'Reactive power' means the imaginary component of the apparent power at fundamental frequency, usually expressed in kilovar ('kVAr') or megavar ('MVAr') - Article 2(28) of the Network Code on requirements for grid connection of generators (NC RfG).
Reactive power control is not an energy balancing function but is required for voltage control (Market Design for Demand Side Response Policy Paper, November 2015, p. 11).
ENTSO-E guidance document for national implementation for network codes on grid connection, Reactive power control modes for PPM & HVDC, Draft for consultation 1 July -15 August 2016, of 30 June 2016 underlines: "[r]eactive power control is a basic requirement for controlling the voltage in electric networks. The voltage is controlled to operate the network within the voltage ranges and to maintain voltage stability. Functions of power system management (e.g. optimal power flow) can only be achieved by specific reactive power provision" (p. 2).
Pursuant to the ENTSO-E Supporting Document for the Network Code on Operational Security of 24 September 2013 2nd Edition Final (p. 30, 31) voltage conditions in a transmission system are directly related to the reactive power situation at the system nodes.
In order to compensate for an excessive consumption of reactive power, Transmission System Operators (TSOs) must make sure that the most efficient and effective producers feed/absorb sufficient reactive power in addition to the reactive power from other sources installed in the transmission system or at demand facilities.
TSOs must further ensure a continuous and locally sufficient reactive power balance to be able in turn to maintain adequate voltage levels.
ENTSO-E guidance document for national implementation for network codes on grid connection of 30 June 2016, Reactive power management at T – D interface, Draft for consultation 1 July - 15 August 2016 (p. 2) underlines the local character of the reactive power management:
"Different system operator for the networks (e.g. distribution or transmission network), network topologies (degree of network meshing), localisation of the connection point at the distribution-transmission interface and load and embedded generation characteristics, lead to the need for different ranges of reactive power. For this reason, the exchange of reactive power at each interface between the two networks strongly depends on the above mentioned local needs. For instance, heavily loaded meshed grids or radial or remote grids typically need more injection of reactive power (production), whereas the same meshed grid in light loading conditions need more reactive power consumption in order to keep the network voltage within the permitted range.
In general it is more cost effective at system level to generate reactive power at the location where it is needed to avoid higher losses and large voltage deviations. Furthermore, the transport of reactive power is possible only over limited distances.
In addition, reactive power is traditionally provided by generating units thanks to the limited marginal investment compared to the delivery of active power only.
A core principle that should underpin all TSO-DSO interactions with regard to reactive power is that each system operator is responsible for ensuring voltage requirements on its network."
Recital 23 of the Network Code on requirements for grid connection of generators
The reactive power capability needs depend on several factors including the degree of network meshing and the ratio of in-feed and consumption, which should be taken into account when establishing reactive power requirements.
When regional system characteristics vary within a systems operator's area of responsibility, more than one profile could be appropriate.
Reactive power production, known as lagging, at high voltages and reactive power consumption, known as leading, at low voltages might not be necessary.
Reactive power requirements could put constraints on the design and operation of power generating facilities.
Therefore it is important that the capabilities actually required for efficient system operation be thoroughly assessed.
In this context, the goal of voltage control and reactive power management is to ensure that:
- voltage levels, reactive power flows and Reactive Power resources are monitored, controlled and maintained in real-time within the Operational Security Limits, in order to protect the equipment of the transmission system and ensure its voltage stability.
- adequate instantaneous reactive power reserve is available in spinning generators, reactors and capacitors in order to secure the technical functioning of the whole electric power system and to be able to restore the Normal State after disturbances.
For this purpose, permanent online monitoring and information exchange that takes place with the TSOs from the TSO's defined observability area is established.
The operational security of the transmission system is closely coupled with the voltage profile.
For the maintenance of a good and viable operational voltage profile and I n order to operate the electrical system (e.g. distribution and transmission network), during all the network conditions (e.g. topology, set of generation unit in service, degree of network meshing), the system operator must have at their disposal reactive power resources that are able to regulate the network node voltage.
These resources are primarily provided by users i.e. generating units, third party HVDC circuits and demand side response.
The main resources of reactive power are the Power Generating Facilities and compensation devices.
Both Power Park Modules (PPMs) and HVDC systems can provide reactive power and in many cases can comply with the network code requirements without the need for additional equipment.
However at low/zero active power additional equipment may be required in order for them to be able to continue to maintain proper reactive capability.
The co-location of the location of renewable generation and demand is generally not as high as that of fossil fuel generation and demand, therefore renewable generation may not be physically able to provide the reactive power required across the network, and supplemental sources may be required.
With the highest level of renewable energy sources (RES) penetration many synchronous generators will be displaced at the times of high RES production (e.g. windy/sunny).
This removes a key source of reactive power (ENTSO-E guidance document for national implementation for network codes on grid connection of 30 June 2016, Reactive power management at T – D interface, Draft for consultation 1 July - 15 August 2016 (p. 4)).
In cases of voltage degradation where the transmission system is in danger and the voltages are outside the ranges for the Normal State or Alert State, the Power Generating Facilities are mostly needed to stay connected to the transmission system, supporting the system voltages and providing Reactive Power for keeping the transmission system intact and reinstating the Normal State.
The ENTSO-E guidance document for national implementation for network codes on grid connection, Reactive power control modes for PPM & HVDC, Draft for consultation 1 July -15 August 2016, of 30 June 2016 ads, moreover, that reactive power is the main measure to control network voltage in high voltage (HV) and extra high voltage (EHV) networks and that with increasing penetration of non-synchronous equipment these devices will be more important for maintaining voltage stability.
Specification of reactive power capabilities may in some contexts require collaboration at regional level (Harmonisation, ENTSO-E guidance document for national implementation for network codes on grid connection, Draft for consultation 1 July - 15 August 2016, 30 June 2016, p. 2).
Article 15 of the Network Code on Demand Connection (DCC)
Reactive power requirements
1. Transmission-connected demand facilities and transmission-connected distribution systems shall be capable of maintaining their steady-state operation at their connection point within a reactive power range specified by the relevant TSO, according to the following conditions:
(a) for transmission-connected demand facilities, the actual reactive power range specified by the relevant TSO for importing and exporting reactive power shall not be wider than 48 percent of the larger of the maximum import capacity or maximum export capacity (0,9 power factor import or export of active power), except in situations where either technical or financial system benefits are demonstrated, for transmission-connected demand facilities, by the transmission-connected demand facility owner and accepted by the relevant TSO;
(b) for transmission-connected distribution systems, the actual reactive power range specified by the relevant TSO for importing and exporting reactive power shall not be wider than:
(i) 48 percent (i.e. 0,9 power factor) of the larger of the maximum import capability or maximum export capability during reactive power import (consumption); and
(ii) 48 percent (i.e. 0,9 power factor) of the larger of the maximum import capability or maximum export capability during reactive power export (production);
except in situations where either technical or financial system benefits are proved by the relevant TSO and the transmission-connected distribution system operator through joint analysis;
(c) the relevant TSO and the transmission-connected distribution system operator shall agree on the scope of the analysis, which shall address the possible solutions, and determine the optimal solution for reactive power exchange between their systems, taking adequately into consideration the specific system characteristics, variable structure of power exchange, bidirectional flows and the reactive power capabilities in the distribution system;
(d) the relevant TSO may establish the use of metrics other than power factor in order to set out equivalent reactive power capability ranges;
(e) the reactive power range requirement values shall be met at the connection point;
(f) by way of derogation from point (e), where a connection point is shared between a power generating module and a demand facility, equivalent requirements shall be met at the point defined in relevant agreements or national law.
2. The relevant TSO may require that transmission-connected distribution systems have the capability at the connection point to not export reactive power (at reference 1 pu voltage) at an active power flow of less than 25 % of the maximum import capability. Where applicable, Member States may require the relevant TSO to justify its request through a joint analysis with the transmission-connected distribution system operator. If this requirement is not justified based on the joint analysis, the relevant TSO and the transmission-connected distribution system operator shall agree on necessary requirements according to the outcomes of a joint analysis.
3. Without prejudice to point (b) of paragraph 1, the relevant TSO may require the transmission-connected distribution system to actively control the exchange of reactive power at the connection point for the benefit of the entire system. The relevant TSO and the transmission-connected distribution system operator shall agree on a method to carry out this control, to ensure the justified level of security of supply for both parties. The justification shall include a roadmap in which the steps and the timeline for fulfilling the requirement are specified.
4. In accordance with paragraph 3, the transmission-connected distribution system operator may require the relevant TSO to consider its transmission-connected distribution system for reactive power management.
DCC Articles 46 and 47
Compliance monitoring for transmission-connected distribution facilities
With regard to compliance monitoring of the reactive power requirements applicable to transmission-connected distribution facilities:
(a) the transmission-connected distribution facility shall be equipped with necessary equipment to measure the active and reactive power, in accordance with Article 15; and
(b) the relevant system operator shall specify the time frame for compliance monitoring.
Compliance monitoring for transmission-connected demand facilities
With regard to compliance monitoring of the reactive power requirements applicable to transmission-connected demand facilities:
(a) the transmission-connected demand facility shall be equipped with necessary equipment to measure the active and reactive power, in accordance with Article 15; and
(b) the relevant system operator shall specify the time frame for compliance monitoring.
Network Code on System Operation
1. Each TSO shall monitor the availability of ancillary services.
2. With regard to active power and reactive power services, and in coordination with other TSOs where appropriate, each TSO shall:
(a) design, set up and manage the procurement of ancillary services;
(b) monitor, on the basis of data provided pursuant to Title 2 of Part II, whether the level and location of available ancillary services allows ensuring operational security; and
(c) use all available economically efficient and feasible means to procure the necessary level of ancillary services.
3. Each TSO shall publish the levels of reserve capacity necessary to maintain operational security.
4. Each TSO shall communicate the available level of active power reserves to other TSOs upon request.
Reactive power ancillary services
1. For each operational planning timeframe, each TSO shall assess, against their forecasts, whether its available reactive power ancillary services are sufficient to maintain the operational security of the transmission system.
2. In order to increase the efficiency of operation of its transmission system elements, each TSO shall monitor:
(a) the available reactive power capacities of power generating facilities;
(b) the available reactive power capacities of transmission-connected demand facilities;
(c) the available reactive power capacities of DSOs;
(d) the available transmission-connected equipment dedicated to providing reactive power; and
(e) the ratios of active power and reactive power at the interface between transmission systems and transmission-connected distribution systems.
3. Where the level of reactive power ancillary services is not sufficient for maintaining operational security, each TSO shall:
(a) inform neighbouring TSOs; and
(b) prepare and activate remedial actions pursuant to Article 23.
Network Code on Demand Connection (DCC), Articles 15, 46, 47
- Article 2(28)
- Article 21(3), Requirements for type C power park modules (Reactive power capability)
- Article 25(5), Voltage stability requirements applicable to AC-connected offshore power park modules
- Recital 23
Network Code on System Operation, Article 108, 109
Commission Regulation (EU) 2016/1447 of 26 August 2016 establishing a network code on requirements for grid connection of high voltage direct current systems and direct current-connected power park modules
- Article 20, Reactive power capability
- Article 21, Reactive power exchanged with network
- Article 22, Reactive power control mode
- Article 38
- Article 40(2), Reactive power and Voltage requirements
- Article 46
- ANNEX 4 – U-Q/Pmax PROFILE [General]
- ANNEX 7 – U-Q/Pmax PROFILE [DC connected PPMs]
- ANNEX 8 – U-Q/Pmax PROFILE [ Remote end HVDC convertors]