Page 373 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

360 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

14.2.5 Evaluation of Architectures
Different architectures are compared with regard to efficiency, reliability, and cost. Mathematical
models are established to analyze the system efficiency and reliability performance.
The system efficiency includes the efficiency in grid normal mode and the one in grid failure
mode. It is assumed that the grid provides the power to load in grid normal mode, and other sources
provide the power to load in the grid failure mode according to the power distribution strategy. The
system efficiency is an average efficiency of all power transferring paths. Efficiency in grid normal
mode is an important indicator for system efficiency because Super UPS operates in this mode for
most of the time. In this mode, the efficiency of the path from the grid to the load represents the
efficiency in grid normal mode. While in grid failure mode, the system efficiency is the average one
of all paths, which then power flows through.
Figure 14.6 is used to explain how to calculate the efficiency and takes star architecture I as an
example. W represents the power of each source, and W is assumed as the grid. M is the number
n
n
1
of converters in path n, and η is the efficiency of the converter in each path. The parameter i indi-
ij
cates which source it belongs to, and j indicates the exact location in path of source i. The value of
i varies from 1 to n, and value of j varies from 1 to M . So η , the efficiency in grid normal mode,
n
n
can be expressed as
∏ M 1
W 1 η j 1 M 1
η n = j=1 = ∏ η j 1 (14.1)
W 1 j=1

If the stage number of converter in the path from grid to load is two, the expression of η can be
n
calculated as
∏ 2
W 1 η j 1
η n = j=1 = ηη 12 (14.2)
11
W 1
When a grid fails, if the power of other sources is represented by W to W , then the efficiency in
n
2
this mode can be calculated as
∑ n i W ∏ M i η ij  

η f = i=2  n j=1  (14.3)
∑ i=2 i W

For instance, if the load power provided by battery, gas engine, PV, and fuel cell is represented
by W to W and each path from source to load contains two-stage converters, hence, the efficiency
5
2
η in grid failure mode is solved as
f
∑ 5 i W ∏ 2 η ij    ∑ 5 i W ( ηη i )

η f = i=2  5 j=1 = i=2 5 i1 2
∑ i=2 i W ∑ i=2 i W

W 2 ( η 221 22) +W 3 η η 4 η η 5 η η
51 52)
(
(
(
η
31 32) +W
41 42) +W
= (14.4)
W 2 +W 3 +W 4 +W 5