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Importance of Voltage Stability in Electricity Supply Networks
Where and Why Does Voltage Stability Play Such an Important Role Today?
Voltage stability is an essential component of the energy supply, as it ensures that the electrical voltage available to various consumers remains constant within precisely defined limits. This is of great importance both for transmission grids at extra-high voltage level and for distribution grids at medium and low voltage level. Failure of voltage stability can have fatal consequences, as overvoltages lead to the destruction of consumers and undervoltages disrupt their function. A high level of reliability of voltage stability is therefore a key aspect of the quality of the power supply.
Challenges of Voltage Stability Today and in the Future – Focus on the Low-Voltage Grid
With the increasing feed-in of renewable energies at all voltage levels as part of the energy transition, voltage stability at all grid levels is becoming increasingly complex. Voltage stability in low-voltage grids is one of the biggest challenges facing energy suppliers and grid operators today due to the fluctuating nature of feed-in and consumption from photovoltaic systems, electromobility/e-charging stations and the increasing number of heat pumps in the grid. At times of low load, strong feed-ins can cause the grid voltage to rise, while at times of increased load, the grid voltage can fall dangerously close to the tolerance bands defined in EN 50160. This requires solutions to keep the voltage in the low-voltage grid stable within the defined limits.
What Are the Guidelines for Voltage Stability in the Low-Voltage Grid?
In Germany, grid operators must take measures to ensure that the grid voltage complies with certain tolerance limits. According to EN 50160, in order to prevent damage to appliances and ensure a reliable power supply, the mains voltage in the low-voltage grid for small consumers must be between +/- 10% – i.e. between 207 V and 253 V – 95% of the time and between 85% and 110% – i.e. between 195.5 V and 253 V – of the nominal mains voltage of 230 V 100% of the time. All appliances must be designed to operate correctly within this voltage range. Narrower tolerances would be even more favourable for some consumers, but would massively increase the effort required for voltage stability to such an extent that the cost/benefit ratio would no longer be ideal.
These guidelines for voltage stability concern the longer-term deviations of the effective voltage values from the nominal values (230 V, or 400 V in industrial applications with heavy current). Short-term phenomena such as transient voltage fluctuations or deviations from a sinusoidal voltage curve (harmonics), which are measured/detected by mobile/portable or permanently installed network analysers in the network, are not covered by voltage stability. Frequency regulation is also a separate task that does not fall under voltage stability.
Methods for Voltage Stability in the Low-Voltage Grid
Methods for Voltage Stability
Compliance with the aforementioned guidelines is already often only possible today through the use of suitable countermeasures and technologies. The need for transparency and controllability of the low-voltage grid will continue to increase in the future due to the rapid expansion of renewable energy sources and electromobility. This is the only way that grid operators can successfully meet the challenges of the energy transition, and voltage stability in particular, and ensure a stable and reliable energy supply. Possible measures for voltage stability are presented below in a brief overview.
- Grid reinforcement/line extension: Grid reinforcement/line extension: Line extension is a classic/conservative approach to solving voltage stability problems. Additional conductors or stronger cables are installed in order to increase the capacity and stability of the grid and thus reduce voltage drops. This requires considerable financial investment, both in materials and labour. Implementation is time-consuming (often many months, sometimes years) and once the lines have been installed, they offer little flexibility for later changes or adjustments to the grid.
- Reducing the feed-in: Temporarily reducing the feed-in when the voltage is too high is quick and easy to implement, but valuable ‘renewable’ energy is lost in the process. A curtailment is also only a potential remedy in the event of voltage increases and therefore only offers a sham solution to voltage stability problems in the low-voltage grid.
- Battery storage/energy storage: Storage systems such as accumulators could store generation peaks and release them later, but are expensive and cause additional energy losses due to efficiency levels. In addition, energy storage systems are still very expensive and cannot yet be used across the board, partly due to the raw materials required.
- Reactive power feed-in: Reactive power compensation systems (PFC) such as chokes and capacitors are used to generate or absorb reactive power and thus stabilise the voltage in the grid. These systems can be installed both centrally and decentrally. This is also increasingly being practised with inverters for photovoltaic systems. However, generating and feeding in reactive power can be technically very complex, requires regular maintenance and monitoring and is associated with additional energy consumption, which reduces the overall energy efficiency of electricity distribution. In addition, reactive power compensation systems generate grid perturbations, which in turn has a negative impact on grid quality.
- »LVRSys® low-voltage control system« from A. Eberle: The »LVRSys® low-voltage control system« was specially developed to solve voltage stability problems caused by the integration of electromobility, photovoltaics and an increasing number of heat pumps into the low-voltage grid. It represents an economical, locally flexible and low-maintenance alternative to alternative solutions such as line extension, energy storage and reactive power feed-in/compensation systems. The system can also be used efficiently and flexibly as a string controller or as a controller directly at the local grid station. Further information on the »LVRSys® low-voltage control system« can also be found here.
LVRSys®
The »LVRSys®-Low-Votage Regulation System« was developed to solve voltage stability problems due to the integration of electromobility, photovoltaics and heat pumps in the low-voltage grid. It represents an economical and flexible alternative to costly and time-consuming line extensions.
Difference/Differentiation Between Static Voltage Stability and Dynamic Voltage Support
1. Static Voltage Stability:
- Normal operation: Static voltage stability ensures that the voltage remains within the permissible voltage band during normal operation. This is achieved through various measures such as the provision of reactive power, the stepping of transformers and the use of voltage regulators.
- Use case: This method is essential for the daily operation of the power grid in order to ensure trouble-free operation of the connected devices.
2. Dynamic Voltage Support:
- Fault: Dynamic voltage support is used in the event of faults, such as short circuits, to ensure stable continued operation of the system. Sufficient short-circuit power is required to limit voltage dips locally and ensure the stability of electrical machines.
- Use case: This method is particularly important for minimising the effects of incidents and quickly restoring the grid to a stable state.
Practical Application Examples and Technical Reports
For Solving Voltage Stability Problems in the Low-Voltage Grid:
Pilot Project: Motorway Car Park With« »LVRSys®«
At the PWC (car park with WC) “Auergründel” on the A6 motorway, the fresh water and waste water pumps were increasingly breaking down. The motorway authority suspected that the reason for this was excessive loads on the low-voltage cable. The short-circuit power of the network at the car park station may not be sufficient for stable pump operation.
Economic Efficiency Analysis: »LVRSys®« vs. Line Extension
The energy transition and the associated changes in the distribution grid pose various challenges for many distribution grid operators. Line expansion in particular is a very cost-intensive and time-consuming investment.
»LVRSys®« in the Low-Voltage Grid in Malta
This application report deals with the integration of the »LVRSys® low-voltage control system« into the low-voltage grid in Malta.
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