Monitoring of Active Distribution Networks in Steady State and Transient Conditions by means of...

Monitoring of Active Distribution Networks in Steady State and Transient Conditions by means

of accurate synchrophasors measurements

Mario PaoloneÉcole Polytechnique Fédérale se Lausanne - EPFL

Outline

Introduction PMU requirements for power distribution

network applications Proposed algorithm for the synchrophasor

estimation PMU prototype based on the NI Compact Rio Application examples and experimental activities Conclusions

Introduction

Evolution of distribution networks passive active major changes in their operational procedures; main involved aspect is the network monitoring; massive use of advanced and smarter monitoring tools

that would result into faster and reliable real-time state estimation;

possible use of distributed measurement of synchrophasors based on the use of phasor measurement units (PMUs).

In transmission networks WAMS (Wide Area Monitoring Systems) are typically based on the measurements of bus voltages synchrophasors realized by means of Remote Terminal Units typically synchronized by means of the Universal Time Code – Global Positioning System.

Peculiar characteristics of distribution networks lower p.u.l. inductances with non-negligible R; low power flows values; high harmonic distortion levels.

Improved accuracy of synchrophasors measurements

PMU requirements for power distribution network applications

60 Hz

Δθ*

Ee

E

2 2

2 2

( ) - ( ) -r r i i

r i

X n X X n XTVE

X X

(**) IEEE Standard for Synchrophasors for Power Systems, IEEE Std. C37.118, 2005

Total Vector Error (TVE) (**):magnitude of the difference between the theoretically true phasor and the estimated one, in per unit of the true phasor magnitude.


synchrophasor #1

111 ,, QPI

1E 2E synchrophasor #2

222 ,, QPIjX Estimated phasor errors:

RMS: ΔEphase: Δθ

1 2,E E

2

2

2E

E

p e

q e

phase angle difference between phasors : δ Δδ=2∙Δθ

1 2,E E


synchrophasor #1

111 ,, QPI

1E 2E synchrophasor #2

222 ,, QPIjX

1 2,E E


Structure of the developed algorithm for the synchrophasor estimation

I. Sampling of the waveforms (voltage/current), within a GPS-PPS tagged window T (e.g. 80 ms, i.e. 4 cycles at 50 Hz), starting in correspondence of the GPS-PPS wave-front.

II. Identification of the fundamental frequency tone within a specific frequency window Δf (i.e. f0 ± Δf).

III. Reconstruction in time-domain of the identified fundamental frequency tone and improved estimation of the dynamic phasor amplitude, phase and frequency.

Proposed algorithm for the synchrophasor estimation

Input signal:

0 0,1

cos , , n

h h t tt Î Th

s t s s h t s DC s t Gaussian noise

-1-

1

sin- ,

sin

Nn iN

h N h Nh

G k f S D k f f T being D eN N

Discrete Fourier Transform

Problems related to the identification of the fundamental frequency tone:a. spectral leakage effects caused by the finite length of time window T;

b. identification of the correct frequency value that may fall between two subsequent frequency values provided by the DFT.


Problem a: simple solution by applying a proper windowing

Problem b:

1

( 1) ( -1)1- , ( ) ( ) -

2 2

nN N

H h N h N Nh

D DG k f S H k f f T H D

0 , 0 1, f m bin f bin m

HP: since the number of samples N per time window T is very large (fsampling=100 kHz >> f0=50-60 Hz); the sine function in the denominator of the Dirichlet kernel DN(ϑ) can be approximated by its argument:

Also, the following approximation applies:-1

--1

Ni

Ne iN


sinN N

These assumption led to a linear expression of GH(kΔf) that allows to express Δbin as:

2a bbin

a b

Where a e b are the highest and the second highest tone magnitudes in the discrete spectrum GH. Once Δbin is known, the complex amplitude of the fundamental frequency tone S1 at frequency f0 is given by:

1

2 11

sini bin

H

bin binS e bin G m f

bin

The knowledge of f0 and S1 allows to reconstruct the fundamental frequency tone in the time domain in order to improve its phase estimation.


1 0 1 1 1

32 ' "

2f T T T

1T i t

1

1'

clock

T kf

1"

1

s i tT t

s i t s i t


PMU prototype basedon the NI Compact Rio

PXI-based system for the generation of reference signals used forthe experimental characterization

Application examplesand experimental activities

Time-Sync accuracy ±100 nswith 13 ns standard deviation

18-bit resolution inputs at 500 kS/s,analog input accuracy 980 μV over ±10 V input range (acciracy of 0.01%)

16-bit resolution, sampling frequency: 100 MS/s, carrier frequencies with 355 nHz resolution and PXI time synchronization skew < 20 ps

2 2

2 2

( ) ( )r r i i

r i

X n X X n XTVE

X X

Accuracy assessment with reference to steady-state signals (phase error)

-6 -4 -2 0 2 4 6

x 10-5

0.0001

0.05 0.1

0.25

0.5

0.75 0.9

0.95

0.99

0.999

0.9999

Phase error [rad]

Pro

ba

bili

ty

-6 -4 -2 0 2 4 6

x 10-5

0.0001

0.05 0.1

0.25

0.5

0.75

0.9 0.95

0.99

0.999

0.9999

Phase error [rad]

Pro

ba

bili

ty

Single tone signal(50 Hz)

Distorted signal(spectrum of std. EN 50160)


Steady-state signals (single-tone and distorted)


Note that the 3σ of the GPS card is ±100 ns 31.410-6 rad @ 50 Hz

Std.dev of the synchrophasor phase estimation with reference to 10 s frequency sweep ramp from 47 Hz to 53 Hz

Frequency-varying signals



Procedure:1. Calculation of signal waveforms (bus voltages) within

the EMTP-RV environment;

EMTP-RVnetwork model

National InstrumentsPXI Arb 6289

cRio PMUs

2. analog-generation of fault transient waveforms by means of the PXI reference signal generator system.

, 1m m ssKP P fsT

, ,m ssP f

1 2

13.8/20

Tr

+ SW

PQ 20kVRMSLL

2MW1MVAR

Load2

PI

+

Overhead_line

SM

13.8kV5MVA

SM

PQ 20kVRMSLL

2MW1MVAR

Load1Governor

BUS_line_startBUS_SM BUS_line_end

Accuracy assessment with reference to electromechanical transients

, 1m m ssKP P fsT

, ,m ssP f

1 2

13.8/20

Tr

+ SW

PQ 20kVRMSLL

2MW1MVAR

Load2

PI

+

Overhead_line

SM

13.8kV5MVA

SM

PQ 20kVRMSLL

2MW1MVAR

Load1Governor



, 1m m ssKP P fsT

, ,m ssP f

1 2

13.8/20

Tr

+ SW

PQ 20kVRMSLL

2MW1MVAR

Load2

PI

+

Overhead_line

SM

13.8kV5MVA

SM

PQ 20kVRMSLL

2MW1MVAR

Load1Governor


Voltagephasorsmeasured

by PMU


80 MW power plant:two aeroderivative gas turbine (GT) units and a steam turbine unit (ST) in combined cycle;

PP connected to a 132 kV substation feeding a urban medium voltage (MV) distribution network;

PP substation is linked, by means of a cable line, to the 132 kV substation that feeds 15 feeders of the local medium voltage (15 kV) distribution network and provides also the connection with the external transmission network throughout circuit breaker BR1.

80

0 m

ca

ble

lin

e

External transmission network

BR1

BR1-GT1

BR-ST

BR1-GT2

BR2-GT1 BR2-GT2

PMU3

PMU2

PMU1


PMU application example (real scale)

Distribution network voltage phasors angles differences during the islanding


80

0 m

ca

ble

lin

e

External transmission network

BR1

BR1-GT1

BR-ST

BR1-GT2

BR2-GT1 BR2-GT2

PMU3

PMU2

PMU1

Conclusions The use of PMUs in active distribution networks requires the

definition of improved performances of these devices compared to those available in the international standard (e.g. IEEE C37.118).

The developed PMU prototype allows to:

i) identify the fundamental frequency tone with accuracy levels adequate for distribution network applications;

ii) obtain accuracy levels not influenced by the harmonic distortion of the analysed signals and by their time-varying characteristics.

Monitoring of Active Distribution Networks in Steady State and Transient Conditions by means of...

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