Weak electrical grids are usually found towards the medium or low voltage feeders of conventional grids. Some of these weak grids are supported by dis-tributed generation resources (DGR) consisting of renewable energy sources (RES) and energy storage facilities. Compared to strong grids, weak grids fea-ture lower inertia and higher impedance, in addition to their typical low power ratings typically in the order of 100 kW [85]. System inertia of 1 kgm2or less can cause significant fluctuations in power system frequency for relatively low vari-ations in loading. Also, the source impedance enforces significant effects on the voltage during loading and shedding of loads, DGRs and energy storage at the PCC. These manifest as distortions and unbalances at the PCC such as voltage dips and voltage glitches. This means that both the magnitude and the phase of the voltage in a weak grid is not as uniform as in a strong grid. Moreover, in a weak grid with only a few single synchronous generators, switching regulators cause high harmonic contamination in the voltage. This is also a concern in this study, as it directly affects frequency detection.
Having understood the need for an impartial comparison of frequency detec-tion techniques in weak grids, the necessity to create a simuladetec-tion model/platform which can mimic a worst-case weak grid can be justified. Such a model has to account for all important frequency and voltage dynamics in a weak-grid repli-cating the voltage and frequency characteristics appropriately and adequately for further design and development of the techniques. This includes the design, development and comparing of frequency detection techniques and the devel-opment and testing of the proposed energy storage control system. In order to satisfactorily perform the above, the model should ideally comprise of;
• The frequency dynamics of the power system, representing the inertial response and the governor controlled primary frequency control response
• The Automatic Voltage Regulation (AVR) characteristics
• Harmonics and unbalances typically found in a weak-grid
• The Effects of Supply impedance
• Connection of loads
CHAPTER 2: LITERATURE REVIEW
Therefore, a survey has been conducted to identify suitable power system sim-ulation models in the existing literature, that may be able to replicate a weak-grid.
Kundur et al. in [85] presents generic control system models of active power control for frequency regulation in power systems. The models are derived for large thermal power systems of large inertias that can supply a significant proportion of the national power generation. According to Kundur et al., there are two speed governing mechanisms practised in power systems to recover the immediate inertial response; i.e,
• Isochronous governors
• Governors with speed-droop characteristics
When a load disturbance occurs, isochronous governors can bring the decreas-ing frequency back to the nominal value, while a governor with speed-droop characteristics can restore the frequency to a set-point below the nominal. The use of isochronous governing is applicable, when a single generator is appointed for the speed regulation. However, when two or more generation units with speed regulation capabilities are present in a system, droop control has to be used to share the load among the generators at the steady state without con-flict. The participating generators in that case are provided with a speed droop to settle in a steady state [85].
Anderson and Mirheydar in [3] published a low-order system frequency re-sponse (SFR) model for large power systems. The main aim of this model was to showcase only the essential dynamics, so that a droop frequency control is replicated using per-unit values of the inertia constant (H), the reheat time con-stant (TR) and the droop gain (R1) as the most dominant time constants. Other factors included are the damping factor (D), the fraction of the power generated by the high power turbine (FH) and the mechanical power gain factor (Km). See Fig.2.3 for an illustration of the SFR model. The SFR model shown regulates the power deficit equivalent to the load disturbance causing a deviation in the angular frequency, which is the output of the model. To realise the model to demonstrate frequency control, the knowledge of the dominant thermal time constants is necessary.
A more recent study by Costabeber in [4] introduces another speed-governing model based on the angular speed ω; so that, instead of active power, torque is considered in the loop. The engine is defined as a first order lag, and the
CHAPTER 2: LITERATURE REVIEW
Figure 2.3:System Frequency Response model by Anderson and Mirheydar [3]
generator is modelled using actual inertia J(Kgm2), which are the dominant time constants of the overall system. The speed governor is a PI controller that is tuned to deliver the desired speed dynamics. As shown in Fig.2.4, the angular frequency error occurring as a result of the load torque disturbance is regulated by the speed governor at the nominal. Instead of the speed difference, this system provides the angular speed at the output of the loop.
Figure 2.4:Speed governor loop by Costabeber [4]
Having considered several power system models for frequency control, the last model proposed by Costabeber was understood as the most suitable model for this work, due to its properties. The emulation of the diesel engine is achieved by representing the engine/prime mover as a first order lag and the generator is represented by its inertia. Note that the torque producing current gain was linearised and included in the PI control speed loop gain. This configuration delivers a simplified representation of the swing equation that can sufficiently indicate the speed change occurring as a result of the load torque disturbance.
In a weak-grid, the speed control is usually performed by a single unit and
CHAPTER 2: LITERATURE REVIEW
due to low inertia, the ROCOF following a load change can be very high (e.g.
−50Hz/s) [41]. Also, the angular frequency makes provision to use angle θ to generate sinusoidal signals, which create the electrical network. These char-acteristics are fulfilled by the low order isochronous speed-control model pre-sented in [4]. Nonetheless, the model needs to be updated with the inclusion of automatic voltage regulation and a supply impedance to adequately replicate the voltage of a weak-grid. The harmonic distortion can also be added manu-ally to the voltage. The process of modifying Costabeber’s model with the new additions can be found in Chapter 3, where a complete weak-grid simulation model is presented.