Microgrid modelling and analysis

Microgrid modelling and analysis

General

The module improves productivity by reducing the amount of manual work required at each step of the analysis. Relying on a simplified or detailed model of the installation, the module allows the creation of various study cases by combining system loading conditions (e.g. peak and minimum load) with minimum and maximum DER contributions (e.g. 0 and 100%), all defined as simulation parameters. Controlled load-flow analyses are then executed on each scenario to assess the impact on the system in terms of steady-state voltage, transient voltage variations (flicker), thermal overloads and reverse power flow.

• The detailed modelling of grid-forming DERs, such as isochronous and droop control modes, considering their operational and/or physical limits
• The ability to perform unbalanced power flows, short-circuit analyses and time-series simulations on islanded and grid-connected microgrids
• A customisable load-shedding and curtailing algorithm imbedded into the power-flow solver for islanded simulations where the load offsets the available generation

This unique combination of detailed system modelling and refined, steady-state analyses facilitates the design, planning and operation of microgrids, allowing, for example:

• The identification of under and over-voltage conditions and overload in multiple scenarios
  
• The verification of power and energy availability for the operation of islanded microgrids at peak and during extended periods
• The impact assessment of starting motors
• The ability to calculate fault-current duties to support protection coordination studies and to evaluate temporary overvoltage (TOV)
• The conduction of arc-flash hazard studies to ensure the safety of utility personnel

Modelling

The new microgrid circuit type is now available for the creation of a state-of-the-art model for both isolated and grid-tied topology. The former is a standalone system without a connection to a feeder or a substation and has all its energy supplied by DERs connected to it The latter typically has a single point of interconnection (POI) to a distribution circuit through a switching apparatus – although having multiple POIs is possible – and can generally operate either grid-connected or islanded. All these configurations are permitted with the module.

As a microgrid needs clearly identified electrical boundaries, it is now possible to define a microgrid simulation zone (MSZ) that includes the sections from the connectivity model located between the microgrid reference node and the delimiting device(s). As such, infinite sources, namely a source equivalent or a synchronous generator in swing mode, are not permitted within the MSZ.

The microgrid-supporting DERs also have their model enhanced to properly emulate the different islanded control modes. In fact, any type of dispatchable DER, such as battery energy storage systems (BESS), inverter-based generators and synchronous and induction generators whose control mode is set to isochronous or droop, will enable a simulation-ready microgrid.

While non-dispatchable DERs can exist on a microgrid, they are normally insufficient to guarantee the active power balance required for viable islanded operation. This is the exact purpose of the isochronous and droop-islanded control modes. While the first fixes the voltage magnitude at the DER’s terminal and provides the necessary active power (thereby keeping the frequency constant), the second adjusts the active and reactive output power based on frequency and monitored voltage, respectively.

Analysis

Whenever the active power balance is not guaranteed, a load-shedding and curtailment algorithm can be implemented by identifying a prioritised list of participating loads and motors. If insufficient active generation is identified during a load-flow simulation, the Newton-Raphson unbalanced solver will start shedding or curtailing loads and motors, one at a time, until enough generation is available or the entire list has been cleared.

Running load flows on microgrids is a simple way to obtain important information regarding the network performance, such as abnormal conditions, losses, generated power per DER, etc.

Fault analyses also take into account grid-connected and islanded microgrids using the same DER short-circuit models as in standard networks. This makes possible the determination of by-phase short-circuit currents for all types of fault at each node of the microgrid.

Remote Load Centre

A new topology detection tool identifying remote load centres, based on a set of user-defined criteria related to downstream load, distance from the substation and presence of circuit ties, complements the module. When cross-referencing the results of the detection with system reliability metrics and a load density heat map, engineers can easily identify circuits where reliability-improving NWA opportunities exist.