Connection of Generators & the European Network codes

In the previous article, we introduced the spirit and structure of the European Grid Code (Grid Code) and gave some insight into how it conditions the grid connection of new generators embodied in the European CoR on Requirements for Generators (RfG), better known as Commission Regulation (EU) 2016/631 of 14 April 2016. In this article, we will provide a more detailed breakdown of the requirements that must be met to achieve this connection, which is conditioned by the size of the plant and its typology, among other aspects.

Traditionally, the requirements of the European CoR for generation connection to the grid have covered aspects ranging from the overcoming of voltage dips to the performance of voltage and speed regulators, as well as simulation models [1]. These requirements have been mostly included in the European Requirements for Generators (RfG) CoR, where a remarkable harmonisation effort can be seen. There are four key elements to consider in understanding grid connection requirements:

1. The significance of the generator: The RfG classifies generators into 4 categories from lowest to highest power: A, B, C and D, according to the power of the generator and its connection point voltage as shown in Figure 1. Thus, each category has to comply with the requirements applicable to its own category and those immediately below it. These thresholds can be modified by the Transmission System Operator (TSO) of each country, as we will see in the following paragraphs.

1. The type of generator: The RfG distinguishes between 3 technologies, namely synchronous power generation modules, power park modules and offshore power park modules. As shown in Figure 2, the first technology corresponds to synchronous generators, while the latter two are connected to the grid via converters and thus constitute non-synchronous generation.

1. Generator location: The RfG modulates the requirements for generators according to the synchronous zone in which they are located: Continental Europe, Great Britain, Nordic countries, Ireland and Northern Ireland and the Baltic States. Thus, different thresholds are established for voltage ranges, frequency or the same separation by generator power as shown in Figure 1. In the case of Continental Europe, these limits are 1 MW, 50 MW and 75 MW for generators B, C and D respectively.
2. The definition of the requirement: The requirements of the RfG can be closed or open, depending on whether they are fully defined or whether they must be completed by the regulations of each country to adapt them to the particularities of their network. For example, in the case of voltage dips or reactive capacity requirements, the RfG establishes ranges within which each TSO must set its limits. Another example is the significance, which in the case of Spain reduces the Continental European limits to 0.1 MW, 5 MW and 50 MW for generators B, C and D respectively, as can be seen in the proposed Operating Procedure (P.O.) 12.2 of October 2018 available here.

The requirements applicable to generators described in the RfG can be classified according to this reference, as shown in Figure 3:

The requirements related to Frequency Stability are intended to cope with frequency disturbances and to keep the frequency within safe margins:

• Frequency ranges: are frequency bands above and below the steady-state range that generators must withstand for a given period of time without tripping.
• P-f regulation: the active power that generators inject in response to frequency variations. It also establishes the power reduction applicable in cases of over-frequency and limitations to power reduction in cases of under-frequency.
• P controllability: adds to the P-f regulation requirements, those of power range, speed, frequency insensitivity, droop and deadband, among others.
• Frequency derivative (ROCOF): defines the maximum frequency variation rate that the generator must withstand without disconnecting.

There are also requirements related to Angular Stability and Robustness whose purpose is to ensure the robustness of the system to disturbances affecting the angular separation of the generators:

• Voltage dip: defines a voltage-frequency envelope that the generator must withstand without disconnecting after a grid fault.
• Capacity to withstand P swings and reclosing: generators must maintain stability in the event of power swings at any point in their operating diagram, as well as withstand single-phase and three-phase automatic reclosing. The damping of oscillations is also regulated.
• Recovery of P after a fault: the generators must be able to provide a magnitude of active power in a given time after a fault in order to strengthen the system.

On the Voltage Stability side, there are requirements that seek to control the voltage within specific ranges and shield it against disturbances in the system:

• Voltage ranges: are bands of voltages above and below the steady-state range that generators must withstand for a given period of time without disconnection.
• Capacity of Q at Pmax and below Pmax: establishes the operating points of the generator as a function of the active power. In the case of maximum power, the voltage is also considered, while below rated power the capacity diagram of the generator is taken into account
• Fast fault current: sets the current injection by the power park modules during the fault to facilitate its identification by the protections, as well as to maintain and recover the voltage during and after the fault.
• Q control modes: sets requirements for setpoints, deadbands, accuracy, speed, slopes and operating ranges.

And of course there are important requirements related to System Management, namely:

• Control and protection schemes: describes the protections that the generators must incorporate, as well as their settings and the capacity to exchange information in real time or periodically with the TSO.
• Instrumentation and simulation models: this covers the magnitudes to be measured, as well as the settings of the monitoring equipment. It describes the static and dynamic simulation models, their validation procedure, format and documentation. The simulations cover most requirements, from overcoming voltage dips to islanding to control performance.
• Synchronisation: regulates the frequency and voltage conditions for synchronisation to the grid, as well as the procedure and equipment to automate this function.

In order to facilitate overcoming network problems, requirements related to System Reset were included as follows:

• Reconnection: regulates the automatic reconnection of generation modules to the grid after a disconnection. It also describes fast resynchronisation and its interaction with protections.
• Autonomous start-up: is the ability of a generation module to recover from total disconnection without using external power supply within a certain period of time. Modules with this capability must be able to maintain voltage and frequency within defined margins and withstand load shocks.
• Island operation: establishes the operating conditions of the power generation modules when they are in a grid that has been isolated from the main grid to feed it while maintaining frequency and voltage.

This is a first approach to the content of the Requirements for Generators (RfG) code approved as Regulation (EU) 2016/631, however, its complexity requires detailed study before tackling any project, the larger the project.

The harmonisation of European electricity systems is an ambitious project with changes that will affect the way we are used to operate our systems. In Spain, many Operating Procedures (O.P.P.) will be affected by the European Generator Requirements code, for example:

• O.P. 12.2 concerning minimum requirements for design, equipment, operation, safety and commissioning.
• P.O. 12.1 for the processing of applications for access, connection and commissioning to the transmission grid.
• P.O. 12.3, whose voltage dip requirement for wind power will foreseeably be integrated into P.O. 12.2.
• P.O. 11.1 and 11.2 for protections and operation of automatisms.
• P.O. 9 on information exchanged by the System Operator (SO).
• P.O. 7.4 for the complementary voltage control service.
• P.O. 1.6 for the establishment of security plans for system operation.
• P.O. 1.4 for energy delivery conditions at grid border points.

At Norvento, we know the grid codes and we also have the simulation tools necessary to overcome the procedures defined for connection to the grid. We help companies overcome the requirements set by transmission system operators, which will undoubtedly become more demanding as the penetration of renewable generation increases.