Installation of storage systems in combination with RES and CHP stations by self-producers with net metering

The installation of storage systems in combination with RES and CHP stations by means of energy-neutral pipelines with net metering was provided for by par. 1 of Article 14A of Law No. 3468/2006 as amended by Article 23 of Law 4513/2018 “Energy Communities and other provisions”, while the technical specifications and conditions of operation and use of storage systems were defined by Article 8 of Ministerial Decision ΥΠΕΝ/ΔΑΠΕΕΚ/15084/382 (GG Β’ 759/5.3.2019) as supplemented by Article 4 of Ministerial Decision ΥΠΕΝ/ΔΑΠΕΕΚ/74999/3024 (GG Β’ 3971/30.8.2021).

The storage systems can be installed in combination with photovoltaic, biomass/biofuel, biogas, small hydro, and CHP stations connected to low-voltage (LV) or medium-voltage (MV) networks, as well as with small wind turbine stations. Specific to MV, storage systems can be combined with a station consisting both of the above technologies.

A storage system refers to a system of electrochemical accumulators used for storing electrical energy, commonly known as a Battery Energy Storage System (BESS). It consists of the following components: a) power converter(s) with the ability to absorb or supply active or reactive power, b) accumulators connected to the DC side of the converter, c) electrical connection equipment (switches, wiring, protection devices, etc.), and d) control and monitoring system.

With the operation of the storage system in net metering installations, it is possible to increase the self-consumption of energy from the generation plant, thereby reducing the amount of energy absorbed from the Network at the self-producer installation. For example, in combination with a PV station, a storage system could store excess energy during the midday hours and deliver it to the load of the self-producer during the evening hours, i.e. the hours when the self-producer would have to absorb energy from the Network.

This operation creates an additional economic benefit for the net metering self-producer, as it reduces the energy absorption from the Network and consequently reduces the charges for the use of the Network, the System (variable part) and the ETMEAR. The charges for public utility services (PUS), because they are calculated on the energy consumed, irrespective of whether it comes from the Network or the power station, do not change.

It is noted that an increase in the self-consumption of the power plant’s energy could also be achieved by “smart” management of the consumption loads, i.e. by shifting their operation in a way that achieves synchronization of the energy consumed and produced by the plant.

During standard operation of the production station and the storage system, the storage system absorbs energy exclusively from the production station and this energy is available exclusively for the purpose of supplying some or all of the loads of the self-producer. Therefore, the storage system does not exchange energy with the Network, i.e. the accumulators do not charge from the Network and do not discharge to the Network.

1 When a power electronic device is used to transfer power in both directions, i.e. from and to the DC side, the term “converter” is used for the device as opposed to the term
“inverter” which is used when power is transferred only from the DC side, as in the case of connecting PV panels.
In order manage the energy in the self-producer installation and ensure that the energy of the storage system is not exchanged with the Network, an “energy flow direction sensor” or a ‘smart energy meter”, single-phase or three-phase as appropriate, is installed at the entrance of the self-producer installation, which, depending on the direction of the energy (injection/absorption), instructs the converter control system to charge/discharge the accumulators.
As an example, the following management strategy could be adopted:

• when energy from the Network enters the electrical installation, as is the case where the energy produced is not sufficient for the load of the self-producer, the discharge of the accumulators to the load will be permitted in order to reduce the absorption from the Network
• when there is an excess of energy produced and energy exits the power plant, the accumulators will be permitted to be charged by the production station in order use the stored energy at a later when there is an increased demand
• in case of high demand, energy from both the power station and the accumulators can be used simultaneously
• in case of low demand and increased production or when the accumulators are fully charged, the excess energy from the production station can be injected into the Network and offset against the energy absorbed at a later time by the Network.

If the accumulators have a sufficiently large capacity to absorb all the excess energy from the production station and/or the station receives a command from the management system to reduce or stop its production, then the installation may be able to zero feed-in to the Network. In this case, it is possible for the station to be connected to a saturated network, provided that the saturation is due to the exceeding of the nominal power of the network elements or the exceeding of the desired limits of the power at network nodes.

In order certify the non-exchange of energy of the storage system with Network, the self-producer is obliged to submit to HEDNO a Technical Description showing compliance with the requirement accompanied by an electrical drawing and a technical manual of the manufacturer of the converter and the “directional sensor”. In addition, the self-producer is bound by a Solemn Declaration according to Law 1599/86 and that during the entire period of operation of the installation the settings of the storage system will not be modified.
During the commissioning of the storage system, HEDNO may carry out appropriate tests to verify the requirement of non-exchange of energy with the Network, while during operation, it may monitor and correlate the metering data of the produced and injected energy in order to detect any violation of the requirement.
In case it is found that the above requirement is violated, HEDNO may deactivate the connection of the self-producer to the Network.

When commissioning a production station combined with a storage system, in addition to the checks, tests and measurements foreseen for the commissioning of the production station, HEDNO shall carry out the following tests on the storage system:

• Storage system coupling protection tests
• Control of non-exchange of energy with the Network: with the load and the production station off, the non-exchange of energy of the storage system with the Network is controlled.

In the case of connection of high capacity storage systems, the Operator may additionally require that tests and measurements be carried out during a trial period of a maximum of thirty (30) days.

The storage system is part of the self-producer’s Internal Electrical Installation, i.e. it is installed “behind the meter” and therefore the responsibility for its installation and its safe and uninterrupted operation rests with the self-producer and legally competent engineer. The installation must comply with the ELOT HD 384 Standard and IEC 62933-5-1, in accordance with which the potential risks that may be caused by the operation of the storage system shall be identified and the necessary measures shall be taken to reduce or eliminate them. The application of more specific standards and rules depends on the type of accumulators, e.g. for lead-acid batteries, compliance with EN 50172 is mandatory. It is clarified that during commissioning, only those elements of the installation that have an impact on the operation of the network and other connected users are checked. In any case, the responsibility for the proper functioning of the entire installation rests with the self-producer and the legally competent engineer.

The following conditions apply to the connection of the storage system to the installation of the self-producer:

• Converters up to 5 kVA are allowed to be connected in one phase, while for converters with higher power a three-phase connection to the Network is required.
• In three-phase systems, the accumulators can be connected through a three-phase converter or through three single-phase converters. Especially in three-phase systems in which a single-phase inverter of a PV station may be installed under certain conditions, a single-phase battery converter may be connected to the same phase.
• When single-phase converters are used in three-phase systems, the power of the storage system connected to the three phases should be at least two-phase.
• In case the storage system is combined with a PV station, the accumulators and the station can be connected through the same power converter (Figure 1b) to the internal electrical installation of the self-producer or the accumulators can be connected through an independent power converter (Figure 1 and Figure 2) to the installation. The independent converter connection scheme can be used for the case of connection of each RES or CHP plant.

Σχήμα 1: Ενδεικτικό απλοποιημένο μονογραμμικό διάγραμμα εσωτερικής ηλεκτρικής εγκατάστασης ΧΤ όπως διαμορφώνεται α) μετά τη σύνδεση σταθμού παραγωγής και συσσωρευτών μέσω διαφορετικών μετατροπέων (ac coupled) και β) μετά τη σύνδεση ΦΒ σταθμού και συσσωρευτών μέσω του ίδιου μετατροπέα (dc coupled), χωρίς δυνατότητα απομονωμένης (off-grid) λειτουργίας. Η διακεκομμένη γραμμή υποδηλώνει την επικοινωνία του «αισθητήρα κατεύθυνσης» (S) με το σύστημα διαχείρισης του μετατροπέα. Στο διάγραμμα δεν απεικονίζονται τα μέσα ζεύξης και προστασίας της υφιστάμενης εγκατάστασης κατανάλωσης.

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• The protection against islanding should be applied to an Automatic Interconnection Switch placed at the input of the general consumption panel, so that, in the event of a power failure, accumulator continue to operate isolated from the Network and supply the load (Figure 2).
• The storage system shall have the technical capability to operate without the Network (off-grid mode).
• In the case where the installation is supplied by the storage system and normal operation of the Network occurs, if the utility wants to avoid a momentary interruption of the supply to its installation, the Automatic Interconnection Switch, in addition to the basic protections against voltage, frequency and islanding limits, should have a synchronization device equivalent to that of modern generators.
• The installation must comply with the requirements of the ELOT 60364 standard both when powered by the Network and when powered by the storage system. The electrical installer shall demonstrate compliance with Standard, considering both supply modes.

To determine the maximum power of the storage system converter, the following limitations must be satisfied simultaneously:

• The nominal capacity of the storage system converter (in KVA) shall not exceed the nominal capacity of the production station (in kW).
• The capacity of the storage system converter shall be such that the maximum current that can be generated during the operation of the production station and the storage system does not exceed the maximum charging capacity of the Supply
• The maximum power limit of the storage system converter is 30kVA.
  The 30 kVA limitation does not apply storage systems installed in conjunction with RES and CHP stations of the legal entity of the state or legal entities under public or private law pursuing public interest purposes of general or local scope in the Interconnected System or in Crete.
  Example: A 15 kWp PV station and storage system is to be installed in three-phase Supply No. 2 (25 kVA) and connected to the electrical installation through an independent power converter, as shown in Figure 1:
• In accordance with the 1st limitation, a storage system converter up to 15 kVA can be installed
• In accordance with the 2nd limitation, assuming simultaneous power injection from both the PV station and the accumulators, a maximum power margin of 10 kVA for the accumulator converter is obtained.

Therefore, in accordance with all three limitations, a three-phase converter with a capacity of 10 kVA or alternatively 3 single-phase converters with a capacity of 3.3 kVA each can be installed.

In the event that there is no commercially available converter with a nominal power rating less than the maximum power limit resulting from compliance with the above limitations, the installation of an inverter with a higher nominal power rating is permitted, provided that the converter is limited to the maximum permissible power limit.

Please note that there are no limitations on the capacity (kWh) of the storage system.

The technical specification of the storage system converter is at least the following:

• The converter shall have protection against voltage, frequency and islanding limits. The good functioning of the converters must be ensured at
  a range of variation from +15% to -20% of nominal voltage and a range of frequency variation from -2.5 Hz to +1.5 Hz. The protection against islanding shall be in accordance with VDE 0126 or other internationally recognized active or passive protection method.
• In case the converter is switched off (automatic decoupling), it can be coupled in at least 3 min.
• The Total Harmonic Distortion (THD) of the converter current must not exceed 5%.
• Since the converter does not have an isolation transformer, the DC injection should be limited to 0.5% of the rated current.
• The converter shall be capable of adjusting the power factor from 0.95 inductive to 0.95 capacitive.

The technical specification of the storage system converter shall be shown either in the manufacturer’s technical manual or in its certificate.

The procedure followed for connecting a production station with net metering combined with a storage system is the same as for connecting a production station with net metering without a storage system, with the following differences:

• The following additional document and data shall be provided with the connection request: a) the technical specification of the storage system and b) the technical manual and certificate of the batteries, inverter and “direction sensor”.
  c) Technical Description outlining the energy management strategy for the indoor electrical installation, while the electrical plan shall show the devices by which the desired management strategy is implemented. The purpose of the above is to document that the storage system does not exchange energy with the Network.
• In the Solemn Declaration of Law 1599/1986 in which the self-producer states that the station’s coupling protection settings will not be modified, it should additionally state that the storage system settings will not be modified.

It is noted that the cost of the works and works of HEDNO for the connection of the station to the LV is not differentiated due to the existence of the storage system.

In case the self-producer wishes to add a storage system to an existing net metering installation, it should apply to HEDNO for the amendment of the connection contract by submitting the following document and items: a) the form with the technical specification of the storage system b) a Solemn Declaration of an engineer of appropriate specialization for the overall installation, with attached electrical plan, technical description, technical manual and certificate of the accumulators, the converter and the “directional sensor” which shall demonstrate compliance the requirements of Ministerial Decision ΥΠΕΝ/ΔΑΠΕΕΚ/15084/382 (GG Β’ 759/5.3.2019) as in force, and c) Solemn Declaration of Law 1599/1986 in which the self-producer will state that they will not modify the settings of the storage system

It is noted that in the case of adding a storage system to an existing net metering installation, due to the way the storage system operates and due to the restrictions imposed on the capacity of the storage converter, no study, network project or power increase of the Supply side is required for the connection of the storage system, while in addition, HEDNO does not require the self-producer to install an additional meter in the internal electrical installation.

Provided that the document and items (a) to (c) submitted are complete and the installation complies with the requirements of the Ministerial Decision, the self-producer is informed of the payment of the storage system activation cost, amounting to €150 plus VAT and agrees
with HEDNO on the date on which the relevant tests are to be carried out. If the tests are successful, the amended connection contract shall be signed. The net metering contract shall not be amended.

Requests for the installation of a storage system in combination with a production station with net metering are submitted to the reception unit of HEDNO which, according to relevant announcements, is responsible for receiving the request of the production station.

In case the Network has been classified as saturated due to exceeding the thermal limit of the upstream LV/MV transformer, the installation of a storage system is allowed as the storage system will operate without injecting energy into the Network.
In the case where the Network is classified as saturated due to exceeding of the short-circuiting level at the MV busbar of the upstream LV/MV transformer, the installation of a storage system is not allowed as the storage system contributes to the fault current (IEC 60909).