Mittigating RES impact on the grid

Intermittency & null PV intertia

Renewable power should be for free but actually, it is far from being such. Moving clouds and gusty winds cause that the renewable power is unstable over time. Since the share of renewable energy sources (RES) grows in the grid, their random power fluctuations and lack of PV inertia pose an increasing risk to the grid's overload and destabilization, and worsen the power quality. To mitigate this impact, the grid's transmission capacity is being reinforced in order to balance the power on a high aggregation level, and over long distances by large-scale water pumps or gass turbines providing sufficient inertia and a reasonably fast power response. Increasing the share of renewable energy requires ever greater investments in the grid infrastructure, which eventually increase the end-user price of electricity and is aesthetically damaging to the landscape.

Power smoothing

A lot of grid investments, transmission losses and landscape could be saved, if the power were balanced on a lower aggregation level, than is currently the case. A substantial portion of the RES balancing can be done by its lossless smoothing on a low aggregation level, or per-RES in case of a utitlity-scale RES. The lossless power smoothing is an instantaneous balancing of power between the grid and an energy storage (ESS). The intermittent power of RES can be smoothed by its low-pass filtering which is almost loss-free, but needs a large and fast ESS. In theory, the low-pass filter (LPF) accumaltes a minimum of energy, if it is excited by the exact future power singal that has to be filtered. Ultra-short-term forecasting (nowcasting) of renewable energy is available, albeit not accurate enough to bring the accumulated energy close to its theoretical minimum. Actually, the lossless smoothing of renewable power is not yet affordable on a commercial basis.

Our first objective was to analyze the ideal power smoothing model IPLPF, exciting the LPF with the exact future PV power signal. In theory, such an excited power filter eliminates the unwanted accumulation of energy othwerwise accumulated due to the time lag of LPF. Based on the measured solar radiation GI(t) over a time span of 1+ year, the ideal PV power smoothing model was numerically simulated. Given a ramping limit of output power, LPF parameters were optimized in order to minimize the storage capacity, the cost of which determines a payback potential of the lossless power smoothing. The simulation has proven that IPLPF smoothing (if existed) would be highly affordable with the current price of electricity and storage technology.
The IPLPF model eventually allows for aggregating the specific accumulation rate on a daily basis, an objective measurement of the solar intermittency in terms of its smoothing.

Next, a novel smoothing method SPLPF has been designed in order to minimize the accumulated energy by LPF when its future input power signal is distorted by a prediction error, since this error is inevitable in the real-time power smoothing. Given the measured solar data GI(t), the accumulation rate of SPLPF smoothing was aggregated while applying a variable prediction error at the LPF input: With low to medium prediction error, the SPLPF smoothing performs close to the IPLPF model, much better than the LPF excited by the same predicted input signal does. The numeric simulation gives a real presumption that the SPLPF smoothing is affordable with the nowcasted PV power signal.

Ramping limit of RES

However low is the energy accumulated by the RES smoothing, its storage is not for free. Smoothing of either local or aggregated renewable power pays-off only with a favourable infeed tariff granted for meeting the prescribed power ramping limits. The goal is to minimize the sum "smoothing costs" plus "costs of other power balancing measures (slower than RES smoothing)" plus "power losses due to the curtailment regulation of RES (ideally, get rid of the RES curtailment)". Given the power ramping limit and the measured signal GI(t) and its prediction error, the specific accumulation rate and the specific cost of the power smoothing can be aggregated. The sum cost criterion can be eventually minimized by iterating the RES power ramping limit, untill this is optimized.

Grid inertia

In standard power plants, turbines and generators are fixed on a common shaft. Actually, their 3-phase alternators are synchronous reciprocating motor-generators. The frequency of induced AC voltage is given by their angular velocity (RPM) and vice versa. Since many alternators are connected to the AC grid in parallel, the frequency and phase of all connected units must be synchronized. Their reciprocity and inertia altogether help keeping the grid in balance: Thanks to inertia, the total generated power in the grid is always equal to the total drawned power. In case of a reduction in power draw, the grid accumulates the excess energy by increasing the angular momentum of turbine-generators, and vice versa. Changes in the power consumption result in the varying grid frequency. As a counter-measure, the turbines must consequently modify their power in order to keep the grid frequency close to constant. The inertia of generators serve as an energy buffer (temporary storage), being irreplaceable part of the grid control system.
PV inverters are instantly synchronized by AC voltage from the grid. PV plants have no inertia because their inverters cannot accumulate energy. They just curtail the PV power infeed in case of its surplus. The more installed PV power, the less relative inertia in the grid and the steeper and deeper random changes in AC frequency. The grid stability is threatened by too much installed PV power without its smoothing.

Smoothing & synthetic inertia

However, not every inverter's oscillating must be fully subordinated to the grid. The installed storage is not always fully occupied by the smoothing (it depends on the solar intermittency). The installed ESS capacity and its two-way inverter can eventually provide both smoothing and angular inertia to the grid: The spare capacity of the ESS can emulate angular inertia if the bi-directional converter between the ESS and the grid oscillates analogously to the rotation of a turbine shaft - being controlled by the Newton's force law. The rate of "synthetic" angular inertia could be dynamically adjusted according to the momentary spare ESS capacity. Prioritizing the smoothing over inertia is correct, since the smoothed PV power has a limited impact on AC frequency in the grid. This way, the installed ESS can be fully utilized even on days with low solar intermittency, including days with low solar radiation.