Strengthening the grid: How a renewables-based energy supply demands new approaches

The transformation of the electrical energy system towards 100% renewables and therefore CO₂-free generation leads to major challenges in terms of stable grid operation, write experts from TransnetBW.

Decentralised renewable energy resources take over the job of energy supply from traditional centralised large-scale power plants. To maximise the energy output, renewables are built at the locations with the best external conditions with respect to wind and sun hours. Mostly these locations are far away from the consumers, making it necessary to transmit the power over long distances.

To maximise renewable generation, system operation is optimised, maintaining only safety margins to the stability limits of the grid. In order to raise these limits to the extent possible, and eventually reach a 100% renewable power system, a sufficient amount of inertia and controlled reactive power sources in the grid play a major role.

Both grid users and TSO-owned equipment will be needed to face this challenge.

Need for reactive power

With a large amount of electricity being transmitted over longer distances, large currents are flowing across large inductive impedances. If a current is flowing over an impedance, the reactive power demand of that impedance depends on the squared current magnitude and the reactance.

Therefore, when operating long lines at high currents, a change in current is leading to a massive change of the reactive power consumption of the grid, which must be compensated dynamically on a local basis.

This compensation can be achieved by reactive power sources with the ability to react dynamically.

Need for system inertia

System inertia and frequency control are needed, when power generation and consumption is not balanced in the power system with the worst case being a system split.

In large systems like the European interconnected power system, bulk transits across large distances are common, because with this, socioeconomic benefits can be created. As these bulk transits can reach high values, the resulting imbalances after a system split can become very large.

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In such a situation, inertia fills the gap by instantaneously feeding the missing power or absorbing the power surplus and with this, preventing the system from a blackout in the first moment.

The power fed or consumed by inertia is slowing or accelerating the rotating parts of the system (synchronous generators) and therefore producing a rate of change of frequency (RoCoF), which becomes higher with less available inertia. This effect can also be emulated by a converter using a grid forming control scheme in combination with an energy storage to buffer the energy in both directions.

A system split would drive the RoCoF to very high values and with this, frequency quickly reaches its limits even before proportional frequency containment schemes can react. In these situations, only an appropriate amount of inertia will save the system from a blackout by slowing down the change of frequency and thus gaining time for frequency control mechanisms.

E-STATCOM – a grid-based element beneficial in two ways

The needs for inertia and controllable reactive power that were identified from a system perspective to ensure stable system operation can be met with a single device. The STATCOM, which was originally intended solely for the dynamic provision of reactive power, is used as a basis for this purpose.

Its principle of a converter-based reactive shunt element, which can create almost every three-phase voltage system in relation to the grid voltage leading to reactive current, is upgraded with a suitable short-term energy storage unit.

This enables the so-called E-STATCOM to provide active power proportional to the RoCoF, analogous to the behaviour of a synchronous generator. Combined with the grid-forming control, the ability to provide inertia qualifies the E-STATCOM to contribute to system strength.

As a result, this single device can be used to provide contributions to controlled reactive power locally, and inertia and system strength globally simultaneously at any time.

Design aspects of an E-STATCOM

Among the various Modular-Multilevel-Converter (MMC) topologies, single-delta full-bridge topology (see Figure 1 (a)) and double-star topology (see Figure 1 (b)) allow controllability of the positive and negative sequence in a wide operating range¹.

While the single-delta MMC can offer inertia by storing the required energy in the submodules², advantages and disadvantages of the topologies have to be evaluated individually. Research³ suggests that the double-star configurations are the most appealing choice for MMC-based E-STATCOMs, due to their robustness and flexibility.

Figure 1: Single-Delta topology as in classic STATCOM projects (a) and Double-Star topology (b) with energy storage

In EHV-grid-applications, where a transformer is obligatory, the advantage of a double-star-configuration to match a certain voltage at the AC-terminals using the least number of submodules⁴ vanishes, therefore classic STATCOMs were mostly designed with single-delta topology.

The dynamic requirement of the E-STATCOM demands instantaneous current contribution up to the current limit to stabilise severe grid events based on a voltage source behaviour. These current leads to an exchange of reactive and active power.

Conventional submodule capacitors do not store enough energy for this on their own, particularly when inertia provision is required over time periods up to 1.25 s. To allow this power exchange with a single-delta MMC, an extension of the submodules typically with parallel connected DC/DC energy storages is used. To integrate a scalable DC energy storage, it appears more favourable to connect it at one common node: The neutral point of a double-star topology as depicted in Figure 1 (b).

In the event of contingencies, the energy stored on the DC side is transferred to the AC side by the MMC converter. As depicted in Figure 1, the converter is connected to the grid via a YΔ or YY transformer. Such a transformer is required to limit the zero sequence current the MMC might generate, especially if over-modulation is applied.

With this principal system design approach, the dimensioning of the electrical storage can be easily derived from the system needs and can be scaled due to the modularity of the aggregated capacitor. The energy storage is designed based on the technical capabilities of the converter to provide inertia within the grid frequency limits of 50 ± 2.5Hz.

The average maximum RoCoF goal of ±1Hz/s according to the ENTSO-E⁵⁶  is also a decisive factor for the electrical storage design, resulting in practice in higher local RoCoFs of up to 2 Hz/s. The E-STATCOM is expected to provide inertia within these limits without reaching its technical limitations.

Taking these premises into account, the four TSOs of Germany defined a common standard for the E-STATCOM of an active power output of 150 MW [7] during an event with a RoCoF of ±2 Hz/s until the frequency limits are reached.

This translates to a total time duration of active power exchange of 1.25s with an energy content of 187.5 MWs in load direction for over-frequency and generator direction for under-frequency events. Having the short-term energy storage idling at a neutral state of charge, the total energy storable for the provision of inertia amounts to 375MWs.

Real-life project experiences

The application of the E-STATCOM is advancing: In 2022 TenneT TSO started the first E-STATCOM project together with Siemens Energy. In early 2024, TransnetBW has awarded Hitachi Energy with the construction of two E-STATCOM units to be put into operation until 2028.

Four further E-STATCOM are currently in tendering at TransnetBW. Building a “first of its kind” technology, several technical challenges must be mastered in the project. The grid forming control must be developed and adapted to the double-star topology plus short-term energy storage.

Secondly, the design of the short-term energy storage has to be optimized, which consists of a vast number of supercapacitors (“supercaps”) installed in racks with combinations of parallel and series connections. The optimized design needs to cover diverse topics such as:

• Failure of one supercap must not lead to an outage of the whole string connected in serial
• DC voltage must be distributed equally across the supercaps to avoid local overvoltages
• Super capacitor racks need to be supplied with auxiliary power

Equally important is the requirement in space. At TransnetBW, the E-STATCOM is built within existing substations, where the area available is limited to 45x85m (approx. half a soccer field), requiring a thoughtful design utilizing several floors.

The transformation towards the full utilisation of renewable energy resources revolutionises the way the power system is planned according the generated and distributed energy. The E-STATCOM technology is a key player to adapt the extra high voltage network to the new challenges. The first E-STATCOM projects are developing the inverter-based approach for system-inertia into a state-of-the-art technology. System stability is continuing.

The authors

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