BY RENATA CARNIELETTO, ˜ O, D AN I L O I G L E SI A S B R A ND A S I DD HA RTH SURYA NA RA YA NA N, F E L I X A . F A R R E T, ˜ ES & M AR CE LO G. S I M O
T
HE PRIMARY GOAL OF this article is to discuss the development of intelligent controls for a power electronic
inverter capable of interfacing a photovoltaic (PV)-based unit to the utility grid. The inverter is designed as a single-phase, full-bridge converter operating at 120 V, 60 Hz ac. The control functionalities of the inverter are defined under the perspective of the Smart Grid Initiative (SGI) of the U.S. Department of Energy and ability to supply real and reactive power to local loads, supply real and reactive power to other utility loads up to the rated capacity of the
© ARTVILLE
inverter, provide voltage support at the point of common coupling (PCC), store energy in a leadacid battery bank, and enable the provision of control options
Integration of distributed generation (DG) sources to
to the consumer based on near real-time electricity information
the electric distribution system has potential advantages
obtained from the utility through advanced metering devices.
including improved supply reliability, custom power
A multifunctional single-phase voltage-source inverter Digital Object Identifier 10.1109/MIAS.2010.939651
IEEE INDUSTRY APPLICATIONS MAGAZINE SEPT j OCT 2011 WWW.IEEE.ORG/IAS
substantiated via case studies in this article as the
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Date of publication: 28 June 2011
1077-2618/11/$26.00©2011 IEEE
1 Grid Capacitor + Banks
Protection Breaker 2
2 C
C 1
2
1
Test Breaker 1 Protection Breaker 1
C 1 2 C 1
Brkr Pro
Brkr
2
Grid Load
Grid Test Breaker 2 Measurements
Outin1 Secondary VSI Load
Block diagram of the voltage-source inverter with smart functionalities.
out2 in2 –
Breakers Control
Brkr Brkr Pro Brkr Zinv Brkr Q Islanding and Reclosure Protection Local Load Schedule STATCOM Function out1 in1 +
Battery Bank
Bidirectional Buck-Boost
VSI IGBT Two-Bridge LCL Filter Measurements Inverter
Primary VSI Load in2 out2 B
in1 out1 A
in2 out2 –
–
in1 out1 +
PV Model
Boost
+
g
Smart Inverter STATCOM Discrete Controllers Function PWM Generator Qref (kW) Qref Pulse Uref pwm-out Pref Pref (kW) Discrete, Ts = 5e–007 s.
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Outin1
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quality, local use of thermal energy, the ability to off-load electric energy from the transmission grid, and the provision of an avenue to meet mandatory renewable portfolio standards (RPSs) [1]. The U.S. federal government has ratified the SGI as its official policy to modernizing the electricity grid, which calls for increased levels of renewable energy sources in the grid; provision of timely information and control options to consumers; deployment of smart technologies, appliances, and advanced metering devices; and real-time pricing of electricity [2]. The perspective of SGI is to develop the functional controls for a power electronic inverter capable of interfacing PV installations to the utility grid. For the purpose of this article, smart controls of a voltagesource inverter are defined as the combined functional ability to supply power to local loads, supply power to other utility loads up to rated capacity of the inverter, provide voltage support at the PCC of the utility, store energy in a local lead-acid battery bank, and provide control options to the consumer based on near real-time electricity information obtained from the utility through advanced metering devices. A general modular design methodology for flexible inverters that may cater to increasing demands in the smart grid is presented in [3]; however, the combined smart functionalities described here are deemed unique. The smart inverter functionalities described in this study look beyond the recommendations of the current national technical standard for interconnecting DG sources to the grid—IEEE Standard 1547 [4]—in providing voltage support at the PCC—thus, offering an ancillary service in case of low-voltage scenarios. Traditionally, voltage sags in distribution systems are corrected using utility-owned (or) -operated capacitor banks; however, with the advent of inverters with smart functionalities, the ability to regulate voltage at the PCC is brought to the customer. The authors have not probed the safety issues stemming from performing voltage control on the grid side using the proposed inverter setup. Based on real-time spot pricing of electricity obtained from the utility using an advanced metering device, the inverter control algorithm determines the optimal operating mode. This algorithm enables the inverter to: 1) schedule local loads and 2) determine either to locally store energy or sell energy to the grid. Description of the Inverter Control The voltage-source inverter is designed as a 5 kVA onephase, full-bridge converter operating at 120 V, 60 Hz ac with current control and voltage control to function in two modes: grid tied and islanded, respectively [5]–[7]. The entire control is developed in the DQ frame with a virtual Q axis (as the application is one phase). In the technical literature, this second virtual quantity is obtained either by the derivative of the fundamental signal [8] or by delaying the real-axis quantity by one quarter of the line period [9]. In this implementation, the latter technique is employed. Figure 1 depicts the block diagram of the inverter simulation on a popular modeling and simulation platform. Phase-locked loops (PLLs) are used to supply the control loops with phase angle information [10]–[12], as shown in Figure 2. The current and voltage control shown use four proportional-integral (PI) controllers: two equal PIs for the inverter current (id and iq ) and two equal PIs for the inverter voltage (vd and vq ). The PI compensator is chosen for its simplicity and ease in implementation, and the respective gains
p_ref
iVSI
(p.u.) 2
q_ref
Current Loop Control Conversion to DQ Frame iVSI i _dq thetaVSI VSI
pq_ref
(p.u.) 1
Id_ref Iq_ref
pq_ref/Idq_ref
vVSI
vVSI thetaVSI
P1
1 pwm_out
Brkr
Conversion to DQ Frame
PLL
m φ
PWM Modulator
Current Control
Voltage Loop Control
vVSI thetaVSI
vVSI
i2dq m id_ref Iq_ref φ (degree)
Switch
v2dq
vVSI_dq
[1 0]
m vdvq_ref φ (degree)
vd_ref vq_ref
Voltage Control
m φ
P1
PWM Modulator
2
Current control and voltage control loops for the inverter with smart functionalities.
The Smart Functionalities of the Inverter The primary intent of the inverter development with smart functionalities is to enable an efficient interconnection and economical operation for dispersed PV-based DG installations to the utility grid. The motivation of this study comes from a pilot program by a local utility in Colorado to implement some of the smart grid recommendations in a candidate medium-sized metropolitan area [13]. Such an implementation is based on the ubiquitous deployment of PV installations at a residential level of the candidate city. Some distinctive aspects of this pilot program are smart metering, the incorporation of smart appliances, the provision of pricing information to consumers, the provision of some control
Voltage Reference 350
Voltage Control Loop +–
12,235×[1 2×683 683×683] s 3 + 2×32,509s 2 + 32,509×32,509s
Out
Inverter Control Algorithm
PWM Generator 1 >
0
PWM Generator 2 0
>
Current Control Loop 2778×[1 1,700] s 2 + 94,248s
Current Reference + –
-C-
g 1
in1 1 S1 2
D2
v+ –
L = 430 µH + i –
C2 = 1.3 mF g 1
dc Link Side (Vdc)
S2 2
in 2 3
D1
C1 = 560 µF
out1 2 Battery Side (VB) out2 4
3 Block diagram of bidirectional dc–dc buck-boost converter subsystem in the inverter setup.
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options to consumers, and information exchange on a fully networked system enabled by massively deployed sensors. It is in this regard that the inverter with the aforementioned smart functionalities is being proposed in this article. The local load served by the inverter is modeled as two components: primary and secondary VSI loads, which distinguishes the critical loads from others that can be scheduled at the location. So if the inverter is operating at the islanded mode and it does not have enough power to supply all local loads, only the VSI primary load will be supplied. Another convenience of this load set is the ability to operate in the economic mode. This will be explained in the following sections. The input to the smart inverter is a steady-state voltage of 350 Vdc , provided by PV panels [14], with a nominal output voltage of 192 V. A dc–dc boost converter has been used in the model to raise the PV voltage level to 350 Vdc. The inverter setup also includes a lead-acid battery storage bank with nominal voltage of 192 V and 24 Ah cells [15] connected to the dc link through a bidirectional dc–dc buck-boost converter, modeled as shown in Figure 3. A
can be tuned through extensive simulations. In the case of the device shown in Figure 2, the PI compensators are used to generate the references for a pulsewidth modulator (PWM). According to Figure 2, the main control consists of two loops: voltage loop, which is enabled when the inverter operates in islanded mode, and current loop for a grid-connected condition.
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the grid based on IEEE Standard 1547 [4]. The functionality is modeled as a subsystem with the following input [Qref] parameters: frequency, phase, and DQ +– PI 1 frame voltage of the grid (denoted as Brkr_Q 1 Discrete Freq_Grid, Theta_Grid, and Vdq_Grid, Discrete Q ref PI Controller 0 PLL-Driven respectively in Figure 5) and inverter Switch Fundamental Value side (denoted as Freq_VSI, Theta_VSI, freqg Freq m and Vdq_VSI, respectively in Figure 5). The algorithm compares the inputs with sin cosg Sin_Cos IEEE Standard 1547 recommendations φ Fix Inverter Apparent v3pu In and generates an output signal (Brkr) in Power at 5 kVA the required time frame according to +– 2 25 sqrt IEEE Standard 1547 to island or to Pref reclose the inverter to the grid. It is [Qref] Pref_kW × pertinent to note that: 1) the 1547 Saturation recommendations are not reproduced 4 in this article and the attention of the curious reader is pointed to [4] and 2) Block diagram of the STATCOM function used in the inverter-based DG setup. while the inverter looks beyond IEEE battery model from a popular library simulation platform Standard 1547 in its ability to regulate the voltage at the [16] was used for this purpose. The storage subsystem brings PCC, it conforms to IEEE Standard 1547 for grid connecflexibility to the system, e.g., the ability to supply local tion and disconnection. loads when the inverter is islanded without enough power, to store cheap energy, and to sell when the price is higher. Bidirectional DC–DC Buck-Boost Converter The bidirectional dc–dc buck-boost converter shown in Figure 3 is responsible for controlling the charge and discharge STATCOM Function As affirmed by [17], if individual distributed energy (DE) sys- processes of the battery setup. This converter can behave either tems are allowed to regulate reactive power, they can also be as buck or as boost converter depending on which switch (S1 used to provide voltage support at the low-voltage single-phase or S2 in Figure 3) is ON. When the battery is charging, switch distribution level. Simulations with inverters for DG systems S1 is ON, and the converter runs in the buck mode. When indicate that the inverter must be connected as a static var com- the battery is providing power to the inverter, the S2 is ON, pensator (STATCOM), where the reactive power is injected and the converter works in the boost mode. The specifications of the dc–dc converter are shown in Table 1. into the ac grid to regulate voltage at the PCC. The inverter control algorithm has been used to decide The STATCOM function block used in this inverter setup is shown in Figure 4; this block measures the voltage sag (grid the buck-boost converter action. For the buck functionality, voltage magnitude minus nominal voltage reference), if any on a current control loop was designed based on the average the grid side, to calculate the necessary reactive power for volt- current method [18]. For the boost functionality, a voltage age compensation. The PI controller keeps the sag voltage control loop was designed based on the K factor [19]. The buck-boost inductor nominal current IL is calcunull. There may be times in the inverter operation when it may not be profitable to perform a voltage compensation, such lated as shown in as during times when the real power spot pricing is higher; P during such times, a controlled switch (see Figure 4) is used : (1) IL ¼ VB for bypassing the voltage compensation functionality of the inverter, and only real power is supplied to the grid.
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Voltage Sag 1 p.u. Measurement
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Islanding and Reclosure Function
The islanding and reclosure function performs the task of connecting (and islanding) the inverter-based DG to (and from)
Freq_Grid Freq_VSI Theta_Grid Theta_VSI Vdq_Grid Vdq_VSI
IEEE Standard 1547 Inputs Output
Islanding and Reclosure Conditions
Brkr
5
Islanding and reclosure subsystem of the inverter-based DG setup.
TABLE 1. SPECIFICATIONS OF THE BIDIRECTIONAL DC–DC BUCK-BOOST CONVERTER USED IN THE INVERTER-BASED DG SETUP. Quantity (Symbol)
Value
Buck-boost nominal power (P)
5 kW
DC link nominal voltage (Vdc )
350 V
Battery nominal voltage (VB )
192 V
Switching frequency (fs )
15 kHz
Resistance of battery bank (rB )
0.2 X
Inductor resistance (rL )
0.02 X
Resistance of the C1 and C2 capacitors (rC )
0.05 X
pffiffiffiffi where the pffiffiffi ffi xz ¼ xGC K , the xp ¼ xGC K ; and K3 is type 3 dc gain, THE PI as shown in Figure 3. K is a constant calculated using the boost-phase adCOMPENSATORS vance [20]. VB ðVdc VB Þ Figure 7 shows the Bode plot of the ARE USED TO : (2) L¼ transfer function for the open-loop comfs Vdc 0:5IL pensated system, Tboost (s) as per unit GENERATE THE system for the type 3 control method. The buck capacitor, C1 , is designed From Figure 6, it is observed that to limit the ripple voltage at the battery REFERENCES FOR PM and the gain margin (GM) are infito 0.1% of the nominal battery voltage A SINUSOIDAL nite and 2.82 dB, respectively, which (VB ), as shown in indicates an unstable system. From PULSEWIDTH 0:5IL Figure 7, it is observed that PM is C1 ¼ , (3) 60.3° and GM is 9.72 dB, which is 8fs 0:001VB MODULATOR. satisfactory for a stable close system because PM is almost in between 30° and the boost capacitor, C2 , is intended and 60° and GM is higher than 6 dB to limit the ripple voltage at the dc link to 0.1% of the nominal dc link voltage (Vdc ), as shown in [21]. This represents good robustness, which is important for future implementation of the device in hardware protoPðVdc VB Þ type. The prototyping of the device is beyond the scope of (4) C2 ¼ 3 : this article. fs 0:001Vdc The inductance (L) of the buckboost converter is designed to limit the ripple current at the dc link to 50% of the nominal current (IL ), as shown in
Current Control Loop
The type 2 controller G2 (s), shown in (5), has been used for this method of current control of the buck-boost converter [20]. G2 (s) ¼
K2 (s þ xz ) : s(s þ xp )
(5)
Voltage-Control Loop
The transfer function of the small signal dynamic boost model, Tboost (s), can be expressed as [20] 1:3 3 104 s2 þ 0:22s þ 3:5 3 104 : 9:4 3 105 s2 þ 1:3 3 102 s þ 47:2
G3 (s) ¼
K3 (s þ xz )(s þ xz ) , s(s þ xp )(s þ xp )
(6)
Open-Loop Bode 80 Magnitude (dB)
Figure 6 shows the Bode plot of the open-loop transfer function for the uncompensated system, Tboost (s), as per the unit proportional control method. The zero-crossing gain frequency, xGC , has been chosen as 1/20th of the angular switching frequency (xs ¼ 2pfs ). The phase margin (PM) has been adopted at 60°. Thus, the boost phase advance was calculated to be 147° [20]. As the boost phase advance is higher than 90°, a type 3 controller G3 (s) shown in (7) was chosen [20] as
Phase (degree)
Tboost (s) ¼
60 40 20 0 360 315 270 225 180 102
103
104
Frequency (rad/s)
(7) Open-loop Bode plot for the uncompensated system.
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6
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The high-frequency pole xp must be chosen close to the switching frequency fs, the zero xz must lie in between one half and one third of the resonant frequency, and the K2 is a type 2 dc gain, as shown in Figure 3.
Premise of Operation of the Smart Inverter The smart functionalities of the inverter are aimed at the provision of real and reactive power support to local loads, the provision of real and reactive power to grid loads up to the rated capacity, the option to control voltage at the PCC during voltage sags, and decision-making ability aided by information of real-time pricing obtained through advanced metering devices from the utility grid. Based on these functionalities, the inverter operation is governed by certain rules, which determine the mode of operation, identified in this article as supermodes and submodes. Depending on the status connection to the grid as determined by compliance with IEEE Standard 1547, there exist two supermodes: stand-alone (S1) and grid-tied (S2) modes. In supermode S1, i.e., the stand-alone mode, the inverter is islanded (isolated) from the electric distribution system, and it is subject to operation under one of the following three submodes, namely, s1, s2, and s3, depending upon the available inverter active power (PINV ) and the
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Magnitude (dB) Phase (degree)
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price to sell active power to grid ($PS ), local active power demand (ZINV ), where PINV represents the total power output spot price to sell reactive power to THE LOCAL LOAD grid ($QS ), and a threshold value of of the PV panels and the battery bank output, and ZINV represents the sum of the grid pricing of an electricity unit SERVED BY THE that will enable the consumer to decide the primary VSI load plus secondary which loads will be powered using the VSI load. INVERTER IS inverter. In this case, it is assumed that In submode 1 under supermode 1 the grid has infinite demand, i.e., the (identified as S1s1), when PINV is lesser MODELED AS grid will purchase whatever the inverter than ZINV , the power output of the PV intends to sell. panels and stored energy are not enough TWO In submode 1 under supermode 2 to supply the full demand of the local COMPONENTS, (identified as S2s1), when $QS is load. In such circumstances, prioritization of local demand is effected, and greater or equal than $PS , the inverter PRIMARY AND selected loads (primary VSI load) are is controlled to provide voltage suppowered by the inverter. If, after the port compensation to the grid. If there SECONDARY VSI selection of loads, there is any remainshould exist additional inverter capaing power from the PV panel, it will be bility to provide real power, then the LOADS. directed to storage in the battery bank. inverter is controlled so that ZINV is Typically, such a redirection to the supplied by the PINV , and any remainenergy storage depends on the level of ing power is sold to the grid or stored the state of charge (SOC). It is important to note that the in the battery bank [22]. SOC control of battery banks has not been developed by In submode 2 under supermode 2 (identified as S2s2), the authors. when $QS is lesser than $PS , the inverter is controlled to fix In submode 2 under supermode 1 (identified as S1s2), the reactive power reference to zero and to supply real power when PINV is greater than ZINV , the available inverter to local loads ZINV , and any remaining power is sold to the power is greater than the local demand. In this case, the grid or stored in battery banks [22]. excess power is routed to the battery banks for storage. There is another submode under this supermode, i.e., In submode 3 under supermode 1 (identified as S1s3), S2s3. This is related to the option of powering local loads PINV is equal to ZINV , wherein the available inverter power using the inverter versus the option of buying active is equal to the local load demand. In this case, the inverter power from the grid when there is power available from powers the local loads without storage. In this case, priori- DG. This can be chosen based on the comparison of the tization of loads may be effected if there is a need to store real-time electricity pricing obtained from the grid ($PB ) some of the energy for later use. with a threshold value, such as the marginal cost of In supermode S2, i.e., the grid-tied mode, the inverter electricity production or a set customer preference, deis interconnected to the electric distribution system and noted as MCP. If $PB is lesser than MCP, then the inverter is subject to operation under one of the following four load is supplied by the grid, and PINV is stored in a batsubmodes, s1, s2, s3, and s4, depending on the available tery storage for consumption or selling to the grid later, inverter active power (PINV ), local active power demand possibly during isolation from the grid or when real-time (ZINV ), and economic considerations for trading active pricing of electricity is conducive to profitability, respecand reactive power with the grid on a spot-pricing basis. tively. Or if there is no PINV available, then the electric The variables for economic consideration include the spot energy can be purchased from the grid and stored in a battery for later use. If $PB is greater than the MCP, then the inverter is so controlled that ZINV is supplied by PINV , and the stored energy in the Open-Loop Bode battery bank and any remaining 60 power is sold to the grid. The use of 40 the production marginal cost may 20 not be applicable in the case of PV 0 systems; however, if the installation –20 considers customer preferences as –40 input as in [22], then the above rationale can be upheld as shown in 360 case study 1. 270 Submode 4 under supermode 2 (identified as S2s4) refers to the oper180 ation at an economic mode; this mode 90 is being proposed as an alternative 101 102 103 104 105 106 under the mode S2s3. This typically Frequency (rad/s) occurs when the cost of purchasing 7 electricity from the grid is relatively Open-loop Bode plot for the compensated system. expensive, i.e., such as described in
Voltage (p.u.), Current (p.u.)
Inverter Side
Voltage (p.u.), Current (p.u.)
Simulation Results Based on the foregoing discussion, three case studies describing the smart functionalities of the inverter are presented in this section. The power and voltage bases used were 5 kWand 120 V, respectively. The gridside quantities are assumedly measured at the PCC.
THE SMART FUNCTIONALITIES OF THE INVERTER ARE AIMED AT THE PROVISION OF REAL AND REACTIVE POWER SUPPORT.
associated with this operation mode are shown in Figure 8. It is noticeable in Figure 8 that the voltage and current waveforms on the inverter side are shifted by 180°; this implies that the inverter is buying electric energy from the grid. As the voltage and current are both at 1 per unit (p.u.), then the inverter-active power is 5 kW, i.e., at the inverter nominal power. At the grid side, the voltage is 1 p.u. and the current is 1.6 p.u. indicating that the grid-active power is 8 kW. This corresponds to summation of the VSI loads (3 kW) and the power supplied to the battery bank (5 kW). The reactive power demands at the inverter loads were set to zero.
1
Voltage (p.u.) Current (p.u.)
0 –1 0.7
0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 Time (s)
0.8
Grid Side
2
Voltage (p.u.) Current (p.u.)
1 0 –1 –2 0.7
0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 Time (s)
0.8
8 Current and voltage at inverter and grid side for Case 1.
1 0.5 0 –0.5 –1 0.58
Voltage (p.u.) Current (p.u.)
0.6
0.62
0.64
0.66 0.68 Time (s)
0.70
0.72
0.74
0.76
Grid Side Voltage (p.u.), Current (p.u.)
In this simulation, the local demand for an hour is 3 kW split into the primary VSI load of 2 kW and the secondary VSI load of 1 kW. As defined in mode S2s3, the real-time electricity pricing obtained from the grid ($PB ¼ US$1/kW) is lower than a set customer preference (MCP ¼ US$2/kW). In this case, the reference points of the inverter are set such that the gridactive power is purchased to power the local load and to charge the leadacid battery bank. Therefore, the inverter is connected to the grid. The bidirectional dc–dc buck-boost converter is operated in the buck mode. The voltage and current waveforms
Voltage (p.u.), Current (p.u.)
Inverter Side
Case 1, Viz., S2s3
1 0.5 0 –0.5 –1 0.58
Voltage (p.u.) Current (p.u.)
0.6
0.62
0.64
0.66 0.68 Time (s)
0.70
0.72
0.74
0.76
9 Current and voltage at the inverter and grid sides for Case 2.
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S2s3; thus, the inverter is set up for powering all its local loads while connected to the grid. However, in such a case, if the inverter power is not enough to supply its loads, the customer has an option to operate at an economic mode, i.e., effecting prioritization of primary VSI loads and shedding the secondary VSI loads (as in S1s1). This submode is achieved in the simulation by the use of a flag variable; if the flag is set to zero, it operates in the economic mode using variable loads; and if the flag is set to one, it operates in an always supplying loads mode, as the inverter power is not enough, power from the grid is purchased to supply the remaining loads. It is pertinent to note that this study does not consider the time line for powering loads or charging storage, i.e., in terms of energy demanded and supplied. Consideration of the energy supplied and demanded is inherently tied to hours of available sunlight and to the PV installation capacity and battery storage [22]. Since this article is aimed at describing the smart functionalities of an inverter, the authors feel that the use of power ratings is adequate for a proof of concept.
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as indicated by the grid voltage (1 p.u.) and grid current (0.2 p.u.). At 0.65 s, the inverter begins operating in the islanded mode with the inverter voltage and current at 1 p.u. and 0.8 p.u., respectively; therefore, the inverter-active power is 4 kW (3 kW from PV panels and 1 kW from batteries). The grid-active power is 0.0 W (grid current is 0.0 p.u.). As in Case 1, the reactive power demand is taken as zero in this case as well. The transients seen around 0.65 s in Figure 9 are caused by transition from the grid-tied mode to the islanded mode.
Grid Voltage Magnitude Voltage Magnitude (p.u.)
1.1
1
0.9
0.8 0.5
1
1.5
2 Time (s)
2.5
3
3.5
In this simulation, the primary and secondary VSI loads are both set at 1 kW. A voltage sag was induced on the grid side between 0.5 and 1.2 s. Operating in the S2s1 mode, Voltage magnitude on the grid side for Case 3. the inverter provides voltage support to the PCC. This phenomenon, including the transient performance, can be observed in Figure 10. The grid-voltage magnitude (Vmag ) Case 2, Viz., S1s1 In Case 2 (S1s1), the primary VSI load is 4 kW, and the is kept at 1 p.u. by the action of the STATCOM function secondary VSI load is 2 kW. The PV panels are set up to block of the inverter. The voltage and current waveforms assoprovide 3 kW. The inverter is initially in the grid-tied ciated with this mode of operation are shown in Figure 11. Notice that the inverter is not islanded by the islanding mode until 0.65 s when the islanding mode is reinforced, possibly because of a far away fault in the grid. In the and reclosure subsystem because the magnitude and duraislanded mode, the PV panels are not able to supply tion of the voltage sag do not fall within the limits recomenough power to the loads, thus the bidirectional dc–dc mended by IEEE Standard 1547 [4]. From the simulation start-up to 0.5 s, the inverterbuck-boost converter will operate in the boost mode, and the lead-acid battery bank will provide 2 kW. As the load active power is 5 kW, and the grid-active power is 3 values are higher than 5 kW, the inverter (or the kW since 2 kW is consumed by the VSI loads. The surcustomer) must buy 1 kW of active power. Initially, in the plus is injected to the grid side. When the grid has a voltgrid-tied mode, the inverter (or customer) purchases this age sag from 0.5 to 1.2 s, the inverter voltage and current 1 kW from the grid. However, when the inverter func- are both at 1 p.u. and 32.4° out of phase (see Figure 11); tions in the islanded mode, the secondary VSI load drops this indicates that the inverter-active power is 4.2 kW as described in mode S1s1. The voltage and current wave- and inverter-reactive power is 2.7 kvar. The grid-active forms associated with this mode of operation are shown power is 2.2 kW, and grid-reactive power is 2.7 kvar. So the inverter is selling active and reactive powers to in Figure 9. When the inverter is connected to the grid, the inverter grid, and it is compensating voltage sag by supplying voltage and current values are both 1 p.u., indicating that reactive power to PCC. After 1.2 s, the voltage sag event the inverter-active power is 5 kW (3 kW from PV panels on the grid is assumed to be cleared; however, the inverter and 2 kW from batteries). The grid-active power is 1 kW keeps providing reactive power; hence, the voltage magnitude on the grid side increased quickly. However, the STATCOM action decreased the inverter-reactive power to 0.0 var and consequently Inverter Side 1 increased the inverter-active power to Voltage (p.u.) 5 kW, helping the stabilization of Current (p.u.) Vmag at 1 p.u. Thus, the postevent 0 grid-active power is 3 kW and the postevent grid-reactive power is 0 var. –1 It is observed in Figure 11 that the 1.1 1.11 1.12 1.13 1.14 1.15 1.16 0.17 1.18 1.19 1.20 voltage waveform on the inverter side Time (s) leads the current waveform; this is Grid Side caused by the injection of reactive 1 Voltage (p.u.) power from inverter to grid side. It is Current (p.u.) also noticeable that the voltage and 0 current waveforms on the grid side are out of phase by approximately –1 231°. This phase difference is not 1.1 1.11 1.12 1.13 1.14 1.15 1.16 0.17 1.18 1.19 1.20 exactly 180° as in Case 1 because in Time (s) this case the inverter injects (selling) 11 both active and reactive powers to the grid. Current and voltage at the inverter and grid sides for Case 3. Voltage (p.u.), Current (p.u.) Voltage (p.u.), Current (p.u.)
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Case 3, Viz., S2s1
Conclusions This article describes the topology, control philosophy, operational algorithm, and simulation results of a voltage-source inverter, interfacing a PV-based DG system to the grid, with certain smart functionalities such as the ability to supply real and reactive power to local loads, supply real and reactive power to other utility loads up to rated capacity of the inverter, store energy in a battery bank, provide voltage support at the PCC, schedule loads, and provide control options to the consumer based on near real-time electricity information obtained from the utility through advanced metering devices. This VSI is capable of automatically choosing the operation mode based on a set of super and submodes corresponding to system conditions and real-time pricing of electricity. The smart functionalities are deemed to look beyond the recommendations of the current national technical standard for interconnecting DG sources to grid, IEEE Standard 1547, i.e., providing voltage support to PCC, thus, offering an ancillary service in case of low-voltage scenarios. This represents one of the tenets of the SGI, namely, enabling active participation of consumers in the demand response using timely information and control options. Several case studies are presented to illustrate the functionalities of the proposed device.
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Renata Carnieletto and Felix A. Farret are with the Universidade Federal de Santa Maria in Santa Maria, Brazil. Danilo Iglesias Branda˜o is with the Universidade Estuadal Paulista in Sa˜o Paulo, Brazil. Siddharth Suryanarayanan is with Colorado State University, Colorado. Marcelo G. Simo˜es (
[email protected]) is with Colorado School of Mines in Golden, Colorado. Suryanarayanan and Simo˜es are Senior Members of the IEEE. This article first appeared as “A Multifunctional Single-Phase Voltage-Source Inverter in Perspective of the Smart Grid Initiative” at the 2009 IEEE Industry Applications Society Annual Meeting.
IEEE INDUSTRY APPLICATIONS MAGAZINE SEPT j OCT 2011 WWW.IEEE.ORG/IAS
Acknowledgments This work was supported in part by the Colorado Clean Energy Project through the Colorado Renewable Energy Collaboratory and the Xcel Energy Foundation and in part by the U.S. National Science Foundation under grant 0757956. D. Branda˜o gratefully acknowledges F. Marafa˜o and D. Colo´n of the Universidade Estadual Paulista for their insightful comments about this article and for their help in designing the controller parameters described in the “The Smart Functionalities of the Inverter: Bidirectional DC–DC Buck-Boost Converter” section.
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