NewsletterVOLUME 1, NO. 4NOVEMBER 2007 EDITION

ElectroMagnetic Transients Program

VERSION 2.1 IS NOW AVAILABLE! EMTP-RV now supports the integration of DLLs, enabling EMTP-RV users to become developers of their own customized version of the program. The new DLL capabilities o�er unsurpassed potential for building user-de�ned models:1. The DLL is totally independent from the EMTP-RV code. You can use any

programming language capable of generating standard DLLs. Any number of DLLs can be used in a given design and use totally arbitrary names and locations on your computer.

2. There is no need to link with the EMTP-RV code. You just follow the DLL plug-in standards and specify in EMTPWorks.

3. The DLL plug-in standard for EMTP-RV is basically a set of speci�c methods that EMTP-RV will try to discover by interrogating the DLL.

4. You can create models that can interact intimately with EMTP-RV computation methods. This means that you can program “true-nonlinear” (iterative) models, models that become fully integrated without any time-step delay into the EMTP-RV main system of equations and control models for receiving and sending control signals from a black-box type device. The DLL can have control pins and power pins and any number of pins! It can use your own symbol and data input design.

5. Basically you can develop and use models as if you were an EMTP-RV developer!

6. The available capabilities will open the door to the development of advanced models in addition to providing an extremely powerful environment for research on new solution techniques and models. It is now possible to build advanced devices and test them within large scale practical networks assembled in EMTPWorks.

(continued...)

INSIDE:

Dr. Jean MahseredjianÉcole Polytechnique de Montréal

1155 Metcalfe Street, Suite 1120 Montreal, Quebec, Canada H3B 2V6

www.ceati.com • info@ceati.com

Phone +1 (514) 904-5546Fax +1 (514) 904-5038

A Word from the Editor

Georgia Johnston

EMTP-RV Sales O�ce at CEATI Email: emtp@ceatech.ca Tel.: 1-888-781-EMTP International Tel.: +1-514-904-5546

Welcome to the November issue of the EMTP-RV Newsletter!

This issue features articles from Electricité de France, Univer-sity of Bologna and Electric Power Research Institute (EPRI) and is dedicated to the demonstration of the advanced customization capabilities available in EMTP-RV, as the program is used to develop new tool layers and indepen-dent environments with GUI and models.

We’d also like to take the opportunity to share with you the success of our recent training seminar held in Montreal in September 2007 and have included some snapshots of the event on the back page.

As always, we are keen to hear your feedback, including your suggestions for future editions. Please do not hesitate to contact the EMTP-RV Sales O�ce with any comments.

SOFTWARE NEWS• Commercialization of Version 2.1 1• Version 2.1 is Now Available! 1• EMTP-RV Toolboxes Interest Group

Meeting – November 2007 2

TECHNICAL CORNER• An Intra-Day Scheduler of Distributed

Energy Resources for the Optimal VoltageControl in a Distribution Network in thePresence of Embedded Generation 2

• Transient Recovery Voltage Toolbox inEMTP-RV 10

• Lightning Surge Impact Evaluation Tool 15

SUPPORT• Training Success! 24• Upcoming Training Opportunities 24

COMMERCIALIZATION OF VERSION 2.1CEATI is pleased to announce the release of EMTP-RV 2.1 – now available to all new customers and those with a valid subscription to the EMTP RV Mainte-nance Program. • Current subscribers to the maintenance program will receive a new access code for 2.1 by email and can then proceed to the EMTP-RV Support website (www.emtpsupport.com) to download the latest release. If your subscription is up to date but you have not yet received your new access code, please contact the sales o�ce at CEATI. • To bring your maintenance subscription payments up to date, please contact the sales o�ce at CEATI for a quotation.

Software News

2

Another important feature in 2.1 is the ability to locate errors directly on the design page. Now when you get an error message, click on it and a device name appears with a dynamic link; click on the device name and it will show you where the device is located on the screen!

EMTP-RV TOOLBOXES INTEREST GROUP MEETING – NOVEMBER 2007 The inaugural meeting of the EMTP-RV Toolboxes Interest Group will be held in Montreal in November 2007. The objective of this group is to take the lead in developing add-ons, also known as toolboxes, for EMTP-RV. Open to all organizations interested in collaborating with other EMTP-RV users to expand the capabilities of the program for their specific needs, the group will be discussing projects in the areas of wind generation, power quality/harmonic analysis, lightning impact analysis and data compatibility. If your organization is interested in receiving more information about participating in this group, please contact the CEATI offices directly at 1-888-72-CEATI (international number +1-514-866-5377).

Technical Corner AN INTRA-DAY SCHEDULER OF DISTRIBUTED ENERGY RESOURCES FOR THE OPTIMAL VOLTAGE CONTROL IN A DISTRIBUTION NETWORK IN THE PRESENCE OF EMBEDDED GENERATION

Authors (left to right): C.A. Nucci, University of Bologna A. Borghetti, University of Bologna M. Bosetti, University of Bologna M. Paolone, University of Bologna

The coordinated operation and management of distribution networks in presence of high penetration of Distributed Energy Resources (DER) can be approached in several ways, varying from a fully decentralized approach to a centralized one. Both approaches have peculiar characteristics and may be more or less appropriate depending on the specific situations [1-4]. Assuming that all the DERs are owned by the same distribution system operator (DSO), the implementation of a coordinated integration of different DERs [5-8] suggests the adoption of a centralized approach. Such an approach, suitably integrated with local control systems to ensure system security during fast transient dynamics (e.g., due to random load or configuration changes), is aimed at optimizing the system operation during, for instance, slow load modification due to daily, weekly and seasonal variations. The optimization process is realized by means of an automatic Energy Resource Scheduler (ERS). The following considerations further support the adoption of an ERS that implements specific optimization functions:

• problems related to high variations in the network steady state voltage profile are often an obstacle for the integration of distributed generation (DG);

• the centralized ERS allows also for a higher DG penetration, in terms of number and size of accepted DERs, by providing generation adjustment actions during critical contingencies, particularly in islanded conditions;

• the current availability of commercial and dedicated measurement/communication instrumentation provides adequate support for achieving monitoring and improved state estimation of distribution networks;

It is not possible, however, to directly transfer centralized transmission network operation concepts to distribution systems because of their specific characteristics. In particular,

3

• typical distribution network topology (radial or weakly meshed) and the reduced number of real-time measurements require the adoption of state estimation techniques significantly different from those implemented in transmission Energy Management Systems (EMS);

• the contemporary presence in active distribution networks of renewable energy resources (RER), and loads characterized by reduced smoothing effect in time variation, makes the management of energy balance more complex;

• for the case of low voltage networks, the typical ratio between line resistances and reactances is higher than that of transmission networks.

This contribution describes an automatic ERS of the DER set points that, on the basis of measurements and short-term load and RERs production forecasts, updates the DER set points for the optimization of the network voltage profile every 15 minutes. The proposed scheduler has been implemented into a computer code based on a JavaScript link between MATLAB and EMTP-RV simulation environments. The presented results are obtained for the case of a network configuration adapted from the IEEE 34-node test distribution feeder. DESCRIPTION OF THE ALGORITHM The developed intra-day scheduler acts at 15-minute time intervals. It collects: (i) information on the network state in terms of DER set points and voltage profile and (ii) short term forecasts of both RES mean production and load requests. Then it calculates the three-phase power flows and, if some technical constraints are violated or if the voltage values are too far from the rated value Vset, it starts an optimization procedure that modifies the active and reactive outputs P and Q of the N selected DERs that participate in the network voltage control. The optimization objective is the minimization of: (i) the square norm of the voltage deviations, with respect to the rated value Vset, in the M selected network buses and (ii) the deviations of each j-th DER active power output, with respect to the corresponding value Pj,set, defined by a different scheduler that takes into account different economical constraints. This last objective is taken into account by means of a specific coefficient α as expressed by the following objective function.

( ) ( )1 1

22 2,... , ... 1 1

minN N

M N

i set j j setP P Q Q i jV V P Pα

= =

⎧ ⎫− + −⎨ ⎬

⎩ ⎭∑ ∑ (1)

The relationship between DER outputs and voltage deviations is not linear. The problem is addressed by using a constrained linear least-squares programming solver included in an iterative procedure. In particular, at each algorithm iteration k, the matrix of the so-called sensitivity coefficients is calculated by means of a power flow calculation. Each element of the matrix, namely Ki,jp and Ki,jq, represents the linear relationship between the voltage variation at bus i due to a small variation of the j-th controlled-DER active or reactive outputs. The constrained linear least-squares problem is formulated as

2minx

Cx d− (2)

The results of the power flow calculation at previous k-1 iteration allow both deviations of the M voltages iVΔ =Vi

k-1-Vset and of

the DER active power outputs jPΔ =Pjk-1-Pj,set to be determined. Therefore, we can write

111,1 1, 1,1 1,

,1 , ,1 ,

1 1

... ............ ...

... ...

... 0 0 ... 0...0 0 0 ... 0 ...

0 ... 0 ... 0

P P Q QN N

P P Q QNM M N M M N M

N N

VPK K K K

PK K K K VCx dQ P

Q P

α α

α α

⎡ ⎤Δ⎡ ⎤⎡ ⎤ Δ ⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥ Δ Δ⎢ ⎥− = +⎢ ⎥⎢ ⎥⎢ ⎥Δ⎢ ⎥⎢ ⎥ ⋅Δ⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥Δ⎢ ⎥⎢ ⎥⎣ ⎦ ⎣ ⎦ ⋅Δ⎢ ⎥⎣ ⎦

M M M M

O

(3)

where ΔP1… ΔPN and ΔQ1… ΔQN are the optimal variations of active and reactive power operating levels at iteration k.

4

The problem is completed by the linear constraints given by the generators’ capability limits and the security reserves requirements. Concerning the power flow calculations, the connection to the feeding network is considered as the slack bus. The limits on the P and Q variations of the controlled DERs take into account not only the unit production capabilities but also the convergence requirements of the iterative process. If the optimal variations are sufficiently small, the iterative process will converge to the optimal solution of (1). The iterative procedure is terminated when the difference between the objective function values calculated at two consecutive iterations is lower than a predefined small threshold. IMPLEMENTATION OF THE ALGORITHM The algorithm of the intra-day scheduler has been implemented by means of an interface between MATLAB and EMTP-RV. Such an interface, whose structure is illustrated in Figure 1, is realized within the JavaScript modeling programming environment that is part of the EMTP-RV [9-11]. For that purpose the MATLAB code, aimed at solving the constrained minimization problem described by (2) and (3), has been compiled as a COM (Component Object Model) object and included as an ActiveX (Active eXtension) control inside the developed JavaScript code. Moreover, specific JavaScript procedures have been also developed in order (i) to simplify the development of the EMTP-RV models relevant to new network configurations and (ii) to provide easy and direct access to all the EMTP-RV network models for their automatic set point updates and power flow output calculations.

Figure 1: Structure of the intra-day scheduler

An extensive analysis has been carried out to investigate the proposed optimization strategies. In particular, we show here the results obtained for the case of a test network adapted on the basis of the 34-node IEEE radial distribution test feeder [12]. The network structure is illustrated in Figure 2 and its data and details on implementation in EMTP-RV follow.

5

CP

+

800_

802

0.79

CP

+

802_

806

0.53

CP

+

808_

806

9.82

CP

+

812_

808

11.4

3C

P+

814_

812

9.06

CP +

810_808

1.78

CP

+

816_

850

0.09

CP +

816_818

0.52CP +

818_820

14.68CP +

TLM1

4.19

CP

+

824_

816

3.11

CP

+

826_

824

0.92

CP +

828_824

0.26

CP

+

830_

828

6.23

CP

+

854_

830

0.16

CP

+

856_

854

7.11

CP +

852_854

11.23CP +

858_832

1.49CP +

864_858

0.49

CP

+

834_

858

1.78

CP

+

860_

834

0.62

CP

+

836_

860

0.82

CP

+

840_

836

0.26

CP +

862_836

0.09CP +

838_862

1.48

CP +

842_834

0.09CP +

844_842

0.41CP +

846_844

1.11CP +

848_846

0.16

1 2YY1

150/24.9

LF

Phase:0

Slack: 150kVRMSLL/_0

LF1

LLoad_810

LLoad_822

LLo

ad_8

26

LLoad_864

LLoad_848L

Load

_856

LLoad_840

V_reg_1

V_reg_2

GWIND

1 2YD_3

24.9/0.69

GPV

1 2YD_4

24.9/0.69

GGen_818

1 2YD_5

24.9/6

LLoad_800

LLoad_838

1 2YD_6

24.9/6

GGen_802

1 2YD_2

24.9/6

GGen_856

816

854

828

862

832

820

846

860

830

842

858

836

834

864

840

848

812

810

850

806

824

814

852

826

844

818

800

838

808

822

802

856

Figure 2: Test network implemented in EMTP-RV, based on the IEEE 34-node test distribution feeder

6

The IEEE 34-node test feeder is composed of branches characterized by different conductor configurations. In order to simplify the simulation results, the following assumptions have been made: (i) all the branches of the network are composed of overhead lines using conductor configuration “ID #500” shown in figure 1 of [12], where the phase sequence a, b and c refers to the line conductors from left to right; (ii) the network load locations correspond to the line terminations; and (iii) the DERs are connected to the network via distribution power transformers (see Figure 2). All the transformers are represented by means of a 50 Hz standard model and the relevant parameters are reported in Table 1.

Table 1: DER power transformer data

Transformer name Rated power (MVA)

Rated dividing ratio (kV/kV)

Short circuit voltage (%)

Tr_1 25 150/24.9 9 V_reg_1; V_reg_2 15 24.9/24.9 8 Tr_2; Tr_3;Tr_5 10 24.9/6 6

Tr_4 2 24.9/0.69 6 Tr_6 5 24.9/0.69 6

Three different dispatchable electric power production units are considered to be connected to the network in correspondence of nodes 802, 818 and 856 (see Figure 2). The relevant power limits and costs are reported in Table 1. Two non-dispatchable electric power production units are also connected to the network, namely a photovoltaic (PV) array for a total peak power equal to 50 kW connected to node 844 and a 750 kW wind generator connected to node 826.

Table 2: Model parameters for the DERs considered to be dispatchable

Dispatchable DER Symbol Rated output (kW) Pmin (kW)

CHP gas-turbine Gen_818 1800 720 Diesel Gen_856 2000 800

Gas turbine Gen_802 4000 1600 Figure 3 illustrates a typical day-ahead scheduling solution over a one-day horizon with 15-minute time intervals. Such a solution has been adopted as the input for the intra-day scheduler.

Figure 3: Electrical load balance adopted as input for the intra-day scheduler

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93

DERs electrical outputs (kW)

15-minute periods

load gas turbineCHP dieselgrid renewable

7

In order to provide an example of the effectiveness of the intra-day scheduler action, we have first considered a high load period from the one-day horizon shown in Figure 3, namely the 39th time period. Figure 4 shows the voltage amplitudes at the various network buses for the different operating conditions using the dispatchable DERs at this particular period, namely, (i) without the available dispatchable DERs, (ii) with only Gen-856, and (iii) with all the DERs available. The voltage profiles in the presence of dispatchable DERs, namely the operating conditions (ii) and (iii), are obtained for two different α-coefficient values from objective function (1), i.e., 0 and 50. Table 3 reports the load requests and the actual renewable production levels during this period, with their corresponding power factor pf values.

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

800

802

808

810

816

818

822

824

826

834

836

838

840

844

848

854

856

858

864

Volta

ge (p

.u.)

Bus

with all DERs (α=0)with all DERs (α=50)with only Gen-856 (α=0)with only Gen-856 (α=50)without dispatchable DERs

Figure 4: Intra-day scheduling solution:

Phase-a voltage profiles at various network buses for different operating conditions using dispatchable DERs at period 39 of Figure 3 (namely, without available dispatchable DERs, with only Gen-856 and with

all the DERs available) for two different α-coefficient values of the scheduler objective function (0 and 50).

Table 3: Loads and RER production levels, with the relevant power factor pf, in the 39th period of Figure 3 Symbol Power level (kW) Pf Symbol Power level (kW) Pf

Load_800 3002.8 0.9 Load_848 200 0.95 Load_810 1290.7 0.9 Load_856 2000 0.95 Load_822 610 0.9 Load_864 141 0.9 Load_826 580 0.95 RERs Load_838 120 0.9 PV 15.98 1 Load_840 160 0.95 Wind 300 0.9

For the case of α=0, only the voltage deviations are minimized; for the case of α equal to a large number (namely 50), the output of each DER j is equal to the corresponding value Pj,set defined by the day-ahead scheduler. The Pset values in this period are 1800 kW (Gen-818), 1647 kW (Gen-856), and 4000 kW (Gen-802). In order to investigate the overall behavior of the intra-day scheduler, it has been applied for the first 15-minute interval of each of the 24 hours shown in Figure 3. In particular, Figure 5 shows the mean absolute phase voltage deviations and Figure 6, the DER outputs’ absolute deviations. The results presented in these figures refer to two different values of the α-coefficient, namely 0 and 1. Figure 5 shows that even where α=1, the mean absolute voltage deviation values are slightly worse than those obtained when α=0 (in particular when limited to a few tens of volts), whilst, as shown by Figure 6a, the DER active power outputs’ absolute deviations can be very different from those provided by the day-ahead scheduler.

8

0

5

10

15

20

25

30

35

40

45

50

55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Mea

n ab

solu

te v

olta

ge d

evia

tions

(V)

hours

phase a - α=1 phase b - α=1 phase c - α=1phase a - α=0 phase b - α=0 phase c - α=0

Figure 5: Intra-day scheduling solution: mean absolute phase voltage deviations for two α-coefficient values: α = 0 and α = 1. Voltage deviation value refers to the maximum phase-to-ground voltage.

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2

DER

s ou

tput

dev

iatio

ns (k

W)

hours

GEN-802GEN-856GEN-818

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2

DER

s ou

tput

dev

iatio

ns (k

W)

hours

GEN-856GEN-818GEN-802

a) b)

Figure 6: Intra-day scheduling solution: DER active power output deviations with respect to Pset for two different α-coefficient values: a) α = 0, b) α = 1.

CONCLUSIONS The intra-day scheduler described in the paper appears to be a useful tool for the correct management of a distribution network with high penetration of DERs. The preliminary simulation tests have indeed shown that it is able to tackle technical objectives with computational time compatible for online applications. The network representation within the EMTP-RV environment allows one to incorporate distributed resources different from those considered connected in the analyzed test distribution network, such as on-load tap changers and power electronic components (SVC, DFACTS, etc.). Further developments of the scheduler, however, will also need to investigate the opportunity to introduce the load flow equations into a non-linear mathematical programming model in order to improve computational performance and to obtain a more compact code.

9

ACKNOWLEDGEMENTS A preliminary version of the scheduler has been developed in the framework of the Italian Electrical Power System Research Program in collaboration with CESI and University of Genova. REFERENCES [1] J.H. Choi and J.C. Kim, “Advanced voltage regulation method of power distribution systems interconnected with

dispersed storage and generation systems”, IEEE Trans. on PWRD, vol.16, no.2, April 2001, pp. 329-334.

[2] S. Repo, H. Laaksonen, P. Järventausta, O. Huhtala, M. Mickelsson, “A case study of voltage rise problem due to a large amount of distributed generation on a weak distribution network”, in Proc. of. 2003 IEEE Bologna PowerTech Conference, vol.4.

[3] N.D. Hatziargyriou, A. Dimeas, A.G. Tsikalakis, J.A. Pecas Lopes, G. Kariniotakis, J. Oyarzabal, “Management of Microgrids in Market Environment”, Proc. of 2005 International Conference on Future Power Systems, 16-18 Nov. 2005.

[4] A. L. Dimeas, N. D. Hatziargyriou, “Operation of a Multiagent System for Microgrid Control”, IEEE Transactions on Power Systems, vol. 20, no. 3, August 2005.

[5] R. Lasseter, A. Akhil, C. Marnay, J. Stevens, J. Dagle, R. Guttromson, A. S. Meliopoulous, R. Yinger, J. Eto, “White Paper on Integration of Distributed Energy Resources—The MicroGrid Concept”. Available at: http://certs.lbl.gov/pdf/50829-app.pdf, 2002.

[6] European Research Project MicroGrids. Available at: http://microgrids.power.ece.ntua.gr/.

[7] A. Bertani, A. Borghetti, C. Bossi, L. De Biase, O. Lamquet, S. Massucco, A. Morini, C.A. Nucci, M. Paolone, E. Quaia, F. Silvestro, “Management of Low Voltage Grids with High Penetration of Distributed Generation: concepts, implementations and experiments”, Proc. of CIGRE general session, Paris, 2006.

[8] A. Borghetti, M. Bosetti, C. Bossi, S. Massucco, E. Micolano, A. Morini, C.A. Nucci, M. Paolone, F. Silvestro, “An Energy Resource Scheduler Implemented in the Automatic Management System of a Microgrid Test Facility”, Proc. Int. Conf. on Clean Electrical Power, Capri, Italy, 21-23 May 2007.

[9] J. Mahseredjian, S. Lefebvre and X.-D. Do, “A new method for time-domain modelling of nonlinear circuits in large linear networks”, Proc. of 11th Power Systems Computation Conference PSCC, August 1993.

[10] J. Mahseredjian, L. Dubé, L. Gérin-Lajoie, “New Advances in the Simulation of Transients with EMTP: Computation and Visualization Techniques”, Proc. of 7th International Conference on Modeling and Simulation of Electric Machines, Converters and Systems, Montreal, August 2002.

[11] J. Mahseredjian, S. Dennetière, L. Dubé, B. Khodabakhchian, “On a new approach for the simulation of transients in power systems” Proc. of the International Conference on Power Systems Transients IPST’2005, Montreal, June 2005.

[12] IEEE Distribution Planning Working Group, “Radial Distribution Test Feeders”, IEEE Trans. on Power Systems vol. 6, no. 3, August 1991, pp. 975-985.

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TRANSIENT RECOVERY VOLTAGE TOOLBOX IN EMTP-RV

Author: S. Dennetière, Electricité de France (EDF)

INTRODUCTION The continuous growth in the consumption of electrical energy and an increased interconnection of the transmission networks lead RTE (French electricity transmission system operator) to install large shunt capacitor banks and series reactors in substations. Shunt capacitors are used for reactive power compensation and series reactors for the limitation of fault currents. When an installation of a shunt capacitor bank or a series reactor is planned, a Transient Recovery Voltage (TRV) analysis is carried out in order to re-evaluate the breaking capability of existing circuit breakers under the new system configuration. The accuracy of the TRV studies requires careful modeling of the system components (lines, sources, transformers…) in EMTP-RV. As these studies are more and more frequent, EDF R&D has developed a special toolbox in EMTP-RV to make these evaluations more practical. THE TRV TOOLBOX The TRV toolbox developed in EMTP-RV by EDF R&D is designed to provide a specialized interface for convenient evaluation of the breaking capability with respect to the IEC 62271-100 standards [1]. The TRV library contains elements frequently used in TRV studies. The user specifies parameters in simplified forms. These elements are:

• Sources and short circuit impedances: the voltage and the RL values are calculated from 1-phase and 3-phase short circuit data.

• Overhead lines: transmission lines are represented by non-transposed frequency dependent line models. • Circuit-breakers: circuit breakers are modeled as ideal switches. Re-ignition is modeled by a second switch placed

in parallel. The specification of the breaker rated voltage will result in the comparison of the TRV with the IEC TRV envelopes.

• Faults: faults are modeled either as solid connections to ground or as some impedance to the ground. The user can choose the faulted phases and the type of fault (floating or grounded).

• Steady-state current, voltage and power meters: these meters are used to display steady-state values during the fault.

11

Figure 1: The "Fault" Element Figure 1 presents the “Fault” element with its simplified form and its internal circuit. A script updates the internal circuit in accordance with the instructions filled in the form. The breaker’s rated voltage is specified in the form shown in Figure 2. For each phase, the user has to choose the standard TRV envelope that will be superimposed on the simulated TRV.

Figure 2: The "Breaker" element and the IEC standards tab

p

+Ra

1

+Rb

1

+Rc

1

+SWaGND

+SWbGND

+SWcGND

+ SWab

+ SWbc

+

SWac

+-1|1E15|0

b

a

c

Form of the "Fault" element

Internal circuit of the "Fault" element

F

Fault Grounded faultImpedance=1

Form of the "Breaker" element & IEC standards tab

+

breaker1

550kV

12

The TRV toolbox adds a button in the toolbar as presented in Figure 3. Simulation options can be set from this menu. The simulation time is defined by the user and the time step can be automatically determined from the time response of the traveling waves of the shortest transmission line. Steady-state results are displayed on the drawn circuit with the usage of current, voltage and power meters. These results can also be displayed in a per-unit format.

Figure 3: The "TRV" menu A SIMPLE TEST CASE The test case presented in Figure 4 deals with the installation of a 30 Mvar shunt capacitor bank (SCB) at the substation. The high capacitance of a SCB influences the transient phenomena occurring in the vicinity of the substation during switching or faults. EMTP-RV results show how the TRV is modified when the 30 Mvar SCB is added to the network (Figure 5).

SHUNT CAPACITOR BANK

3-phase fault

1 km 30 km

2

3

1

225/64.5/10

+

72.5kV

F

Fault

+RL2

P Q

Load2

10MW1MVAR

+

AC3

245kVRMSLL /_0 P Q

Load1

50MW10MVAR

+

24uF

Liaison 225kV+

20km

liaison1Liaison 90kV+

1km

23

1

225/64.5/10

Liai

son

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V+

40km

+

+

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Liaison 90kV+

30km

Liaison 90kV+

30km

liaison5

Iia=7.91e+3ib=7.95e+3ic=7.92e+3

CT1

1 km

+

72.5kV

F

Fault

Liaison 90kV+

1km

Iia=7.91e+3ib=7.95e+3ic=7.92e+3

CT1

Steady-state results

13

Figure 4: Shunt capacitor bank insertion Saturation and stray capacitances to earth (4nF) of autotransformers are taken into account in transformer models. Once the network has been modified to reflect the configuration of the system to be studied, the TRV toolbox:

• sets the time-step, • starts the simulation with steady-state initialization, • displays steady-state results as shown in Figure 4, • displays TRV waveforms superimposed with the IEC standard envelop as shown in Figure 5.

0.012 0.013 0.014 0.015 0.016 0.017 0.018

0

2

4

6

8

10

12

14

x 104

time (s)

Vol

tage

(V)

IEC standard TRVTRV with SBCTRV without SBC

Figure 5: TRV of the CB, which clears a 3-phase fault in the vicinity of the substation As explained in [2] a damping circuit for SCB switching was developed at EDF in order to attenuate TRVs not covered by standards. The damping circuit consists of a reactor in parallel with a resistor which is in series with a metal-oxide arrester.

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0.012 0.013 0.014 0.015 0.016 0.017 0.018 0.019

-4

-2

0

2

4

6

8

10

12

14

16

x 104

time(s)

Volta

ge (V

)

TRV with damping circuitIEC standard TRV TRV without damping circuit

Figure 6: TRV of the CB with and without damping circuit The TRV toolbox generates the result given in Figure 6. The user can quickly determine the effect of adding the damping circuit on the TRV. CONCLUSION The TRV toolbox developed in EMTP-RV by EDF R&D is designed to provide a specialized interface for convenient evaluation of the breaking capability regarding the IEC 62271-100 standards. Other expert toolboxes are under development for:

• lightning analysis and design, • electromagnetic compatibility analysis, • short circuit calculations.

REFERENCES [1] IEC standard 62271-100, “High Voltage switchgear and control gear”, Part 100: “High-Voltage alternating-current circuit

breakers”

[2] A. Sabot, C. Morin, C. Guillaume, A. Pons, J. P. Taisne, G. Lo. Pizzo, H. U. Morf, “A unique multipurpose damping circuit for shunt capacitor bank switching”, IEEE Transactions on Power Delivery, Vol. 8., No. 3, July 1993

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LIGHTNING SURGE IMPACT EVALUATION TOOL

Authors (left to right): Anish Gaikwad, EPRI Mark McGranaghan, EPRI Harish Sharma, EPRI

Lightning-induced transients can be an important cause of customer equipment damage, as well as one of the most important causes of utility distribution transformer failures. There are many factors that affect the potential for problems associated with lightning transients – distribution design, grounding practices, distribution transformer design, surge protection at the transformer, secondary circuit configurations and lengths, customer grounding, customer circuits and loads, and customer surge protection. All these variables make the analysis of potential problems quite complex. This article describes a simplified module that has been developed using EMTP-RV for the EPRI Power Quality program that allows convenient analysis of lightning-induced transients, protection options, and potential impacts on both the utility and the customer. Use of the module is illustrated with an example case study. LOW-SIDE SURGE CONCERNS It is common practice for distribution systems exposed to lightning to protect transformers with primary side surge arresters (see Figure 1). The arresters are located as close as possible to the transformer to prevent excessive transient voltages across the transformer primary winding. However, these arresters do not provide complete transformer protection and they do not protect customer loads from transients induced by lightning strokes to the distribution line.

Figure 1: Typical surge arrester protection of distribution transformers

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Transients coupled to the secondary circuit when there is a lightning strike to the primary distribution line are known as “low-side current surges” because it is the lightning current surge in the secondary circuit that causes high transient voltages. An example circuit illustrating this phenomenon is shown in Figure 2. The lightning strikes the primary line and the current is discharged through the primary arrester to the pole ground lead. Some of this current will flow towards the load ground -- the current division is determined by the ratio of the utility and customer grounds. This current will also couple transient currents into the secondary phase conductors, forcing a transient current through the transformer secondary windings and the customer loads (or, hopefully, surge suppressors). These transients can cause the failure of the distribution transformer and/or customer equipment damage.

PRIMARYARRESTER

DISTRIBUTIONTRANSFORMER

+ -

CURRENT SPLITSBETWEEN UTILITY ANDCUSTOMER

UTILITY GROUNDCUSTOMERGROUND

SERVICE CABLEVOLTAGE DROP

LIGHTNINGSTROKEDISCHARGED THROUGHPRIMARY ARRESTER

Figure 2: Current division between utility and customer grounds DESCRIPTION OF LIGHTNING SURGE IMPACT MODULE The Lightning Surge Impact Module can be used to perform lightning surge analysis of single-phase residential and three-phase commercial distribution systems. The module has template circuits for single- and three-phase which can be used by experienced or novice EMTP-RV users with equal ease. Advanced users can modify or build new circuits based on the templates. Salient features of the modules are as follows:

• Single-phase transformers can be modeled as interlaced or non-interlaced type. • For three-phase circuits, different transformer winding configurations (delta-delta, wye grounded-wye grounded etc.)

can be simulated. • A library of commonly used conductors for phase and neutral wires is provided in the module. The user also has a

choice to give sequence impedance data if conductor geometry is not available. This information is used to model incoming line and service entrance drop.

• A library of arresters and spark gaps is provided. Users can model different protection devices at transformer primary, transformer secondary, service entrance, and load location.

• Utility and building grounding can be modeled by specifying grounding resistance, lead wire type (from the conductor library) and length.

• Load can be specified in kilowatts. In addition, wiring inside the facility can be modeled by specifying conductor type and length.

• Multiple service drops and loads can be modeled in parallel. • Three stroke locations can be chosen (transformer primary, transformer secondary, service drop). • The characteristics of the lightning surge can be selected by the user.

During the design phase, special care was taken to ensure the user friendliness of the tool. EMTP-RV has a library of functions that can be used to develop advanced GUI using JavaScript and HTML languages. These functions were used to develop advanced GUIs for non-standard components in the tool. For example, GUI for Service Drop is shown in Figure 3. Standard

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Windows GUI controls like text boxes, radio buttons, check-boxes, drop-down lists, etc., can be used for entering the system data. This approach is an improvement on the script-based masks for individual components as it is more user-friendly and less prone to data entry errors. The main user interface for the module is shown in Figure 4. It allows users to open default circuits for single-phase and three-phase applications. The interface also allows users to make a copy of an existing circuit and rename it as desired. A screen shot of the single phase circuit template is shown in Figure 5. Each block represents a component (for example transformer, building, primary protection etc.). A template for plotting voltages and currents at different locations was created in ScopeView. A sample output of the module showing phase-ground voltages at different locations (transformer primary, transformer secondary, service entrance and load terminals inside the house) resulting from a primary lightning stroke is shown in Figure 6.

Figure 3: GUI for “Service Drop” component

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Figure 4: Main interface of LSIM

Figure 5: Single-phase template

LSIM Version 1.0

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Figure 6: Module example output CASE STUDY: SINGLE PHASE RESIDENTIAL CUSTOMER This section illustrates the use of the lightning surge impact evaluation module with an example case study. For the base case, it is assumed that the primary side of the transformer having non-interlaced winding has arrester protection. In addition, a 6 kV gap is protecting the service entrance. The peak transient voltages at three measurement locations are shown in Table 1. The transients illustrate that transient overvoltages caused by secondary surges can be quite high without surge protection. The transient voltage across the transformer secondary winding is of particular concern and the meter gap is not likely to protect loads in customer facilities (many electronic loads can be damaged by transients higher than 2 kV).

Table 1: Base case results

Peak Transient Voltage (kV) Transformer Type Secondary Service Entrance Equipment

Non-Interlaced 142.23 6.03 3.43 The following sections illustrate the effect of different circuit parameters and surge protection options. IMPACT OF INTERLACED WINDING Interlaced windings in the transformer significantly reduce the inductance of the winding seen by the surge current, thus reducing the surge voltage across the secondary winding. The impact of interlacing the windings on the resultant surge voltages is shown in Table 2 and Figure 7. The transient voltage across the secondary winding is considerably reduced. The transient at the customer service entrance is still being limited by the meter gap.

Table 2: Results for transformer with interlaced windings

Peak Transient Voltage (kV) Transformer Type Secondary Service Entrance Equipment

Non-Interlaced 142.23 6.03 3.43 Interlaced 24.38 6.01 2.16

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Transformer Secondary Line-Neutral Voltage

-160-140-120-100-80-60-40-20

020

0 10 20 30 40 50

Time (uS)

Volta

ge (k

V)

Non-interlaced Interlaced

Figure 7: Impact of interlaced winding on transformer secondary IMPACT OF SERVICE DROP CABLE The effect of the type of service drop cable on the transient voltage levels is tabulated in Table 3. It is seen that the use of triplex cable can also reduce the transient voltages in the secondary circuit but the transient magnitudes could easily still cause failure of the secondary winding.

Table 3: Effect of the service drop

Peak Transient Voltage (kV) Service Cable Secondary Service Entrance Equipment

Vertically-spaced open 142.23 6.03 3.43 Triplex 48.54 6.01 4.94

IMPACT OF CUSTOMER LOAD The effect of the customer load characteristics on the transient voltage levels is tabulated in Table 4. Only the resistive portion of the load needs to be modeled as the inductive loads are effectively an open circuit to the high frequency transients caused by the lightning strike. The load is effectively being shorted out by the meter gap when it operates. As the voltage is rising, the load affects the peak transient magnitude. It can be seen that the higher the load, the lower the peak voltage that the load is exposed to. However, all of these transient voltages are high enough to cause equipment failures.

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Table 4: Effect of the load

Peak Transient Voltage (kV)

Load Secondary Service Entrance Equipment 1 kW (Base Case) 142.23 6.03 3.43

5 kW 142.23 6.00 2.54 0.5 kW 142.23 6.07 3.93

IMPACT OF UTILITY GROUNDING The effect of the utility grounding resistance on the transient voltages is shown in Table 5. It is seen that better grounding (lower resistance) at the utility pole (in relation to the grounding at the customer service) reduces the transient voltages due to the reduction in current flowing into the transformer secondary winding.

Table 5: Effect of the grounding

Peak Transient Voltage (kV)

Utility Grounding Secondary Service Entrance Equipment 50 ohm (Base Case) 142.23 6.03 3.43

10 ohm 83.63 6.00 4.84 PROTECTION AT THE CUSTOMER LOADS OR THE SERVICE ENTRANCE Use of MOV surge suppressors at individual loads in the customer facility results in reduced surge voltages at the customer loads (Table 6). The meter gap is still operating, absorbing much of the energy in the transient for this case. Cases with lower surge currents could cause operation of surge suppressors at the load without operation of the gap at the meter.

Table 6: MOV across the load

Peak Transient Voltage (kV) Arrester At Load Secondary Service Entrance Equipment

None 142.23 6.03 3.43 650 V MOV 142.23 6.01 1.59

The effect of using MOV protection at service entrance is shown in Table 7. It is seen that this arrangement provides protection for downstream loads in the house. Also, it may be noted that service entrance arrester will provide protection for the whole house only as long as no parts of the secondary circuit can come into contact with a ground other than service entrance ground.

Table 7: MOV at service entrance

Peak Transient Voltage (kV) Arrester At Service

Entrance Secondary Service Entrance Equipment

None 142.23 6.03 3.43 650 V MOV 141.08 2.19 2.16

PROTECTION AT THE DISTRIBUTION TRANSFORMER The only way to provide the adequate protection to the distribution transformer is by placing MOV across the secondary winding (see Table 8). The resultant voltage transients in the secondary circuit at the three measurement locations are shown in Figure 8. Protection at the transformer alone would not protect the downstream load devices. But, the use of transformer protection in conjunction with the protection at service entrance will provide protection to the entire secondary circuit.

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Table 8: MOV across transformer secondary

Peak Transient Voltage (kV) Arrester At Xfmr Secondary Secondary Service Entrance Equipment

None 142.23 6.03 3.43 650 V MOV 2.72 6.01 2.07

Figure 8: Transients corresponding to arrester at transformer secondary and spark gap at service entrance

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CONCLUSIONS A user-friendly tool has been developed that can be used to study the phenomenon of “low-side surges”. The tool is simple to use and has an easy-to-use GUI. The default case templates have a wide repertoire of data for conductor types, protection devices, etc., so that the user is not burdened with supplying/finding the input information. The tool allows evaluation of various solutions for protection and can be set up for a customer-specific case study or can be used as a generic template to study the effect of a number of parameters, including transformer design, type of service drop to the customer, loading, grounding, and surge arrester application. FUTURE WORK EPRI previously developed a capacitor switching evaluation tool using EMTP-RV as the platform and the same had been featured in the last year’s newsletter. Through EPRI’s program on Power Quality, EPRI plans to continue to use EMTP-RV for designing similar simulators to study problems related to power quality and other transient phenomenon. During 2007 EPRI is working on developing the following modules:

• Motor Starting o Simulate starting of different types of induction motors (based on NEMA types) o Simulate different starting methods to reduce starting inrush

• Flicker Analysis

o Develop a flicker meter in EMTP-RV that can calculate instantaneous as well as Pst values o Develop a test case to measure flicker due to an arc furnace

ACKNOWLEDGEMENTS The authors would like to acknowledge Roger Dugan at EPRI for his technical inputs during the implementation of this module.

VOLUME 1, NO. 4NOVEMBER 2007 EDITION

ElectroMagnetic Transients Program

1155 Metcalfe Street, Suite 1120 Montreal, Quebec, Canada H3B 2V6

www.ceati.com • info@ceati.com

Phone +1 (514) 904-5546Fax +1 (514) 904-5038

Support

TRAINING SUCCESS!

CEATI is pleased to report the success of the recent training seminar – the Simulation and Analysis of Power System Transients using EMTP-RV – held in Montreal in September 2007. The event brought together engineers from North and South America, Europe and Asia and was lead by Dr. Jean Mahseredjian, creator and main developer of EMTP-RV.

We would like to take this opportunity to thank all those who contributed to event, in particular the instructors – Dr. Jean Mahseredjian, Mr. Bahram Khoda-bakhchian P.Eng., Dr. Vijay Sood – and extend our congratulations to the students for their successful completion of the course!

Upcoming Training OpportunitiesThe EMTP-RV training schedule for 2008 will be available shortly and will tentatively include courses in North America, Europe, India and Australia. For updated information on training, please visit http://www.emtp.com/services/seminars.html.

On-demand training with our team of power systems experts at your location is available to organizations with specialized needs. Contact the EMTP-RV Sales O�ce at CEATI if you are interested in learning about customized training opportunities.