The main parameter of the ES control as witnessed is the proportional gain KES. Up to now KES has been a deterministic factor in stability, and it was understood that for best frequency recovery KESneeds to be as high as possible.
However, this is valid when there is no restriction on the size of the ES or the rating of the power converter. In order to determine an optimum KES, a study is now conducted to observe the power and energy usage when KESis varied.
The power rating of the ES corresponds to the maximum active power required (PMAX (kW)), while the energy capacity corresponds to the total energy sup-plied (Etotal (kJ)). Hence,the peak active power and the total energy spent were
CHAPTER 4: FREQUENCY STABILISATION INWEAK GRIDSUSING
INDEPENDENTENERGY STORAGE
recorded for varying KES, while imposing a current limit of 27 A on the d-axis current to perceive the size requirements.
As one can see in Fig.4.45a, PMAX increases when KES is increased, for both rated and half rated load disturbances. For the rated load, the peak power does not increase much with KES as the current limit affects the maximum peak power. For the half-rated load, the peak power increases significantly for smaller KES, but levels off for KES >50 as a consequence of reducing frequency drop. Nonetheless, one can select any power converter and limit the current accordingly, while selecting a high KESbelow the stability margin.
(a)Maximum active power supplied (b)Total energy supplied
(c)Maximum frequency drop (d)Settling Time
Figure 4.45:ES performance against various KES, for rated and half rated loads The size requirement of the ES can be determined from the total energy spent in delivering the required frequency response. As shown in Fig.4.45b the to-tal energy supplied increases with KES as expected. The total energy used has a stronger dependence on KES as the capacity should increase by nearly 20%
when KES is increased from 30 to 70. Therefore, KES has to be determined ac-cording to the available capacity of the ES.
CHAPTER 4: FREQUENCY STABILISATION INWEAK GRIDSUSING
INDEPENDENTENERGY STORAGE
Consequently, higher KES produces lower initial frequency drops as displayed in Fig.4.45c. Even though for higher KES, the initial peak power is limited at a constant value, for higher KES, the ES provides energy longer at this high peak power, reducing the actual frequency drop. The settling times, as shown in Fig.4.45d increase with KES. When using higher KES, the frequency error seen by the governor becomes slightly smaller, lengthening the time taken for the governor to increase the engine torque up to the required torque.
In conclusion, in selecting a KES value, one must consider both the stability margin and the energy capacity. If one has a large enough ES, KESshould be as high as possible without violating the stability margin to decrease the frequency drop.
4.11 Conclusion
In a weak electrical grid, due to low inertia, frequency drops are eminent dur-ing load disturbances. The slow dynamics of typical speed governors exac-erbate the response and usually produce frequency responses outside of the desired regulatory limits levied by the grid code. This situation may be disad-vantageous to emerging distributed power generation, as failure to abide by the frequency regulations of the main utility may result in islanding and cascading disconnection of weak counterparts of the grid, causing undesirable blackouts.
As a solution, this research proposes a novel technique to independently con-trol an ES, for the specific purpose of frequency stabilisation in weak electrical grids. It was successfully achieved by employing power system frequency as the key control input. In brief, the proposed technique compares the instan-taneous power frequency with a set frequency threshold and as soon as the frequency falls below this threshold, the ES is automatically activated. The re-sultant frequency deficit proportionally actuates the ES to generate additional active power required to restore the power frequency above the threshold.
In this way, a non-zero frequency error is maintained throughout the load dis-turbance; thereby, not interfering with the power system’s usual speed govern-ing, while supplementing active power. The prime mover (engine) experiences a gradual torque increment compared to the rigorous action required without ES support, as a combined result of the proposed ES control and speed gov-erning. Once the power frequency reaches the threshold, the ES is deactivated
CHAPTER 4: FREQUENCY STABILISATION INWEAK GRIDSUSING
INDEPENDENTENERGY STORAGE
and smoothly hands over the rest of the power frequency control to the speed governor, meaning the prime mover continues to supply the added load de-mand in steady state without further support. This indicates and assures the rest of the grid that the DG is self sufficient to accommodate the new demand.
An equal and opposite scenario was also observed during load shedding. Us-ing detected frequency completely eliminated the need for communication be-tween the speed governor and the ES.
In a practical situation, detected frequency is associated with inevitable tran-sient delays and steady state ripple. It was also revealed that a delay in quency detection causes an oscillatory response in the recovered power fre-quency. On this basis, a smaller delay and a higher attenuation of the steady state ripple were declared as desirable for the proposed technique. Such criteria in frequency detection could be realised using a DSOGI-FLL.
To explore the properties of the new control technique, application of the ES with the proposed method in a weak grid was approximated in the s-domain using control systems analysis. The stability criteria for the proposed control was derived using the complete approximated system, which implies that the stability of the method depends on the inertia and the impedance of the power system it is coupled with. In other words, a higher value of KES may be desired for larger power systems.
The ES support was represented as a controlled current source, which injects d-axis current in order to supply the additional active power as calculated by the ES control. This simulated environment was used to examine and validate frequency recovery in more practical situations.
The purpose of frequency recovery for both loading and load-shedding was validated using the simulated electrical system. Consistent performance could be observed for different load conditions. The proposed ES control was suc-cessfully able to restore frequency within the ±1 Hz in 4s for the loading and shedding of the rated load and in 2s for the half the rated load. The method was successfully able to maintain the d-axis voltage, Vdnearly a constant, given that no reactive power control is specified.
The investigation of power and energy usage showed that the choice of KES depends on the power and energy rating of the ES, provided that the stability margins are maintained.
A saturation limit on the current defines the maximum power injection possible
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INDEPENDENTENERGY STORAGE
under practical conditions. The simulated results suggest that having satura-tion limits only affects the responses with respect to larger load disturbances, which commands the ES current to exceed the nominal limits. This was found to have a positive effect on the recovered frequency by further damping the initial oscillatory response for larger loads.
Since the proposed method works by supplementing the active power, the most significant improvement was seen in terms of frequency. While the frequency following AVR effect on the PCC voltage was eased during loading, drawing power at the PCC slightly disturbed the voltage during load shedding. Since the voltage control is associated with reactive power, an analogues control tech-nique may be suggested to control reactive power at the PCC as a future study.
On the whole, compared with the conventional governor controlled frequency regulation, the proposed ES control technique was successfully able to deliver a much better frequency regulation in a worst case weak grid, by independently controlling an ES that uses detected frequency as the main control input and trigger. The proposed technique improves the viability of weaker parts of the grid, subsequently aiding in sustaining a safe and assured DG power supply in places where the strong grid is unable to reach.
C
HAPTER5
Comparison of Methods For Accurate Frequency Trend Estimation Following Load
Transients in Weak Electrical Grids
5.1 Introduction
Estimating the frequency trend following a load disturbance accurately is of utmost importance in the scope of this research, which proposes using energy storage for frequency support in weak grids. The particular energy storage control technique presented in Chapter 4 exploits the detected frequency sig-nal as the main control input, thereby calling for unique frequency detection requirements, as discussed in the section 4.3. In support of this, Chapter 5 in-vestigates selected frequency detection techniques with the aim of discovering the best suited method(s) for the energy storage control applied specifically to weak electrical grids.
In this chapter, three candidate frequency detection methods were chosen for comparison, from different technical backgrounds; namely, a phase locked loop (PLL) based Synchronous Reference Frame PLL (SRF-PLL), a Discrete Fourier Transform(DFT) based Generalised Modified DFT and an adaptive filtering technique called Double Second-Order Generalised Integrator Frequency-Locked Loop (DSOGI-FLL). The methods will be first investigated individually against a set of specifications for frequency detection requirements derived specific to weak grids. Then, the three methods will be compared on an unbiased criteria
CHAPTER 5: COMPARISON OF METHODS FORACCURATEFREQUENCY TREND
ESTIMATIONFOLLOWING LOAD TRANSIENTS IN WEAKELECTRICAL GRIDS
based on bandwidth-matching. After declaring suitable method(s) to be used in the energy storage control strategy, a further optimisation technique to avoid spurious frequency variations in the estimation due to abrupt changes in volt-age will be presented. Then, the comparison of methods are validated using experimentally acquired voltage data from the weak-grid facility presented in Chapter 3. Finally, conclusions are drawn.