Distributed Secondary Control for Islanded Microgrids—A Novel Approach
Summary (3 min read)
Introduction
- The time latency and data drop-out limits of the communication systems are studied as well.
- The conventional secondary control approach relays on using a MicroGrid Central Controller (MGCC), which includes slow controls loops and low bandwidth communication systems in order to measure some parameters in certain points of the MG, and to send back the control output information to each DG unit [1], [2].
- Tertiary control exchanges information with the distribution system operator (DSO) in order to make feasible and to optimize the MG operation within the utility grid.
- Although secondary control systems conventionally have been implemented in the MGCC, in this paper the authors propose to implement it in a distributed way along the local control with communication systems.
II. PRIMARY CONTROL FOR MICROGRIDS
- Power electronics based MG consists of a number of elements that can operate in parallel either in islanded mode or connected to the main grid.
- As depicted in Fig.1, each DG system comprises a renewable energy source (RES), an energy storage system (ESS), and a power electronic interface, which normally consist of a dc-ac inverter.
- Each DG can be connected to a predefined load or to the AC common bus directly in order to supply power.
- The reference of the voltage control loop will be generated, together with the droop controller and a virtual impedance loop.
- Furthermore, a virtual impedance loop is also added to the voltage reference in order to fix the output impedance of the VSI which will determine the P/Q power angle/amplitude relationships based on the droop method control law.
III. CENTRALIZED SECONDARY CONTROL FOR MICROGRIDS
- Since the primary control is local and does not have intercommunications with other DG units, in order to achieve global controllability of the MicroGrid, secondary control is often used.
- Conventional centralized secondary control loop is implemented in MGCC [2].
- Hence, those variables are compared with the references in order to be compensated by the secondary control, which will send the output signal through the communications channel to each DG unit primary control.
- The advantage of this architecture is that the communication system is not too busy, since only unidirectional messages are sent in only one direction (from the remote sensing platform to the MGCC and from the MGCC to each DG unit).
- The drawback is that the MGCC is not highly reliable since a failure of this controller is enough to stop the secondary control action.
A. Frequency control
- Taking the idea from large electrical power systems, in order to compensate the frequency deviation produced by the local P- droop controllers, secondary frequency controllers have been proposed [26].
- The approach needs communications in order to avoid instability in the MG system caused probably by different stories of each local inverter.
- Here, , , N is the number of packages (frequency measurements) arrived through communication system and n is number of DG units.
- According to the proposed average method, secondary control is able to remove voltage deviations caused by primary control level in every DG unit as shown in Fig. 6 (b). IEEE TRANSACTIONS ON POWER ELECTRONIC 7.
IV. PROPOSED DISTRIBUTED SECONDARY CONTROL
- The problem of using the MGCC for implementing secondary control is that a failure can result in a bad function of the whole system.
- In order to avoid a single centralized controller, a distributed control system approach is proposed in this paper.
- Primary and secondary controls are implemented in each DG unit.
- The secondary control is placed between the communication system and the primary control.
- Secondary control in each DG collects all the measurements (frequency, voltage amplitude, and reactive power) of other DG units by using the communication system, average them and produce appropriate control signal to send to the primary level removing the steady state errors.
C. Line impedance independent power equalization
- It is well-known that in a low R/X MicroGrid the reactive power is difficult to be accurately shared, and the same effect occurs when trying to share active power in high R/X MicroGrids.
- This way, reactive power sharing can be obtained independently from voltage sensing mismatches or line impedances in the MG.
- The small signal model of the secondary control for voltage restoration and reactive power sharing has been derived by using equations (13) and (14), and has been depicted in Figs. 9(a) and 9(b) respectively.
- These models allow us to set the control parameters of secondary control properly.
V. EXPERIMENTAL RESULTS AND DISCUSSION
- An experimental MG setup as shown in Fig. 10 was used to test the performance of the proposed approach, consisted on two DG inverters forming as an islanded MG.
- All the parameters are the same for both DG units.
- The secondary control parameters have been selected so that its response at least six times is slower than primary control [25].
- In the subsections C and D, the effects of communication latency delay and data drop-out on the proposed secondary control is investigated and the results are compared with the conventional secondary control.
- All the electrical and control parameters are the same for both distributed and central controllers as listed in Table I.
A. Black Start Process for the Proposed DSC
- If a blackout occurs in a MG, a sequence of actions and conditions must be checked during the restoration procedure which called black start process.
- Conventionally, the MG black start will be performed centrally by the MGCC based on the information stored in a database about the last MG load scenario.
- Fig. 12 shows the black start process for the islanded MG setup.
- After activating synchronization process (t=20s), DG units are connected at t=25s and then they works as an islanded MG.
- Finally, DSC is activated at t=40s, which remove deviations and shares reactive power between two DGs.
B. Frequency/Voltage Restoration and Q Sharing
- The performance of DSC applied to a MG has been depicted in Fig. 13. Fig 13(a) and Fig 13(b) showing how the new secondary control strategy restores frequency and voltage deviation of the DGs.
- As seen, DSC restores frequency and voltage amplitude properly after changing the load.
- Results show restoration process of frequency and amplitude for DG1 as result of its own local secondary control effort.
- This figure verifies the concept of Fig. 6 that primary control determines the power rate of MG units, and secondary control is responsible for recovering the deviations of the units.
- As can be seen, both controllers have good performance for the time delay of 200ms.
D. Effect of Data Drop-Out
- In the real communication system, there may exist data drop-out or pocket losses which can affect the system output performance.
- The performance of proposed secondary control in the presence of data drop-out is illustrated in Table III, comparing to the central one.
- Results have been shown for different amount of pocket losses, 50% and 95%, considering 100 ms communication delay.
- When data drop-out is up to 95%, the central controller is not able to control the system and system goes to instability after a while.
- The proposed distributed controller is still stable and restores deviations properly.
VI. CONCLUSION
- This paper has introduced a distributed control strategy for droop controlled MGs.
- A decentralized secondary control encompasses every DG unit local controller and the communication system.
- Thus, adding more DG units is easy, making the system expandable.
- Still having a MGCC is mandatory to achieve some other purposes like coordination of the MG units in black start process or energy management.
- The results experimental showed that the proposed control strategy has a good performance in removing frequency and voltage steady state errors and can share reactive power between DG units perfectly.
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Citations
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Cites background from "Distributed Secondary Control for I..."
...In microgrid applications, droop control usually plays the role of primary control in a hierarchic structure, in which secondary control is designed to keep the frequency and voltage around the nominal value [10]–[15], and/or to obtain accurate reactive power [14]–[19], harmonic and unbalance power sharing [17] and voltage harmonic compensation [15], [20] in islanded mode, and tertiary control is designed to manage synchronization to the main grid, power-flow control in grid-connected mode and optimal operation [12]–[13]....
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751 citations
Cites background or methods from "Distributed Secondary Control for I..."
...In [90], a distributed networked control system is used to restore the frequency and amplitude deviations and ensure reactive power sharing....
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...The distributed control is often applied to parallel converters [24], [32], [33], [90]–[93]....
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715 citations
Cites background from "Distributed Secondary Control for I..."
...In [52], a communication network is spread all over the microgrid and the functionality of the centralized secondary controller is embedded in each converter....
[...]
600 citations
References
4,145 citations
"Distributed Secondary Control for I..." refers background or methods in this paper
...On the other hand, this MGCC also can include tertiary control, which is more related to economic optimization, based on energy prices and electricity market [1]....
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...However, this control strategy can be used to share active power in high R/X MGs as well....
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...The conventional secondary control approach relays on using an MG central controller (MGCC), which includes slow control loops and low-bandwidth communication systems in order to measure some parameters in certain points of the MG and to send back the control output information to each DG unit [1], [2]....
[...]
...In order to analyze the system and to adjust the parameters of DSC for frequency restoration, a small signal model has been developed for low R/X MGs [1], [30], according to (3) and Pdroop control law....
[...]
...Recently, hierarchical control for MGs has been proposed in order to standardize their operation and functionalities [1]....
[...]
2,276 citations
"Distributed Secondary Control for I..." refers background or methods in this paper
...Conventional centralized secondary control loop is implemented in MGCC [2]....
[...]
...The conventional secondary control approach relays on using an MG central controller (MGCC), which includes slow control loops and low-bandwidth communication systems in order to measure some parameters in certain points of the MG and to send back the control output information to each DG unit [1], [2]....
[...]
...This concept was used in large utility power systems for decades in order to control the frequency of a large-area electrical network [14], [15], and it has been applied to MGs to restore frequency and voltage deviations [1], [2], [9]–[13]....
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1,702 citations
1,550 citations
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Frequently Asked Questions (15)
Q2. What is the effect of the proposed distributed secondary control method?
The proposed distributed secondary control is able to keep the reactive power shared between DG units when the load changes frequently as well.
Q3. What is the main advantage of a centralized secondary control system?
secondary controllers for large power systems are based on frequency restoration, since the frequency of the generator-dominated grids is highly dependent on the active power.
Q4. What is the effect of the droop control on the MG?
It can be seen that frequency and voltage values are slowly and successfully regulated inside the islanded MG, removing the static deviations produced by the droop control.
Q5. What is the procedure for restoring the voltage in a MG?
When the voltage in the MG is out from a certain range of nominal rms values, a slow PI control that compensates the voltage amplitude in the MG, pass the error through a dead band, and send the voltage information by using low bandwidth communications to each DG unit.
Q6. What are the main steps to be considered for the islanded MG?
The main steps to be considered include building the islanded MG, connecting distributed generations (DGs) which feed their own protected loads, controlling voltage and frequency, synchronizing DG units inside islanded MG, connecting controllable loads and MG synchronization with the LV network [31].
Q7. What is the power requirement for the proposed DSC using the average method?
It is worth noting that power change requirement for the proposed DSC using the average method depends on the power rates of the MG units.
Q8. What is the role of communication latency in the MG?
Impact of Communication LatencyCommunication has a predominant role in providing the infrastructure that enables data to be exchange among the different elements of the MG.
Q9. What is the advantage of the secondary control method in front of the conventional one?
The advantage of this method in front of the conventional one is that the remote sensing used by the secondary control is not necessary, so that just each DG terminal voltage, which can be substantially different one from the other, is required.
Q10. Why is the voltage not common in the whole MG?
The reason is that as opposed to the frequency, the voltage is not common in the whole MG as well as the impedance between the DG units and common point is not the same.
Q11. What is the performance of DSC applied to a MG?
The performance of DSC applied to a MG has been depicted in Fig. 13. Fig 13(a) and Fig 13(b) showing how the new secondary control strategy restores frequency and voltage deviation of the DGs.
Q12. What is the way to restore the voltage of a DG?
a possible solution is to implement a secondary control for power sharing locally, so that each DG unit sends the measured Q (or P in high X/R MicroGrids) to the other DG units in order to be averaged.
Q13. What is the main structure of a dc/ac MG?
As depicted in Fig.1, each DG system comprises a renewable energy source (RES), an energy storage system (ESS), and a power electronic interface, which normally consist of a dc-ac inverter.
Q14. What is the drawback of the MGCC?
The drawback is that the MGCC is not highly reliable since a failure of this controller is enough to stop the secondary control action.
Q15. How much of the data drop-out is seen in the graph?
It can be seen that both controllers has an acceptable performance in restoring frequency and voltage deviation for 50% of data drop-out.