Transcript Taajuusmittaus FPGA:lla

Frej Suomi
Frej Suomi
Education and career
Swedish vocational institute 1995-1998
 Swedish polytechnic
2000-2004
 Vaasa university
2004-2008


ABB Distribution Automation 2008->
Frequency measurement
challenges
Computational challenges
Accuracy vs. Speed
 Precision vs. Frequency range
 Disturbances

 Random
 Harmonics
 Transients
Frequency measurement
challenges
Implementation resources

CPU
+ Complex functions easily made
- Limited parallel processing (multi-core)

FPGA
+ Paralel processing
- Complex functions possible but consumes a
lot of resources
Why use a FPGA in grid
frequency measurement





Does not require complex logic
Straight forward ”number crunching”
suitable for FPGA
Frees the resources of a possible CPU
in the same design for more complex
logic functions
Fast due to parallel processing
Deterministic execution time
Choosing the frequency
measurement method
1
 f [ Hz ]
T

Zero-crossing
X
+ Simple
- Slow
- Sensitive to disturbances that moves the zero
crossing point
Choosing the frequency
measurement method
n
2m
N
X n  1  xn  xn  N 
 ImX n 

 arctan
 ReX n 
X m [ n]  e
j
m
m
m
Fs n   n 1 
f 
2

DFT
X
+ Fast
- Very limited frequency range
Choosing the frequency
measurement method
a
xr  x r  2 
 xr  xr  2 
2
 4 x 2 r 1
2 xr 1
fs 

f  arccos Rea  
2 


SDFT (Yang, Jun-Zhe & Liu Chih-Wen
2000)
X
+ Wider useful frequency range compared to
DFT
+ compensation of harmonics possible
- Square root, division, arccos and solving a
2nd degree polynom is difficult and resource
consuming on a FPGA
Choosing the frequency
measurement method


Fs arccos Q  1
f 
10 π

”Amplitude response” method (Tevfik Sezi 1999 )
+ Wide frequency range
- Bad accuracy at low frequencies
V
Implementation
Front end calculations FIR filters, easily
implemented in a FPGA
 Arccos(sqrt(Q)-1)could be solved
by a lookup table and linear interpolation
 Alternatively the filters on the FPGA and
Arccos(sqrt(Q)-1) on a soft-core
CPU

Conclusion

Choosing the right frequency
measurement method is always a
compromise, the last method presented
is however a good candidate for
implementation in a FPGA.
References
Yang, Jun-Zhe & Chih-Wen Liu (2000). A
Precise Calculation of Power System
Frequency and Phasor. IEEE
Transactions on Power Delivery, vol. 15,
no. 2
April 2000.
 Sezi, Tevfik (1999). A New Method for
Measuring Power System Frequency.
0-7803-5512-6/99/$10.00 1999 IEEE.
