designed in accordance with the applicable TAB (currently TAB 2007). Infeed to the meter panel is always routed through the upper connection area. Storage units with a connected active power rating of 30 kw, which are used to optimize own consumption or reduce infeed power, can also be connected to sub-distributors. Particular care must then be taken by the system developer when checking the wiring size and protection for the electrical installation. Under no circumstances is connection to a final circuit permissible. The maximum connected active power rating approved by the network operator for drawing power from, or feeding power into, the grid (from which can be derived the maximum apparent power by considering cos φ) may not be exceeded at the network connection point. The rated power fed into the public grid must be monitored by the system operator and limited if necessary. Section 5 summarizes the possible connection variants and applicable metrology designs for storage units and/or storage systems. Note 1: This version of the Technical Information initially only deals with concepts that are relevant for /1/, i.e., storage units and/or storage systems with a power rating of 30 kw that are connected to the low voltage network. Page 8 Connecting and operating storage units in low voltage networks VDE FNN

15 Note 2: In the case of deviations, the requirements in terms of billing measurement and accounting must be examined and agreed with the network operator. It is not permissible to draw electricity from the grid and then feed it back in order to claim statutory reimbursement, e.g., as provided for in EEG or KWK-G. The system operator is responsible for providing validation (e.g., manufacturer's certificate). If reimbursement of stored electricity is provided by law, it must be stored separately by primary energy source and split into different feed-in tariffs. 4.4 Symmetry and monitoring of infeed power Storage units must always be designed and connected to the grid as symmetrical, three-phase rotary current units. Since the infeed quantity from generators and the own consumption quota according to VDE-AR-N 4400 are determined using the Ferraris measurement principle, this same principle is assumed to be equally applicable when settling energy quantities among generators, consumers and storage systems. Single-phase connections of generators and storage units are generally possible up to 4.6 kva. This limit applies to the aggregate of infeed power and power drawn at the network connection point. Compliance with the maximum specified asymmetry of 4.6 kva between two external conductors is mandatory when operating single-phase generators in conjunction with single-phase storage units, i.e., particularly when the generator and storage unit feed in simultaneously. When connecting the storage units and/or storage systems, the network operator must be consulted in respect of the phase/phases to which the storage units and generators must be connected in each case. Various measures can be adopted to ensure compliance with the maximum permissible asymmetry of 4.6 kva between 2 external conductors. Establishing communication between generator and storage unit and limiting the aggregate power to the maximum permissible asymmetry. Measuring and regulating the power exchange at the network connection point for each phase in respect of the maximum permissible asymmetry. Note: The 1 minute mean value is used for determining satisfaction of the asymmetry condition. VDE-AR-N 4105 currently only describes the requirements for generators that feed electricity into the public grid or in-house network, while TAB 2007 describes the connection of electrical consumers. Both documents stipulate a maximum asymmetry of 4.6 kva between two external conductors for single-phase connections. This can, however, produce an aggregate asymmetry of 9.2 kva between two external conductors (example: single-phase generator on one 4.6 kva external conductor and a consumer up to 4.6 kva on another external conductor). So far, this has been distributed purely stochastically in the absence of active intervention, e.g., by an energy management or storage system, and generally evens out again over multiple house connections or at the local power transformer. Often, both energy management or the use of a storage unit cause this stochastic distribution to result in systematic asymmetry. Looking ahead, these considerations imply that the requirements for connection of such systems to networks must change in future. Since the explanations above establish a link between the asymmetry related in each case to consumption and generation for the first time (discharge mode as per VDE-AR-N 4105 and charge mode as per TAB VDE FNN Connecting and operating storage units in low voltage networks Page 9

16 2007), a transition period will be necessary to allow manufacturers to adjust their systems to the revised requirements. After consultation with the operator of the distribution network, one of the following exceptions is permissible for systems that are put into service before June 30, 2014: Connection of a storage unit up to an infeed and charge capacity of 4.6 kva to the same phase as the single-phase generator up to 4.6 kva. Alternatively, the storage unit may be connected to a different phase than the single-phase generator and may then charge at up to 4.6 kva with simultaneous single-phase generation of up to 4.6 kva. 4.5 Impacts on network loads The load on the network may be relieved if the power infeed from the customer's system into the grid is reduced by storing the power in the customer's system. In the case of storage units and/or storage systems with single-phase (or possibly two-phase) connections in the customer's system, this effect can only be produced to a certain extent and cannot be reliably calculated by the operator of the distribution network. A meter measures the power balance within the customer's system at the network connection point using the Ferraris measurement principle (in accordance with VDE- AR-N 4400). This principle nets the power flows across all phases. Physically, the power generated on one phase is, however, only compensated by consumers on the same phase. Excess power in this phase is therefore fed into the grid and must be distributed through it. As such, the external conductors of the connecting lines and the network operating equipment close to the connection point must be designed to cope with absorbing the maximum possible infeed power. Additionally, asymmetrical power flows increase the load on the neutral conductor; this must also be taken into account when planning a system. In order to avoid further increases of the load on the conductors, storage systems must therefore always be designed as symmetrical, three-phase rotary current units (see also Section 4.4). 4.6 Reactive power The provisions of VDE-AR-N 4105 apply in "Energy supply" (discharging) mode. In "Energy consumption" (charging) mode, cos φ=1 applies. 4.7 Active power limitation An appropriate technical solution must ensure compliance with the specified limitation of active power at the network connection point (e.g., EEG 6 /3/, BMU guidelines /1/). This can be achieved, on the one hand, by setting the system components to a fixed active power rating, or by using metrology equipment (sensors) to control the equipment, on the other. Information is provided below on how to implement active power limitation with storage systems. The control unit specifications can be found in FNN publication: "Information on technical / operational implementation of infeed management". The following requirements apply: If a sensor reading is not available, the system component setting must be fixed at the appropriate active power rating. Adherence to the permissible mean over a period of 10 minutes must be assured. Page 10 Connecting and operating storage units in low voltage networks VDE FNN

17 4.8 Active power reduction in the event of overfrequency In "Energy supply" (discharge) mode, the provisions of VDE-AR-N 4105 (Section ) apply for active power reduction in the event of overfrequency. Currently, there are no specifications governing active power control in the event of underfrequency. Future requirements in respect of intelligent load management (e.g., from the ENTSO-E Network Code-Demand Connection) must be taken into account, if necessary. 4.9 Storage system design / NC protection Compliance with the requirements of VDE-AR-N 4105 Section 5 must be assured in "Energy supply" and "Energy consumption" modes Technical balance requirement In order to comply with the requirements for claiming reimbursement as specified in 16 EEG 2 in conjunction with 3 (1) EEG, the following conditions must be observed when operating a generator and a storage unit at the same network connection point: Storage unit that does not draw power from the public grid If the storage unit is intended to feed into the public grid, electricity must not be drawn from the grid to charge the storage unit. Storage unit that does not feed into the pubic grid If the storage unit is intended to charge from the public grid, a technical solution must assure that the electricity drawn from the grid is not fed back into the public grid. Note: From the point of view of storage units and/or storage systems, a key technological issue would be to permit top-up charging of storage units from the grid without sacrificing entitlement to statutory reimbursement. It may be possible to use suitable and appropriately tested measuring and metering systems in future to ensure compliance with the technical balance requirements by encompassing all individual units, e.g., in the form of virtual power plants Validation of compliance with technical requirements Proof of conformity must be provided to validate compliance with the requirements. Note 1: Certification must be provided to validate compliance with the requirements of Section 5, letters a - d of the Guidelines for promoting stationary and decentralized battery storage systems for use in connection with photovoltaic systems, published on Friday, April 19, 2013 /1/. A manufacturer's declaration suffices as long as certification is not available in the marketplace. In addition to the validation procedures specified in VDE-AR-N 4105 and DIN V VDE V , proof of functional reliability of the energy flow direction sensor (EFDS) must be provided. Manufacturers must perform function tests (type approval) on the sensors (for the storage system) and confirm as much with a type-specific declaration of conformity. Prior to putting the sensor (for the storage 2 Network operators must reimburse system operators for electricity produced by systems that only use renewable energies or methane; such minimum reimbursement to be dictated by EEG VDE FNN Connecting and operating storage units in low voltage networks Page 11

18 system) into service, the system developer must verify installation in accordance with the manufacturer's instructions. Note 2: This requirement must be taken into consideration when revising test standard DIN V VDE V , which forms part of VDE-AR-N Note 3: We recommend the use of the "PV storage pass". Page 12 Connecting and operating storage units in low voltage networks VDE FNN

19 5 Connection and operating concepts 5.1 General Section 5 summarizes the valid concepts for connecting, operating and measuring storage systems in context to the preceding sections. This version of the Technical Information focuses primarily on concepts that are relevant for implementing the BMU guidelines /1/ (Sections ). 5.2 Storage units without generator plant and consumption system, directly connected to the grid The technical conditions for connection (TAB), VDE-AR-N 4105, and bilateral agreements apply between network and system operators. 5.3 Generator with storage unit without consumer Fig. 1: Generator with storage unit without consumer This connection variant (Fig. 1) describes the principle of full infeed. The storage unit can be incorporated into the AC or DC path. Sensor S1 monitors the same load flow as meter Z1. Technical balance requirement Storage unit that does not draw power from the public grid: Electricity may only be drawn from the grid to supply the generator plant. As such, the storage unit may not be charged when active power is flowing in the direction of the generator/storage unit (Z1 P+>0). Technical solution for limiting active power VDE FNN Connecting and operating storage units in low voltage networks Page 13

20 Active power is controlled on the basis of the readings from sensor S Storage system in the generation path Fig. 2: Storage system in the generation path In this variant, the storage unit is fixed to the generator plant and measured by meter Z2, together with the generator. The storage unit can be incorporated into the AC or DC path. Sensors S1 and S2 (see Fig. 2) monitor the same load flows as meters Z1 and Z2. Sensor S3 monitors the load flow in the consumption path. Technical balance requirement The following variant must be selected and registered to operate the storage system. Storage unit that does not draw power from the public grid: In the generation path, electricity may only be drawn from the grid to supply the generator plant. As such, the storage unit may not be charged when active power is flowing in the direction of the generator/storage unit (Z2 P+>0). Technical solution for limiting active power Active power is controlled on the basis of the readings from sensor S1 on the consumption side, or by S3 in the consumption path and S2 in the generation path. Note: Active power can also be controlled on the basis of S2 readings if all affected generator plants and storage units are monitored by S2 in the generation path. Page 14 Connecting and operating storage units in low voltage networks VDE FNN

21 The aforementioned requirements are generally transferable to concepts comprising multiple generator plants with storage systems in the customer's system. Figure 3 shows an example of a PV system with storage system and and a CHP system with storage system. Fig. 3: Storage system in the generation path with CHP system Storage unit that does not draw power from the public grid: Active power must not flow in the direction of the storage units (i.e., S2 and S3 measure the direction of energy flow to the storage unit (see Fig. 3) VDE FNN Connecting and operating storage units in low voltage networks Page 15

22 5.5 Storage system in the consumption path Fig. 4: Storage system in the consumption path The storage unit is not fixed to the generator plant in this variant. Sensor S1 monitors the same load flow as meter Z1. Technical balance requirement One of the two variants must be selected and registered to operate the storage system. Storage unit that does not draw power from the public grid If the power generated by the generator plant and interim stored in the storage system is intended to be fed into the public grid, the storage unit must not charge from the public network. As such, the storage unit may not be charged when active power is flowing in the direction of the generator/storage unit/consumer (Z1 P+>0). Storage unit that does not feed into the pubic grid If the storage unit is intended to charge from the public grid, steps must be taken to ensure that the storage unit does not feed back into the grid when discharging. As such, the storage unit may not be discharged when active power is flowing into the grid (Z1 P->0). Technical solution for limiting active power Active power is controlled on the basis of the readings from sensor S1 on the consumption side. Page 16 Connecting and operating storage units in low voltage networks VDE FNN

23 Fig. 5: Storage system in the consumption path with CHP system Storage unit that does not feed into the pubic grid: The storage unit must not feed into the public grid in this variant. Stored electricity from the EEG generator plant and CHP generator plant is no longer eligible for reimbursement. As such, active power must not flow from the storage unit towards the meter (Z4 P+>0) (S4 monitors the flow of energy toward the public grid). If the energy flows in the direction shown in Fig. 5, the storage unit must not discharge. 5.6 Isolated network operating mode (in the low voltage network) Technical balance requirement The customer's system with storage unit is disconnected from the public grid. The storage unit is charged from, or discharged into, the customer's own grid. Technical solution for limiting active power Active power management must be based on the customer's load conditions. Operation in parallel with the public grid is permissible for a maximum period of 100 ms VDE FNN Connecting and operating storage units in low voltage networks Page 17

24 References [1] BMU guideline Guidelines for promoting stationary and decentralized battery storage systems for use in connection with photovoltaic systems, April 19, 2013 [2] NAV Ordinance governing the general terms and conditions for network connections and the use thereof for supplying low voltage electricity, September 3, 2010 [3] EEG Act on the Priority of Renewable Energy Sources (Renewable Energy Sources Act - EEG), December 20, 2012 This Technical Information was compiled by the FNN Network of storage experts. Page 18 Connecting and operating storage units in low voltage networks VDE FNN

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