synchronizing microgrids to the utility through a … microgrids to the utility through a series...

18th Power Systems Computation Conference Wroclaw, Poland – August 18-22, 2014

Synchronizing Microgrids to the Utility Through a

Series Compensator

André A. P. Lerm, Luciano A. Braatz, Michel F. da Cruz, Tiago L. Riechel, Sérgio L. S. Severo

GPSE – Energy Systems Research Group

IFSul – Federal Institute of Science and Technology

Pelotas, Brazil

aaplerm,lucianobraatz,cruz.michel,[email protected], [email protected]

Abstract—This paper presents a novel approach to deal with

the synchronization of microgrids to the main grid. It is

based on the use of a power electronic device derived from

the original concept of the SSSC (Static Synchronous

Series Compensator). The proposed scheme considers the

connection of this device at the point-of-interconnection

(POI) of the microgrid to the low voltage side of the

substation transformer (main grid). The voltage originated

by the series compensator is fully controlled in both

amplitude and phase angle ensuring the conditions for the

synchronization. The proposed scheme allows a smooth

transition between the islanded operation and the grid

connected operation overcoming typical problems

observed when conventional methods are applied.

Numerical results obtained for a hypothetical microgrid

show the effectiveness of the proposed approach.

Keywords—FACTS, Microgrids, series compensator,

synchronization

I. INTRODUCTION

The widespread use of microgeneration has become one of the most remarkable facts nowadays in electric systems worldwide. Aspects related to the current restructuring of electrical systems have encouraged the implementation of microgenerators on a scale never before imagined. This fact is reinforced by the need to reduce carbon emissions throughout the use of renewable energy sources like those based on solar photovoltaic (PV) and wind turbines. In particular, the PV generation has a standout feature due to its ability to be implemented in small-scale units, next to urban residential consumers. Distributed generation can offer considerable benefits to the electric system such as reduction on power networks losses, smart power management, increase on reliability to final consumers and reduction for future grid reinforcement and expansion.

When a particular electrical subsystem has a strong capacity for distributed generation compared to its installed load it may be characterized as a microgrid. In this sense, microgrids are small-scale versions of the centralized electricity system, embedding a variety of distributed

generation (PV panels, small wind turbines, fuel cells, diesel and gas microturbines etc.), distributed energy storage devices (batteries, super- capacitors, flywheels etc.) and customer loads. These systems are interconnected to the medium voltage distribution network, but they can be also operated isolated from the main grid [1].

The existence of an outstanding generation within the microgrid makes its operation not trivial. As a result, it is necessary to perform several analyzes involving different transient and steady-state operative conditions of microgrids. These studies are conducted to improve the control and operation of microgrids connected to the main power grid, the islanding transition, the isolated operation and its resynchronization to the main grid. Synchronization is one of the issues that deserve special emphasis on studies of microgrids and this is the issue addressed by this article.

The synchronization process of microgrids to the utility (main grid) plays a very important rule on its whole control strategy. When reconnecting a microgrid operating in the islanded mode to the utility grid, voltage, frequency and phase criteria must be satisfied. Synchronizing a microgrid consisting of several generators with different characteristics and loads is more challenging than synchronizing a single generating unit to the grid. There are three possible methods of microgrid synchronization accordingly to IEEE standard 1547.4-2011 [2][3]:

active synchronization, where there is a control mechanism to match the voltage, frequency, and phase angle of the island system to the utility grid,

passive synchronization, which employs a synchronization check for paralleling, and

open transition transfer, where loads and DGs in the island are de-energized before reconnecting to the grid.

The active synchronization is mostly focused on inverter based DGs. In this case, a proper control is applied individually to each one of the distinct technologies of DG installed in the microgrid. The passive synchronization depends not only on an efficient algorithm to detect the appropriate instant for the reconnection but also on fast static switches, e.g., circuit breakers (CB)-based, silicon controlled rectifiers (SCR)-based,

Paper submitted to Power Systems Computation Conference, August 18-22, 2014, Wroclaw, Poland, organized by Power Systems Computation

Conference and Wroclaw University of Technology.


and insulated gate bipolar transistors (IGBT)-based. On its turn, open transition transfer means a black start that brings a decrease in the power system reliability and in the power quality performance.

This paper proposes a novel approach to deal with the synchronization of microgrids to the main grid. It is based on the use of a power electronic device derived from the original concept of the SSSC (Static Synchronous Series Compensator). The proposed scheme considers the connection of this device at the point-of-interconnection (POI) of the microgrid to the low voltage side of the substation transformer (main grid).

The voltage originated by the series compensator is fully controlled in both magnitude and phase angle. By controlling this series voltage, the proposed scheme is able to guarantee the conditions for the synchronization in terms of same instantaneous values of the voltages on the microgrid and on the utility sides. Once these conditions are fulfilled, the set microgrid/series controller is connected to the main grid by a circuit breaker. After this synchronization process the developed control acts to reduce the series voltage until the voltages on the microgrid and on the utility sides become the same ones. The proposed scheme allows a smooth transition between the islanded operation and the grid connected operation, overcoming typical problems observed when conventional methods are applied.

II. THE MODELED SYSTEM

A. Network and load modeling

In order to demonstrate the effectiveness of the proposed method to synchronize a microgrid to the main grid this work considers a distribution system as indicated in Fig. 1. This microgrid comprises two microsources (MS1 and MS2) connected to a distribution system and loads at busbars 3 and 4, in a total of 2.0 +j2.0 kVA. The considered nominal frequency is 60 Hz while the nominal voltage at the distribution system is 380 V (phase-to-phase). The point-of-interconnection (POI) of the microgrid to the low voltage side of the substation transformer is located at busbar 5.

All the loads are modeled as constant impedance for both active and reactive component (dependent on voltage and frequency) under three-phase balanced operation.

The system was implemented and simulated in Simulink/MATLAB environment on a platform developed by the authors.

B. Microsource modeling

The microsources are modeled accordingly to Fig. 2. Much of the current technologies involved in distributed generation are based on the use of a DC link. This is the case of microturbines, flywheels, fuel cells, wind generators, and photovoltaic arrays. Particularly for the cases in which a proper storage scheme is used to maintain an adequate DC energy level, it is possible to replace the original models by only the mentioned DC interface regardless of the technology used in the power generation process. For this work, there is no loss of generality since it is focused on the synchronization of the

MS 1

MS 2

1.5+j1.5 kVA

POI

Series

Compensator

1

2

3

4

5

60 Hz

Yg Δ

10 kVA

0.38/13.8 kV

0.5+j0.5 kVA

R13 = 0.02 Ω

L13 = 0.02 mH

R25 = 0.01 Ω

L25 = 0.01 mH

R45 = 0.04 Ω

L45 = 0.04 mH

R24 = 0.01 Ω

L24 = 0.01 mH

6 7

Fig. 1. The system considered in the simulations, including the microgrid

(busbars 1 to 5), the series compensator and the main grid (busbars 6 and 7).

Rf Lf

CfΔ Yg

microturbines

flywheels

fuel cells

wind generators

PV arrays

DC boost

converter

DC-AC

inverter

Filter TransformerDC link

Fig. 2. Structure adopted for the microsources.

microgrid to the main grid. The mentioned DC link is presented on the left in Fig. 2.

A DC boost converter is applied to the DC link. This step-up converter has an output voltage greater than its input voltage. The implemented DC boost converter controls its output voltage accordingly to the duty cycle imposed to its internal MOSFET. The output voltage Vo follows the input signal Vref, as shown in Fig. 3, and is used in the voltage control scheme of the microsource as pointed out in the following.

The DC-AC inverter is presented in Fig. 4. This inverter is composed by IGBT modules connected in an H-bridge configuration. These modules are switched by an optimized PWM (OPWM) scheme as suggested in [6]. The fires angles obtained from this modulation technique are obtained through an optimization technique that minimizes an objective function related to the resulting total harmonic distortion (THD) of the resulting voltage waveform. By applying a switching frequency about 3 kHz, the resulting THD is approximately 1%. Since these fires angles are kept constant along the inverter operation the output voltage of the inverter is controlled by modifying the DC voltage level at its input (output signal Vo from the DC boost converter, accordingly to Fig. 3).

Despite of the low THD originated by the IGBTs switching, it is included a filter composed by RLC elements. In order to adjust the voltages of the microsource and the microgrid it is added a transformer in the output of the inverter.

The microsource is controlled by means of a voltage source inverter (VSI) by emulating the behavior of a synchronous machine. The VSI acts as a voltage source, with the magnitude and frequency of the output voltage controlled through droops, as described by

QmVV

Pm

Q

P

0

0


where P and Q are the inverter active and reactive power outputs, mP and mQ are the droop slopes, and ω0 and V0 are the reference values of the angular frequency and terminal voltage at no load conditions.

The control applied to the active power, as indicated by (1), is delighted in the block diagram presented in Fig. 5. From the resulting angular frequency, it is possible to obtain the phase angle of the output voltage of the microsource.

Fig. 3. The DC boost converter modeled in Simulink.

Fig. 4. The DC-AC inverter together with its filter and output transformer

implemented in Simulink. The inverter is represented by the IGBTs and PWM

switching.

The resulting value for the magnitude of the microsource voltage is applied to the input of the DC boost converter (signal Vref, Fig. 3) while the frequency and phase angle are applied to the input of the inverter as the signals freq and phase, respectively (Fig. 4).

mP1

s

+p

-p

P0

P

0

D d

Fig. 5. Block diagram of the active power droop.

III. THE PROPOSED ELECTRONIC SYNCHRONIZER

The synchronization process to the main grid is one of the most important challenges in the operation of microgrids. In fact, synchronizing a distribution system containing several microsources is more challenging than synchronizing a single generating unit to the grid. The use of conventional methods for microgrids synchronization can lead to undesired problems such as those already mentioned in the first section of this paper.

This work proposes the use of a series compensator in order to synchronize a microgrid operating in the islanded mode to the utility grid. The proposed methodology consists on the use of a power electronic device derived from the original concept of the SSSC (Static Synchronous Series Compensator), a controller that belongs to the Flexible AC Transmission Systems (FACTS) [8].

In general, any synchronization process requires some conditions: the nodes across the interconnection point must have the same voltage, frequency and phase angle. As microgrids operating in the islanded mode have in general a frequency slightly smaller than the nominal value, the mentioned condition of same frequency must be outlined by the adopted synchronization control.

The schematic circuit of the proposed system in shown in Fig. 6. In this figure, the Converter 1 is a rectifier responsible to maintain the voltage Vdc at the DC link. The synchronization process of the microgrid to the main grid is based on the application of a controlled series voltage Vc by a transformer at the output of the inverter module (Converter 2). The voltage originated by the series compensator is fully controlled in both magnitude and phase angle. By controlling this series voltage, the proposed scheme is able to guarantee the conditions for the synchronization in terms of same instantaneous values of the voltages on the microgrid and on the main grid sides.

Vdcac ac

Converter 1 Converter 2

Microgrid Main grid

Supply

transformer

Series

transformer

ControlInput

signals

Reference

signals

CB1 Vc VgVmg

CB2

Fig. 6. Schematic circuit of the proposed system.


The control signal to close the circuit breaker (CB1) must be done when there is a minimal electrical potential difference between its poles. This condition is exemplified in the phasor diagram shown in Fig. 7. The parallelism goal is achieved when

cmgg VVV

where Vg is the main grid voltage, Vmg is the microgrid voltage, both at the POI, and Vc is the inverter controlled voltage.

Vmg

Vc

Vg

qVc

Fig. 7. Voltages involved in the synchronization process.

The block diagram of the Vc regulation is shown in Fig. 8. The determination of the voltage Vc is performed by a digital phase-locked loop (PLL) as proposed by the authors in [5]. From the previous experience of the authors on this topic a digital PLL demonstrates more robustness if compared with other techniques like Park and Clark decomposition when the involved signals have a significant distortion on their waveforms. The implemented Vc computation module in Simulink is presented in Fig. 9. This module has as input the voltage signals from the microgrid and the main grid, and as output the module and phase angle of the controlled voltage Vc.

Once the conditions are fulfilled for the synchronization, an internal loop based on a PI controller reduces the voltage Vc from its original value to zero. After the total reduction of the voltage Vc the series compensator can be bypassed by the circuit breaker CB2 as indicated in Fig. 6.

The inverter module indicated in Fig. 8 is composed by IGBT modules connected in an H-bridge configuration with the same structure adopted for the microsources (Fig. 4).

Vg

Vc

Vmg

Inverter(PWM)

Vc computation

|Vc | qVc

Microgrid Main grid(PLL)

Fig. 8. Block diagram of the proposed electronic synchronizer.

The proposed scheme allows a smooth transition between the islanded operation and the grid connected operation as shown in the following section.

IV. NUMERICAL RESULTS

The present section shows numerical results obtained from simulations carried out for the system indicated in Section II (Fig. 1). For the microsource 1 it is assumed mP = 0.020 pu/pu and mQ = 0.020 pu/pu, and for microsource 2, mP = 0.025 pu/pu and mQ = 0.025 pu/pu. These values of droop slopes consider as reference values of the active and reactive power at full load conditions, for each microsource, 5 kW and 5 kVar, respectively. The reference values of the frequency and terminal voltage at no load conditions are set to 60.05 Hz and 385 V (phase-to-phase), respectively.

A first simulation is performed by considering the microgrid initially deenergized. The microsources are started from null initial conditions with the loads at busbars 3 and 4 already connected. The results for this simulation are presented in Fig. 10, where it is possible to visualize the initial transients arising from the microsources start process. In steady state, the frequency is approximately 59.83 Hz while there is more power dispatched on MS1, as expected from the adopted droop slopes.

Fig. 9. Block diagram of the Vc computation module based on PLL, as modeled in Simulink.


Fig. 10. Behavior of the microgrid with a start of microsources from null

initial conditions (MS1 and MS2 stands for microsources 1 and 2,

respectively).

A second simulation is carried out to analyze the resulting voltage Vc from the series compensator without considering the synchronization process. In other words, the CB1 indicated in Fig. 6 is open in the entire analyzed time interval. The resulting magnitudes and phase angles for each phase are depicted in Fig. 11. The points ‘A’ and ‘B’ at this figure represents the ideal time instant to perform the connection of the microgrid to the main grid. At these points there is a minimum electric potential difference across the CB1, what means that the voltage phasors from both sides of the CB1 are in phase. However, by considering a practical range for this as 0 to 5 degrees (with the microgrid leading the main grid), this range of timely represents only about 7 milliseconds (less than half a cycle of 60 Hz).

In the sequence, it is performed a simulation in which a direct connection is made of the microgrid to the main grid without the use of the proposed synchronizer. This is just to compare this behavior with the case in which the series compensator is used to synchronize both systems. The results are presented in Fig. 12. This simulation considers a microgrid leading the main grid by 10o el. It must be stressed that this is not a practical situation by synchronizing both systems. It is possible to detect from this figure the unacceptable behavior of the active and reactive power dispatched by the microsources under this situation.

Fig. 11. Behavior of the voltage originated by the series compensator for a

wide range of time.

Fig. 12. Behavior of the microsources when the microgrid is synchronized to

the main grid directly by the CB1 (microgrid is leading the main grid by 10o).


A final simulation is performed by applying the proposed synchronizer with the same conditions as in the previous case (the microgrid leads the main grid by 10o). The results are presented in Fig 13. The voltage originated by the series compensator is reduced in a convenient range of time allowing the conditions for a smooth transition during the synchronization process.

Fig. 13. Behavior of the microsources when the microgrid is synchronized to

the main grid directly by the proposed synchronizer (microgrid is leading the

main grid by 10o).

V. CONCLUSIONS

This paper presented a novel approach to deal with the synchronization of microgrids to the main grid. It is based on the use of a power electronic device derived from the original concept of the SSSC (Static Synchronous Series Compensator). The proposed scheme considers the connection of this device at the point-of-interconnection (POI) of the microgrid to the low

voltage side of the substation transformer (main grid). The voltage originated by the series compensator is fully controlled in both amplitude and phase angle ensuring the conditions for the synchronization.

The main advantage of the proposed synchronization technique to other methods relies on a resulting smooth transition between the islanded operation and the grid connected operation. This aspect overcomes typical problems observed when conventional methods are applied.

Numerical results obtained for a hypothetical microgrid show the effectiveness of the proposed approach.

Future technical developments on this topic include a detailed analysis under various operation conditions, type of loads, position of microgrids and performance under disturbances among others. In addition, it must also be addressed the cost wise feasibility of the proposed series compensator and its reliability.

REFERENCES

[1] R.H. Lasseter and P. Piagi, “Control and Design of Microgrid Components,” Final Project report: PSERC Publication 06-03, January 2006. Available at http://certs.lbl.gov/pdf/microgrid-control.pdf

[2] Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003.

[3] Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems, IEEE standard 1547.4-2011.

[4] H. Laaksonen, and K. Kauhaniemi, “Synchronized re-connection of island operated a LV microgrid back to utility grid,” in Innovative Smart Grid Technologies Conference Europe (ISGT Europe), 2010 IEEE PES, 2010, pp-1-8.

[5] L.A. Braatz, S.L.S. Severo, A.A.P. Lerm, and W.F. Ciarelli, “A Digital PLL for Synchronization of Single Phase Sources with the Utility Grid,” in IEEE PES PowerTech 2013, Grenoble, France, June 2013.

[6] V.N. Obadowski, A.A.P. Lerm, and W.F. Ciarelli, “A Nonlinear Optimization Technique Applied to PWM Signals,” in 2012 VI ANDESCON (Andean Region International Conference) 2012, Cuenca, Equator, Nov. 2012.

[7] J.A. Peças Lopes, C.L. Moreira, and A.G. Madureira, “Defining Control Strategies for MicroGrids Islabded Operation,” in IEEE Trans on Power Systems, vol. 21, no. 2, May 2006.

[8] N.G. Hingorani, and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems,” IEEE Press, ISBN: 0-7803-3455-8, Piscataway, NJ, 2000.

http://certs.lbl.gov/pdf/microgrid-control.pdf

synchronizing microgrids to the utility through a … microgrids to the utility through a series...

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