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Australia and other electricity grids are facing many issues with the increase of asynchronous generation commonly associated with renewable energy. One of the concerns is that renewables will displace synchronous plant and associated inertia. Demand-supply imbalance is compensated by rotational energy stored using inertia. As inertia is reduced in the power system, the rate of change of frequency (RoCoF) following a system contingency or fault increases and at some point, the existing frequency standards cannot be maintained. Fast RoCoF has the potential to compromise the stability of the power system. Some reasons for this include: • Mechanical torque on rotors increases for rapid changes in frequency, which may cause damage or a turbine trip and this further exacerbates frequency control problems • Historical frequency control mechanisms such as load shedding and turbine governor control may be inadequate in low inertia systems due to their relatively slow speed of response in comparison to the rate of change of frequency. The transition from the current operating regime to low inertia power systems poses challenges and many utilities around the world including Australia are seeking to mandate minimum system inertia to ensure the grid continues to operate as it always has. Whilst this is one way to solve the problem, it incurs economic costs on all users of the power system and there are likely to be alternative solutions. As inverter-based generation systems are able to rapidly increase or decrease active power, they could potentially be made to respond quickly to frequency variations. Faster frequency control plant may be able to provide sufficient 'synthetic inertia' or act as an alternative for inertia. In terms of load response, fast communications systems could potentially be used to assist in load shedding when the detection of rapid rate of change of frequency is too slow. Inverter controlled load such as air conditioners, pumps and fans could also be programmed to respond to frequency variations and provide load relief that would otherwise not be available. This paper investigates the operation of a low inertia grid to examine the impacts on the frequency standard, system operations and viable options to ensure continued operation of the interconnected power system as inertia is reduced. The authors seek to investigate what would be required to operate a practical power system, consisting of a variable mix of new and old technology generation, with very low inertia compared with current levels. An interconnected 2 region grid will be configured to resemble a practical real-world example. The objective of the paper is to facilitate economic generation deployment and grid operation by offering an alternative to mandatory rotational inertia levels provided by synchronous machine technology. [ABSTRACT FROM AUTHOR]

The DC voltage on the photovoltaic (PV) array connected to an inverter plays an important role in the operation of the PV inverter system. The DC voltage is controlled by the maximum power point tracking (MPPT) controller to deliver active power based on the PV array V-P curve. This varies with solar irradiance and PV cell temperature. The DC voltage needs to be kept higher than the peak AC voltage at the inverter terminal for the inverter to operate correctly. It is also used to regulate the PV array output. The DC voltage is thus a function of both the PV array design (solar irradiance and cell temperature) and the inverter side (peak AC voltage). The need for a margin between the DC voltage and the peak inverter terminal voltage creates situations that may limit the active power and deliver unexpected responses. These can have regulatory compliance issues because, in Australia, the technical standards require 'continuous uninterrupted operation' (CUO). In brief, CUO has been interpreted to mean that the active power remains unchanged following power system disturbances, and that reactive power is only changed as required for voltage control for connection point voltage variations within an operating band of 90%-110%. In practice this means that additional inverters are required to meet CUO and the ratio of inverter capacity to rated active power typically exceeds 1.2. In normal operating conditions, the power plant controller must limit inverter operation to a value below the MPPT optimum at high irradiance levels, reducing both active power and yield to a level that allows active power and reactive power capability to be maintained at 90% nominal connection point voltage, without tap changer operation. For some irradiance/high temperature conditions, active power may reduce if the inverter ac voltage increases to within the acceptable margin of the DC voltage. Under these conditions, the DC voltage is increased to maintain the required margin between AC peak and DC voltage, consequently curtailing active power. Depending on the AC design of the generating system, the collector voltage can be significantly affected by system voltage variations. Under some feasible conditions, a decrease in inverter voltage caused by a change in the system conditions, can result in an increase in active power output when the active power is initially below maximum available level. This is not usually a power system security issue but in Australia it creates compliance issue, which has the potential to exclude otherwise high performing inverter systems or have high compliance costs. [ABSTRACT FROM AUTHOR]