Patent Grant
10804704
A claim for priority under 35 U.S.C. § 119 is made to Korean Patent Application No. 10-2015-0128955 filed Sep. 11, 2015, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.
The present document concerns a microgrid control system and method. More particularly, the present document concerns a microgrid converter droop control system and method for assisting frequency adjustment of power transmission system.
Microgrids are small-scale power networks independently supplying power and heat, coming to as new paradigms for power systems along with the rising of small-scale distributed generators using renewable energy in recent years. Microgrids, as a set of a multiplicity of small-scale distributed generators and loads, may be called a small-scale power network which can be connected with or driven independent from an existing power network.
Microgrids are now employed as a power system continuously supplying power into loads of the microgrid by simply using distributed generators in separation from a main system when the main system is in trouble with suspension/short-circuit and serious degradation of power quality.
In consideration of energy self-generation and systemic reliability, the microgrid as a small independent system has a problem that a power transmission system becomes weak when there is frequency fluctuation because loads are incapable of providing electrical inertia and frequency-load reaction to the power transmission system in the case of increasing the engagement of DC distribution or microgrid members connected with a system through a DC converter.
As shown in
In the case of connecting two or more controllable generators, the droop control mode is a control mode setting load allotment rates of respective generators to be automatically set in accordance with a droop tilt against load variation, preventing a hunting effect and allowing a stable operation of the distributed generators. An active power control amount is determined in Equation 1, as follows, by detecting frequency fluctuation from the microgrid 20 in the system of
f−f0=R(P0−P) [Equation 1]
In Equation 1, f0 denotes a reference frequency of a microgrid, f denotes a current frequency of the microgrid, R is a droop set value, P denotes active power of a target to be controlled, and P0 denotes active power that is currently being transmitted.
However, the droop control mode does not contribute to adjustment of a systemic frequency because it considers only a frequency operating in a microgrid. For example, controlling active power through such a droop control mode at each of a multiplicity of microgrids connected to a single power transmission system 10 affects the power transmission system 10 and causes a frequency of the power transmission system 10 to fluctuate. Such frequency fluctuation of the power transmission system 10 may also affect a frequency operating in each microgrid.
The present document concerns a microgrid control system considering both a power transmission system and a microgrid in regard to frequency fluctuation.
The microgrid control system may include a power transmission system configured to supply power to a microgrid, a DC converter, and a converter control unit. The microgrid is configured to receive power from the power transmission system and to supply power into at least one or more distributed generators or into at least one or more loads. The DC converter is configured to convert power that is supplied to the microgrid. The converter control unit is configured to measure a first power frequency variation of the microgrid and a second power frequency variation of the power transmission system and to control active power that is transmitted to the microgrid by the DC converter.
The microgrid control method (performed by a converter control unit controlling an active power amount supplied into a microgrid) may include: measuring a first power frequency variation of the microgrid; measuring a second power frequency variation of a power transmission system; calculating an active power control amount (which is to be supplied to the microgrid) from the first power frequency variation and the second power frequency variation; and controlling a DC converter with the active power control amount.
The above and other objects and features will become apparent from the following description with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified.
Hereafter, embodiments of the inventive concept will be described in conjunction with the accompanied figures. Embodiments of the inventive concept may be variable in various forms and the scope of the inventive concept may not be construed as restricting the following embodiments. These embodiments will be provided to help those skilled in the art to fully comprehend the inventive concept as possible. Shapes or patterns of the accompanied figures are exaggerated for more clearly illustrating structural configurations of elements in the figures.
As illustrated in
The power transmission system 100, generally including lines and facilities for power transmission, may be referred to as a power system formed of many generators and transmission lines.
As illustrated in
Power to be transmitted to the microgrid 200 through the power transmission system 100 may be converted through the DC converter 300 and then the converted power may be supplied to the microgrid 200. The microgrid 200, as a smart grid system allowing self-sufficiency of power in a small-scale local area, may be regarded as a new-generation power system fused and composed with renewable energy resources, for example, solar or wind power, and energy storage units, thus being simply called a small-scale independent power network in a word.
The converter control unit 400 may measure a first power frequency variation, which is a frequency variation of the microgrid 200, and a variation of a second power frequency variation that is a frequency of power transmitted to the DC converter 300 from the power transmission system. The first power frequency variation and the second power frequency variation, which are detected, may be used to control an active power amount of the DC converter 300 (a) to define a frequency variation width of the microgrid 200 within a regulated frequency variation range and (b) to define a regulated frequency variation width of the power transmission system 100 within a regulated frequency variation range of the power transmission system.
The converter control unit 400 may control an active power amount by the droop control described above, and as illustrated in
As illustrated in
The converter control unit 400 may calculate an active power control amount by using the first power frequency variation and the second power frequency variation. The storage module 430 equipped in the converter control unit 400 may store a specific initial droop value, a microgrid frequency-load characteristic invariable, and a droop control invariable for calculating an active power control amount.
The converter control unit 400 may use factors, which are calculated by the first measuring module 410 and the second measuring module 420 or stored in the storage module 430, to decide an active power control amount with. Such an active power control amount may be calculated by Equation 2 as follows.
In Equation 2, ΔP denotes an active power control amount, R denotes an initial droop set value, Δfs denotes a frequency variation of the microgrid (a first power frequency variation), Δfm denotes a frequency variation of the power transmission system (a second power frequency variation), Cr denotes a droop control invariable, and Ds denotes a frequency-load characteristic invariable of the microgrid.
The initial droop set value R means a tilt of a linear graph showing a correlation between frequencies and power as like a general droop control mode. A tilt of the linear graph may be set by a device operator.
The droop control invariable Cr may be calculated by Equation 3 as follows.
In Equation 3, Co denotes a specific frequency adjustment contribution set value, Δfsreg denotes a regulated frequency variation range of the microgrid, and Δfmreg denotes a regulated frequency variation range of the power transmission system.
Co is determined by an operator. A larger Co may positively affect the power transmission system. While a regulated frequency variation range of the microgrid and a regulated frequency variation range of the power transmission system may be also set by an operator, it is general to regulate the ranges in the scope not to affect each system.
For example, it may be permissible to set a regulated microgrid frequency range of the microgrid (i.e., microgrid frequency variation range) in ±0.1 Hz and a regulated frequency variation range of the power transmission system (i.e., power transmission system frequency variation range) in ±0.5 Hz.
The microgrid frequency-load characteristic invariable Ds is valued by entirely reflecting characteristics of loads practically connected to the microgrid. For example, assuming that a load connected to the microgrid is a rotation motor, Ds may become a value entirely counting a reaction, such as a decrease of the number of rotations of the rotation motor according to frequency fluctuation of the microgrid, into the characteristics of a system. Namely, Ds means a value showing physical characteristics of a distributed generator or load according to frequency fluctuation of the microgrid. The microgrid frequency-load characteristic invariable Ds is a factor set on operation in accordance with the number or characteristics of distributed generators or loads which are connected to the microgid, and may be calculated by an operator who knows the characteristics of a system.
A microgrid control system may control the DC converter 300, which is connected with the microgrid 200, to prepare power frequency fluctuation of the microgrid, and additionally, may decide an active power control amount of the DC converter 300 in consideration of the power frequency fluctuation of the power transmission system 100 at the same time.
In the same manner with the aforementioned example, a microgrid frequency variation range may be set in ±0.1 Hz and a power transmission system frequency variation range may be set in ±0.5 Hz.
As shown in
During this, in the case that a general droop control mode causes the microgrids to consume active power regardless of frequency degradation in the power transmission system, it can be seen that the frequency is out of the power transmission system frequency variation range ±0.5 Hz. Differently, it can be seen that a systemic control mode results in allowing the frequency to be defined within the regulated variation range.
Additionally, as shown in
A microgrid control system may be implemented as described above. A microgrid control method will be now described hereafter in conjunction with
As shown in
An active power frequency control amount may be calculated (S300) through Equation 2 and Equation 3 by measuring the first power frequency variation and the second power frequency variation (S100 and S200). The calculated active power frequency control amount may be used to control the DC converter (S400).
Accordingly, the microgrid control method may consider frequency fluctuation in a microgrid as well as in a power transmission system to decide an active power amount that is to be supplied into the microgrid, thus reducing a frequency adjustment reserve power that is secured against frequency fluctuation.
According to a microgrid control system, it may be allowable to reduce frequency adjustment reserve power that is secured against frequency fluctuation, by fairly deciding an active power amount, which is to be supplied to a microgrid, in consideration of a power transmission system and the microgrid together with respect to frequency fluctuation.
While the inventive concept has been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the inventive concept. Therefore, it should be understood that the above embodiments are not limiting, but illustrative.
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|---|---|---|---|
| 10-2015-0128955 | Sep 2015 | KR | national |
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