Frequency control

As electricity cannot be stored easily in large quantities, the production al-ways equals the system load (which includes the line losses). This balance guarantees a stable operation of the electricity grid at a constant frequency of 50 Hz¹. When there is a sudden change in the production or consump-tion, frequency deviations occur. In order to maintain and/or to restore the frequency, the output of the generators must be rapidly increased or decreased.

In the European synchronous electricity grid, the frequency regulation is achieved in three stages: primary, secondary and tertiary control [5, 8–10].

These different stages are discussed next.

2.1.2.1 Primary frequency control

Primary control maintains the balance between power generation and con-sumption when a deviation occurs. Its main task is to limit the frequency deviation∆f from the nominal value fnom and stabilize the grid frequency after a disturbance. It is a local and automated control and within seconds (∼ 15 s - 30 s) the active power output of the participating generators can be increased or decreased. The change in active power output due to the functioning of the primary control,∆Pprim, is proportional to the deviation of the grid frequency:

∆Pprim= Kprim(fnom− f) (2.1) where f is the actual grid frequency, fnom is the nominal grid frequency (50 Hz) andKprim is determined by the slope of the droop characteristic, shown in Fig. 2.1. In order to achieve this change in active power output, primary or frequency containment reserves have to be maintained at any

1In a large part of the world, including Europe, 50 Hz is the nominal frequency. In some countries (e.g., USA, Canada, parts of Japan, etc.), 60 Hz is used instead of 50 Hz.

However, this has no impact on the operation of the frequency control.

P0

Primary reserve

fnom

∆Pprim

∆f

f P

Figure 2.1: Frequency control: P /f droop function.

time. After the activation of these reserves, the power balance is restored at a frequency deviating from the nominal value.

2.1.2.2 Secondary frequency control

Secondary control is used to restore the grid frequency to the nominal fre-quency. Furthermore, the desired energy exchange between the different control areas is maintained by this control. A control area is a part of the network that is controlled by a transmission system operator (TSO). Sec-ondary control is only activated in the control area where a power deficit exists and is activated automatically. It starts after a few seconds and is usually completed after 15 min. In this case, secondary or frequency restora-tion reserves are needed. An automatic PI-control funcrestora-tion that is applied at the transmission system operator control center activates the secondary reserves. After about 15 min, secondary control has finished and the sec-ondary reserve are replaced by the tertiary control.

2.1.2.3 Tertiary frequency control

The main function of the tertiary control is to restore the required level of operating reserves (primary and secondary reserves). Tertiary or replace-ment reserves are maintained at any time and are activated manually by the TSO about 15 min after the initial deviation. Tertiary control copes with persistent control deviations after production outages or long-lasting load changes. The tertiary reserves can be situated in the control area where the power deficit exists or in other areas of the synchronous area.

2.1.2.4 Operational reserves

Operational reserves are active power reserves located in the generation units or loads to maintain balance between generation and demand and

Figure 2.2: Activation of power reserves (based on [10]).

restore the frequency to its set point value in the synchronous system.

As already stated, operational reserves are classified as frequency contain-ment reserves, frequency restoration reserves and replacecontain-ment reserves [11].

Sometimes, a distinction is made between spinning reserves and standing reserves [4, 6, 12]:

• spinning reserves: increase or decrease in generation that can be pro-vided at short notice, carried out by partially-loaded generating units that are synchronized to the grid.

• standing reserves: increase or decrease in generation that can be pro-vided by those generating units that are not synchronously online.

In Fig. 2.2, the timing of the activation of the different power reserves is shown. First the primary reserves are deployed, which have to be completely available after30 s. Then, the secondary reserves start to take over to free the primary reserves and to restore the frequency to the nominal value.

After about15 min, tertiary reserves are manually activated.

2.1.2.5 Time control

If the mean system frequency in the synchronous zone deviates from the nominal frequency of50 Hz, this results in a discrepancy between the syn-chronous time and the universal coordinated time (UTC). Therefore, the synchronous time is calculated and its correction is organized centrally. Cor-rection involves the setting of the set-point frequency for secondary control at 49.99 Hz or 50.01 Hz, depending upon the direction of correction, for full periods of one day.

The different functions of primary, secondary, tertiary and time control are summarized in Fig. 2.3.

2.1.2.6 New terminology

In recent grid codes, new terminology is introduced concerning frequency control [13, 14]. However, the operating principle remains the same. The new control structure is as follows:

System

Figure 2.3: Overview of frequency control (based on [10]).

• Frequency containment process (FCP): stabilizes the grid fre-quency to a steady state value after a disturbance by activation of frequency containment reserves (FCR). This process corresponds to the primary frequency control.

• Frequency restoration process (FRP): restores the frequency to the nominal frequency (50 Hz) by activating frequency restoration re-serves (FRR). Some of the frequency restoration rere-serves are automat-ically activated, which corresponds to the secondary control reserves.

The reserves that are activated manually activated are part of the tertiary control reserves.

• Reserve replacement process (RRP): replaces the FRR by ac-tivation of replacement reserves (RR). This is part of the tertiary frequency control.

Furthermore, the imbalance netting process is added to reduce the amount of simultaneous counteracting FRR activation of different control areas.

In the remainder of this work, the ‘old’ terminology is used, as it is well known and still widely used.