With the increased penetration of renewable resources, and the available flexibility in terms of control provided by converter interfaced generation and load, it is not far-fetched to envision a future electric grid in which all sources of power and load are interfaced to the network through power electronic converters. Consequently, the system has zero inertia. This raises a number of important technical questions associated with system operation in the steady state and in the dynamic state, and regarding system protection.
Based on the long-term experience with conventional synchronous generators, the concept of load/generation balance and frequency control in electrical grids is closely linked to the rotating inertia in the system. When the topic of a zero inertia power system is discussed, a question always arises whether the system could be satisfactorily operated and controlled with either reduced inertia or zero inertia. This is a valid concern and necessarily must be addressed in order to demonstrate the viability of a zero inertia power system. Additionally, the ability to appropriately control voltage and frequency is an absolute necessity. The requirements of voltage and frequency control raise questions of how multiple converter interfaced units would coordinate control functions and what specific aspects of control need to be deployed to guarantee control performance and robustness.
Power electronic inverters also happen to be expensive in terms of cost and like any other equipment in the system need to be operated within their ratings. A critical aspect of this function is the response of the inverter to system disturbances. Significant precaution has to be taken to ascertain that the current capabilities of the inverter are not violated post-disturbance. In experimental verification in a laboratory setting or associated with a single unit in the field, the current capability violations could be monitored and guarded against. In a large scale verification using simulation, it would be imperative to adequately model all the critical components and protection functions to faithfully represent the performance of the device via simulation.
In a recent project completed by the Power Systems Engineering Research Center, the viability of a zero inertia power system has been systematically examined and analyzed. This viability analysis has been conducted on a large-scale system using positive-sequence time domain simulation. A key initial step in this study was to develop an appropriate model for converter interfaced generation which would capture the essential characteristics of the actual device. Careful analysis using a complex point-on-wave simulation model led to the development of a versatile controlled voltage source positive sequence converter model for use in commercial time domain simulation packages. Numerically, this model is more robust than the prevalent boundary current injection model which represents the converter as a controlled current source. It should, however, be recognized that these positive sequence converter models are meant to represent the essential features of the response and not the minute and exact characteristics of the device.
In analyzing the viability of the system, an assumption was made that an adequate reserve margin is available, as the primary focus of the study was on the possibility of control and operation of a zero inertia system. It was found that the principles of power-frequency droop, coupled with the availability of fast response from the converter devices, can serve ably in both arresting frequency change and in the recovery of frequency. Further, with adequate current carrying capability in the inverter switches, both voltage and frequency can be simultaneously controlled limitlessly. It goes without saying that some form of priority will have to be established (depending on system characteristics) to ensure reliable and compliant operation in the presence of limited current carrying capability. This was another aspect studied in the analysis but its results are, however, in the preliminary stages. The developed converter model was also found to be numerically robust to varying short circuit capacity at the point of interconnection.
Though the converter decouples the energy source from the network, it is not practical to expect a stiff dc bus for large converter interfaced generation sources. Additionally, if these sources are required to participate in primary frequency response, the dynamics of the dc bus and the characteristics of the source behind the rectifier-inverter would impact the behavior of the network. In conventional power systems, active power and voltage magnitude are decoupled. However, with large converter interfaced generation, the non-stiff dc bus and the grid side pulse width modulated inverter can cause the active power and voltage magnitude to be dependent on each other.
The analysis conducted has shown that the control and operation of a zero inertia power system is viable. However, a commentary is required with regard to the assumptions made in the analysis. The primary assumption is the adequacy of available reserve margin. It follows that renewable sources of energy would need to be dispatchable in order to maintain system reliability. Consequently, market mechanisms that allow this operation mode across the system would need to be devised. Alternatively, if the wide spread use of energy storage proves to be more reliable and economical, the renewable sources could be relieved from operating in a dispatchable mode.
The second assumption deals with the ability of converter interfaced generation to provide local voltage support. This would require optimal sizing of the converter to ensure that sufficient current margin is available. Immediately following a disturbance, the limited short time over current capacity of the converter could be insufficient to maintain the required voltage magnitude level causing the voltage level at load buses to drop by a large amount. In the analysis conducted, it was assumed that every converter interfaced generation source was capable of providing voltage support.
Existing converter control mechanisms require the presence of a moderately strong grid at the point of interconnection. The converter source then follows the grid during operation. Consequently, converter interfaced sources behave as current sources while relying on the grid to maintain an adequate voltage level. In zero inertia systems, it would be imperative that large converters be operated as a voltage source in a grid forming mode. They would have to be able to set the voltage level which other small converters can follow. This would require a shift in the control ideology.
A pure converter system (both generation and load side) has many advantages. The importance of system frequency, and frequency limits, in a zero inertia system in terms of system dynamic performance criteria are yet to be explored in detail and need to be carefully investigated.
Vijay Vittal
Ira A. Fulton Chair Professor, School of Electrical, Computer and Energy Engineering, Arizona State University
Deepak Ramasubramanian, Engineer Scientist II, Electric Power Research Institute
Leave a Reply