Ensuring stable power

The energy efficiency of a typical gas turbine-driven main refrigerant compression (MRC) train is around 40%, whereas electric motor-driven trains have up to 95% efficiency. Despite the advantages, developing an e-LNG facility’s power supply system presents unique technical challenges that are somewhat new to developers, given the industry’s past reliance on mechanical drives. This is particularly the case for large capacity plants that require complex electrical infrastructure in a smaller footprint.

One specific concern that must be addressed very early on is harmonic distortions in the electrical system caused by the large power loads of the MRC trains and variable speed drives (VSDs). Failure to understand the impact of transient scenarios and implement proper mitigation measures can lead to disruptions in the electrical supply (whether from the utility or self-generated). As a result, utilities (i.e., grid operators) require e-LNG facilities to adhere to stringent power quality and power factor regulations.

Ensuring compliance can be challenging, as traditional VAR compensation technologies used for power factor correction are bulky, slow to react with limited control capability, and expensive to maintain. Newer technologies, such as frequency converters, have been used to develop modular static synchronous compensator (STATCOM) solutions that have a much smaller footprint, can react rapidly and precisely to maintain high power factors, and provide additional benefits in terms of reliability and lower maintenance costs.

Designing electrical supply networks for e-LNG plants

The liquefaction process consumes a large amount of energy. To liquefy 1 kg of natural gas, around 0.25 kWh of electrical power is required. For a 5 million tpy liquefaction facility, this translates to roughly 140 MW of power supply (not including grid losses). For an 18 million tpy plant, as much as 800 MW could be needed.

While many of the e-LNG plants in operation today are supplied by self-generated power using dedicated gas and/or steam turbine generators (i.e., microgrids), an increasing number of developments are considering grid connections to capitalise on clean energy sources, such as wind, solar, hydro, and nuclear. Others are considering hybrid designs, using a combination of on-site generation and ‘across-the-fence’ power from utilities. Hybrid configurations offer the potential for the e-LNG facility to export power to the external grid under low utilisation conditions.

The type of electrical source and its distance from the plant significantly influence the design of the electrical system. In all cases, transmission and distribution infrastructure is required to feed power from the generation source to the liquefaction plant. This includes transmission lines, step-up and step-down distribution transformers, high-voltage switchgear, compensators, and remotely monitored and controlled protection devices and/or relays.

Generally speaking, as the power supply increases, so does the complexity of the electrical system. It is not uncommon for 132 kV or higher transmission lines to be used as feeders to e-LNG facilities (Figure 2).

Interactions and feedback from various consumers in the system mean that each component must be carefully selected and designed to ensure that power remains stable under all operating conditions. Maintaining high power quality is critical in this regard. Power quality can be affected by various events and component behaviour within the distribution network (e.g., switching speeds, arc flash and short circuit management and protection, lightning, voltage recovery speed, etc.).

Transient conditions caused by the start-up or shutdown of large induction electrical motors and VSDs (with either LCI or VSI converters) driving MRC trains can also impact the power quality of the supply network. For many e-LNG plants, electrical motors (ranging in size from 35 – 85 MW) can represent as much as 90% of the plant’s overall electrical demand. The harmonic distortions they create upstream and downstream (particularly in the case of LCIs) can lead to several undesirable effects, including:

• Overheating of transformers, motors, and other electrical equipment.
• Voltage distortion, leading to malfunctions in sensitive electronic equipment and reduced power quality.
• Resonance in electrical systems, leading to excessive current flow and equipment damage.
• Power losses, leading to reduced efficiency and increased energy costs.
• Non-compliance with power quality standards, resulting in potential fines and legal liabilities.

The importance of grid studies and dynamic modelling

Harmonics and electrical system stability are issues that the LNG industry needs to become more deeply familiar with, given its history of mechanically driven MRC trains. As a result, stakeholders often tend to view e-LNG development as a simple driver swap (i.e., replacing a gas turbine with an electric motor). However, the reality is far more complex.

Performing grid studies and dynamic modelling is crucial to predicting how electrical systems will behave under the various operating conditions of the e-LNG plant, including during faults and transient scenarios. The objective is to develop a digital twin of the plant’s electrical infrastructure to be tested and optimised before physical implementation.

Grid studies involve analysing the entire electrical supply system – from the generation source to the LNG facility’s point of consumption. Studies normally encompass a steady-state (i.e., static) analysis of the network and, for complex distribution systems like those required for e-LNG plants, dynamic modelling, and simulation.

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