Enhanced Protection for Inverter Dominated

8/12/2019 Enhanced Protection for Inverter Dominated

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ENHANCED PROTECTION FOR INVERTER DOMINATED

MICROGRID USING TRANSIENT FAULT INFORMATION

X. Li*, A. Dyko*, G. Burt*

*University of Strathclyde, UK, [email protected]

Keywords: Transient based protection, inverter dominated

microgrid, Mathematical Morphology, travelling wave, rateof change of current.

Abstract

Protection of an inverter dominated microgrid is always agreat challenge, as inverters are well known for their

insufficient contribution to the fault current, undermining theaccuracy and viability of traditional overcurrent protectionschemes. Based on the wide review of solutions developed in

the past, this paper proposes a novel protection strategy, withthe main protection method based on the time and polarityfeatures of initial current travelling waves using mathematicalmorphology (MM) technology and backup protection strategybased on the rate of change of current. Simulation tests inPSCAD/EMTDC considering different fault resistances, fault

positions and fault inception angles prove this protectionapproach to be sensitive and reliable.

1 Introduction

High penetration of distributed generation (DG) is expected tobe a permanent feature of future power systems. One way ofachieving efficient integration of large and diverse amounts of

DG is to control and regulate a cluster of DGs and customerloads integrally as a microgrid, which can be run in both grid-connected and islanded mode of operation. Studies have beenfocused on both control and protection of the microgrid.Referring to the study of protection, the lack of fault currentcontribution from inverter-interfaced generators (IIG)

becomes one of the main concerns [1].

During normal grid-connected operation, the s low fault

current contribution generally does not interfere with theexisting current grading of the distribution network. Duringislanded operation, however, the lack of effective fault current

sources weakens the effectiveness of the existing overcurrentrelays. In order to resolve this issue the protection schemeshould ideally be independent of the fault to load current

ratio. Existing solutions can be classified into three groups:1. Adaptive protection based on overcurrent principles [2];2. Implementation of additional Fault Current Source

(FCS) devices [3][4];3. Unit type protection current differential [5] or phase

comparison [1].

Although efforts have been made to implement adaptiveprotection schemes [2], this group of solutions faces a number

of challenges such as time consuming pre-planning of settings

and complicated validation process. As the protectioncontinues to be based on overcurrent principle, the lowcurrent magnitude and time duration still appear to be themajor obstacles. On the other hand, FCS based protectionarrangement can be seen as unreliable from a system

protection view, as the reliability of the whole protectionsystem relies on a single electrical device, an energy storage

device with high short-circuit capacity. Even if several FCSsare connected to ensure required reliability, the cost of such

protection scheme is likely to be prohibitive. Similar to FCSbased protection conventional unit protection approach can

also face a significant cost in building and maintaining highbandwidth communication channels.

Very few people have discussed the idea of introducing faultgenerated travelling wave as a guiding principle of themicrogrid protection. The authors of the paper [6] applied this

idea to a zonal DC marine system. However, with nodedicated signal analysis, fault transients are not properlyextracted, and their arrival time and polarity information are

vague. Furthermore, the method requires very highcommunication bandwidth to transport high frequencysampled real time current measurements. Reference [7]introduced a hybrid protection idea using fault generated

current travelling wave and superimposed power frequencyvoltage using multi-resolution wavelet analysis. However, the

method is not validated through realistic simulation.

This paper proposes and discusses an efficient microgridprotection strategy combining fault generated current

travelling wave based primary protection and rate of changeof current based backup protection. The primary protection

introduces a very efficient and engineering-application-

friendly signal processing technology mathematicalmorphology (MM). The algorithm is modified by the authorsto meet the requirements of the proposed protection schemewith the capability of extracting polarity features. The casestudy considering different fault conditions is presented toassess the sensitivity of the proposed scheme. The paper is

organised as follows: Section 2 explains the basic principlesof MM technology and presents the modified MM filter;Section 3 discusses the main travelling wave based algorithm

and introduces the backup protection principle based on therate of change of current; In section 4 a 20KV microgridbenchmark model is set up to verify the proposed protection

scheme using PSCAD/EMTDC simulator.


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2 Modified mathematical morphology filter

(MMF)

Mathematical morphology (MM)[8][9][10] uses a structuralelement (SE) to extract the necessary features of the original

signals. Dilation ( ) and erosion ( ) are two basicoperations in MM. Assuming a signal f(n)(0


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In a radial network, this method is sufficient as the routes ofthe travelling waves are one way only. If the topology of themicrogrid is a meshed network, mere polarity information

may not correctly indicate the faulty line. In this case, acomparison of the time information between the local and

adjacent units can solve this issue. Therefore, for meshednetworks the main fault detection algorithm is blocked unless:

(7)Where is the time information from the local unit, and are the times from the adjacent units.This method has the advantages of ultra-high speed faultdetection and low requirement of the communicationbandwidth. However, as an inherent issue within thetravelling wave based methods, fault inception angle is still alimiting factor affecting its performance. Although most of

the faults occur when voltage is around its peak, backupsolution is still needed to detect the faults occurring at thepoint of voltage zero crossing (POW = 0). In this paper, abackup solution based on the rate of change of current is

proposed. The indicator is defined as (8):

(8)

Where calculates the mean value of the magnitude ofthe current signal f(n) within a window of most recent Numsamples. A fixed threshold considering the maximum rate ofchange of the current IN_Diff_max is assumed. Unlike the main

protection, this indicator is not affected by the fault inceptionangle. On the other hand, similar to the main protection,polarity information of can also be used to easilydetect the faulty zone.

The diagram of the proposed protection strategy is depicted in

Figure 3.

MMF based

main protection

Diff based

backupprotection

f(t)OR Trip

ANDTime delay

Figure 3: Proposed protection strategy based on two

independent principles

4 Simulation case studies

For the evaluation of the performance of the proposedprotection scheme PSCAD/EMTDC is employed to build a20kV benchmark MV microgrid model [1] as shown in

Figure 4. The simulation time step is 1S.

In this microgrid system, four IIGs are connected to buses 2,4, 6 and 8; two microgrid loads (induction motors) are

connected to buses 3 and 7. The lines are 20kV ABB XPLEcables [13]. The line models were derived using geometricand material cable data as shown in Figure 5. It should be

noted that PSCAD does not take into account thesemiconducting layer presented in real cables. To simulate the

cable through a large frequency range, the proper way is toadd semi-conducting screen to the conductor layer [14], sinceit shows good appearance of fault transients in the frequencyrange of 0~4MHz. The power of IIGs is provided by the ideal

DC sources, which sufficiently emulates the presentmicroturbines and fuel cell systems, or any other source withstorage on the DC side [15].

Four study cases are conducted to test the protection strategy:

CASE 1 - Solid fault in zone (line 4-5); CASE 2 - Solid fault out of zone (line 1-4); CASE 3 - High resistance fault (Rf= 50 Ohms); CASE 4 - Fault with POW of 0(a), 5(b) and 90(c); CASE 5 - Close-up fault near bus4 on line 4-5(a), and on

line 1-4(b).

Figure 4: The travelling route of the fault transients within a

radial microgrid

In the microgrid model, CB1 and CB2 are open to form a

islanded radial network, and each line is equipped withcurrent instruments at both ends. In practice, high bandwidthRogowski coil current transducers can be used as the current

instruments. To simplify the test scenarios, all the faultsapplied are single phase-to-earth faults. The final results arelisted in Table 1 and Table 2. For CASE 4b the indicator

waveforms of both main and backup methods are presented inFigure 6 and Figure 7. It can be seen that the main protectionis able to detect the fault with POW as low as 5, however, itis significantly affected by the zero fault inception angle. Inthis case, the backup protection using rate of the change of

current detects the fault features within several ms.


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Figure 5: Identical geometric cable parameters of 20kV ABB

XPLE cable in PSCAD/EMTDC

Figure 6: The detection of the initial traveling wavefrontsgenerated by fault with POW = 5 using the main protection

Figure 7: The detection of abrupt change of current by faultwith POW = 5 using backup protection

Test scenariosPeakvalue

Polarity Result

CASE 1Ins. 4-5 0.8248 1

TripIns. 5-4 0.7162 1

CASE 2 Ins. 4-5 0.3174 1 Non tripIns. 5-4 0.2834 1

CASE 3Ins. 4-5 0.1005 1

TripIns. 5-4 0.1339 1

CASE4a (0)

Ins. 4-5 N/A N/A BackupprotectionIns. 5-4 N/A N/A

CASE4b (5)

Ins. 4-5 0.0093 1Trip

Ins. 5-4 0.0123 1

CASE

4c (90)

Ins. 4-5 0.7645 1Trip

Ins. 5-4 1.0183 1

CASE

5a

Ins. 4-5 0.4290 1Trip

Ins. 5-4 0.3371 1

CASE

5b

Ins. 4-5 0.4290 1Non tripIns. 5-4 0.3371 1

Table 1: Results using MMF based main protection

Test scenariosPeakvalue

Polarity Result

CASE4a (0)

Ins. 4-5 79.2653 1Trip

Ins. 5-4 121.6911 1

CASE

4b (5)

Ins. 4-5 71.5028 1Trip

Ins. 5-4 134.1181 1

Table 2: Results using Diff based backup protection

A system consisting of the protection strategy stated above is

composed of mainly three parts:1) Rogowski coil based current instruments connected

through an integrator to a high speed data acquisitionboard with a sampling rate of 1MHz or more.

2) Signal transforming block. The three phase currentsignals are transformed into small voltage signals

before the band-pass filters. After sample-and-holdunits, the signals are transferred to digital signals byhigh speed ADC before processed by MMF algorithm

and Diff calculation.3) Communication channels with capability of

transporting signals time and polarity information,

only activated when disturbance is detected.

It is believed that with increasing availability and continuingcost reduction of modern data acquisition and processingtechnologies, high sampling rate requirements of the proposedscheme should not be prohibitive in the deployment of

travelling wave based protection schemes in microgridenvironments. Further work will make an attempt to assessthe overall cost of this type of scheme through building of a

laboratory prototype demonstrator.

5 Conclusion

Protection of inverter dominated microgrids is always a

challenge. This paper proposes a hybrid approach consistingof the main protection based on travelling wave

615.77 615.775 615.78 615.785 615.79 615.795 615.8-5

0

5

10

15x 10

-3

X: 615.8

Y: 0.01229

Current(kA)

T(ms)

X: 615.8

Y: 0.009261

i45

i54

615.6 615.7 615.8 615.9 616 616.1 616.2-50

0

50

100

150

X: 615.8

Y: 134.1

dCurrent(kA)/dt(s)

T(ms)

X: 615.8

Y: 71.5

i45i54


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measurements and backup protection based on the rate ofchange of current. The main protection scheme uses only theinitial travelling wavefronts. The wavefronts are extracted by

the modified MMF algorithm. This protection is ultra-fast asit can detect a fault within several micro seconds. The dead

zone dealing with low inception angle faults, which is themain shortcoming of all travelling wave based methods, isaddressed by the backup protection based on the rate ofchange of current. Simulation results using PSCAD/EMTDC

show that this protection strategy is able to rapidly andreliably detect the fault regardless of fault impedance, faultposition or fault inception angle.

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Enhanced Protection for Inverter Dominated