Transients in Distribution Systems on DG Energization

1

Transients in Distribution Systems on Distributed Generation Energization J. A. García-Pérez, Student Member, IEEE,

E. V. Morales-Irizarry, UPRM Graduate Student

L. R. Orama-Exclusa, Member IEEE Abstract-- In the last decade or so electrical systems worldwide have experienced dramatic configuration changes. Distributed generation (DG) emerged as an option to deal with growth in the demand, advances in technology, sensitive loads, and new energy policies. In DG systems, alternative energy sources, such as wind turbines, photovoltaics, fuel cells, and other new technologies are integrated to the system near the loads, i.e., at the distribution level. The option of DG provides grid robustness and reliability. The major goals of this work are: (1) study transient effects when the DG is connected to the grid; and, (2) study the interaction of the DG with the distribution system. The Alternative Transients Program version of the Electromagnetic Transients Program (EMTP/ATP) has been used as the transient simulation tool to accomplish the above goals. This work shows that the connection of DG always impacts the grid no matter at which voltage level it is connected. However, it is been noted that the impact is higher at lower voltages levels. Index Terms — Distributed Generation, EMTP/ATP, Transients, Distribution System, Power Quality, Unbalance Voltage, IEEE 13 Node Test Feeders, Interconnection

I. NOMENCLATURE DG EPS PCC EMTP ATP THDv THDc

– – – -

Distributed Generation Electrical Power System Point of Common Coupling Electromagnetic Transients Program Alternative Transients Program Voltage Total Harmonic Distortion Current Total Harmonic Distortion II. INTRODUCTION

I

N the last ten years electrical systems worldwide experienced dramatic configuration changes. Distributed generation (DG) has emerged as an option to deal with growth in the demand, advances in technology, sensitive loads, and new energy policies, [1-5]. DG systems using alternative energy sources, such as wind turbines, photovoltaics, fuel cells, and other new technologies are integrated to the system near the loads, i.e., at the distribution level. The option of DG

This work is part of the final report of the course INEL 6077: Surge Phenomenon of the University of Puerto Rico , Mayagüez Campus, PR 00680 J. A. García-Pérez is with University of Puerto Rico, Mayagüez Campus, P.R. 00680 (e-mail: [email protected]). E. V. Morales-Irizarry is with University of Puerto Rico, Mayagüez Campus, P.R. 00680 (e-mail: [email protected]). L. Orama-Exclusa is with University of Puerto Rico, Mayagüez Campus, P.R. 00680 (e-mail: [email protected]).

provides grid robustness and reliability. The purpose of this work is to develop a transient simulation and analysis for distribution systems with DG to: (1) perform transient analysis when the DG is connected to the grid; and, (2) study the interaction of the DG with the distribution system. The three phases power distribution system used in this work was the IEEE 13 Node Test Feeder, [7].

III. DISTRIBUTED GENERATION There are many definitions for distributed generation (DG). Within this work DG is defined as any external energy source connected to the distribution system near to the load, and with a capacity below 10 MW at the point of common coupling (PCC), [8]. A. Advantages of the DG DG systems offer multiple advantages in the electric power system (EPS). Most notably: 1. modularity 2. may use renewable energy sources 3. peak power supply 4. spinning reserve supply 5. improve voltage profile 6. reduce the losses in the transmission and distributions systems 7. decrease demand from the grid 8. enhance reliability 9. may improve power quality 10. may reduce or eliminate emissions (environmentally friendly) B. Disadvantages of the DG Although many advantages exist, DG systems also have some disadvantages. Some of them may be caused by poor system planning. Most common disadvantages are: 1. re-design of protection system and schemes 2. the DG has to be synchronize to the system 3. high initial cost 4. islanding issues 5. when poorly planned it has a high $/kWh C. Requirements Some of the energy sources, either alternative or other sources produce direct current (dc) power while the electrical power system (EPS) uses alternate current (ac) power. A power electronic device (inverter) has to be used to

2 conversion dc-ac in order to interconnect the DG to the EPS. References [8-11] bring the requirements to appropriately connect a DG to the EPS. Those requirements follow: • Integration with the Area EPS grounding – “The DG shall not cause overvoltage, and shall not disrupt the coordination of the ground fault protection on the Area EPS”, [8]. • Synchronization – “The DG shall not cause voltage fluctuation at the PCC greater then ±5%”, [8]. • Inadvertent energization of the Area EPS – “The DG shall not energize the Area EPS when it is de-energized”, [8]. • Voltage disturbances – “The DG shall cease to energize the Area EPS within the clearing time as indicated”, [8]. • Frequency variation – “The DG shall cease to energize the Area EPS within the clearing time as indicated”, [8]. • Power Quality – “The DG shall cease the harmonic and dc current injection established. The DG shall not create objectionable flicker for other customers on the Area EPS”, [8]. The Total Harmonic Distortion (THD) shall not exceed 5.0% of the fundamental in the PCC, Table 1. TABLE 1: MAXIMUM HARMONIC CURRENT DISTORTION IN PERCENT OF CURRENT

Individual Harmonic Order h (odd harmonics) h < 11 11 ≤ h < 17 17 ≤ h < 23 23 ≤ h < 35 35 ≤ h Total Harmonic Distortion (THD)

•

Percent (%) 4.0 2.0 1.5 0.6 0.3 5.0

Islanding – “For an unintentional island in which the DG energizes a portion of the Area EPS through the PCC, the DG interconnection system shall detect the island and cease to energize the Area EPS within two seconds of the formation of an island”, [8].

Fig. 1: IEEE 13 Node Test Feeders.

V. SIMULATION The Alternative Transients Program (ATP) is the simulation tool used in this work. It is a royalty free version of the Electromagnetic Transients Program (EMTP), [12]. These programs are considered the industry standard used for simulations of electrical systems transients. EMTP/ATP is used for digital simulation of transient phenomena of electromagnetic and electromechanical nature. ATP can also calculates variables of power electronics systems as a function of time. The initial conditions can be specified by the user or can be determined automatically by the program as a steady state solution before the transient simulation actually begins. The trapezoidal rule of integration is used to solve the differential equations of the system components in the time domain. A. Steady State Condition Figure 2 shows the schematic circuit of the IEEE 13 Node Test Feeder applied in EMTP/ATP. The DG system was simulated as an AC source with equivalent impedance and connected in two different buses. The selected buses are Bus 634 and Bus 680.

IV. IEEE 13 NODE TEST FEEDER The system for the simulations must has characteristics that bring the conditions needed to realize a realistic analysis of the DG interconnection. For the impact of the DG to be quantifiable in the feeder, the DG capacity must be a considerable percentage of the total feeder load. The IEEE 13 Node Test Feeder system was selected to illustrate this interconnection Test Feeder, Figure 1, [7]. Figure 2 shows the ATPDraw schematic of the circuit with no DG connected. The original configuration of the feeder had a capacity of 5 MVA, the DG unit to be installed provided an additional 10 MVA. The installation of this unit should have a substantial impact on the system. This system has different voltage levels, 115 kV, 4.16 kV, and 480 V. Our objective is to simulate a DG connected to an industrial consumer’s bus (4.16 KV) and to a small commercial consumer’s bus (480 V).

Fig. 2: ATPDraw Schematic Circuit.

3 Figures 3 and 4 show the voltages and currents in the Bus 634 without a DG connected, respectively. We may describe this as a “normal scenario” or the benchmark for the study; which describe a secondary distribution system with a normal unbalance, as reflected by the current signal.

4000 [V] 3000 2000 1000 0

400 [V]

-1000

250 -2000

100

-3000 -4000

-50

5

15

25

35

(file bus13chvsinDG.pl4; x-var t) v:680A

v:680B

45

55

[ms]

65

v:680C

Fig. 5: Voltage in the Bus 680 without a DG connection, the first positive peak is the phase B, ABC sequence..

-200

6

-350

[mA]

-500 10 20 30 40 50 (file bus13chvsinDG.pl4; x-var t)v:634A v:634C v:634B

4

60

70

[ms]

80

Fig. 3: Voltage in the Bus 634 without a DG connection, the first positive peak is the phase C, ABC sequence.

2

0

500 [A]

-2

375 -4

250 125

-6 10

0

20

30

40

(file bus13chvsinDG.pl4; x-var t) c:671C -601C

50

60

c:671B -601B

70

80 [ms] 90

c:671A -601A

Fig. 6: Current in the Bus 680 without a DG connection, the first positive peak is the phase A, ABC sequence..

-125 -250 -375 -500 0.04

0.06

0.08

(file bus13chvsinDG.pl4; x-var t) c:6000A -634A

0.10 c:6000B -634B

0.12

[s]

0.14

c:6000C -634C

Fig. 4: Current in the Bus 634 without a DG connection, the first peak positive is the phase B, ABC sequence.

Figures 5 and 6 show the voltages and currents in the Bus 680 without a DG, respectively. Also a “normal scenario” or benchmark for the bus. The conditions are similar but at different voltage level, primary distribution system with normal unbalance.

These plots show the steady state conditions of the system before the DG connection, and the initial conditions for the different cases studied. It is important to recall that both systems present an unbalance condition. B. Switch Operation To quantify and appreciate the impact of the DG connection in the EPS, it must be connected at t > 0 to observe the transient phenomena involved. To control the connection of the DG in milliseconds, three voltage-controlled switches were used, and set at ~15ms. Figure 7 shows the voltage across the switch phase A during the operation. The voltage across the switch is the DG voltage before they are closed at 15ms, afterwards the voltage is zero.

4 4000 [V]

400

3000

300

2000

200

1000

100

0

0

-1000

-100

[V]

-2000

-200

-3000 -4000 0.00

-300 0.05

0.10

0.15

0.20

0.25

[s]

0.30

(file bus13chvfinal2.pl4; x-var t) v:X0009A-X0190A

-400 0.13

0.14

0.15

0.16

(file bus13chvfinal.pl4; x-var t) v:X0134A

Fig. 7: Voltage in the switch during the DG connection.

0.17

v:X0134B

0.18

0.19

0.20 [s] 0.21

v:X0134C

Fig. 9: Voltage in the Bus 634 with a DG connected in Bus 634, the first peak positive is the phase A, ABC sequence.

The current through the switches is zero before the connection of the DG and increases to 40A peak after the switching operation. Figure 8 shows the current in phase A during the DG connection at 15ms.

MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004 File bus13chvfinal.pl4 Variable v:134A [peak] Initial Time: 0.2833 Final Time: 0.3 400 [V]

40 [A]

250

30 100

20 10

-50 0

2

4

0

6 harmonic order

8

10

12

8

10

12

200

-10 -20

-150 0

2

-30 -40 0.00

0.05

0.10

0.15

0.20

0.25

[s]

0.30

4

6 harmonic order

Fig. 10: Harmonic Magnitude and Harmonic Phases for voltage in the Bus 634 with a DG connected in Bus 634

(file bus13chvfinal2.pl4; x-var t) c:X0009A-X0190A

Fig. 8: Current in the switch during the DG connection 500 [A]

VI. RESULTS The DG was connected at two voltage levels: 480 V (Bus 634) and 4.16 kV (Bus 680). The energy injected in the different buses was 10 MVA using a Thevenin equivalent circuit. Also, the DG will be injecting currents at the 5th and 7th harmonic, which represent common harmonics that power electronics devices (like inverters) produce at interconnection points. EMTP/ATP computes the THD of the signals, it was the parameter to be compare and to measure the impact of the DG in the EPS.

375 250 125 0 -125 -250 -375 -500 0.11

0.13

0.15

(file bus13chvfinal.pl4; x-var t) c:X0134A-634A

A. Case 1: DG connected in the 634 node at 480V The DG was connected at 15 ms; each phase has a voltage control switch. Figures 9 and 11 show the voltages and the currents in the Bus 634 during the DG connection.

0.17 c:X0134B-634B

0.19

[s]

0.21

c:X0134C-634C

Fig. 11: Current in the Bus 634 with a DG connected, the first peak positive is the phase C, ABC sequence.

5 MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004

MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004

File bus13chvfinal.pl4 Variable c:134A -634A [peak] Initial Time: 0.2833 Final Time: 0.3

File bus13chvfinal.pl4 Variable v:632A [peak] Initial Time: 0.2833 Final Time: 0.3

400

3500

[A]

[V] 2500

250 1500 100 500

-50

-500 0

2

4

6 harmonic order

8

10

12

40

0

2

4

6 harmonic order

8

10

12

8

10

12

100

0

0 0

2

4

6 harmonic order

8

10

12

Fig. 12: Harmonic Magnitude and Harmonic Phases for current in the Bus 634 with a DG connected

The DG voltage has Voltage Total Harmonic Distortion (THDv) of 1.781%, Figure 10. The voltage and the current in the load connected in the PCC do not exhibit large disturbances with the DG connection. The preview PCC voltage and current graphics (Figures 5 and 6) are similar to the voltage and current graphics when the DG is not connected, except for a small spike in the voltage, caused by DG injected harmonics. Figure 13 shows the voltage in the Bus 632. The current injected to the system is shown in Figure 15.

0

2

4

6 harmonic order

Fig. 14: Harmonic Magnitude and Harmonic Phases for voltage in the Bus 632 with a DG connected in Bus 634 70.0 [A] 52.5 35.0 17.5 0.0 -17.5 -35.0 -52.5

4000 [V]

-70.0 0.10

3000

(file bus13chvfinal.pl4; x-var t) c:632A -X0130A

0.13

0.16

0.19

0.22

[s]

0.25

Fig. 15: Current injected to the system by a DG connected in Bus 634

2000

MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004 File bus13chvfinal.pl4 Variable c:632A -X0130A [peak] Initial Time: 0.2833 Final Time: 0.3

1000

65 [A]

0

51

-1000

37

-2000 23

-3000 9

-4000 0.11

0.13

(file bus13chvfinal.pl4; x-var t) v:X0130A

0.15 v:X0130B

0.17

0.19

[s]

0.21

-5 0

v:X0130C

Fig. 13: Voltage in the Bus 632 with a DG connected in Bus 634, the first peak positive is the phase C, ABC sequence.

2

4

6 harmonic order

8

10

12

8

10

12

90

-90 0

2

4

6 harmonic order

Fig. 16: Harmonic Magnitude and Harmonic Phases for current injected to the system by a DG connected in Bus 634

The DG notably injects harmonics to the EPS. The voltage in the Bus 632 has a THDv equal to 1.413%, small distortion, but the current magnitude increase and it has a THDc equal to 11.066%. In the substation (Bus 650) the harmonic distortion is less than the load branch, only 3.757%. The THDc of the Bus 634 is equal to 4.23%, which is consider acceptable. But if in the Bus 634 the THDc is

6 calculated from the 1st to 11th harmonic, Figure 16, it is equal to 4.14%, the DG can not be connected because exceed the Individual Harmonic Order Percent permit in IEEE Std. 15472003, refer to Table 1.

50.0 [A] 37.5 25.0 12.5

B. Case 2: DG connected in the 680 node at 4.16KV The same DG (same power an harmonic order) was connected to the Bus 680 with a different voltage magnitude. Figures 17 and 19 show the voltages and the currents in the Bus 680, respectively.

0.0 -12.5 -25.0 -37.5 -50.0 0.17

7000 [V]

0.18

0.19

0.20

(file bus13chvfinal2.pl4; x-var t) c:680A -X0195A

5250

0.21

0.22

c:680B -X0195B

0.23

0.24

[s]

c:680C -X0195C

Fig. 19: Current in the Bus 680 with a DG connected in Bus 680, the first peak positive is the phase A, ABC sequence.

3500

MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004

1750

File bus13chvfinal2.pl4 Variable c:680A -X0196A [peak] Initial Time: 0.2833 Final Time: 0.3

0

40

-1750

[A] 30

-3500 20

-5250

10

-7000 0.11

0.12

0.13

0.14

(file bus13chvfinal2.pl4; x-var t) v:DGA

v:DGB

0.15

0.16

0.17

0.18 [s] 0.19

v:DGC

0 0

Fig. 17: Voltage in the Bus 680 with a DG connected in Bus 680, the first peak positive is the phase C, ABC sequence.

2

4

6 harmonic order

8

10

12

8

10

12

50

MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004 File bus13chvfinal2.pl4 Variable v:DGA [peak] Initial Time: 0.2833 Final Time: 0.3

-90 0

6000 [V]

2

4

6 harmonic order

Fig. 20: Harmonic Magnitude and Harmonic Phases for the current in the Bus 680 with a DG connected in Bus 680

4600 3200 1800 400 -1000 0

2

4

6 harmonic order

8

10

12

8

10

12

150

-100 0

2

4

6 harmonic order

Figure 17 shows the harmonic distortion of the voltage injected to the system by the DG, THDv = 4.66%. The THDc injected was equal to 8.77%, Figure 19. The distortion appreciated at the node 650, Figure 21, was a THDc = 3.241% and a THDv = 1.759%, Figure 22. The magnitude of the current is higher and it causes more impact. 250.0 [A] 187.5

Fig. 18: Harmonic Magnitude and Harmonic Phases for voltage in the Bus 680 with a DG connected in Bus 680

125.0 62.5 0.0 -62.5 -125.0 -187.5 -250.0 0.14

0.15

0.16

(file bus13chvfinal2.pl4; x-var t) c:XS1A -650A

0.17 c:XS1B -650B

0.18

[s]

0.19

c:XS1C -650C

Fig. 21: Distortion of the current of the Bus 650, the first peak positive is the phase B, ABC sequence.

7 MC's PlotXY - Fourier chart(s). Copying date: 6/30/2004 File bus13chvfinal2.pl4 Variable v:650A [peak] Initial Time: 0.2833 Final Time: 0.3

2) Underground Line Configuration Data: Configuration Phasing 606

3500 [V]

Cable

A B C N 250,000 AA, CN

607

AN

1/0 AA, TS

Neutral

Space ID

None

515

1/0 Cu

520

2500

3) Line Segment Data: 1500

Node A

Node B

Length(ft.)

Configuration

632

645

500

603

632

633

500

602

633

634

0

XFM-1

645

646

300

603

650

632

2000

601

684

652

800

607

632

671

2000

601

671

684

300

604

671

680

1000

601

671

692

0

Switch

VII. CONCLUSIONS

684

611

300

605

This work demonstrated the impact of a DG source in an unbalanced EPS. The connection of the DG source always has an impact on the EPS no matter at which voltage level it is connected. An appreciated difference is noted on the magnitudes of current and voltages. When the DG was connected to a 480 V Bus, the percent of the magnitude of the current distortion of the system was more than when the DG was connected to a 4.16 kV bus by 2.3%. The integration of a DG connection to the EPS may cause a re-design of the protection system, because the magnitude of the currents increases and the imbalance condition with distortion may cause neutral currents. It becomes an additional cost in the implementation of the DGs to the EPS. If the EPS is unbalanced the DG neither mitigate nor damage the unbalance condition, it only increases the magnitude of the currents in the circuits. An important aspect that was not mentioned here is the switching technology of the DG inverter. It was the reason to integrate the harmonic source in the simulations, to represent the power electronic used in the DG interconnection. The aspect of harmonic in the connection of DG in the EPS has to be one of the reasons to determinate the optimal PCC. To obtain a more realistic scenario is necessary to integrate the power electronic technology in the simulations.

692

675

500

606

500

-500 0

2

4

6 harmonic order

8

10

12

70

0 0

2

4

6 harmonic order

8

10

12

Fig. 22: Harmonic Magnitude and Harmonic Phases for the current of the Bus 650

APPENDIX A. IEEE 13 Node Test Feeder parameters

kVA Substation: 5,000 XFM -1

500

kV-high

kV-low

115 - D

4.16 Gr. Y

Phase

Neutral

Spacing

ACSR

ACSR

ID 500

601

BACN

556,500 26/7

4/0 6/1

602

CABN

4/0 6/1

4/0 6/1

500

603

CBN

1/0

1/0

505

604

ACN

1/0

1/0

505

605

CN

1/0

1/0

510

R-% X-%

4.16 – Gr.W. 0.48 – Gr.W.

1

8

1.1

2

5) Capacitor Data: Node

Ph-A

Ph-B

Ph-C

kVAr

kVAr

kVAr

200

200

200

200

200

675 611

100

Total

300

6) Regulator Data: Regulator ID:

1

Line Segment:

650 - 632

Location:

50

Phases:

A - B -C

Connection:

3-Ph,LG

Monitoring Phase:

A-B-C

Bandwidth:

2.0 volts

PT Ratio:

20

Primary CT Rating:

700

Compensator Settings:

Ph-A

Ph-B

Ph-C

R - Setting:

3

3

3

X - Setting:

9

9

9

122

122

122

Voltage Level:

1) Overhead Line Configuration Data: Configuration Phasing

4) Transformer Data:

7) Spot Load Data: Node

Load

Ph-1

Ph-1

Ph-2

Ph-2

Ph-3

Ph-3

Model

kW

kVAr

kW

kVAr

kW

kVAr

634

Y-PQ

160

110

120

90

120

90

645

Y-PQ

0

0

170

125

0

0

646

D-Z

0

0

230

132

0

0

652

Y-Z

128

86

0

0

0

0

8 671

D-PQ

385

220

385

220

675

Y-PQ

485

190

68

60

290

212

692

D-I

0

0

0

0

170

151

611

385

220

Y-I

0

0

0

0

170

80

TOTAL

1158

606

973

627

1135

753

8) Distributed Load Data: Node A Node B 632

671

Load

Ph-1 Ph-1 Ph-2 Ph-2 Ph-3 Ph-3

Model

kW

kVAr

kW

kVAr

kW

kVAr

Y-PQ

17

10

66

38

117

68

VIII. ACKNOWLEDGMENT The authors gratefully acknowledge the contributions of Bienvenido Rodríguez-Medina, Doeg Rodríguez-Sanabria, and Carlos Ramos-Robles for their help on the original version of this document. IX. CITED REFERENCES [1]

[2]

[3]

[4]

[5]

[6] [7]

[8]

[9]

[10]

[11]

[12] [13]

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X. BIOGRAPHIES E. Verónica Morales-Irizarry received her BS degree in Theoretical Physics from University of Puerto Rico at Mayagüez (UPRM). She is working towards an MSc in Electrical Engineering on the impact of land use on lightning frequency. The research is carried out at the Atmospheric Phenomena Laboratory at UPRM. José A. García-Pérez was born in Ciales, Puerto Rico. He graduated from the Juan Antonio Corretjer High School of this town. Later began his studies in the University of Puerto Rico at Mayagüez, where obtain his B.S. degree of Electrical Engineering with concentration in Power in 2002. Today is a graduate student of the same institution and expects to obtain his M.S. degree in May 2005.