Download Research of a New Line Protection Measurement and Controlling

Research of a New Line Protection Measurement and Controlling
Device Based on DSP
LI Yujin, ZHANG Qing
Mechanics Electric Engineering College, Agricultural University of Hebei, P.R.China, 071000
[email protected]
Abstract This paper presents the design of a new line protection measurement and controlling device,
including its hardware system, software function and algorithms. The core component of the device is
TMS320LF2407A DSP, which is rapid and strong. Through advanced high-speed DSP (Digital Signal
Processing) technology, serial A/D, serial EEPROM, CAN bus, surface craft and backward structure, the
device integrates protection with far action function. It can fulfill protection, fault record, four
functions(YX,YK,YC,YT), communication, electrical power calculating and collecting, and so on. The
design of the device’s software system is modular. Through combining hardware analogue record wave
and software digital record wave, Fourier algorithm and software compensation power angle algorithm
32 points per cycle, it guarantees the device’s reliability, advancement and measurement precision.
Key words DSP, Line protection measurement and controlling, Serial A/D, Serial EEPROM, CAN
network, Fourier algorithm, Software compensation power angle algorithm, Two-watt method,
Three-watt method
1 Introduction
With the rapid development of the first power network, the requirement of relay protection
becomes higher and higher. The integrated automation for substation becomes the inevitable trend.
Those original devices, including electromagnetic type and transistor-type ones, can’t meet the demand
of new substation because they haven’t the fault and event memorizing function and communication
function. So they will be certainly replaced by produces based on microcomputer. But the
microcomputer based products also can’t meet the demand of new substation because their CPUs are
usually slow, their data and address lines are usually few and their integration degree is usually low. The
application of microcomputer based relay protection in the low voltage power system is relatively
prevalent in western countries. Some large corporations, such as SIMENS, ABB, GE, ALSTOM and
SEL, have provided microcomputer based produces since 1980’s, and these products becomes more
perfect. But these overseas products are too expensive to stand for most domestic users. Some domestic
corporations, such as NanRui, NanZi, BeiJing SiFang, XuJi, have developed congeneric produces
recently, but these products can’t meet the requirement of whole market because of their various
functions and limited outputs. The new line protection measurement and controlling device based on
high-speed DSP integrates protection with measurement and controlling functions, its communication
function is powerful and it is very flexible, so it draws the merits of equivalent products to increase new
features for line protection and promote the integrity of automation products.
2 Design of Hardware System
The new line protection measurement and controlling device based on DSP adopts 6u chassis,
embedded setting, entire panel form, backward structure and connection wires behind box. Terminals
are directly installed on the plates to reduce intermediate links and improve anti-jamming ability. It
supplies internal power supply plug, CPU plug and relay plug. Motherboard is at the front of the chassis
and a Chinese keyboard and LCD are on the front plate.
CPU plug adopts return circuit protection converter which is very precise and effectively isolated,
return circuit measurement converter and voltage converter. As core of the device, it adopts high-speed
digital signal processor as the CPU, namely TMS320LF2407A DSP, coupling with high-speed serial
analog-digital convert device TLC2543. It can fulfill analog inputs in eleven channels and realtime data
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acquisition and transformation in three self-inspection channels. It uses serial EEPROM x5163 with low
voltage detection and watchdog function to store protection parameters and settings. It has sixteen
switching inputs that are photoelectric isolated and nine switching output circuits with rapid
photoelectric isolation function. It uses CAN (Controller Area Network) interface to communicate. It is
so simple that it connects outside with only two lines. Its Transmission speed is up to 1 Mbps and its
transmission distance is up to 10 KM. It uses CRC to verify data and it can also provide corresponding
error correcting ability. With non-destructive bus arbitration technique, it improves the communicating
efficiency inside station and anti-jamming capability.
Relay Plug receives controlling orders downloading from CPU and accomplishes switching on,
switching off and anti-jumping functions. It gets and uploads switch’s signal that indicates on or off state.
It fits for various switching on/off current with the unique design of the operation circuit.
Front motherboard has human-machine interface. With the convenient keyboard and LCD, it can
facilitate the certain operating, export experimenting, measuring data, reporting accident, inquiring state
and other operations.
3 Design of Software System and Its Algorithms
3.1 Design of Software System
Analog: analog inputs (IA, IB and IC indicate protection current, IA and IC indicate measuring
current, UA, UB and UC indicate voltage, I0 indicates zero-sequence current, U0 indicates
zero-sequence voltage, Uxl indicates line extracting voltage, one self-inspection channel) are acquired
(32 points sample per cycle), stored and calculated after converted by serial A/D converter;
Input volumes: it is responsible for data acquiring and changing of address space of fourteen road
inputs;
Remote pulse volume: it is responsible for data acquiring of remote pulse volume and calculating
of pulse value in two roads;
Output volumes: it includes altogether nine roads, they are one startup road, one trip protection
road, one remote control switching off road, one remote control switching on road, one general accident
road, one general alarm road and two remote control backup roads;
Certain value protection and fault reporting reading/writing: it uses inquiry method to accomplish
reading and writing operation through the SPI interface of DSP;
Protection function: using tri-phase and syllogistic current protection with the low-voltage and
direction atresia function, it can seize no pressure and three-phase reclosing in the same period. It can
accelerate two or three phase latter acceleration function, grounding line selection function of small
current, load decreasing function in low-frequency (slip atresia, low flow atresia, low pressure atresia),
fault recording function. Each function and atresia condition can be set by control word for retreat;
Measurement function: UA, UB, UC, UAB, UBC, UCA, IA, IB, IC, U0, I0, Uxl, P, Q, COS ф, F,
etc;
Communication function: it combines active uploading method with asking/answering method to
transmit date remotely;
Human-machine interface function: with six-key tangible keyboard and large LCD whose language
is Chinese, along with readable and flexible C programming language, it realizes humanized design.
3.2 Algorithms
Algorithm is an important research aspect of microcomputer based protection measurement and
controlling device. The main algorithms used by this device are shown as follows.
1) Fourier algorithm
It supposes that the sample analog signal is a periodic time function, in addition to fundamental
wave, it also contains non-decay DC component and various hypo-harmonic. That can be expressed as:
（ 1）
∞
x(t) = ∑ [bncosnw 1t + an sinnw 1t ]
n =0
In formula (1), n is a natural number, n = 0,1,2,... , a n and b n are the various harmonic sine and
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cosine value of the amplitude. As each harmonic phase may be arbitrary, they are decomposed into sum
of arbitrary amplitude of the sine and cosine item. a 1 and b1 are respectively fundamental component
of sine and cosine amplitude items. According to the principle of Fourier series, we can get a 1 and b1 ,
namely:
（ 2）
（ 3）
T
a1 =(2/T)∫ x(t)sinw1tdt
0
T
b1 =(2/T)∫ x(t)cosw1tdt
0
When dealing them with computer, the integral value of formula (2) and (3) can be gotten with
trapezoidal rule:
（ 4）
（ 5）
N −1
a1 =(1/N )[2 k∑=1 x k sink(2π/N)]
b1 = (1/N )[x 0 + 2
N −1
∑
k =1
x k cosk(2π/N)+ x N ]
In the above formulas, N is a sample point, x k is the k-th sample value, x 0 and x N are
respectively the sample value when k is equal to zero and N .
Rewriting formula (4) and (5) as the following expressions, it can obtain amplitude and phase of
random harmonic.
N −1
an = (1/N )[2 k∑=1 x k sinkn(2π/N )]
6
bn = (1/N )[x 0 + 2
N −1
∑
k =1
（）
（ 7）
x kcoskn(2π/N) + x N ]
2) Software compensation algorithm of power angle
The algorithm is used to improve the accuracy of power based on the specific status of this device’s
sampling module. The device in this paper uses TLC2543 Serial A/D converter as sampling module and
uses switched-capacitor technology to complete successive approximation A/D conversion process. As it
has 10us conversion time within the operating temperature range, and it needs time to implement the
process, so it makes a certain angle between the sample phase voltage and the its corresponding current
phase, thereby it will affect the accuracy of power calculation.
Bellow we take U a and I a as example to illustrate how to make U a and I a ’s phase angle
nearly zero with software compensation algorithm. Supposing sampling I a firstly, and then sampling
U a , we set two software clock counter, namely count1 and count2 (us), in interrupt timer, then put them
respectively on sampling time I a and sampling time U a , finally we can get the sample angle of I a and
Ua :
a = [(count2 − count1)/20000 ] * 2π
8
If we take I a as benchmark, then A-phase compensation voltage is:
(− ) +
(− )]
a = ( ar +
ai )[
= (U ar cosα + U ai sinα) + j (U aicosα − U ar sinα)
9
From I b , U b and Ic , U c , we can get formula (10) and (11):
U b = (U brcosβ + U bisinβ) + j (U bicosβ − U brsinβ)
10
（）
U U jU cos α jsin α
（）
（ ）
（ ）
U c = (U crcosδ + U cisinδ) + j (U cicosδ − U crsinδ)
11
In formula (9), (10) and (11), U ar , U ai , U br , U bi , U cr and U ci respectively indicate the real
and imaginary part of U a , U b and U c , α , β and δ respectively indicate sampling angel of I a
and U a , I b and U b , Ic and U c .
It can effectively improve the accuracy of power calculation with this method.
3) Two-watt method and three-watt method
The original sample values of microcomputer based protection measurement and controlling device
are current and voltage, so it must get active power and reactive power value by calculating known
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sample values. The device in this paper is flexible and it can automatically adopt two-wall method or
three-watt method to measure power according to different external wiring.
Two-watt method:
P = Re[ UabIa ∗ + UcbIc ∗ ]
(12)
Q = Im[UabIa ∗ + UcbIc ∗]
(13)
In the above formula, Uab and Ucb respectively indicate the vector of line voltage, Ia and Ic
respectively indicates the vector of a and c current’s common yoke.
Three-watt method:
P = Re[UaIa ∗ + UbIb ∗ + UcIc ∗]
(14)
Q = Im[UaIa ∗ + UbIb ∗ + UcIc ∗]
(15)
In the above formula, Ua, Ub and Uc are respectively the vector of a, b and c voltage, Ia , Ib
and Ic are respectively the vector of a, b and c current’s common yoke.
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4 Conclusion
The new line protection measurement and controlling device, whose core is rapid and strong
TMS320LF2407A DSP, adopts new computer technology and modular design. It is highly reliable with
both hardware and software watchdogs. It makes full use of advanced computer technology and
effectively simplifies the user's work with large humanized LCD interface whose language is Chinese. It
integrates the protect and far action function into one device, and it can fulfills protection, recorded
waves, four distances, communication, degree measurement and collection, and other functions. It is
sure that the new device will replace the original electromagnetic type and transistor type devices. It is
also the ideal substitute of old computer based line protection device.
References
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[3] Yang Qixun. Base of Microcomputer Relay Protection[M]. China Waterpub Press: Beijing, 1994,
p48.
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