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Abstract — Nowadays, a large part of the world population lives without access to electricity, principally due to the location, too far from the main grid. Renewable power sources connected in an autonomous local grid are a real opportunity for these communities. These weak electrical grids composed by several microsources, energy storage systems and loads can be considered as islanded microgrids. As it is disconnected from the main grid, the frequency and the voltage of the microgrid have to be controlled. The frequency is generally controlled maintaining the power balance between generation and consumption, using energy storage or dump loads. The voltage regulation is approached in different ways, according with the controller design objectives and the nature of the loads connected to the grid. This paper proposes a review of several voltage and frequency control strategies in islanded microgrids.
I. INTRODUCTION
According to a 2009 IEA (International Energy Agency) report, there are still 1.3 billion people who have not access to electricity. This lack of energy access is due to the fact that a large part of population in developing countries lives in rural areas far away from the main utility grid.
When the customers are too far from the main grid, small power sources (typically diesel generators or renewable power sources) are used to produce the local needed energy. Generally, such small-scale electrical grid can operate isolated or interconnected with a main grid and is called microgrid (MG). It is mainly composed by one or several micro-sources, multiple loads and potential energy storage systems connected together [1]. A MG is typically connected to a main grid trough a switch, which allows it to work both in grid connected as well as island mode when the static switch is open (figure 1).
In islanding operation mode, the MG can be considered a weak electrical grid and a special attention have to be paid for the frequency and the voltage control in order to supply a good power quality to the consumers.
This paper proposes a review of different voltage and frequency control strategies used for islanded microgrids.
II. THE ISLANDING OPERATION MODE OF A MICROGRID
The islanding mode of a MG can occurred in a planned way or unintentionally, and, in both cases, the isolated part have to continue providing energy to its loads. The review will focus on the intentional islanded mode.
The authors are with ESTIA, F-64210 Bidart, FRANCE (corresponding author to provide phone: +33(0)559438461; e-mail: s.baudoin@estia.fr).
In an islanded microgrid, the energy storage systems play an important role. It is usually formed by two complementary storage devices:
- Long-term energy storage, with high energy density used to balance the power between the production and the demand. For this kind of storage, it is important to have the power capacity properly sized according to the power sources and loads.
- Short-term energy storage, with high power density used to hedge against grid defaults as it has a faster response than long term storage.
Figure 1. Architecture of a Microgrid
Usually, each microsource and storage system is connected to the grid through a power conversion system, controlled by different strategies that ensure good power quality and grid stability. One of the most important challenges with this kind of system is the control of voltage and frequency.
III. CONTROL STRATEGIES OF ISLANDED MICROGRIDS
The energy sources of a MG are mostly synchronous and induction generator. The output voltage of a synchronous generator can be controlled directly modifying the rotor excitation current through an automatic voltage controller [2]. However, an induction generator is more suitable due to his price, robustness and simpler starting. With this type of generator, different control strategies have to be used in order to regulate the voltage and the frequency of the grid. In an autonomous MG, active power fluctuations affect its frequency. In order to control this effect, the power excess
A review of voltage and frequency control strategies for islanded
microgrid
Sylvain Baudoin, Ionel Vechiu, Member, IEEE and Haritza Camblong, Member, IEEE
can be dissipate on dumping loads or/and using energy storage devices.
The induction generator voltage and frequency depend on its speed, capacitance and load [3]. Thus, if the turbine speed is considered constant and the excitation capacitances are well chosen, loads fluctuations will directly cause voltage and frequency variations.
The goal of control strategies of an isolated MG is to balance the generation/demand power with high performances during acceptable range of frequency/voltages amplitude. In the literature two main control strategies are used for this purpose: PQ inverter controller and Voltage source Inverter (VSI) controller. A PQ controller keeps the active power constant at a desirable power factor and a VSI controller regulates frequency and voltage amplitude. Two main control strategies are mentioned in [4] for coordinated control of the isolated MG sources. A Single Master Operation uses one VSI as a master to set the voltage reference whereas all the other power sources inverters operate in PQ mode as slaves. The Multi Master Operation uses several VSI with pre- defined voltage and frequency characteristics.
In [5], the experimental results show that the output voltages of the sources controlled with a VSI controller are stable during load variations whereas their active and reactive powers change significantly. On the contrary, the PQ controller keeps the output power of the controlled source constant but its output voltage varies according to the load. This study shows that both controllers are effective but one alone cannot control the whole parameters of a MG.
If there are several RES, the frequency and amplitude voltage reference must be set to one of the sources. This can be made directly with the parameters of a synchronous generator or with a VSI control if there is no such generator in the microgrid. The other RES can have a PQ controller to regulate their active and reactive power generation.
There are two main types of control strategies: central control strategy and peer to peer strategy, where every sources of the MG have to ensure good power quality to the grid. This second kind of control is preferable as communications between the sources are unnecessary and new microsources can be added to the MG without changes to the control strategy. That is why in this paper we will focus on this kind of strategy.
A. Frequency control
In islanding mode, the frequency stability is no longer ensured by the robustness of the main grid. The islanded MG must then control the grid frequency by itself. The frequency of the power sources dictates the power unbalance between generation and demand. Eq. (1) represents the conventional frequency vs. power droop control.
where f is the measured MG frequency and f 0 is the reference frequency. The slope Kp depicted in Eq. (2) is chosen in order to obtain slight frequency variation when the power varies between zero and its maximum value Pmax. The chosen gain is made of a compromise between a high value which reduce the stability margin and a small value which increases the
response time. In [6], the frequency droop gain is selected to have the best tradeoff between stability and dynamic performance.
When a load connected to an isolated MG increases, the generated power has to increase as well. The microsource droop controller detects the power augmentation and decreases the generated frequency. The contrary occurs when the local load needs less power.
This simple control is used in [5] where 3 microsources are connected to static loads in an isolated MG. For a variation of load of 2kW, the frequency is stabilized in some milliseconds. When the load increases by only 4kW, the permanent frequency decreases by 0,05Hz, which is acceptable.
However, a big load variation can change the permanent frequency to a value too far from its reference and the system can become unstable. The controller in [7] changes automatically the droop position to restore the frequency to its reference. When the measured frequency exceeds a chosen limit, the droop position is changed turning on a switch which activates the second part of the controller. With this strategy, the frequency becomes stable in 0.5s whereas the time needed with the conventional droop control is 0.1s. The frequency stability is thus improved despite of its dynamic behavior.
Instead of a simple gain, a PI corrector, which eliminates the static error, is often preferred to control the frequency of isolated MG [4] [8] [9] [10] [11] [12] [13]. Eq. (3) describes the frequency control strategy which uses a PI corrector to provide an estimation of the active power unbalance.
The controller parameters must be optimally chosen. It is generally pre-determined by the controller designer. The PI gains parameters presented in [14] are selected dynamically by a fuzzy controller which takes into account the changing parameters of the microsource. In case of important overload, the fuzzy controller disconnects arbitrarily a user load to keep the frequency at a constant value within 2s.
The PI controller in [15] is tuned using Particle Swarm Optimization (PSO) technique. It is shown that PSO based controller improve dynamic performance of an autonomous power plant frequency controller better than conventional PI controller. According to simulation results, the frequency deviation is reduced by half comparing with a conventional PI controller.
The frequency controller in [2] uses the same principle that the grid frequency depends on the power unbalance. The use of a smart load allows the system to keep the same permanent frequency value modifying the output active power. Moreover, a battery energy storage system is integrated in the MG to manage better the active power balance. The smart load ensure the active power control in one direction (ΔP>0) while the battery controls the power flow in both direction. The frequency is then controlled either by the smart load or by the battery according to the controller strategy (Figure 2).
An unconventional model of the islanded microgrid shows that the voltage can be controlled by the active power [17], as the frequency can be determined by the reactive power. A virtual resistance RV is used to provide damping to the controller represented in Figure 5. This model allows controlling the voltage unbalance using the active current.
Figure 5. Acive power / Voltage controller
The voltage balancing controller in [16] maintains at zero the negative sequence of the voltage vd,q neg in unbalanced load condition. The controller is illustrated in Figure 6 where Id neg is the negative sequence of the active current at the output of the voltage source inverter and C the output capacitor filter. The same principle is applied to the negative sequence of the reactive current.
Figure 6. Active negative current compensator of voltage balancer
Simulation and experimental results show the robustness of this controller in unbalanced and purely inductive loads. In worst case scenario, where a three phase inductive load is connected while the grid is supplying a single phase inductive load between a phase and the ground, the voltage is stabilized within 10s.
The voltage balancing of this unconventional control is also made in [21]. As in the previous case, the same worst case scenario is tested experimentally, except that the single phase load is connected between two phases, which is less common in practice. In this test conditions, the voltage is stabilized in 30ms. The authors have then applied this control strategy to several voltage source converters to operate in parallel in islanded microgrid [22].
IV. ANALYSES OF THE V/F CONTROL STRATEGIES
The VSI controller in [5] is the simplest controller but is not able to correct the variation between the frequency and voltage grid and their reference. The variations shown in the experimental results are acceptable but the study does not take into account high power demands. The frequency could in that case reach a value that is too different than the reference frequency, which can be a problem for the supplied loads. The change of the droop position in [7] can bind the frequency to its reference despite the fall of the dynamic performances.
Despite of its cost and lifetime, a storage system is essential to control the active power balance in order to maintain the frequency at its reference. A dump load is a waste of energy, unless it can be fully recovered as heat.
Another way to avoid storage system in case of overload is to disconnect user loads as made in [14]. Except in some cases where loads of the grid can be disconnected without undue disruption to consumers, this solution should be avoided.
For the same power variations, the H∞ frequency controller in [19] appears to be 10 times faster than the one used in [2], and its deviation almost 10 times smaller. However, the voltage control is not carried out and it doesn’t show the performances in unbalanced loads conditions, whereas the controller in [2] makes the voltage unbalance compensation through dump loads. This strategy is limited by the maximum power that the loads can dissipate.
It is not the case for the voltage balancer used in [16] which controls the negative sequence of the voltage at zero. This control is effective in worst-case scenario but the rapidity of the controller, which may be improved tuning the PI parameters, is not acceptable. The stabilization of the voltage in [21] is 100 times faster for the same condition. However, the grid frequency reaches a value that is 10Hz more than its reference.
Through the analyzed papers, two main kinds of strategies have been identified: the conventional method where the frequency is controlled by the active power when the voltage is controlled by the reactive power (FP/VQ), and the contrary (VP/FQ). The first strategy type is more common, simpler to design and has often good dynamic performances. The second one seems to be more appropriate to regulate unbalanced voltage. Moreover, VP/FQ strategies don’t need frequency generators to set the frequency reference.
The controller characteristics of the principal strategies viewed in this paper are compared in Table I. It is shown that every strategy has benefits (+) and disadvantages (-) regarding the dynamic performance, the strategy complexity and the management of unbalanced loads. Indeed, the authors’ objectives are different so the improved controller characteristics are not the same.
TABLE I. COMPARISON BETWEEN THE STRATEGIES VIEWED
Strategy Principal characteristics Type Ref Stability Precision Speed Simplicity unbalanced FP/ VQ
[5] + - - + + + - - [2] + + - + + [19] + + + + - - VP/ FQ
[16] + + - - - + + [21] - + + + - +
V. CONCLUSION
The configuration of the isolated MG is an important part of the system control optimization. The integration of storage systems is essential to control the active power unbalance. The excess power that cannot be carried out by a storage system can be dissipated in a dump load, providing heat power for the local customers.
The control strategy depends on the type of the power conversion system and their situation in the isolated MG. It also depends on the type of loads that will be connected to the grid. If the loads can be unbalanced or connected to a single phase, a voltage unbalance must be integrated to the controller. According to the microgrid configuration and the
dynamic performances wanted, the controller designer must choose the appropriate strategy making a trade-off between rapidity, accuracy and stability.
ACKNOWLEDGMENT
This study has been carried out with financial support from the RURAL GRID project. It has also been supported by the group SI+E of the University of the Basque Country.
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