Research on Operation Control of Micro Sources within a Microgrid

Wang Jing1 , Wang Xiaohui1 , Wang Jia1 Electrical and Information Engineering

Beijing University of Civil Engineering and Architecture Beijing, China

wjhust10@163.com

Du Hong2 Beijing Xinnenghuizhi Micro Grid Technology Co., Ltd

Beijing, China dhseeker@163.com

Abstract—According to characteristics of the micro sources and operating mode of a microgrid, the control methods of the micro sources applied in grid-connected mode and islanding mode are designed in this paper. The control of micro sources is realized conveniently and feasibly through electronic inverters. The constant power (P/Q) control strategy is utilized to realize that the active and reactive power of the micro sources can be controlled as constant values respectively. The voltage and frequency (V/f) control strategy could ensure the stability of voltage and frequency of microgrid operates in islanding mode. The corresponding controller is designed according to the methods, and the simulation results show the effectiveness of the proposed control strategy. The study on control strategies of micro source provides a platform for further studies of microgrid.

Keywords- microgrid; micro sources; control strategy; constant power (P/Q) control; voltage and frequency (V/f) control

I. INTRODUCTION To address technical, economical and environmental issues

of conventional power systems, as a promising type of power grid, microgrid as a new type of power grid is being paid more and more attention [1-2] . It has been proposed that one solution to solve the reliability and stability problems is to take the advantages of microgrid technologies. The currently developing technologies for microgrids are based on renewable sources and micro sources with the characteristics of very low emissions. Microgrid can improve the power supply reliability and power quality, meanwhile, it can guarantee the power supply in special periods such as natural disasters and the state of war. The combination of microgrid and power grid can also enhance the safety of power grid, and microgrids access to a centralized grid is the trend of the power grid [3] .

The term “microgrid” is still vaguely defined, and the micro sources and their associated loads are indispensable elements in a microgrid [4] . Microgrid can operate in both grid-connected mode and isolated mode.

Whether the microgrid operates in grid-connected mode or in islanding mode, the effective control of micro sources is necessary to guarantee the voltage and frequency of the system within the standard extent. The study on the control methods of micro sources is the core technologies to develop microgrid.

II. CONTROL STRATEGY OF THE MICRO SOURCES In simulation analysis, the micro sources can be divided

into two categories [5] : some micro sources whose output power are randomness and intermittent, greatly affected by the weather, such as wind power, photovoltaic power generation, etc; and the others whose output power are regolabile and governable such as micro turbine and fuel cells, etc. The control strategy of micro sources is related with the respective categories and functions in a microgrid, at the same time, the operating state of the system should be concerned [6] .

The control strategy of micro sources is the key to achieve stable operation of a microgrid. For most micro sources are connected to the microgrid via power electronic inverters, the control of micro sources is realized through the control of power electronic inverters. Since the micro source can be equivalent to a voltage source inverter (VSI), the control strategies of the microgrid are realized by controlling the current. The control of power electronic inverters in microgrid should ensure the voltage and frequency of the microgrid within the standard range and consequently guarantee the power quality in both grid-connected and isolated operating modes [7] . There are two effective kinds of basis control strategies of electronic inverters, constant power control (P/Q control) and voltage and frequency control (V/f control) based on droop characteristics.

A. Constant power control As the interface between micro sources and a microgrid, the

basic function of the electronic inverters is to achieve the control of output active power and reactive power from the micro sources. The method of constant power control means that the active and reactive power of the micro sources can be controlled as constant values, in this way, the micro source is equivalent to the current-controlled voltage source [8] . This control method is generally used in grid-connection operating status, and the reference voltage and frequency are provided by the power grid. Meanwhile, the load fluctuations, frequency and voltage disturbances of the grid are regulated by the power grid. When the micro sources participate in voltage regulating of the grid feeder directly, it can produce negative impact on the system. However, in this way it can be avoid effectively [9] .

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To furthest utilize the secondary energy, the constant power control strategy is usually applied to the intermittent micro sources. According to this control strategy, micro sources exchange active power and reactive power with the system respectively with predetermined value. The reference active power and reactive power are input externally, the appropriate coordinate system selected in this paper can realize that the active and reactive components are decoupled and controlled respectively, and coordinates system transformation from three phase static synthetic coordinate (a, b, c) to two phase synchronous rotating coordinate (d, q), and the transform object is the output voltage from the inverter [10]. After coordinate rotation transformation, the three-phase static coordinate system in the fundamental sine variables is transformed into DC variables in the synchronous rotating coordinate system, The abc component of the output voltage from the inverter is converted to dq0 component by means of the Park transformation. Based on the transformation the active power and reactive power is represented as

*d d q dP V I V I= ∗ + (1)

*d q q dQ V I V I= − ∗ + (2)

To simplify the analysis, the q axis component of voltage set to zero (V q=0), the equations are

d dP V I= ∗ (3)

d dQ V I= − ∗ (4)

Schematic diagram of constant power control is shown in Fig. 1.

1

-1

abc /dq 0

Ugabc

PI

ω Lf

Pref

Qref

Udd

Udq

Iqref

Idref

-

-

-++

+

++

Ud

Uq

Id

Iq

+

+

+

+

ω Lf

PI

Figure 1. Schematic diagram of constant power control

The constant power control strategy is implemented by regulating the active power current and reactive power to track the reference current. By setting the reference active power and reactive power, the micro sources can inject active power and reactive power into the system with predetermined value. This control strategy is to be a suitable option while the system operates in grid-connection mode, however, it may be not work while the grid is separated from the power grid. Once the system operates with huge vacancy, the microgrid may be collapsed, for there is no reference source to provide reference voltage and frequency in it.

B. Droop characteristic The microgrid connected with micro sources via inverters,

is essentially a system of power inverter in parallel, and the control of the micro sources could be realized by the way of simulating the droop characteristic of traditional generator, which is called voltage versus frequency droop control [6] . The voltage difference of transmission line in the medium and low voltage distribution grid is relative with active power, as well as the frequency to reactive power. The droop control method can be used to realize the control of micro sources, similar with the primary frequency regulation of the synchronous generator, and each micro source uses the frequency, instead of the power angle or phase angle, to control the active power flows since the units do not know the initial phase values of the other units in the stand-alone system. By regulating the real and reactive power flows through a power system, the voltage and frequency can be determined. The frequency versus active power droop (f-P) characteristic curve is shown in Fig. 2(a), and voltage versus reactive power (u-Q) characteristic curve is shown in Fig. 2(b), where f0 and V0 are the base frequency and voltage respectively, and P0 and Q0 are the temporary set points for the active and reactive power of the machine.

Fig. 2(a) f-P droop characteristic Fig. 2(b) u-Q droop characteristic

Figure 2. droop characteristics shared by micro sources

In this method the voltage and frequency are determined by the rate of droop characteristic and the power exchanged between the micro sources and loads. The unbalanced power can be allocated to each micro sources dynamically in this method, which is simple, reliable and easy to be implemented [1] .

However, this control method does not consider the recovery of voltage and frequency in the system, which is similar to the second frequency adjustment in conventional generators. Therefore, the frequency of the system may not be guaranteed once the microgrid encounters severe disturbance [11] .

C. Voltage and Frequency (V/f) Control The basic function of voltage and frequency control applied

in microgrid is to sustain the voltage and frequency of the isolated microgrid, to ensure stability of the output voltage and frequency, and to inherit a certain load power following characteristics [9] . When the load changes, the steady-state frequency and voltage will fluctuate because of the droop characteristic. By adjusting output power of the micro sources, the frequency and voltage of the microgrid could be returned to the rated values, that is to say, the droop characteristic curves can be shifted left and right to maintain the frequency and

voltage of the system, which is shown in Fig. 3 (a) and Fig. 3 (b).

Fig. 3(a) f-P droop characteristic Fig. 3(b) u-Q droop characteristic

Figure 3. V/f Control

The V/f Control strategy is realized by regulating the feedback voltage of inverters to ensure the output voltage in bus. The dual-loop structure is applied in this paper, that the outer ring is used to realize voltage control and the inner ring to realize current control. The schematic diagram is shown in Fig.4.

Figure 4. Schematic diagram of voltage and frequency control

III. SIMULATION RESULTS In order to verify the validity and correctness of the control

strategies, relevant simulation modules based on the control scheme proposed in this paper are built. A micro grid platform is designed, which could operate in grid-connected mode with P/Q strategy and in isolated mode, the micro source models built in Matlab/Simulink is shown in Fig. 5.

Figure 5. Simulation platform for microgrid

For the control strategy is the key point of the research, the micro sources are simulated by VSI. A microgrid with two sources is set up in the simulation platform. The micro sources and their control circuits are packaged in “MS1” and “MS2”

modules, and the LC filter relates to three phase LC filter circuit. The main simulation parameters in this part are shown as follows: the DC voltages of the “MS1” and “MS2” are both 700V; Load1: active power is 10kW and reactive power is 1kvar; Load2: active power is 1kW and reactive power is 1kvar; Power Grid: the voltage is 380V and the frequency is 50Hz.

In the initial status, breaker K1, K2, K3 and PCC are closed, and the microgrid operates in grid-connected mode. Both the two micro source modules are controlled by the constant power control strategy. The reference active power is 6kW and the reactive power of MS1 is 1.5kvar and active power is 5kW and the reference reactive power of MS2 is 0kvar in this part. At the time t=0.3s, K3 and PCC opens, and the microgrid operates in isolated mode. The reference micro source is module “MS1”, which is controlled by the V/f Control strategy, and module “MS2” is still controlled by the constant power control strategy.

The simulation results are shown as follows: the output power of MS1, MS2 and the power grid are shown in Fig. 6 to Fig. 8. The voltage and frequency of Load1 are shown in Fig. 9 and Fig. 10.

Figure 6. Active power and Reactive power of MS1

Figure 7. Active power and Reactive power of MS2

Figure 8. Active power and reactive power of MS2

Figure 9. Voltage on load1

Figure 10. System frequency

Before the time t=0.3s, the system operates in grid-connected mode and the constant power strategy is applied in the micro sources. As shown in Fig. 6 and Fig. 7, the output active power and reactive power of the MS1 are about 6 kW and 1.5kvar respectively, and the output active power and reactive power of the MS2 are about 5kW and 0kvar respectively. The insufficient power needed for the loads is provided by the power grid. It can be indicated from the simulation results that the constant power control method used in the microgrid can ensure the micro sources inject active power and reactive power as constant values respectively. At the same time the voltage and frequency are well maintained by the power grid.

After the time t=0.3s, the microgrid operates in isolated mode. As shown in Fig. 9 and Fig. 10, the fluctuation of frequency is under the tolerant and the voltage experiences a fluctuation at the time of PCC breaks, and then the voltage starts to maintain stability again. It also can be indicated from the simulation results that the voltage and frequency control method can guarantee the stable operation of microgrid in isolated operation mode.

IV. CONCLUSIONS The control strategies of micro sources within a microgrid

are reached in this paper. The control methods suitable for micro sources in grid-connected operation mode and isolated islanding operation mode respectively are simulated in a simulation platform. When the microgrid operates in grid-connected mode, all the micro sources are controlled by the constant power control strategy, which can exchange active power and reactive power with the system with predetermined value respectively. Once the microgrid operates in islanding operation mode, one of the micro sources controlled by V/f control strategy operates as the reference power source, and the

simulation result indicates that it can provide the reference voltage and frequency in the microgrid, and the system operates stably.

The micro sources are simplified by DC voltage sources in the simulation platform built in this paper, which ignores the dynamic characteristics of the micro sources. A more detailed dynamic simulation platform of microgrid will be set up in the future work, and the more appropriate control strategies will also be studied in the future.

ACKNOWLEDGMENT This work was supported by Science Research Fund of

Beijing University of Civil Engineering and Architecture and Foundation for Beijing municipal Talents.

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