Virtual Inertia Control of Variable Speed Heat Pumps for the Provision of Frequency Support

Abstract

The growth in the integration of converter interfaced renewable energy has reduced the system inertia, which threatens system stability due to high rate of change of frequency (RoCoF) and frequency nadir issues unless steps are taken to mitigate it. There is a need to provide sufficient fast frequency response to maintain adequate inertia in the system. This paper investigates the capabilities of a variable speed heat pump to provide an emulated inertial response. This paper presents a virtual synchronous machine control for a variable speed heat pump that provides support for grid frequency regulation over the inertial response time frame. A small-signal model with the transfer function of the variable speed heat pump is developed to analyse the effectiveness and feasibility of providing virtual inertia at the device and grid level, respectively. Furthermore, the small-signal model is validated using hardware in the loop simulation. Finally, the aggregated frequency response and virtual inertia contribution by a population of the heat pumps are evaluated and quantified in an urban distribution system.

Modelling of variable‐speed refrigeration for fast‐frequency control in low‐inertia systems

“…refrigeration, air conditioning and heat pumps) are usually represented by simplified dynamic models to assess the overall response of an aggregation of devices with the same technology. In particular, the authors in [11,12] include the induction machine inertia, torque control and DC-link dynamics and derive a thirdorder model for a VSD heat pump system. The authors in [6] model the electrical side of VSD heat pumps by employing firstorder motor dynamics and a first-order transfer function for the VSD, whereas Hui et al [7] derived a third-order model for an inverter air conditioning system.…”

“…Some of the aforementioned drawbacks are addressed in [8, 10–12]. The authors in [8] estimate the potential of industrial induction motors in Great Britain for providing FFR using local droop control.…”

“…However, achieving a coordinated response within such a population of conventional devices, that can only operate at a fixed power or be turned off, is difficult as it requires considering and estimating the behaviour of all contributing units. In contrast, emerging variable-speed drive (VSD) technologies, studied in [6][7][8][9][10][11][12], offer a continuous controllable range for a single unit, hence simplifying the aggregated control.…”

“…More precisely, the entire load unit is typically subsumed within a single low-order transfer function and the dynamic interactions between the units and the grid are usually neglected. Some of the aforementioned drawbacks are addressed in [8,[10][11][12]. The authors in [8] estimate the potential of industrial induction motors in Great Britain for providing FFR using local droop control.…”

“…Furthermore, Malekpour et al [11] showcased a reduced efficacy of the control scheme for a lower inertia scenario. In contrast, Ibrahim et al [12] presented a small-signal model (SSM) of higher-order for a VSD, induction-machine-based heat pump that can provide inertial response and FFR. The model is verified through hardware-in-the-loop simulations, and time-domain simulations are performed for a sample distribution system.…”