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Power Electronics Platforms for Grid-Tied Smart Buildings

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Renewable energy sources (such as sun, wind, water, or fuel cells) are attracting great interest for either grid-tied or off-grid arrangements in smart green buildings. It must be either used when generated, stored for future use on-site, delivered to the power grid, or shared among combination of these. Grid-tied buildings are connected to the utility grid service lines. Off-grid buildings have no connection to utility service lines. Both types employ inverters to convert power from direct current (DC) to alternating current (AC), and most off-grid systems have batteries to store energy for use when needed. Accordingly, power electronics systems are playing an important role as the enabling technology for smart grid. In addition, smart meter represents the interface part between the green building and the utility grid. In order to realize the interaction between both systems, a bidirectional power conditioning module is needed. This chapter introduces the different power electronics platforms suitable for grid-tied smart green buildings (such as residential homes, commercial, and industrial) as well as its integrative functionality with advanced metering infrastructure (AMI). In order to show the superiority of these platforms in conjunction with smart meters, a hardware case study with one of the most popular power electronics topologies is presented. A voltage/current sensor module is connected through the AC-bus common coupling point between the MISO system and utility grid. This module is used to acquire the power measurements as a part of smart meter functionality for real-time energy management operation. The stability of DC-bus voltage is the main parameter to decide on energy flow scenario. Then, we can set upper and lower limits for DC-bus voltage in order to maintain system balance operation while connected to AC grid. For example, if Vdc is higher than Vdc_up, then it means that there is excess power available from wind and PV generators. Accordingly, the battery will operate in the charging mode to store energy while supplying local load demand and sending power back to grid based on smart building contract agreement. In this case, net meter measurement will show a negative reading which indicates feeding energy into utility grid. On the other hand, if Vdc is lower than Vdc_low, then it means that either one or both wind and PV generators is supplying less power into MISO system. As a result, the battery will operate in the discharging mode to support local load demand while receiving a portion of the needed power from grid. In this case, net meter measurement will show a positive reading which indicates receiving energy from utility grid. Finally, if Vdc is within the upper and lower stability limits, then it means that both wind and PV generators are balanced and sharing power with utility grid to supply building loads. As a result, the battery will operate in the rest mode which improves battery life time. The detailed parameters and specifications of MISO platform are listed in Tables 1 and 2.
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Chapter 6
Power Electronics Platforms for Grid-Tied Smart
Buildings
Mahmoud Amin
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/62920
Abstract
Renewable energy sources (such as sun, wind, water, or fuel cells) are attracting great
interest for either grid-tied or off-grid arrangements in smart green buildings. It must
be either used when generated, stored for future use on-site, delivered to the power
grid, or shared among combination of these. Grid-tied buildings are connected to the
utility grid service lines. Off-grid buildings have no connection to utility service lines.
Both types employ inverters to convert power from direct current (DC) to alternating
current (AC), and most off-grid systems have batteries to store energy for use when
needed. Accordingly, power electronics systems are playing an important role as the
enabling technology for smart grid. In addition, smart meter represents the interface
part between the green building and the utility grid. In order to realize the interaction
between both systems, a bidirectional power conditioning module is needed. This
chapter introduces the different power electronics platforms suitable for grid-tied smart
green buildings (such as residential homes, commercial, and industrial) as well as its
integrative functionality with advanced metering infrastructure (AMI). In order to show
the superiority of these platforms in conjunction with smart meters, a hardware case
study with one of the most popular power electronics topologies is presented.
Keywords: power electronics, smart meters, green buildings, grid-tied systems, net
meters
1. Introduction
In the future smart green buildings, there will be increasing connection to the distribution
network of renewable energy sources, electric vehicles, and heat pumps. Most of alternative
energy sources generate direct current (DC) power (for example, photovoltaic and fuel cell
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
systems). As a result, a power electronics inverter is required to connect them into the utility
grid. On the other hand, the alternating current (AC)-based alternative energy sources (for
example, wind, wave, and hydro systems) can be either directly connected to the AC grid or
indirectly through AC-DC and DC-AC conversion interface stages. Power electronics convert‐
ers are then represents rapid independent control means of real-reactive power to satisfy grid-
connected alternative energy conversion system needs [1]. In this chapter, we will explore the
different power conversion topologies and energy link integration methodologies based on the
renewable energy system structure (single or hybrid).
Grid-tied buildings are classified as either feed-in-tariff (FIT) or net metered (NM). In FIT
systems, utilities purchase renewable energy at variable rates, which are usually higher than
the sales price. This is tracked by using two meters: one to measure electricity going to the grid,
and another for electricity coming from the grid. NM systems buy and sell electricity at the
same rate, using a single meter, which runs either forward or reverse, depending on the
direction of power flow. Most NM systems do not provide homeowners with credit for any
electricity they generate beyond what they use. FIT systems, however, provide 100% credit for
power put into the grid, allowing homeowners to receive a check from their utility when their
production exceeds their use. In USA, regulations for grid-tied systems are established by the
local municipality or state. Not all utilities allow FIT or net metering [2].
In fact, both FIT and NM systems can operate with either analog or digital (smart) meter. It
became feasible to most people that smart meters are considered an integral part of any
intelligent-based green building. In the past, analog regular meters can only provide distrib‐
utors with power flow data at the substation level. With smart meters, it can deliver detailed
in-depth real-time information about load energy consumption which extends visibility down
to the consumer level. In addition, smart meter can help to manage and control customer loads
remotely by involving more IQ functionalities into metering system design [3].
2. Home smart grid
In general, smart grid is considered an integrated and interactive power network. It brings
generating units closer to consumers. On the other hand, consumers may also act as generators.
A smart home involves three new components: smart control and measuring devices, digital
communications systems, and computer software programs [4, 5]. Figure 1 shows the smart
grid simple structure that allows power to be fed into it from different energy sources and
provides real-time management to maximize efficiency. The home smart grid makes individ‐
ual homes energy efficient by reducing total energy consumption and lowering peak demand.
Homeowners can install renewable energy generating components to supplement power they
draw and feed power back when production exceeds home requirements. Home energy
efficiency requires three devices: smart meter that measures power in real time, home receiver
that allows homeowner to monitor power use over time, and power usage monitor (watt
meter) that accurately measures power consumption by individual devices.
Smart Metering Technology and Services – Inspirations for Energy Utilities
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Figure 1. Home smart grid infrastructure.
2.1. Home meters
There are two common types of metering home systems: analog and digital meters, as shown
in Figure 2. The analog (electric, gas, and water) meter measures flow over a given period of
time but cannot transmit measurement data. Utility companies are unable to charge according
to production costs. Users have no incentive to minimize use during peak demand. Digital
metering system represents an advanced smart meter that can be used to monitor the energy
consumption remotely and transfer this information to the control center through secured
communication network [5]. In order to facilitate monitoring functionality, we can add a digital
display to show the energy consumption and its corresponding cost to the customers. The in-
home display (IHD) is mainly used for this purpose. Another type of smart meters is the power
strip one. It contains one or more channels with voltage/current transducers to monitor the
amount of the power usage and relay contact to control major equipment in the building [6–
8]. The smart meter has the following features:
- Read electric use at least once per hour
- Transmit data to electric company
- Monitor for disturbances or interruptions
- RF- or IP-based wireless network
- Net meters allow homeowners to produce electricity and sell excess to utility company
Figure 2. A typical residential analog and digital (smart) electric meter with a digital display showing cumulative kilo‐
watt hours used.
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2.2. Smart meter and smart building
Giving everyone a smart meter will not deliver a smart building. Although smart meters are
an essential component of a truly smart home, there is more to smart buildings than just smart
meters (hardware, software, communication links, and controllers). It is called advanced
metering infrastructure (AMI) that measures, analyzes, and regulates energy usage. The IHD
plays an important role in real-time information monitoring. It may communicate directly with
appliances. It also provides simple tools for cutting energy consumption by 5–10% [9, 10].
Figure 3 shows different popular IHD units that allow electrical consumers to read data from
their home’s smart meter and other measuring devices.
Figure 3. The PowerPortal Home display unit and real-time power, cumulative use, and other data.
3. Renewable energy sources
Renewable energy sources are being developed in many countries to reduce CO2 emissions
and provide sustainable electrical power. The balance of particular technologies and their scale
changes from country to country. However, hydro, wind, biomass, tidal stream, and photo‐
voltaic (PV) are common choices. Substantial equipment cost investment is generally recov‐
ered and cost savings are realized over time [11, 12]. Figure 4 shows photographs for different
renewable energy sources.
Figure 4. Renewable energy sources.
Smart Metering Technology and Services – Inspirations for Energy Utilities110
3.1. Power converters platforms
Power electronics represents the enabling technology for green alternative energy sources in
smart buildings. It involves sophisticated conversion topologies as an interface integral part
between smart power distribution network and local microgrid sources [5]. The installation of
the new advanced alternative energy technologies (e.g., PV and wind) is playing a vital role
in residential and commercial smart buildings. Typically, installed capacity ranges from few
kilowatts for distribution-customer level to several megawatts for high-voltage transmission
grid. The following characteristics are important for power electronics systems for smart grids:
high efficiency, optimal energy transfer, bidirectional power flow, high reliability, synchroni‐
zation capabilities, EMI filtering, smart metering, real-time information, communications, and
fault tolerance/self-healing. Figure 5 shows different power electronics layers to integrate a
cluster of prosumers (an entity in the future grid capable of both producing and consuming
electric power) into the smart building.
Figure 5. Power electronics control and communication structure in smart buildings.
Power electronics converters can operate renewable sources on either off-grid standalone basis
or grid-connected basis. Grid-tied platforms can be classified into two main categories: single
input single output (SISO) platform and multi-input single output (MISO) platform. As an
example, a building that involves only one DC renewable energy source such as PV modules
mounted on the roof, then, the platform structure needed to connect the grid-tied FIT building
is SISO as shown in Figure 6. The output of a PV system is DC and therefore a DC-AC converter
is essential for grid connection.
Figure 6. DC source-based SISO platform for grid-tied FIT system.
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Another SISO platform can be used if the source was AC-based variable speed turbines such
as wind, small hydro, and tidal power generation as shown in Figure 7. Typically, this platform
uses two stages of power conversion. The first stage is AC-DC to convert from the variable
frequency AC generator power output to controlled/uncontrolled DC. The second stage is then
DC-AC to convert from DC into synchronized 50/60 Hz AC utility grid. To capture the
maximum power, the turbine rotational speed is set to the optimum range.
Figure 7. AC source-based SISO platform for grid-tied FIT system.
There are several other renewable technologies that use steam or gas turbines with synchro‐
nous generator such as biomass energy. This technology can be directly connected to utility
grid and is not discussed in this chapter. In general, power converter interface units can be
used to control the amount of the reactive power injected from renewable energy systems into
main AC grid. Then, network voltage is controlled while managing active power to satisfy the
requirements of the utility grid. On the other hand, Figure 8 shows hybrid connectivity to
provide larger energy production to the building as well as the utility grid which is called as
MISO platform. Additionally, MISO platform is sometimes called as multi-port power
converter (MPPC) since it combines more than one input port based on the number of the
micro energy sources and storage installed in the building.
Figure 8. Hybrid source-based MISO platform for grid-tied FIT system.
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3.2. PV system in home
Energy from sun is transformed into an energy source that can do useful work or provide
useful heat. We pay cost of converting stored sun energy into useable energy form. Most energy
sources originated from sun except geothermal energy. The sun’s free energy can be utilized
in three forms:
Passive use; for example, allowing natural sunlight in for heat.
Direct heat transfer; for example, water heater collects solar heat and transfers it to water
PV effect; causes some substances to generate an electric current when exposed to sunlight
Many countries such as USA, Spain, and Germany, have extended their PV system installation
to larger capacities. FITs are finding great interest with PV as it provides guaranteed payment
per energy unit (p/kWh) generated from local renewable power sources. Professional help is
essential to install a grid-connected solar power system in home. It requires “balance-of-
system” equipment such as power conditioning, safety, meters, and instrumentation. On the
other hand, installing a standalone PV power system is usually chosen for homes in remote
areas. Successful systems employ a combination of power-generating technologies and
techniques to reduce electric energy requirements. Standalone system is still not as complex
as creating grid-connected system. Figure 9 shows both grid-connected and standalone PV
systems in home. The grid-connected generating system feeds power into the grid through
one meter while the home’s electric current is brought into the home from the grid through
another meter. The standalone solar generating system with PV panels connected through a
controller to a battery stack and an inverter.
3.2.1. Maximum power point tracking (MPPT) control
Figure 10 represents the maximum output power locus for an individual PV cell. It is inter‐
esting to observe that neither the maximum voltage nor the maximum current are the same
for different levels of illumination at the maximum power point. As the output power (Po) is
given as:
o o o
P V I=
(1)
Then the peak power may be given from the following condition:
0
o o
o o
o o
dP dV
V I
dV dI
= + =
(2)
that is:
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o o
o o
dV V
dI I
= -
(3)
The meaning of this expression is that the dynamical internal resistance of the source should
match the external load resistance, leading to special power peak tracking control approaches.
Figure 9. A schematic diagram shows installation of a grid-connected and standalone PV power system in home.
Figure 10. Conditions of maximum power for various illumination (insolation) levels.
3.2.2. PV-based SISO platform
Figure 11 shows the schematic of SISO-based grid-connected PV system. The main elements
of this system are:
1. Boost DC-DC converter to increase the output voltage and extract the max. power
2. Single-phase DC-AC voltage source inverter (VSI)
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3. Grid interface output filter with/without isolation transformer
4. Voltage/power controller
There are several other DC-DC conversion topologies that can be used in the first stage such
as flyback, half bridge, full bridge, and push-pull [13]. Generally, the DC-DC converter output
is maintained to be constant as an input of the inversion stage. On the other hand, the MPPT
technique is continuously used to find the proper PV voltage that allows most power to be
extracted while PV system parameters (for example, insolation and cell temperatures) changes.
The DC voltage obtained from the DC-DC converter is inverted to 50/60 Hz AC. A VSI is widely
used. Typically, VSI uses a pulse width modulation (PWM) switching pattern to reduce the
output harmonic contents. Then, a final stage of grid-tie-filter is connected between VSI output
and AC grid in order to minimize the current harmonics injected into main power network.
An isolation transformer is sometimes placed at the VSI output to prevent DC injection into
the grid.
Figure 11. Domestic grid-connected SISO PV system block diagram.
3.3. Wind energy system in home
Wind is a manifestation of solar radiation. Air currents are winds that power weather systems
continuously across the surface of the planet. The annual average wind speed is larger than or
equal to 12 mph considering terrain and wind gusts. Favorable locations have wind resource
potential of 510 kWh or more. We should also consider other factors such as investment, zoning
codes, location, and so on. For home-scale wind turbines, residential typical range is 500 W–
100 kW. It is determined based on amount of electricity to generate, budget, and wind resource.
The 5–15-kW turbine should provide 75–100% of the typical home’s power requirements.
Figure 12 shows how a wind turbine works for small-scale 2-kW home model and the main
components for grid-connected wind turbine generating system [14].
According to Betz theory, the turbine mechanical power (Pt) is given by:
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3
0.5 /
t p
P C AV kg m s
r
= ×
(4)
where ρ is the air density, Cp is the power coefficient, V is the wind speed, and A is the area of
the rotor blades. Considering the wind speed, wind generation system (WGS) can be classified
into no, partial, and full load conditions. The wind turbine operates in the no load condition
if the measured wind speed is not within the cut-in/cut-out wind speed range. On the other
hand, WGS output power is regulated at full load power region by changing the pitch angle.
In addition, the maximum extracted power can be achieved by controlling the tip speed ratio.
As the rotor speed must change according to the wind intensity, the speed control of the turbine
has to command low speed at low winds and high speed at high winds, so as to follow the
maximum power operating point as indicated in Figure 13. It is observed that the maximum
power output occurs at different generator speeds for different wind velocities [15].
Figure 12. Hummer Wind Power’s 2 kilowatt wind turbine and components for a grid-connected wind turbine gener‐
ating system.
Figure 13. Turbine rotation versus power characteristic related to wind speed.
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3.3.1. Wind turbine generators
For variable-speed wind turbine (VSWT), there are several types of generators that can be used.
Figure 14 shows the doubly fed induction generator (DFIG) which employs a wound rotor
induction machine with back-to-back converter-inverter connected to its rotor terminals. The
real-reactive power flow of the rotor circuit is controlled via power converters. The rotor side
converter is also used to change the rotor speed through the active power absorbed or injected
into rotor. Furthermore, the wind generator may be self-excited induction (SEIG) or external
excited synchronous (EESG) machine with power converter. The maximum power is captured
by controlling the generator speed while stator output frequency varies with wind speed
conditions. The first stage of the power converter is used to convert the variable frequency
power to DC power. Then, the DC power is inverted to 50/60 Hz AC grid power via the second
conversion stage. Additionally, multi-pole permanent magnet synchronous generator (PMSG)
represents another wind turbine design with direct-driven gearless advantage. It introduces
better mechanical efficiency as well as higher power density.
Figure 14. Variable-speed wind turbine generation technologies.
3.3.2. Wind-based SISO platforms with SEIG
Figure 15 shows a simple structure of the SEIG-based SISO platform for wind generation
system. The system contains three cascaded stages (AC-DC, DC-DC, and DC-AC). The first
stage employs a diode bridge rectifier to convert the variable generator AC output into
unregulated DC. Then, the variable DC is converted to constant voltage DC link as an input
of the inverter stage. However, diode rectifier stage causes undesirable effects of generator
operation through voltage/current low-order harmonic distortion. Another option is to use
two stages back-to-back converter/inverter as shown Figure 16. The first stage uses a PWM
controlled rectifier which achieves high power factor and less harmonic distortion in the
generator side. However, this structure needs the use of bootstrap driver circuits as the
occurrence of short circuit through each phase is possible. Figure 17 shows another variation
of modified back-to-back power electronic converter structure that only uses four switches in
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each stage instead of six switches. On the other hand, the use of this topology is limited for
loss optimization due to voltage balance problem across DC-link capacitor and less power
capability.
Figure 15. WECS with power factor correction using DC-DC stage.
Figure 16. WECS with back-to-back converter.
Figure 17. WECS with modified back-to-back converter.
3.3.3. Wind-based SISO platforms with PMSG
Low-speed high-torque PMSGs are used as the preferred solution in variable-speed high-
power direct-driven wind generators. However, low-speed operation produces low voltage
profile at the generator terminals. As the electrical power available from the wind generation
system cannot be delivered directly to the grid, power electronics plays a decisive role in
overcoming this limitation. Figure 18 shows a bridge boost rectifier (BBR) in cascaded with
VSI topology. A step-up transformer is used for grid connection; since VSI produces low
voltage profile due to low-speed direct-driven generator operation. This topology has same
features as the topology that was previously shown in Figure 15 in addition to the grid-tie-
transformer.
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Another option to achieve high power factor in the generator side is to use a voltage source
converter (VSC) in cascaded with VSI topology in the WECS as shown in Figure 19. The
transformer stage still required to boost the inverter output voltage level to the grid voltage
level. In Figure 20, a semi-controlled rectifier topology was connected to achieve same VSC
topology advantages with higher efficiency; since less switching losses can be obtained
through reducing the number of switching devices. However, it will produce larger current
harmonic distortion because only one half cycle is controlled. The common drawback for all
previous configurations is the use of the transformer for grid connection which increases the
size of the power converter circuit. Then, this structure becomes feasible for small WECSs.
On the other hand, the same rectifier topologies can be efficiently utilized with current source
inverter (CSI) topology feasible for low-speed direct-driven PMSGs. Figure 21 shows same
configuration as the other in Figure 18 but with CSI working as a boost inverter stage. This
configuration has the same drawbacks as illustrated previously. However, omitting the step-
up transformer represents an additional advantage which reduces the size and weight of the
system.
In Figure 22, the VSC topology can be used with CSI topology in order to achieve same
advantages as the configuration in Figure 19. Dual boost feature is obtained with low harmonic
distortion. However, larger losses are expected by increasing the number of switching devices
(12-switches) which reduce the overall efficiency. The CSI increases the voltage towards the
mains by itself, so the output voltage of the VSC must be lower than the lowest rectified line-
to-line voltage.
Figure 18. PMSG with BBR and VSI structure.
Figure 19. WECS with VSC-VSI converter structure.
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Figure 20. WECS with semi-controlled rectifier and VSI structure.
Figure 21. WECS with BBR stage and CSI structure.
Figure 22. WECS with VSC-CSI structure.
3.4. Hybrid energy system in home
In hybrid energy systems (wind, PV, fuel cell, and battery storage) and its application such as
plug-in hybrid vehicles (PHEVs), MISO converters became widely used to enhance the stability
and reliability. Figure 23 shows an application example for the MISO (sometimes called multi-
input boost converter MIBC) platform feasibility with PHEVs. In MISO, the AC grid is
connected to the DC-Bus via boost rectifier, the PV/fuel cell is connected to the DC-Bus via
boost converter and the energy storage system is connected to the DC-Bus through a bidirec‐
tional DC/DC converter.
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Figure 23. The block diagram of the PHEV drive train with MISO structure.
3.4.1. MISO with vehicle to grid (V2G)
PHEVs have an advantage of AC power grid connectivity compared to self-contained hybrid
electric vehicles (HEV). The vehicle-to-grid (V2G) can be utilized when the vehicle battery is
in the discharge mode while grid-to-vehicle (G2V) is used when the battery is in the charge
mode. The V2G mode is considered as a promising concept for electric power grid stability via
large storage vehicular system. However, the availability of electricity supplies to recharge
vehicle battery is still a significant challenge to enable this concept. Figure 24 shows an example
of the V2G system components. The system involves six main parts: (1) AC power grid and
alternative energy resources; (2) master-based independent control; (3) charging stations; (4)
bidirectional power flow and point-to-point Plug-in electric vehicle (PEV) communication; (5)
smart metering and control system; and (6) the vehicular technology including its battery
charging management.
In general, PEVs with V2G interfaces can charge or inject energy into the grid when parked
and connected. The concept requires three elements: a power connection to the grid, a
communication connection with the grid operator, and suitable metering. Communications
must be bidirectional to report battery status and receive commands. In fact, the current
challenge is how to make smart metering system aware of the battery’s state of charge (SOC)
and its capacity in real time. In [16], smart metering system with both on/off boards was
proposed to support V2G concept. As a result, the PHEV can represent a controllable load
while integrating it with green energy sources via smart meter information. GPS locators are
also useful in the large-scale real-time energy management operation. In addition, field area
network is used to realize monitoring and communication functionalities among PHEV
charging stations.
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Figure 24. V2G system with smart meter.
3.4.2. SISO with wireless charging electric vehicle (EV)
Another feasible option for EV is that we can use the simple SISO platform to operate a wireless
power transfer charging station as shown in Figure 25. There are several stages for wireless
charging of EV. Firstly, an AC-DC rectifier with PFC ability is used to convert the AC utility
grid power into DC power source. Secondly, the converted DC power is then inverted into a
high-frequency AC power as the transmitting primary coil. The insulation failure of the
primary coil is possible. As a result, a high-frequency isolation transformer may be connected
between the inverter output and the primary side of the transmitting coil to ensure charging
system protection. The magnetic field of the primary transmitting coil induces an AC voltage
on the secondary receiving EV coil. Finally, the secondary transmitted AC power is then
rectified via AC-DC converter to charge the EV battery [17].
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Figure 25. Typical wireless power transfer EV charging system.
4. Case study
VSI is considered one of the most efficient power conversion topologies for AC/DC-source-
based MISO platform. A bidirectional power flow control strategy is suggested here to enable
the smart operation between local building and electric utility service lines. The primary intent
of the inverter development with smart functionalities is to enable an efficient interconnection
and economical operation for dispersed PV-based building installations to the utility grid.
Some distinctive aspects of this case are smart metering, the provision of pricing information
to consumers, the provision of some control options to consumers, and information exchange
on a fully networked system enabled by massively deployed sensors [18]. It is in this regard
that the inverter with the aforementioned smart functionalities is being proposed in this
section.
The PV panels provide the input of the DC-DC converter with 132 V. Then, the step-up
converter boosts the PV voltage level to 350 Vdc which serves as the input of the smart DC-AC
inverter. Moreover, the inverter setup is a part of MISO system structure that involves lead-
acid battery energy storage bank with 160 V nominal voltage and 100 Ah rated capacity. The
battery storage is connected to the DC link via a bidirectional buck-boost DC-DC power
converter.
The case study was performed through Matlab/Simulink software program and real-time
experimental hardware implementation in order to verify the validity of this platform with
smart meters for grid-tied FIT systems. The test results were acquired through a laboratory-
scale MISO frame hardware setup in the sustainable energy systems (SES) laboratory at
Manhattan College. The overall system schematic and photograph is shown in Figure 26. The
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system comprises multiple converters used to control power flow under harsh ambient
conditions. Different operating modes are possible, for example, the system can be supplied
with electric power by way of a 3-phase wind generator, a PV, and batteries. The system is
housed in an air-cooled case (with thermal protection) and communicates with the master
controller (TMS320F2812 DSP platform) via a D-connector bus. The signal interface features
analog and digital I/Os to allow for the connection of a wide variety of sensors, for example,
temperature sensors, resolver inputs.
A voltage/current sensor module is connected through the AC-bus common coupling point
between the MISO system and utility grid. This module is used to acquire the power meas‐
urements as a part of smart meter functionality for real-time energy management operation.
The stability of DC-bus voltage is the main parameter to decide on energy flow scenario. Then,
we can set upper and lower limits for DC-bus voltage in order to maintain system balance
operation while connected to AC grid. For example, if Vdc is higher than Vdc_up, then it means
that there is excess power available from wind and PV generators. Accordingly, the battery
will operate in the charging mode to store energy while supplying local load demand and
sending power back to grid based on smart building contract agreement. In this case, net meter
measurement will show a negative reading which indicates feeding energy into utility grid.
On the other hand, if Vdc is lower than Vdc_low, then it means that either one or both wind and
PV generators is supplying less power into MISO system. As a result, the battery will operate
in the discharging mode to support local load demand while receiving a portion of the needed
power from grid. In this case, net meter measurement will show a positive reading which
indicates receiving energy from utility grid. Finally, if Vdc is within the upper and lower stability
limits, then it means that both wind and PV generators are balanced and sharing power with
utility grid to supply building loads. As a result, the battery will operate in the rest mode which
improves battery life time. The detailed parameters and specifications of MISO platform are
listed in Tables 1 and 2.
Figure 26. The schematic diagram and photo of the MISO experimental hardware test setup.
Smart Metering Technology and Services – Inspirations for Energy Utilities
124
4.1 Pure P-Q injection capability
An inverter grid-tie system has been designed to verify the effectiveness of the MISO injection
capability for AC power into the grid. The inverter has been tested under various power factor
and command step conditions with 12 mH grid filter. The switching frequency of the devices
is set to be 5 kHz. The grid voltage is 120 V, 60 Hz. The system controller and PWM generation
are conducted by DS1103 PPC Controller Board. Figure 27 shows the grid-tie inverter control
under pure active power, pure leading, and lagging reactive power conditions. The output is
5 kW active power (0°), 2.5 kVar lag reactive power (−90°), or 5 kVar lead reactive power (90°).
4.2 Hybrid P-Q injection capability
Figure 28 shows the grid-tie inverter control with equal active power and reactive power
command. The active power is 3 kW with either lagging (1.5 kVar) or leading reactive power
(3 kVar). We can notice that the grid voltage-current angles are always larger than and
smaller than 90° (lag or lead). The testing results and measurements are confirming the validity
and superiority of the developed MISO platform with smart inverters interconnecting green
buildings to utility grid network.
Figure 27. Pure active-reactive power injection test results for 5 kW active, 2.5 kVar lag reactive, and 5 kVar lead reac‐
tive powers.
Power Electronics Platforms for Grid-Tied Smart Buildings
http://dx.doi.org/10.5772/62920
125
Figure 28. The test results for the hybrid active-reactive power injection for 3 kW active with 1.5 kVar lag reactive pow‐
ers and 3 kW active with 3 kVar lead reactive powers.
5. Conclusion
In this chapter, a detailed study of the different power electronics platforms for grid-tied smart
buildings was presented. Both analog and digital (smart) metering systems were discussed
including FIT and NM infrastructures. The main power electronics converter platforms were
categorized into two types as SISO and MISO structures. Each platform was introduced in
regards to several types of alternative energy resources (wind, PV, etc) involving battery
storage and its use in our home applications. PHEV technology with (V2G-G2V) concepts and
its wireless power transfer operation were discussed. A hardware experimental case study
with MISO platform involving smart bidirectional grid-tied inverter was performed to verify
the validity of the developed power electronic structure for smart systems. The new power
electronics platforms were studied to prove its superiority for grid-tied smart metering
systems.
Symbol Quantity Value
fsGrid frequency 60 Hz
VsGrid voltage 120/208 Vrms
Vdc DC-link voltage 350 V
Cdc DC-link capacitance 1200 μf
LfFilter inductance 12 mH
fsw Switching frequency 5 kHz
kpiProportional current controller 10
kiiIntegral current controller 100
kpcProportional voltage controller 0.07
kicIntegral voltage controller 0.7
Table 1. PV and battery storage specifications and control parameters.
Smart Metering Technology and Services – Inspirations for Energy Utilities126
PV Battery bank
Parameter Specification Parameter Specification
Output power 0–6 kW Rated voltage 160 V
OC and SC points (10 A, 140 V) Connection 10-series
Max, operating point (9 A, 132 V) Rated capacity 100 AH
Switching frequency 50 kHz Battery type Lead-acid
Input inductor 0.33 mH Total power 12 kW
Table 2. Smart bidirectional inverter specifications and control parameters.
Author details
Mahmoud Amin*
Address all correspondence to: mahmoud.amin@manhattan.edu
Department of Electrical and Computer Engineering, Manhattan College, Riverdale, NY, USA
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