By Phil Kreveld
The continual political manoeuvring and propaganda associated with renewable energy sources and associated energy costs render little light, generate a lot of heat and take the national gaze off the truly urgent matter of securing the stability and proper functioning of the national electricity grid.
These notes regarding the transition to renewable energy sources in the NEM are not tinged with propaganda; rather they express a well-founded concern for the viability of the national electricity system, which is heightened by political agendas. The recent Australian Energy Week Convention in Melbourne in June provided sufficient grounds to question the development of the transition.
Related article: Barking up the wrong tree—an engineer’s perspective
The worrying elements in addition to climate considerations are twofold; economic and technical. The concentration here is on technical aspects, and this focus is based on the belief that technical viability of the grid underpins the proper functioning of the entire Australian economy. The economies associated with various forms of electricity generation, be it wind, solar, gas, hydrogen, hydro or nuclear, are not addressed here.
It is not because discussion on the relative energy costs is not important to owners and consumers. Rather, it is that a national grid subject to instability, islanding and blackout has a far more negative effect on the entire Australian economy then the individual profit motives of market participants.
The continual political manoeuvring and propaganda associated with renewable energy sources and associated energy costs render little light, generate a lot of heat and take the national gaze off the truly urgent matter of securing the stability and proper functioning of the national electricity grid.
A speaker at the AEW Convention cut to the chase by stating that the National Electricity Law is no longer fit for purpose, and that the independently formulated renewable energy zones and transmission links by various jurisdictions are counterproductive to the integration process.
In the following discussion, the elements of constituting the technical viability of a large renewable grid are unpicked.
- The NEM is, other than for two DC links, an alternating current (AC) system of interconnected generators and energy consumption (load) centres.
- The various states are interconnected and an important question that must be answered is if nationally it is considered important that each state be energy-wise, self-sufficient. Note: the question is basically a design condition, i.e., is this a must or not.
- The basis of an AC system is stability of voltage, frequency and synchronicity. These must be maintained in a contiguous grid. Inter alia: a design condition can be that instability, resulting in the grid becoming a number of island grids, can be tolerated because each island can safely continue operating.
- Voltage stability is a result of having strong networks. Note: the concept of grid strength is explained further down. Frequency stability results from two important factors; (a) stable or slowly varying electrical loads, (b) having sufficient inertia, (c) having sufficient generator capacity available at all times (note: capacity is expressed in MVA or GVA).
Let’s turn our attention to renewable sources, specifically wind and solar. Where these make up the bulk of all energy we encounter weak grid strength, unless we remediate this, for example by utilising synchronous condensers. Note: our attention here is on high-voltage, transmission grid-connected solar and wind generators.
A weak grid is one in which voltage varies significantly with electrical power being generated. Weak grids not only suffer voltage variability, but the connected solar and wind generators face difficulty in remaining synchronised.
Note: the bulk of these generators are of the grid-following type. Voltage variability tends to ‘confuse’ their voltage tracking ability (achieved through circuitry—the phase locked loop) that can only be ‘tuned’ to a small extent. As explained below, the criterion for connection is that there be sufficient grid strength at the point of connection (PoC) of the generator in question by measurement of the short circuit ratio (SCR) at the PoC.
We will examine two examples to clarify the grid strength issue. (i) a coal or gas-fired synchronous generator providing some, but not all of the energy needs of a zone substation, i.e. an energy consumption centre, a large distance from the synchronous generator. A solar farm near the zone substation is connected to the end of the transmission line and would meet the energy shortfall of the zone substation.
(ii) The case of the synchronous generator being replaced by a battery energy supply system (BESS) and grid forming inverter.
To determine grid strength, in order to gain approval from AEMO to connect the solar farm to the grid at the PoC near the zone substation, the SCR at the PoC must be established. This requires a calculation: first, the transmission line is assumed short-circuited at the PoC (the solar farm is assumed not be connected) and the output of the generator at the other end of the transmission line—i.e., (i) the synchronous generator and the (ii), the BESS-grid forming inverter) short circuit power capacity, usually measured in MVA, is determined.
In the case of (i), the synchronous generator, the short circuit MVA is four times higher than its rated power output. For the sake of a numerical example, assume the rated power to be 150MVA. Its short circuit power is therefore 4 x 150MVA, i.e., 600MVA if the short circuit takes place at the generator. However, at the far end, i.e., at the PoC, a short circuit there reduces the synchronous generator short circuit power because it faces a long transmission. Let us assume the reduction is 50%. Therefore, at the PoC, the short circuit amount 600MVA divided by 2, equals 300MVA.
In the case of (ii), the BESS-grid forming inverter, it is also rated at 150MVA. However, its short circuit capacity is 1.3 times its rated power, i.e., 195MVA. At the PoC, this is reduced by 50% as well, i.e., approx., 85MVA.
Assume that AEMO specifies that the SCR, that is the ratio of the short circuit capacity at the PoC divided by the proposed solar farm rated power, must be three or higher. In the case of (i), the synchronous generator case, the solar farm can have a maximum power of 300MVA divided by 3, equals 100MVA, whereas for (ii), the BESS-grid forming inverter, it is 85 divided by 3, equals 28MVA.
What would happen if a second solar farm also wants to connect? According to the above, it would be limited to 28MVA, adding to the already existing generation capacity, and thus making the SCR for the BESS-grid forming inverter case equal 85 (its short circuit capacity at the PoC) divided by 56, i.e., 1.5, and therefore too low. Both plant owners may now be called on to install a synchronous condenser, to boost short circuit capacity at the PoC.
It is now clear as solar and wind generators replace synchronous generation that grid strength declines, therefore requiring augmentation by synchronous condensers or measures such as var compensators, static synchronous compensators, or thyristor-controlled series capacitors. Decline in grid strength will require major augmentation capital expenditure.
Finally, we look at inertia. More inertia means more energy stored in rotating masses, i.e., the rotor-turbines of synchronous generators and synchronous condensers plus flywheels, and all direct-on line AC motors. The rotating (kinetic) energy can provide sufficient time for generator controls to restore frequency when imbalances between demanded and generated power occur. Changes in climatic conditions can make the changes sharper when we have a lot of wind and solar generation, as well as loads such as air conditioners.
Statically stored energy, for example in batteries, is subtly different: it can, through the employment of inverters perform the same task of rotating, kinetic energy. However, whereas spinning rotors directly provide frequency, this is not case for inverters. The latter rely on control functions, generally commercial-in confidence, to respond frequency-wise.
This lack of uniformity, in contrast to that provided by the immutable laws of physics governing synchronous generators and other electro-dynamic loads, provides a great challenge in the protection engineering regime of renewable grids. We are thus facing a large knowledge gap, to wit:
- Finding economical solutions to increase grid strength
- Creation of sufficient uniformity in grid following and grid forming inverters, in order to have predictability of delta power-delta frequency and delta-time response
- Reparameterisation of protection relays including power oscillation blocking and out-of-step relays, also in regard to planned retention levels of synchronous generator MVA capacity.
- Configuring dynamic restraint of under-frequency, load-shedding relays to take account of increased rates of change of frequency and frequency nadirs.
Related article: Fibs about the renewables transition—and the cost of energy
In conclusion, we are not an impossible transition-to-renewables journey; rather we are on one with no effective overall system engineering direction and supervision. We are thus given a choice between slowing the transition or facing increasing grid failures, or retaining synchronous capacity in one form or another.
It would be best if this were recognised by state and federal governments, as well as their loyal oppositions, and in doing so abdicating their authority to a national, central Australian grid engineering company with the appropriate planning and coordination charter.
