By Phil Kreveld, CT Lab, Stellenbosch, South Africa
Synchronised, GPS-based monitoring and the use of Big Data analysis is the way towards flexible AC transmission as renewable penetration continues on its seemingly unstoppable growth.
Given the divisions in ownerships of distribution, transmission and generation assets, and the separate jurisdictions of the Energy Security Board, the Australian Energy Regulator, the Australian Energy Market Commission and the Australian Energy Market Operator there is an urgent need for mandated, overall monitoring and control of the NEM.
CAUSE FOR CONCERN
The Australian Energy Market Operator’s (AEMO) integrated renewable study of April 2020 gives cause for concern because apart from clearly indicating the problems caused by solar photovoltaic (PV) growth in distribution networks, there are no remedies mentioned.
On page 40 of the study, a list of problems is presented, resulting from inputs from network providers. Voltage regulation and high voltage are chief concerns, but also imbalance and thermal ratings in feeders.
Transformer ratings being exceeded under reverse power flow are not mentioned but undoubtedly are occurring as decreasing solar panel costs are driving higher kW rating PV systems to be purchased.
Given the ambitious green energy plans of many state jurisdictions, upgrading in distribution networks is inevitable, requiring expenditure on reconductoring, higher rating distribution transformers, voltage regulators, and on-line, tap changer (OLTC) transformers.
Given renewable targets of 50% penetration, major capital expenditure would be involved and therefore increased network charges. For many network companies, usually vertically integrated, expenditure to accommodate the growth in PV systems is unattractive as energy sales are at best level but more likely declining. Also, the Australian Energy Regulator (AER) is likely to be gimlet-eyed on new investment approvals.
The above sets the scene for this article which fleshes out an overall control concept for the National Electricity Market (NEM) and the need of a mandate for its inception.
So far, any integrated approach involving not only distribution networks but also transmission line operators (TSO), baseload generators, solar farms, and wind farms has been absent.
Piecemeal solutions include forcing synchronous condensers (syncons) and static var compensators (SVG) to be installed on weak links connecting solar and wind farms, and ‘tuning’ inverters of solar farms, as has been the case of Victoria’s northwest solar farms operating on 50% curtailment of their capacities.
In distribution networks, capital expenditure for new investment resulting from solar PV has been largely avoided by restricting access to high kW capacity installations, forcing ‘no power export’ rules, inverters switching off due to upper voltage limits being reached and making substation voltage adjustments within allowable voltage limits.
Overall control requires a comprehensive, time-synchronised monitoring system. Prior to renewable energy sources becoming common in power networks, the ‘poles and wires’ and the generators were subject to highly predictable demand patterns, other than for network faults.
Therefore, rather than having to rely on parameters, such as instantaneous power flows at generator busbars, transmission line connection points, and substations, reliance could be placed on well-established diurnal patterns with an overlay of contingency planning. This was reflected in physical control features, primarily concerned with protection. The legacy system from yesteryear is now subject to highly variable (short term latency) power generation.
Limited Power Flow Information Available
At present, AEMO still relies on previous day generation data from distribution networks.
As to transmission grid operators, there is limited power flow information available with the ‘comfort zone’ from decades ago being subjected to unplanned instances requiring sudden ‘unloading’ of transmission lines (demand response) at times of excess demand
The need for instantaneous measurement of power flow throughout the NEM is necessary, not only because of the high variability of generation but also because of the imbalance between consumption and generator locations. The latter implies the likelihood of blackouts in regions that are disconnected (islanded) because of congestion limits being reached on their transmission links to the main NEM system.
Having instantaneous real and reactive power flow data, gathered synchronously, permitting correlation of events, throughout networks enables power flow and voltage control by real and reactive power flow balancing, and aids frequency control.
The control task is complex, ideally accomplished by machine learning via neural network schemes and therefore in practice, best achieved organically: starting with networkwide, synchronous measurements utilising geostationary satellite (GPS) timing, observing and recording patterns of voltage, power, reactive power and voltage angles making use of Big Data. The latter facilitates greatly the devising of ‘breadboard’ control schemes and the translation from there to real-life environments in stages.
Stability of Inverters
Distribution networks at their substations, at the very least, should be monitored on the same basis as the transmission/generator system.
Substations are now subject to large, rapid changes in voltage phase, as, in the first instance, real power will fluctuate more because of solar PV generation than reactive power as the latter is largely influenced by aggregate load, and therefore voltage angle fluctuation will feed through the system downstream as well as upstream. This then can affect the stability of inverters.
Extensive studies conducted by UNSW on voltage angle stability of inverters demonstrates the major effect it can have on inverter power output.
In transmission systems, loads are assumed to be aggregated, and in distribution systems, substation bus voltage is assumed to be constant but in systems like the NEM this is not the case now and even less so as solar PV continues its penetration growth.
Extensive synchronous monitoring throughout distribution networks provided via Big Data storage and analysis, economic planning of any network changes resulting from changes in consumption patterns, and in distributed generation.
Big Data analysis on multi-nodal information of micro-synchrophasors and power data, provides invaluable data on desirable upgrades to transformers and accentuation of network sections requiring reconductoring. It also permits planning for changes in protection and for delineation of future inclusions of microgrids as a means of further improving the overall reliability of networks.
Source: Phil Kreveld, CT Lab, Stellenbosch, South Africa. Printed in Transmission & Distribution (T&D). Download the Australasian Power Technologies Transmission & Distribution June – July 2020 Issue where the article was printed (on page 48) – 40MB files size.