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ACCURATE REAL-TIME DISTRIBUTION POWER FLOW MODELING

SOLUTION FOR ENELVEN

I. Roytelman, A.Ilo P. Parra, H. Socorro, L. Rodriguez, T. Romero R. Cespedes

SIEMENS ENELVEN KEMA Consulting

Introduction

ENELVEN is a utility company responsible for the generation, transmission and distribution of

electric energy in Zulia State, Venezuela. ENELVEN has more than 600,000 users anddistributes approximately 1,700 MW of power. Due to the quality of service requirements and

the need to improve the operation efficiency (power and blackout losses were approximately

24% in the year 2001), ENELVEN started an ambitious plan to manage and automate theirextensive distribution power system.

The ENELVEN distribution system contains an excess of 300 distribution feeders at nominalvoltages of 34.5, 23.9, 13.8, 8.3 and 6.9 kV. ENELVENs distribution system includes eight

types of distribution transformer connections, some of which are not very common. The

uncommon transformer connections are not only wye grounded / delta and open wyegrounded / open delta banked transformers with different capacity sizes in the legs, but also

transformers connected in delta / delta, open delta / open delta and delta / wye grounded

configurations as well as single-phase transformers connected phase-to-phase. Accuratesimulation of the loading for each banked transformer leg, in said connections, was required as a

part of the correct power flow solution.

In order to achieve the project goals, several steps were taken including the development of a

corporate GIS-based utility database and the implementation of a Distribution Management

System (DMS). The main tool used in the network analysis portion of the DMS applications isthe real-time multiphase Distribution Systems Power Flow (DSPF).

The distribution power flow solution is used for two main purposes:

a) Check loading and voltage constraints - quality service requirements.

b) Power loss calculation and identification of the lines and transformers where losses

are the highest - operation efficiency.

This paper presents an overview of the adopted solution approach with emphasis on theextensive transformers and loads modeling developed as a part of DSPF for ENELVENs

distribution system. It also shows some uncommon DSPF results from the ENELVEN

distribution feeders which prove a real need for such extensive modeling.


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I. Project objectives

To supply the electricity service in its area, ENELVEN and its sister company ENELCO, part ofthe same holding, have distribution networks as described hereinafter:

Table 1.1a ENELVEN Distribution System

Type Voltage (kV) Distribution

feeders

Length (km)

Urban 24 77 1,684.2

Urban 8 47 256.3

Rural NE 6.9 2 55.4

Rural NW 24 24 2,854.3

Rural SW 24 22 3,671.2

Total 172 8821.4

Table 1.1b ENELCO Distribution System

Voltage (kV) Distribution feeders Length (km)

13.8 104 4220

34,5 19 1550

Total 123 5770

Starting in 1997 when an overall strategic plan was defined by ENELVEN with the assistance of

KEMA Consulting for improving the Distribution system operation, several steps were taken

with company goals which received the total support of ENELVEN management.

Since project inception, ENELVEN has defined general and specific project objectives. Some ofthem are of qualitative nature, among which the most important are:

General Objectives:

! Improve quality of service.

! Make management of the distribution system more efficient under normal

operating conditions.

! Respond more adequately to the service outages.

! Maintain the technical losses at a minimum level.

! Improve the distribution system operation security and reliability.

! Efficient substation management.


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! Personalize customer service from trouble call to service restoration.

! Detect and follow-up the evolution of non-technical losses.

Specific Objectives:

! Reduce the time dedicated to outage repairs.

! Reduce the service restoration times.

! Reduce the number of users affected by outages and service suspensions.

! Reduce the investment costs by more efficient use of the system equipment.

! Reduce operation and maintenance costs.

! Reduce non-technical losses.

! Increase customer satisfaction.

! Generate competitive advantages by improved customer service.

! Improve distribution system operation planning and analysis.

! Increase the overall company benefit to cost ratio related with distribution

operation.These objectives have been maintained as short-, medium- and long-term objectives and have

been attained in accordance with the project phase development.

II. Distribution Management System (DMS)

The information system for Distribution Management includes a hardware and software

infrastructure for the distribution network operation and control (based on existing SCADAsystems) and information exchange equipment between the DMS and other ENELVEN and

ENELCO systems.

The system comprises the following:

1) A system located at a control center with SCADA and DMS functions sharing the

same user interface.

2) A complete model of the distribution network (Distribution System Operation

Model, DSOM), which maintains current conditions of ENELVEN/ENELCOdistribution systems for the support of analysis and study functions.

3) A Trouble Call System (TCS) with corresponding interfaces to the DMS and other

systems.

4) Interfaces with other ENELVEN/ENELCO systems as indicated in Figure 1.


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Figure 1: DMS Main components and Interfaces

As shown in Figure 1, some applications are part of the DMS including: Outage Management,

Switching Order Procedure, Crew Management, Distribution Power Flow, etc. All are supported

by a user interface and other support functions together with a Historical Management System.The remainder of this paper is dedicated to the Distribution System Power Flow (DSPF) and its

particular characteristics as required for the ENELVEN solution.

III. Accurate Distribution Power Flow, Why?

a) General Consideration

DSPF is used as a routine for monitoring the distribution feeder loading and voltages. It is also a

part of the application known as Volt-Var Control, which task is to enhance the use of the

reactive power. This last application is normally among the top of those DMS functions that

produce direct benefits to the company showing high benefit to cost ratio.

For verifying operation constraints, (currents and voltages) it was not required until recently tohave an accurate transformer modeling. The main reason for this is an absence of information on

exact values of the loads connected to these transformers. American type distribution systems, towhich ENELVENs is similar, have a large number (hundreds) of relatively small rating single-phase and three-phase transformers (mainly in the 10 100 kVA range) connected to the same

feeder. Small distribution transformers normally do not have any measurements. DSPF used

mainly for planning, not for real-time, had almost no sources to get additional load information.

In addition, currents were checked for overload conditions on the main feeders only and voltageswere verified at the primary voltage level side.

AM

FM

/

GIS

RTUs

Distribution

Automation

IEDs

TroubleCallSystem

TroubleCallSystem

D S O MD S O M(Tools + Methods)(Tools + Methods)

UserInterface

UserInterface

FaultLocation,

Isolationand

FaultLocation,

Isolationand

ServiceRestoration

ServiceRestoration

HistoricalInformationSystem


Database

Distribution Management System

CrewManagement

CrewManagement

OutageManagementSystem


SwitchingOrderManagement


DistributionPowerFlow


SupportFunctions

SupportFunctions

External Systems

A P I sA P I s

SCADA

CIS

ERP

AM

FM

/

GIS

RTUs

Distribution

Automation

IEDs

TroubleCallSystem

TroubleCallSystem

D S O MD S O M(Tools + Methods)(Tools + Methods)

UserInterface

UserInterface

FaultLocation,

Isolationand

FaultLocation,

Isolationand

ServiceRestoration

ServiceRestoration



Database

Distribution Management System

CrewManagement

CrewManagement







SupportFunctions

SupportFunctions

External Systems

A P I sA P I s

SCADA

CIS

ERP


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It is also obvious that an accurate three-phase DSPF requires a large amount of information on

the network topology and phase connections, and electrical parameters of the distribution lines,

transformers, loads and capacitors.

All mentioned reasons historically developed a very approximate approach to the power flow

calculations used in distribution systems. In some cases, positive sequence power flow is used tocalculate only main three-phase feeders with loads lumped together into large equivalents. In

other cases, power flow is multiphase and loads are presented individually. However

distribution transformers are not included in the model loads are connected directly to theprimary (medium) voltage network. This approach is justified by the fact that very often most

single-phase distribution transformers are connected phase to neutral wire (ground) and three-

phase transformers wye grounded/wye grounded. In this case, transformer phase loading at the

primary side differs from the loading at the secondary side by power loss value only. Thisobservation, together with the fact that mutual impedance is large, makes it possible to estimate,

although with a certain error, feeder loading for each phase based on the knowledge of the

transformers installed capacity per phase.

b) ENELVEN Specific Distribution System Characteristics

The ENELVEN Distribution System widely uses standard for American networks transformers

connected in wye/wye and phase to neutral wire. In addition, ENELVEN also utilizes a varietyof different transformer connections including wye grounded/delta and open wye

grounded/open delta banked transformers with different capacity sizes in the bank legs,

transformers connected in delta/delta, open delta/open delta and delta/wye groundedconfigurations, as well as single-phase transformers connected phase-to-phase (Table 2). For all

these connections, it is absolutely impossible to predict primary transformer side phase loading

based on the knowledge of secondary side load, as is possible for the aforementioned phase-to-ground and wye/wye transformer connections [1].


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Table 3b. The same for 50/100/50 kVA transformer bank

Phase P kWwye/delta

P kW wye/wye

Q kVArwye/delta

Q kVArwye/wye

S kVAwye/delta

S kVAwye/wye

Loading %wye/delta

Loading %wye/wye

A

(ab)

15.09 16.41 10.91 9.46 18.62 18.94 37.24 37.88

B

(bc)

34.81 32.76 19.43 19.04 39.86 37.89 39.86 37.89

C(ca)

16.21 16.40 7.56 9.45 17.89 18.93 35.78 37.86

Total 66.11 65.57 37.90 37.95 76.37 75.76

Therefore, the ENELVEN distribution network variety of different transformer connectionscombined with an aforementioned strategic requirement of maintaining power loss at a minimum

level (which is only a few percent of the total load), creates the requirement for accurate DSPF.

There is also no choice to base accurate DSPF on a full-scale input data model.

Implementation of the Distribution Management System opens the possibility to improve the

quality of DSPF by moving to a Distribution Real-Time Power Flow (DRTPF). DRTPF usesreal-time information about network topology and analog measurements at some distribution

network key points. In combination with using billing information on customer consumption, it

gradually removes the main obstacle on approximate nature of the DSPF. The latter progress inthe creation of a corporate GIS database, which can be used for distribution system data

preparation, makes the task of input data preparation less laborious through automation. Even

under these circumstances however, the decision to use an accurate three-phases unbalancedDSPF for network with a few hundred thousand transformers and close to one million nodes was

carefully justified.

IV. DSPF input data preparation

The large volume of DSPF input data makes data import automation absolutely necessary. The

input data is coming from two different sources: definition of the network elements (lines,

transformers, switches, capacitors) and their connectivity is imported from the corporate GISdatabase. Electrical parameters (impedances, phases, loads values, and etc.) are imported from a

different source. It must be stressed that GIS stores the distribution network by geographical

areas, not by electrical connectivity. One geographical area may include a few distribution

feeders connected to the different supply sources. This information is fed into the SCADAdatabase where network electrical connectivity is built and checked by tracing functions.SCADA also connects the distribution network to the Supply Substations, which already exist in

the ENELVEN Energy Management System (EMS).


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As soon as all electrical elements (lines, transformers, capacitors, switches) of the same

distribution feeder are connected and the feeder topology is established in SCADA, electrical

parameters of these elements are imported through a different database. A set of filters is

established in order to test the feasibility of each imported value (impedance, admittance, ratingand etc.) and to identify possible errors.

These two mentioned data flows (topology and electrical parameters) are merged in the DSOM

as shown in Figure 2. Real-time measurements, statuses of the switches and tap positions are

transmitted to SCADA from the field RTU and are updated in the DSOM.

The DSOM serves as a real-time database located in the computer shared memory. DSPF is

running continuously as a UNIX process. In addition to all standard power flow features, DSPFprovides two additional functions: final checking of the network topology including phase

connectivity and serving as a Distribution State Estimator. It checks consistency of the

measurements and scales loads according to these measurements. Load values, calculated byDSPF, are written back to DSOM to be used by other DMS network application programs.

Figure 2: Structure of Distribution System Operational Model Interaction

V. Load modeling

As previously mentioned, lack of information on load values is a general problem for distribution

systems. Active and reactive power measurements are available for very few ENELVEN loads.For the majority of the distribution transformers, the connected load value can be only estimated

based on the expected (designed) transformer peak loading. In several cases, an average load

DistributionNetwork

Applications

Load Modeling

D R T P F

Distribution System

Operational Model

(DSOM)

Network Elements

Electrical Parameters

Line/transformer

impedances, phases, taps,

loads, capacitors

SCADA

Networkdescription and

topology

Field equipmentstatuses,measurements

GIS Data

Network elementsgeographical

location

DistributionNetwork

Applications

Load Modeling

D R T P F

Distribution System

Operational Model

(DSOM)

Network Elements

Electrical Parameters

Line/transformer

impedances, phases, taps,

loads, capacitors

SCADA

Networkdescription and

topology

Field equipmentstatuses,measurements

GIS Data

Network elementsgeographical

location


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consumption (during 24 hours) can be calculated from the monthly billing data. This data is more

reliable than peak load. In combination with typified 24 hours load profiles for the main types of

customers (residential, commercial, industrial etc.), this information serves as the basis for load

modeling. ENELVEN is currently in the process of researching and developing a mathematicaltool to calculate transformer loads based on the invoiced (billed) energy.

The main source of load data improvement is real-time measurements taken from the supply

transformers, feeder heads and occasionally along the feeder. These measurements are used by

DSPF for load scaling in such a way that calculated active and reactive powers at the pointswhere measurements are taken are equal to the measured values. The procedure of load

adjustment, according to the measured values, is called Load Scaling and serves as a sort of

distribution systems state estimator without the complexity of a real estimation function.

The ENELVEN distribution system has both active P and reactive Q measurements at almost all

feeder heads and supply transformers. The Load Scaling procedure is done in two consecutivesteps: once before power flow (step 1) and iteratively inside power flow (step 2);

1) Load active powers are set according to the closest P measurement upstream(feeder head in most cases) as constant and non-dependent of voltage values.

Power loss is assumed to a default percentage of the measured P. Load reactive

powers are calculated from the active power through individual load power factor.

2) Both load P and Q are scaled iteratively by the power flow until the calculated Pand Q are equal to the measured values.

As a result of step 2, initial load power factors are changed. In the case of significant power

factor change, a special warning is generated. Very often, wrong shunt capacitors connections

(connected through non-telemetered switches) are the reason for the warning, which helps toidentify blown capacitor switches.

Additional load modeling problems arise from the loads connected to the secondary

transformers side in delta with uneven legs. In spite of the fact that only one three-phase load is

normally described in input data, physically there are two loads: three-phase balanced and single-

phase, connected to the largest transformer bank. Inside the DRPF, these loads are split intothree-phase load and single-phase load by using the following equations:

TXG= K_1 * LOAD 1 + K_2 * LOAD3 !!!!!!!

TXP= K_3 * LOAD 1 + K_4 * LOAD3

where TXG, TXP are the largest and the smallest transformer ratings in the bank.

Coefficients K1-K4, currently used by ENELVEN, are shown in Table 4. They are consideredconfigurable parameters and can be changed in the future. They are different for delta and for

open delta connections.


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Table 4. Load Distribution Coefficients

Connection K_1 K2 K3 K4

Delta 1,000 0,333 0,000 0,333

Open Delta 1,000 0,333 0,000 0,667

VI. DRTPF Algorithmical Solutions

The Distribution Power Flow model used in DRTPF is based on phase coordinates with single-

line circuit presentation. Each circuit element (line, transformer, load, capacitors) is representedas an admittance matrix with size 3*3, 2*2 or a single value. The advantage of this approach is

that a standard sparse matrix technique for ordering, factorization and forward/backward

substitution can be applied.

The Current Injection (CI) method is used for the Power Flow solution. The main idea of themethod is described by the following two matrix equations:

[V] = [Y-1] *[I] [I] = [S] / [V]

where: [V], [I], [S] are vectors of nodal voltages, currents and powers,

[Y-1] is a factorized nodal admittance matrix.

The CI method is used in applications where the majority of the nodes may be represented as

loads and the number of PV buses is limited. The distribution systems satisfy these conditions.

Different transformer connections modeling is one of the most challenging requirements for

ENELVEN DRTPF. According to the general approach of the circuit element simulation, eachtransformer type has its nodal admittance matrix [Y_t] included in the general circuit matrix [Y].

The basic steps for computing this matrix are as follows:

1. Build the branch admittance matrix [Y_b] for two or three transformer banks.

2. Build the branch to bus connection matrix [C] for given bank connection type.

3. Compute the nodal admittance matrix for the transformer bank as:

[Y_transformer] = [Ct] [Y_b][C]

Phase-coordinated power flow methods have a common problem with Wye/Delta transformers.These transformers create energized islands isolated from the ground (common bus) [2]. A

special technique is used to factorize nodal admittance matrix bypassing a peculiar matrix

problem.


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VII. Testing the DRTPF Solution Feasibility

In spite of the fact that DRTPF is solved independently for each distribution subsystem, the sizeof one subsystem, which includes part of the network supplied from the same injection source

(from one up to ten feeders), is very significant (from a few hundred up to a few thousand

transformers). That is why the double checking of input data by analyzing the feasibility of thePower Flow results immediately after the data was prepared became extremely important. The

following algorithm formalizes the main steps of the engineering analysis used for Power Flow

results testing:

1. Solve subsystem with no load scaling and capacitors connected.

1.1 Check substation transformer loading. If it is not feasible, check loads.

1.2 Check substation transformer low side voltage, voltage drop, tap increment.

1.3 Check power factors for the injection source and feeder heads. If these values are

more than 10% different from the average load power factor (0.87 in most cases),return to the input data and check power factors for the individual loads.

1.4 Check the ratio of the total power loss to the total injected power for the whole

subsystem and for each feeder. If any of these ratios is not feasible:

1.4.1 Compare total transformer power loss with total line power loss.

1.4.2 If transformer power losses are not feasible, compare transformer no-load losseswith transformer load losses. Check transformer impedances or admittances.

1.4.3 If line power losses are not feasible, check line impedances.

1.5 Select all distribution transformers with loading above and below average. Ininput data, compare these transformer ratings with connected load nominal

powers.

1.6 Select all buses with voltage violations.

1.6.1. If violated bus is at the secondary transformer side, check transformer voltage

drop and tap increment.

1.6.2. If violated bus is at the primary side, see voltages on the trace upstream from this

bus to the injection source.

2. Solve subsystem with P scaling only and no capacitors connected.

2.1 Compare injection source and feeder head P, Q flows and power factors to those

with no-scaling. Calculate correspondent ratios. If the difference is significant:

2.1.2 Check statuses for normally opened and sectionalized switches.


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2.1.3 Check non-conforming load input data and measurements.

3. Solve subsystem with P scaling only but capacitors connected.

3.1 Compare injection source and feeder head power factors with those from step 2.

4. Solve subsystem with both P and Q scaling and capacitors connected.

4.1 Check individual load power factors. If they are significantly different from the

initial factor, check current status of the capacitor switches.

VIII. Conclusions

The implementation of an accurate three-phase unbalanced power flow is required by

ENELVEN in order to satisfy quality of service requirements and improve operation efficiency.The main reason for this requirement is the extremely unbalanced nature of the distribution

circuit due to a wide variety of unbalanced transformer connections and the need to estimate

losses at feeder and transformer levels which are small compared with the feeder load.

Data preparation for the ENELVEN volume distribution network is extremely laborious and

requires a high level of automation. The various proposed steps are a first approach to implementa fully automated data check preprocessor. The results obtained with the DRTPF are promising

both in terms of performance and results that are used in the real-time day-to-day operation at

ENELVENs control center. As more results become available, the use of DRTPF will certainlyprove to be of great interest for utilities with similar problems. The power factor and served

energy related to distribution transformers are good references in the short-term to validate the

feasibility of power flow results.

References

1. W.H. Kersting Distribution System Modeling and Analysis, CRC Press, 2001, 314 p.

2. D. Anderson, B.F. Wollenberg Solving for Three Phase Connectivity Isolated Busbar Voltages Using Phase

Component Analysis, IEEE Trans. On Power Systems, Vol. 10, No.1, 1995, pp.98-105.

3. J. B. Patton, D. T. Rizy, J. S. Lawler, Applications software for modeling distribution automation operations on

the Athens Utilities Board, IEEE Transactions on Power Delivery, Vol. 5, No. 2, April 1990.

4. W. G. Scott, Automating the restoration of distribution services in major emergencies, IEEE transactions on

Power Delivery, Vol. 5, 1990

5. Guidelines for Evaluating Distribution Automation, EPRI Report EL-3728, Nov. 1984.

6. D. L. Brown, J. W. Skeen, P. Daryani, F. A. Rahimi, Prospects for Distribution Automation at Pacific Gas &

Electric Company, IEEE Transactions on Power Delivery, Vol. 6, No 4, October 1991.

7. R. Cspedes, L. Mesa, C. Hoyos, Practical Experiences Of The Implementation Of Substation And Distribution

Automation At Empresas Publicas De Medellin, Distributech, Miami, Fla., 2000.

Acknowledgement: The authors wish to thank the ENELVEN management for their continuous support during the entiredevelopment of the project which has concluded in a full operation DMS located in Maracaibo, Zulia, Venezuela.

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