Short circuit ratio (electrical grid)

In an electrical grid, the short circuit ratio (or SCR) is the ratio of the short circuit apparent power (SCMVA) in the case of a line-line-line-ground (3LG) fault at the location in the grid where some generator is connected to the power rating of the generator itself (GMW). Since the power that can be delivered by the grid varies by location, frequently a location is indicated, for example, at the point of interconnection (POI):


 * $$SCR_{POI} = \frac {SCMVA_{POI}} {GMW}$$

SCR is used to quantify the system strength of the grid (its ability to deal with changes in active and reactive power injection and consumption). On a simplified level, a high SCR indicates that the particular generator represents a small portion of the power available at the point of its connection to the grid, and therefore the generator problems cannot affect the grid in a significant way. SCMVA is defined as a product of the voltage before the 3LG fault and the current that would flow after the fault (this worst-case combination will not happen in practice, but provides a useful estimation of the capacity of the circuit). SCMVA is also called a short circuit level (SCL), although sometimes the term SCL is used to designate just the short-circuit current.

Grid strength
The term grid strength (also system strength) is used to describe the resiliency of the grid to the small changes in the vicinity of the grid location (“grid stiffness”). From the side of an electrical generator, the system strength is related to the changes of voltage the generator encounters on its terminals as the generator's current injection varies. Therefore, the quantification of the system strength can be done through finding the equivalent (Thévenin) electrical impedance of the system as observed from these terminals (the strength is inversely proportional to the resistance). SCR and its variations provide a convenient way to calculate this impedance under normal or contingency conditions (these estimates are not intended for the actual short-circuit state).

Strong grids provide a reliable reference for power sources to synchronize. In a very stiff system the voltage does not change with variations of the power injected by a particular generator, making its control simpler. In a traditional grid dominated by synchronous generators, a strong grid with SCR greater than 3.0 will have the desired voltage stability and active power reserves. A weak grid (with SCR values between 2.0 and 3.0) can exhibit voltage instability and control problems. A grid with SCR below 2.0 is very weak.

Importance of overcurrent
Grid strength is also important for its overcurrent capabilities that are essential for the power system operations. Lack of overcurrent capability (low SCR) in a weak grid creates a multitude of problems, including:
 * transients during the large load changes will cause large variations of the grid voltage, causing problems with the loads (e.g., some motors might not be able to start in the undervoltage condition);
 * the grid protection devices are designed to be triggered by a sufficient level of overcurrent. In a weak system the short circuit current might be hard to distinguish from a normal transient overcurrent encountered during the load changes;
 * during a black start operation after a power outage, large inrush current might be needed to energize the system components. For example, if some loads in a weak system remain connected, an inverter-based resource might not be able to start.

Presence of inverter-based resources
Large penetration of the inverter-based resources (IBRs) reduced the short circuit level: a typical synchronous generator can deliver a significant overcurrent, 2-5 p.u., for a relatively long time (minutes), while the component limitations of the IBRs result in overcurrent limits of less than 2 p.u. (usually 1.1-1.2 p.u.).

The original SCR definition above was intended for a system with predominantly synchronous generation, so multiple alternative metrics, including weighted short circuit ratio (WSCR), composite short circuit ratio (CSCR), equivalent circuit short circuit ratio (ESCR), and short circuit ratio with interaction factors (SCRIF), have been proposed for the grids with multiple adjacent IBRs to avoid an overestimation of the grid strength (an IBR relies on grid strength to synchronize its operation and does not have much overcurrent capacity).

Henderson et al. argue that in case of IBRs the SCR and system strength are in fact decoupled and propose a new metric, grid strength impedance.

Integrating renewable energy sources often raises concerns about the system's strength. The ability of different components in a power system to perform effectively depends on the system's strength, which measures the system variables' sensitivity to disturbances. The short circuit ratio (SCR) is an indicator of the strength of a network bus about the rated power of a device and is frequently used as a measure of system strength. A higher SCR value indicates a stronger system, meaning that the impact of disturbances on voltage and other variables will be minimized. A strong system is defined as having an SCR above three, and the SCRs of weak and very weak systems range between three and two and below two, respectively.

Power electronic applications often encounter issues related to SCR, particularly in renewable energy systems that use power converters to connect to power grids. When connecting HVDC/FACTs devices based on current source converters to weak AC systems, particular technologies must be employed to overcome SCR of less than three. For HVDC, voltage-source-based converters or capacitor-commutated converters are utilized in applications with SCR near one. Failing to use these technologies will require special studies to determine the impact and take measures to prevent or minimize the adverse effects, as low levels of SCR can cause problems such as high over-voltages, low-frequency resonances, and instability in control systems.

Wind farms are commonly linked to less robust network sections away from the main power consumption areas. Problems with voltage stability that arise from incorporating large-scale wind power into vulnerable systems are crucial issues that require attention. Some wind turbines have specific minimum system strength criteria. GE indicates that the standard parameters of their wind turbine model are appropriate for systems with a Short Circuit Ratio (SCR) of five or higher. However, if connecting to weaker systems, it is necessary to carry out further analysis to guarantee that the model parameters are adequately adjusted. Specifically designed control methods for wind turbines or dynamic reactive compensation devices, such as STATCOM, are required to ensure optimal performance.

Example
An experience at ERCOT in early 21st century provides a prime example of how the wind turbine's performance is affected by a weak system strength. The wind power plant, linked to the ERCOT grid through two 69kV transmission lines, worked efficiently when the SCR was around 4 during normal operations. However, when one of the 69kV lines was disconnected, the SCR dropped to 2 or less, leading to unfavorable, poorly damped, or un-damped voltage oscillations that were documented by PMUs at the Point of Interconnection (POI) of the wind plant. After a thorough investigation, it was determined that the aggressive voltage control used by the WPP was not appropriate for a weak grid environment and was the primary cause of the oscillatory response. Due to the low short circuit level detected by the wind generator voltage controller and the high voltage control gain, the oscillation occurred. When compared to the normal grid with high SCR, the closed loop voltage control would have a faster response under weak grid conditions. To replicate the oscillatory response, the event was simulated using a detailed dynamic model representing the WPP.

Impact on grid
The SCR can be calculated for each point on an electrical grid. A point on a grid having a number of machines with an SCR above a number between 1 and 1.5 has less vulnerability to voltage instability. Hence, such a grid is known strong grid or power system. A power system (grid) having a lower SCR has more vulnerability to grid voltage instability. Hence such a grid or system is known as a weak grid or a weak power system.

Grid strength can be increased by installing synchronous condensers.