Voltage

'Voltage' means the difference in electrical potential between two points measured as the root-mean-square value of the positive sequence phase-to-phase voltages at fundamental frequency (Article 2(2) of the Network Code on Requirements for Grid Connection of Generators (NC RfG)).

The voltage is controlled to operate the network within the voltage ranges and to maintain voltage stability. 

Voltage requirements are critical to secure planning and operation of a power system within a synchronous area.

As ENTSO-E guidance document for national implementation for network codes on grid connection, Selecting national MW boundaries, Draft for consultation 1 July - 15 August 2016, 30 June 2016 underlines, voltage regulation provided by the generation represents an important service to the electricity system.

Voltage issues have a cross border impact as disturbances can propagate widely and, in the worst case, can cause significant disconnection of Power Generating Modules (PGMs), either directly or because of the consequence of a large disturbance on the system frequency (Parameters related to voltage issues, 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) 

Voltage ranges are defined for grid users connected above 110 kV.

For the Continental Europe and Nordic synchronous areas, the non- exhaustive parameter to define at national level is the time period for operation in the high-voltage ranges.

The limits of these time periods for operation are defined in all the Connection Network Codes (NC RfG, Network Code on Demand Connection (DCC) and NC HVDC).

Automatic disconnection

The aforementioned ENTSO-E document of 30 June 2016 refers to the issue of automatic disconnection as follows:

"Automatic disconnection is required from HVDC links, Power generating modules type C and D (but not mandatory for type D) and transmission connected demand facilities, transmission connected distribution facility, transmission connected distribution systems at specified voltage levels. During national implementation, the voltage level for disconnection and the technical parameters shall be defined.

For transmission connected demand facilities, transmission connected distribution facility, transmission connected distribution systems, HVDC circuits and power generating modules type D, this requirement is not mandatory. However in the event that there is a risk to voltage stability (notably collapse) system operators may require additional protection to disconnect demand or generation necessary to permit the timely connection of new facilities or generators and maintain security of supply.

For power generating modules, the voltage criteria will depend on whether the PGM contributes actively to voltage regulation. In this case, the PGM shall stay connected within the whole voltage range defined at national level (or in the code for type D) in order to contribute to voltage restoration for as long as possible. On the contrary, if the reactive power contribution of the PGM isn't linked to the voltage at the connection point, the PGM shall disconnect when its contribution increases the voltage disturbance.

For HVDC circuits, transmission-connected demand facilities, or transmission-connected distribution facilities/systems the same principle shall apply as with PGMs."

Voltage ranges

Obligations of Transmission System Operators (TSOs) regarding voltage limits are stipulated in Article 27 of Network Code on System Operation, Article 27 and Annex II thereto (see box and tables below).

Network Code on Requirements for Grid Connection of Generators (NC RfG), Article 2(2), Recital 23

Demand Connection Code (DCC), Recital 15

Network Code on System Operation, Article 27, Annex II

Parameters related to voltage issues, ENTSO-E guidance document for national implementation for network codes on grid connection, 16 November 2016