Weak AC Systems

Short circuit ratio or SCR is a measure of the stability of an electromechanical generator. It is the ratio of field current required to produce rated armature voltage at the open circuit to the field current required to produce the rated armature current at short circuit.
The SCR can be calculated for each point on a grid. Where the SCR is above one, a grid has good ''grid strength''; it will be less affected by the variations in frequency and can provide more short circuit current.

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. SCR is defined as the ratio of the short circuit capacity at the bus the device is located to the MW rating of the device. 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

The recent occurrence in ERCOT 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 system condition rating (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.

Generator SCR

The larger the SCR, the smaller is alternator reactance (Xd) and inductance Ld. This is the result of larger air gaps in generator design (As in Hydro generators or Salient Pole Machines). It results into Machine loosely coupled to the grid, and its response will be slow. This increases the machines’ stability while operating on the grid, but simultaneously will increase the short circuit current delivery capability of the machine (higher short circuit current) and subsequently larger machine size and its cost. Typical values of SCR for Hydro alternators may be in the range of 1 to 1.5.

Conversely, the smaller the SCR, the larger is alternator's reactance (Xd), the larger is Ld. It results from small air gaps in machine design (As in Turbo generators or Cylindrical rotor Machines). Machines are tightly coupled to the grid, and their response will be fast. This reduces the machine’s stability while operating on the grid and will reduce the short circuit current delivery capability (lower short circuit current), smaller machine size, and lower cost subsequently. Typical values of SCR for turbo alternators may be in the range of 0.45 to 0.9.

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.

See also

*Transformer
*Alternator
*Synchronous motor

References

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Electromechanical engineering