What are the primary causes of voltage sags and swells in microgrids, and how can they be mitigated?

Voltage sags and swells are power quality disturbances characterized by temporary reductions or increases in the root mean square (RMS) voltage from its nominal value. In microgrids, these voltage variations can disrupt the operation of sensitive equipment and negatively impact overall system stability. Primary causes and mitigation techniques are:

Voltage Sags (Undervoltages):

1. Faults: Short circuits (line-to-ground, line-to-line, three-phase) cause a sudden increase in current demand, leading to a voltage drop at the fault location and throughout the microgrid. The severity depends on the fault impedance and location.
2. Starting Large Motors: The inrush current drawn by induction motors during startup is several times their full-load current, causing a temporary voltage sag, especially on weaker parts of the distribution network.
3. Transformer Energization: When a transformer is switched on, it draws a large magnetizing inrush current, creating a voltage dip.
4. Sudden Load Increase: A large, abrupt increase in load demand can overwhelm the available generation capacity, leading to a voltage sag until generation can respond.
5. Intermittent Renewable Generation: Rapid drops in renewable generation, such as cloud cover reducing solar photovoltaic (PV) output, can cause a sag if other sources cannot immediately compensate.

Voltage Swells (Overvoltages):

1. Sudden Load Rejection: Quickly disconnecting a large load causes a voltage swell as the stored energy in the system's inductance is released.
2. Capacitor Bank Switching: Switching on capacitor banks for power factor correction can create a temporary overvoltage, especially if not properly controlled.
3. Lightly Loaded Feeders (Ferranti Effect): In lightly loaded distribution lines, the capacitive charging current can cause a voltage rise at the receiving end, particularly in long cables.
4. Surplus Renewable Generation: A sudden surge in renewable energy production, such as a gust of wind increasing wind turbine output, can create a voltage swell if the system cannot absorb the excess power.

Mitigation Techniques:

1. Static VAR Compensators (SVCs): These shunt-connected devices inject or absorb reactive power to rapidly regulate voltage, compensating for both sags and swells.
2. Dynamic Voltage Restorers (DVRs): These series-connected devices inject voltage into the system to correct voltage sags, maintaining a constant voltage at the load.
3. On-Load Tap Changers (OLTCs): These adjust the transformer turns ratio to maintain voltage levels, but their response time is slower compared to SVCs and DVRs. They are suitable for long-term voltage regulation rather than mitigating rapid sags and swells.
4. Energy Storage Systems (ESS): Battery energy storage systems (BESS) and supercapacitors can quickly inject active and reactive power to stabilize voltage, mitigating both sags and swells. They are particularly effective for handling intermittent renewable generation.
5. Smart Inverter Control: Advanced inverter control strategies can enable DERs to provide voltage support by injecting or absorbing reactive power, contributing to overall voltage regulation.
6. Proper Grounding: A well-designed grounding system helps limit overvoltages during fault conditions, reducing the magnitude of voltage sags.
7. Load Management: Demand response programs and load shedding schemes can reduce peak demand and prevent voltage sags during periods of high load.
8. Improved Forecasting: Accurate forecasting of renewable energy generation and load demand allows for proactive control of generation and reactive power resources to minimize voltage fluctuations.

For example, a solar farm integrating a BESS and smart inverters can use the BESS to smooth out solar generation fluctuations and the inverters to provide dynamic voltage support, mitigating voltage sags and swells caused by intermittent sunlight. Similarly, installing a DVR at the point of connection of a sensitive industrial load can protect it from voltage sags caused by motor starting or upstream faults.