Angular frequency variations at microgrids as a challenge for ...

These are excerpts of the paper of IET Generation, Transmission & Distribution vol. 10, no. 13, 2016, pp. 3234–3240, 10.1049/iet-gtd.2015.1572

Angular frequency variations at microgrids as a challenge for developers of measuring instruments Tomasz Tarasiuk Department of Marine Electrical Power Engineering Gdynia Maritime University Gdynia, Poland [email protected] Abstract— The paper’s aim is to discuss challenges facing designers of future measuring instruments in the wake of microgrids proliferation. In this context, the fluctuations of angular frequency of voltage fundamental component and resulting changes of duration of measurement windows are to be considered. The paper chief aim is to consider the above mentioned phenomenon of angular frequency variations from IEC Standard 610004-7 [1] point of view, using real signals. The numerous results of experimental investigation at eleven electric power networks, chiefly marine microgrids, are presented and discussed. Further, comparative analysis of two measuring instruments behavior is provided. Both instruments are based on the different signal processing principles. First device utilizes signal processing method recommended in IEC 61000-4-7 standard. Whereas, the second one is based on application of chirp z-transform principle. Keywords— microgrids, power quality, measurement instruments I. INTRODUCTION “The microgrids are capable of inlanded operation mode, becoming, at least temporary, isolated power systems. The typical feature of the isolated systems is significant susceptibility to load variations, due to relatively low available power of energy sources. As a results the significant variations of angular frequency of fundamental component can be observed. Therefore, it is considered in quite a number of works. The most attention is paid to the problem of frequency fluctuations and its proper control in microgrids [6-7]. A few papers deal with the proper measurement methods for microgrids [6],[8]. For example, in ref. [8] phasor measurement unit has been presented, which is capable of accurately tracking harmonic phasors subject to varying nominal frequency. In shorthand, the proposed solution is capable of relatively fast adaptation to frequency changes. Its effectiveness has been demonstrated on the exemplary simulated microgrid with large frequency drift in the range 62-57 Hz with slope -1 Hz/s. As complement to the other works, this paper is focused on microgrid operation during constant load and relatively small angular frequency variations, considered from the IEC Standard 61000-4-7 [1] point of view. This phenomenon also leads to variations of basic measurement window width as defined in the standard. In order to assess scope of angular frequency variations and possible impact on measuring instruments accuracy, existing isolated power systems can be the best testing ground.” “The paper is based on extensive research of real microgrids supplied by diesel-driven generators during seemingly steady-states. The chief aim of this paper is to present results of experimental investigation of variability of power frequency in various systems, expressed as measurement time window changes. It is completed by comparative study on performance of two measuring instruments, based on two principles of signal sampling and subsequent processing. First instrument is commercial power quality analyzer, which implements synchronization of sampling frequency by PLL and its design directly implements IEC Standard 61000-4-7 provisions [1]. Whereas second one is power quality estimator-analyzer which operates with constant sampling frequency and it is based on other signal processing principle. Both instrument have been used for concurrent measurement of distortion parameters of signal, which angular frequency randomly varies.”

These are excerpts of the paper of IET Generation, Transmission & Distribution vol. 10, no. 13, 2016, pp. 3234–3240, 10.1049/iet-gtd.2015.1572 II. CHANGES OF MEASUREMENT WINDOW WIDTH – RESULTS OF EXPERIMENTAL RESEARCH AT CHOSEN ELECTRIC POWER SYSTEMS “The experimental research has been carried out at eleven various electric power systems, eight isolated systems (microgrids) and three industrial networks for comparison reasons. Seven isolated systems represent marine systems of various kinds, whereas the eighth isolated system is emergency supply of office building with sensitive data centre. The last system represents land microgrid operating in islanded mode. This consisted of two diesel-driven synchronous generators and two UPS devices. The system topology is similar like in the case of five considered marine systems (the systems without shaft generators). Generally, all considered microgrids represent systems with diesel-driven generators, commonly used on shipboard or emergency supply of banks, hospitals, data centers etc. The bulk load of marine systems have been electric motors, sometimes supplied via power converters, whereas the chief loads at office building have been computers and servers. Other three investigated systems represents typical industrial networks, considered from supply point of view (gas electric power plant and wind farm) as well as consumer point of view (network at the factory of medium size). The loads in the case of factory have been similar like in ship systems. The common feature of all investigated microgrids is angular frequency variations of voltage fundamental component during islanded operation due to limited power of energy sources [6]. The variation is notably greater than in the case of supply by electric utility. In order to exemplify the fact, the angular frequency variations have been calculated for particular microgrid and two cases: islanded operation (supply by emergency two dieseldriven generators working in parallel) and supply by electric utility. For both considered cases the load remained the same. The angular frequency has been calculated by zoom-DFT with Kaiser window, refreshed every 1 ms and with frequency resolution 0.005 Hz. Afterwards, the low-pass filter with cut-off frequency equal to 30 Hz has been used for smoothing the obtained results. Finally, the results for both cases are shown in Fig. 1.” 314.9

314.5

315

island utility [rad./s] 314

0

time [s]

150 314.5

314.1

Fig. 1. Angular frequency variations of voltage fundamental component at network of office building during normal supply (blue line) and during supply from emergency generators (islanded operation mode) (red line).

“It has to be added that example shown in Fig. 1 is rather mild. The worst observed by author angular frequency variations of voltage fundamental component are presented in Fig. 2. It has been registered in ro-ro ship during shaft generator (1500 kVA) operation and heavy sea.” 316

ro-ro ship [rad./s ] 308 0

time [s]

57

Fig. 2. Angular frequency variations of voltage fundamental component at network of ro-ro ship during shaft generator operation and heavy sea.

These are excerpts of the paper of IET Generation, Transmission & Distribution vol. 10, no. 13, 2016, pp. 3234–3240, 10.1049/iet-gtd.2015.1572 “If one consider the observed angular frequency variations in microgrids form IEC Standard 61000-4-7 [1] point of view, the obvious result is change of duration of measurement windows width understood as duration of 10/12 periods. Therefore its impact on performance of typical measuring instruments used at microgrids should be carefully analyzed. The further analysis of consequences of the angular frequency variations at investigated microgrids is carried out on basis of changes of standard measurement window widths.” “In order to quantify differences between results obtained for various systems following indices have been adopted:  mean value , standard deviation , maximum observed value max, minimum observed value min of measurement window widths,  mean value , standard deviation , maximum observed value max, minimum observed value min of relative difference wC between widths of two consecutive measurement windows,  mean value , standard deviation , maximum observed value max, minimum observed value min of voltage,  relative number of wC indices, with values above 0.03% (required accuracy of sampling frequency synchronisation according to IEC Standard 61000-4-7). The relative difference wC between widths of the two adjacent windows is calculated according to the formula:

wC 

wC  wC 1 100 wC

(1)

where: wC – duration of the considered window, wC-1 – duration of previous window.”

RESULTS OF EXPERIMENTAL RESEARCH - MEAN VALUES , STANDARD DEVIATIONS , MAXIMUM OBSERVED VALUES MAX, MINIMUM OBSERVED VALUES MIN OF WINDOW WIDTH W, R.M.S. VALUE OF VOLTAGE URMS AND RELATIVE DIFFERENCES BETWEEN TWO ADJACENT WINDOWS WC

TABLE I.

wC

>0.03 % all-electric ferry Urat=400 V frat=50 Hz 3x610 kVA researchtraining ship Urat=380 V frat=50 Hz 2x376 kVA ro-ro ship (with shaft generator) Urat=400 V frat=50 Hz 1X1500 kVA navy ship Urat=380 V frat=50 Hz 2x500 kVA chemical tanker (with shaft generator ) Urat=440 V frat=60 Hz 1x1187 kVA ferry Urat=380 V

63.5

25.5

79.9

13.5

3.6

w [ms]

403.5 0.22 404.1

199.26 0.163 199.62

max min

0.000

402.7

198.81

  max

0.020 0.015 0.079

399.6 0.13 400.0

201.52 0.040 201.64

min

0.000

399.2

201.40

  max

0.090 0.065 0.307

399.0 0.48 400.3

201.50 0.730 203.31

min

0.001

398.0

199.97

 

max

0.015 0.012 0.051 0.000 0.061 0.043 0.255

379.5 0.08 379.8 379.3 440.5 0.88 441.9

199.17 0.121 199.51 198.92 200.28 0.153 200.71

min

0.002

438.4

199.91

 

0.009 0.009

380.0 0.18

200.01 0.053

 

max min

  70.2

Ums [V]

[%] 0.047 0.035 0.196

These are excerpts of the paper of IET Generation, Transmission & Distribution vol. 10, no. 13, 2016, pp. 3234–3240, 10.1049/iet-gtd.2015.1572 frat=50 Hz 2x1400 kVA

max min

car carrier Urat=440 V frat=60 Hz 2x1750 kVA

 

emergency supply of office building Urat=230 V frat=50 Hz 2x500 kVA industrial plant Urat=230 V frat=50 Hz electric power plant with gas turbines Urat=230 V frat=50 Hz wind generator Urat=400 V frat=50 Hz

1.4

max

0.052 0 0.008 0.006 0.037 0.000 0.010 0.008 0.049

380.3 379.4 442.2 0.12 442.5 442.0 228.4 0.20 229.1

200.19 199.92 200.03 0.077 200.17 199.81 199.62 0.022 199.70

min

0.000

228.2

199.54

 

max

0.002 0.002 0.009 0.000 0.001 0.001 0.005

225.0 0.43 225.6 224.0 222.3 0.04 222.4

199.91 0.019 199.96 199.87 200.03 0.016 200.06

min

0.000

222.2

199.99

 

0.003 0.003 0.009 0.000

407.4 0.55 408.0 406.5

199.96 0.008 199.98 199.94

max min

  2.3

0

0

0

max min

 

max min

The results in bold rectangle concerns microgrids, whereas remaining results concern typical industrial grids. 206

w [ms]

202

198

194

time [s]

0

44.3

Fig. 5. Changes of duration of basic measurement window widths at electric power system of all-electric ferry, sea-voyage (red line) and maneuverings (blue line). TABLE II.

RESULTS OF EXPERIMENTAL RESEARCH - COMPARISON OF CONSIDERED INDICES ON ALL-ELECTRIC FERRY DURING HER SEAVOYAGE AND MANEOUVRIG AS WELL AS ON RO-RO SHIP DURNIG ROUGH AND CALM SEA

wC

>0.03 % all-electric ferry sea-voyage 3x610 kVA all-electric ferry manoeuvrings 3x610 kVA

63.5

78.7

ro-ro ship (with shaft generator) rough sea 1x1500 kVA

79.9

ro-ro ship (with shaft generator)

47.1

  max min

  max min

  max min

 

[%] 0.047 0.035 0.196 0.000 0.209 0.317 1.783 0.0005 0.090 0.065 0.307 0.001 0.035 0.030

Ums [V] 403.5 0.22 404.1 402.7 402.3 1.41 407.8 397.6 399.0 0.48 400.3 398.0 399.0 0.20

w [ms] 199.26 0.163 199.62 198.81 200.67 1.216 204.25 195.70 201.50 0.730 203.31 199.97 199.93 0.236

These are excerpts of the paper of IET Generation, Transmission & Distribution vol. 10, no. 13, 2016, pp. 3234–3240, 10.1049/iet-gtd.2015.1572 calm sea 1x1500 kVA

TABLE III.

max min

0.152 0.000

399.6 397.5

200.76 199.51

RESULTS OF EXPERIMENTAL RESEARCH - COMPARISON OF CONSIDERED INDICES ON RESEARCH-TRAINING SHIP FOR THREE ELECTRIC POWER PLANT CONFIGURATIONS

> 0.03 % research-training ship one generator working alone 1x376 kVA research-training ship two generators working in parallel 2x376 kVA research-training ship three generators working in parallel 3x376 kVA

40.7

25.5

12.2

wC [%]

Ums [V]

w [ms]



0.030

401.2

200.41



0.023

0.18

0.053

max

0.098

401.7

200.56

min

0.000

400.7

200.28



0.020

399.6

201.52



0.015

0.13

0.040

max

0.079

400.0

201.64

min

0.000

399.2

201.40



0.015

399.1

200.7



0.011

0.13

0.028

max

0.053

399.4

200.76

min

0.000

398.8

200.62

III. COMPARATIVE STUDY ON MEASUREMENT INSTRUMENTS PERFORMANCE “Finally, three instruments were used to measure characteristic of the very same testing signals concurrently: estimator-analyzer, commercial power quality analyzer of class A (according to IEC Standard 61000-4-30 [16]) and reference instrument. The controller NI PXIe-8106 equipped with two data acquisition boards NI PXIe-6124 has been used as reference instrument. This was used for signal registration and the acquired samples were postprocessed by means of DFT as reference procedure (in this case off-line analysis has been adopted due to computational burden). Widths of measurement windows were adjusted to actual duration of integer number of input signal cycles (10 periods) by software in the case of reference measurements. Several test for signals with constant frequency were carried out, including some standard testing states. They revealed any differences between devices under test. Next the arbitrary testing signal has been used, with harmonic content similar like registered in exemplary microgrid. So it can be considered as more realistic scenario than standard testing signals.” IV. FINAL REMARKS “The work has been considered as case study and its chief contribution is presentation and profound discussion of results of extensive, comparative survey carried out at eleven power systems. The novelty of this paper lays in considering the problem of frequency fluctuations in microgrids from IEC Standard 61000-4-7 point of view, particularly if synchronization of sampling frequency of measuring instrument is used. The research is completed by comparative laboratory study on two instruments performance under angular frequency variations. Both instruments have been based on different signal processing principles and the device which directly used IEC Standard 61000-4-7 [1] method has not produced accurate results.” “Next, the problem of reliable testing of measuring instruments is looming. The current procedures should be supplemented by signals with varying angular frequency (time dependent phases of all components) and concurrently varying magnitudes. So, additional models of testing signals has to be developed. The author’s further research is concentrated on proper design of the models.“

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