This section introduces some frequency events that occurred in different countries. The idea is to illustrate the main challenges that the TSOs cope with when a frequency event
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occurs. Furthermore, it is intended to show the frequency response and the frequency control actions the TSOs took to restore the frequency inside permissible limits.
2.5.1 Italian blackout: 28
thSeptember 2003
The Italian blackout of 28th September 2003 was mainly caused by the tripping of transmission lines connected the Italian network with the Continental Europe synchronous area through Austria and Switzerland. The transmission lines disconnection produced the Italian network to lose synchronism with the Continental Europe synchronous area and be isolated around 12 seconds. After the Italian network isolation, the primary frequency control and the automatic load shedding acted and stopped the frequency drop at 49 Hz [39]. However, during the last stage of load shedding, additional generation units were tripped due to inappropriate under-voltage load shedding operation. The frequency continued declining until it reached the UF threshold at 47.5 Hz, and the system collapsed in around 150 seconds after the separation of the Continental Europe synchronous area [40]. The measured frequency during the frequency event is depicted in Figure 2.2.
Figure 2.2. Frequency response of the blackout of Italian network on 28th September 2003.
2.5.2 Mongolian under-frequency event: 29
thJune 2018
The steady-state operation of the Mongolian power system (MPS) has been very close to the stability limits in recent time, which makes it vulnerable when a disturbance occurs. This stressed operation mode mainly contributed to the latest significant UF-event in the Mongolian power system. During the night of 29th June 2018, the MPS had
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a power demand of approximately 535 MW and power production of approximately 380 MW coming from coal-fired and wind power plants. During that night, strong wind gusts produced a single phase to ground fault in two transmission lines that interconnect the MPS to the Russian power system. At 23:02, the MPS is separated from the Russian power system causing a power deficit of around 155 MW. Following the isolation of the Mongolian power system, the frequency starts to decline rapidly, reaching a minimum value of 48.4 Hz at 23:03. The low values of the frequency caused the activation of the UFLS scheme, and the frequency was recovered to 49.7 Hz. After a while, the frequency begins to decline again, reaching a value of 48.7 Hz at 23:05. Consequently, the TSOs manually disconnected several loads resulting in an over-frequency condition. The track of the frequency response during the frequency disturbance is depicted in Figure 2.3.
The frequency decline after the activation of UF-relays reveals the poor performance of the current UFLS scheme of the Mongolian system power system, as demonstrated in publications [P9] and [P13].
Figure 2.3. Measured frequency in Mongolian power system during the under-frequency event of 9th August 2019.
2.5.3 Great Britain under-frequency event: 9
thAugust 2019
The most recent frequency event in the Great Britain power system occurred on the afternoon of 9th August 2019. The report of the frequency event provided by the National Grid Electricity System Operators indicated that the cause of the frequency declining was the sudden disconnection of two power generation sources. At 16:52, a lightning strike produced the unexpected disconnection of a wind farm and power plant
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based on a steam turbine. The total power generation was 1,131 MW and produced a frequency deviation of around 1.3 Hz [41].
The frequency decline caused the disconnection of several DERs due to the activation of its ROCOF protection. These DERs disconnections maximised the frequency deviation, leading the frequency to reaches a minimum frequency of 49.08 Hz. The National Grid Electricity System Operators (NGESO) activated and used most of the power system frequency reserves, including the energy storage system, to recover the frequency to around 49.27 Hz at 16:53. Nevertheless, two power generation units based on a gas turbine of approximately 400 MW were disconnected due to high-pressure conditions.
After the disconnection, the frequency starts to drop, reaching a value of 48.7 Hz and activating the UFLS scheme. The total power demand disconnection after the action of the UFLS scheme was around 1.0 GW. Finally, the frequency was recovered inside ΔfSS
at 15:01. The measured frequency response during the frequency event is shown in Figure 2.4.
Figure 2.4. Measured frequency in Great Britain power system during the under-frequency event of 9th August 2019.
2.5.4 Continental Europe synchronous area separation: 8
thJanuary 2021.
On 8th January 2021, at 14:04:25 CET, the disconnection of several transmission network elements, starting in Croatia and going through Serbia, the border between Romania and Ukraine down to the Mediterranean Sea, place Europe close to a major blackout.
The sequence of cascading disconnections causes the Continental Europe synchronous area to separate into two areas: North-West Area and South-East Area. During the event, the control actions were carried out in joint coordination between the frequency
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leaders of each area, EMS (Serbia) in the South-East Area and Amprion (Germany) in the North-West Area [42].
In North-West Area, the frequency dropped down to 49.74 Hz within around 15 seconds.
Consequently, TSOs initiate the disconnection of around 1.7 GW of the contracted interruptible services in France and Italy to reduce the frequency deviation. Moreover, the automatic supportive power was automatically activated, and the North-West Area received 595 MW from Nordic and Great Britain synchronous areas. These control actions produced a frequency recovering at a steady-state value of around 49.84 Hz.
Meanwhile, in South-East Area, the frequency initially increased to a value of up to 50.6 Hz, producing a large over frequency condition. The automatic and manual corrective actions were activated to reduce the power supply of the generation units and return the steady-state frequency between 50.2 Hz and 50.3 Hz. Finally, the contracted interruptible services in North-West Area were reconnected at 14:48 CET, and the separated areas are reconnected at 15:08 CET. The frequency measured during the event is shown in Figure 2.5.
The frequency recovery into safe operational range on both North-West South-East Area was due to the correct performance of frequency control schemes. However, other techniques, like identifying generator clustering and islanding conditions, establish in publication [P15], maybe took place to successfully separate the Continental Europe synchronous area and preserve the continuity of electrical service.
Figure 2.5. Measured frequency in North-west and south-east areas of continental Europe synchronous area during the event of 8th January 2021.
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