Y. XU ET AL.

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harm are the problems that hinder the application of mi-

crogrid. Factors affecting the stable operation of micro-

grid include:

a) Microgrid itself is a small capacity power supply

system. Its power capacity is limited, and is comparable

with the load. While the different sorts of loads have

wide and frequent dynamic range, the bus voltage will

fluctuate in wide range. In order to k eep the stability, the

energy storage equipments have to provide a quick re-

sponse. Power supply and load matching is an essential

condition for the operation of power system, especially in

microgrid. However, traditional load statistical methods

are based on average power theory. They do not apply to

nonlinear loads, which introduce harmonics into the mi-

crogrid[7]. What’s more, the present theory does not con-

sider the effect of harmonics on load and power supply.

For an example, while a certain power supply system is

rich in harmonics, the actual output of active power of a

diesel generator of which the rated power is 120 KW, is

only 55 KW. The action principle between the harmonic

and Synchronous motor is not clear. In the engineering

application, the resolution is to configure larger power

motor leading to wasting of resources. Under the condi-

tion of large harmonics, power matching has been a

problem. At the moment, the study methods are based on

the average modules of converters and control system.

But the average module cannot describe the time-variant

characteristic of converters exactly[8]. According to the

analysis requirement of nonlinear electric power system,

the instantaneous power theory provides a feasible me-

thod to solve the time-variant and nonlinear problems. At

home and abroad, the scholars have applied the instanta-

neous power theory to make a lot of researches on the

nonlinear pow er syst em.

b) Distributed generation is based on power electronics

technology. Its impact on the power quality is mainly

caused by the voltage flicker and the introduction of a

large number of harmonics. Part of the distributed power

is greatly influenced by natural conditions, so that it is

difficult to generate stable power[9]. The great changes

in output and the interaction between distributed units

and medium voltage feedback devices both can cause

voltage flicker and impact to the power grid. To enhan ce

the reliability of distributed generation and the applica-

tion of energy storage equipment are effective ways to

solve voltage fluctu ation problems such as voltage pulse,

voltage flicker and power supply interruption etc. It pro-

motes the energy storage device development in energy

density, response time and cost performance.

c)The application of power electronic devices in mi-

crogrid, bring the problem of converter cascade stability.

as nonlinear switching circuits, its Impedance is

time-varying and nonlinear[10]. When the structure of

the power electronic devices is complicated, it is difficult

o predict the stab ility with conventional impeda nce ratio

methods. Besides adjusting the impedance ratio, adding

Intermediary filter and designing bus controller are ef-

fective methods to improve the cascade stability. At the

moment, the domestic study on cascade stability is very

limited. At abroad, only a few scholars are doing the re-

levant theoretical analysis and application research, such

as F. C. Lee of VT, M. Ehasani of Texas A&M Univer-

sity. In microgrid, converters cascade stability still needs

a lot of research work.

t

4. Conclusions

Microgrid based on distributed generation is powerful

supplement and effectively support of utility grid. Also,

it’s one of the important trends of power system in the

future. Power matching theory, energy storage technol-

ogy, converter cascade stability and harmonic suppres-

sion are the bases of operation control and energy man-

agement[11]. With the development of nonlinear power

system, microgrid will play an important role in applica-

tion of new energy.

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