Power system component modelling capabilities

The different types of power system component that the testing environment can model are described below. A summary of each component type is presented, including their name (such asbus), an overview of their main parameters and some essential features of their operation. Appendix A provides a full listing of the parameters for each component type.

Busbars (bus)

Each busbar has an assigned nominal voltage and has voltage magnitude and angle available as measurements. The bus type for load flow (Vθ, PQ or PV) is determined automatically by the types of components that are connected to the bus. The number of components that can be connected to each bus is unlimited.

Circuits (cct)

Circuits can represent either overhead lines or cables, depending on the parameters entered (resistance, reactance and charging). Each circuit has a static power rating that can be overridden by a dynamic rating value. The real and reactive power flows at each end of a circuit are available as measurements.

Transformers (tx)

These are modelled in a similar way to circuits, but with the addition of having a variable voltage ratio and phase shift. The voltage ratio and phase shift can be adjusted in discrete tapping steps. The transformer representation can limit the number of voltage or phase shifting tap steps taken within a time step, simulating lock-out delays. The real and reactive power flows at each end of a transformer are available as measurements.

DC interconnectors (dcx)

These represent point-to-point DC links, with four-quadrant AC/DC converters on both ends. The real power transfer across the link is defined as a controllable parameter and is subject to losses, which are proportional to the power transfer. The reactive power at each end is independently controllable, with each end being in one of three control modes:

1. Q: reactive power import or export is set directly

2. PF: a fixed power factor is assumed, so reactive power import or export is proportional to the real power import or export at the end in this control mode

3. V: voltage-control mode, with reactive power import or export modulated automatically to control the voltage at the connecting bus.

Changes in real power transfer and reactive power at each end are subject to ramp rate limits. At for circuit (cct) components, dcx components have a static power rating that can be overridden by a dynamic rating value. The real and reactive power flows at each end of the link are available as measurements.

In thePyPower electrical model, each end of a dcx is represented in the same way as a gen component.

Circuit breakers (brk)

Each circuit breaker can be associated any number of circuits, which are taken out of service if the circuit breaker is set to “open”.

Loads (ld)

Loads represent aggregated demand, such as at a metering point, and include a block of demand side response (DSR) that can be activated by the control algorithms. The real and reactive power of both the demand and the DSR can be varied over throughout a simulation. Generators (gen)

These components have the flexibility to represent a variety of different generation units, including fully dispatchable plant, plant with limited dispatchability – such as a wind farm that can whose output can be curtailed – and non-dispatchable plant – such as domestic solar photovoltaic (PV). The “prime mover” power of each generator can be time-varying, as is the amount that this can be adjusted “up” or “down”. This allows plant with different dispatching abilities to be represented. For example, for a wind generator, the “prime mover” power would be the power available from the wind, the “up” adjustment of real power would be zero – as the power output cannot exceed the amount of power collected from the wind – while the “down” adjustment of real power can be equal to the current “prime mover” power – representing a wind generator that can be curtailed to zero real power output.

Control algorithms that control generators can apply one of two real power operating modes, which are illustrated in Figure 3.3 and explained below:

1. CAP: as shown in Figure 3.3a, the real power output matches the prime mover power within “cap” limits set by the control algorithm. If the prime mover power is greater than the upper cap limit, then the real power output is set to the greater of the upper cap limit value, or the prime mover power minus the down power adjustment value. Similarly, if the prime mover power is less than the lower cap limit, then the real power output is set to the lesser of the lower cap limit value, or the prime mover power plus the up power adjustment value.

2. SET: in this mode, shown in Figure 3.3b, the control algorithm sets a desired real power output value. This set point value will be used as the real power output of the generator

so long as it is within the bounds of the up and down adjustments around the current prime mover power. If the real power set point value is outside the adjustment bounds, the generator output is set to the value of the closet bound.

The reactive power output of each generator can be set to one of three operating modes previously defined for DC interconnectors (dcx, namely Q, PF or V. The reactive power mode is set independently from the real power mode, and separate real and reactive power ramp rates can be set.

Storage devices (sto)

These represent storage devices that have a power electronics interface. They operate in almost exactly the same way asgen components, with the same operating modes and ramp rate limitations, except that the “prime mover” power is further limited by the state of charge of the storage. The storage capacity is fixed, and a bi-directional conversion efficiency can be specified. Furthermore, the charge and discharge rates can be set to different values. Capacitor/reactor banks (qbk)

These consist of fixed steps of reactance, either inductive or capacitative. A control algorithm can activate any number of steps, though the number of steps that can be changed at one time can be restricted, in the same way that tap operations for a transformer can be time limited. Slacks (slk)

These component transform the busbar they are connected to into a slack bus. Their voltage magnitude and angle are adjustable parameters, whilst the amount real and reactive power they infeed are available as measurements.

Zones (zn)

Zones are virtual components that define which components each control algorithm may act upon. Each zone contains a list of busbars that are considered to be within the zone. Each busbar can only be assigned to a single zone so that no zones overlap, although it is not necessary for zones to be contiguous. Control actions determined by a control algorithm are only applied to the components connected at the busbars within that algorithm’s zone.

Within each power system model a “slack” zone is defined containing the slack bus. This zone is in addition to the zones required for the control algorithms, and ensures the algorithms are not allowed to control the slack bus.

Up adjustment Down adjustment Prime mover power Output real power Time R ea l powe r

Upper cap limit

Lower cap limit

(a)CAP mode

Time R ea l powe r Up adjustment Down adjustment Prime mover power Output real power

Real power set point

Real power ramp rate limitation

(b)SET mode