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Voltage and Frequency Grid Support Strategies | Electrical Grid | Ac Power
voltage and frequency control in grid supporting strategies are of vital importance in power systems analysis.
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fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2016.2539343, IEEE
Emanuel Serban, Senior Member, IEEE, Martin Ordonez, Member, IEEE, Cosmin Pondiche, Member, IEEE
AbstractIn recent years, with a higher penetration of
renewable distributed generators into the ac grid network,
the risk of grid instabilities and vulnerabilities has
increased. This paper proposes grid support strategies that
can be used to alleviate the grid frequency-voltage
variations, which are indicators of the imbalance between
power generation and consumption within an ac network.
The proposed strategies to support grid stability in the event
of frequency-voltage variation go beyond recent standards
and provide extended functionalities. The pre-configured
set-points of active power-frequency P(f) strategy features
the control of the converter's power flow direction with an
adjustable power gradient transition between grid-feeding
and force charge as grid-loading, in response to grid
frequency deviation. The distributed power generators are
represented by an energy storage converter, with the
capacity to discharge and charge the Electrical Energy
Storage (EES) element, primarily for grid support purposes.
The EES energy storage is configured for later usage when
the grid requires active power support, under line frequency
deviation, used under normal conditions, or during on-peak
tariff high demand daytime hours (energy shifting for TOU,
ac load shave). In addition to grid support enhancement, the
P(f) strategy is combined with reactive power-voltage Q(v)
control to attempt to correct frequency and voltage
deviations in a collective impact of distributed generators
operation for grid stabilization purposes. The P(f)-Q(v)
strategy provides an automatic active/reactive power
generate/receive pre-configured control, in response to
frequency-voltage deviations, in order to support the grid
and prevent network instabilities. The proposed control
strategy for local frequency- and voltage-assist is applicable
for single and three-phase grid converters interfaced with
energy storage systems. Simulations and experimental
results, obtained using a single phase 6kVA four-quadrant
EES converter, are presented to validate the proposed P(f)
and Q(v) grid support strategies.
Index Terms Grid support, grid-loading, frequency- and
voltage-assist, grid power balancing, active and reactive
power grid support, electrical energy storage (EES)
converter power system.
fHrec
fLstart
fLstop
fmin, fmax
Depth of discharge.
Instantaneous grid frequency.
Frequency high recovery, inverter resume
operation at rated power.
Frequency low start limit configuration for
converter operation from EES SoC power
Frequency low stop limit configuration for
converter operation at nominal power.
Minimum and maximum converter operation
mP, mr
P(SoC)
Q(v)
v, Vac
VacHyst
VHmin
Nominal grid frequency.
Converter output ac current.
PV array dc current.
Grid code.
Gradient of active power with frequency(W/Hz).
Active power generated or received.
Nominal active power.
Momentary active power operation.
Active power-frequency control strategy.
Active power generation from SoC reserve.
Point of common coupling.
Active power receive from grid to EES.
Reactive power supplied to ac grid.
Reactive power-voltage control strategy.
Minimum (critical) level of SoC (%).
Instantaneous grid ac voltage.
Converter ac voltage configurable hysteresis
EES dc voltage.
Converter ac configurable high limit for q=0.
Converter ac voltage configurable maximum
limit for q=qmax (lag).
Converter ac voltage configurable low limit for
Converter ac voltage configurable minimum
limit for q=-qmax (lead).
Nominal grid ac voltage.
PV array dc voltage.
Grid phase angle.
Time-of-use.
Transmission system operators.
Line impedance ratio.
Distributed generation systems are designed to
supply power for utility ac grid networks within specific
voltage and frequency limits. The power generators must
cease energizing the ac grid when operating outside of the
voltage, frequency and time limits imposed by countryspecific standards (e.g. UL, CSA, VDE, BDEW) [1]. The
transmission system operators (TSOs) responsible for
power dispatch management, develop grid codes [2] - [6]
that are necessary in order to adopt and integrate
renewable power systems. The grid codes (GCs) are
continuously revised, and new supplements (e.g.
California Public Utilities Commission: Rule 21, Hawaii
Electric Companies: transient over-voltage, frequencyvoltage ride-through, requirements) are issued to ensure
the compliance of grid inter-connection power generators.
0885-8993 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2016.2539343, IEEE
Fig. 1. Active power control under grid line frequency variation.
(a) Standard requirement for the converter power curtailment.
(b) Proposed P(f)-strategy for grid support with the converter active power balancing for generation (P) and receiving (Pr) in response to line
frequency variation.
requirements for renewable power systems are related to
the low- voltage and medium-voltage grids. The new
generations of power converters must be designed with
grid support features, which operate cohesively while
voltage/frequency ride through [8] - [11] and
active/reactive power controls [12] - [16] under grid
faults [17] - [19].
The utility grid codes have specific grid support
requirements for grid-connected power generators [2] [4]. The grid code requirements normally addresses one
active feature at the a time. The requirements include:
a) Response to frequency variation to reduce the active
power for over-frequency and increase active power for
under frequency. For example, the power generator must
reduce its active power by a certain power gradient when
the line frequency increases over a specified value (e.g.
50.2Hz, Fig. 1(a)). b) Static grid support, where the
power generator is required to have configurable reactive
power set-points. The power factor (PF) or reactive
power (Q) are set to a constant value; reactive power
value is a function of the grid voltage variation, Q(v);
power factor is variable, with the active power operation,
cos(P). c) Dynamic grid support (e.g. low and high
voltage ride through) in which the power generator
delivers reactive current on a pre-determined voltage
transient profile.
Compliance with the relevant national and local codes is
mandatory for renewable energy systems. The standard
grid connection requirements for distributed generation
impose configurable grid support features [20]. For
example, the main factors that cause ac network grid
frequency deviations are loss of main power generation
(e.g. due to an old power system), lack of grid support
capability in PV power generators, and sudden
interruptions in the productivity of PV generator caused
by high dynamics in irradiance [21].
The renewable energy sources and battery energy storage
elements are grid-interfaced through distributed power
converters. The power converters systems can be
integrated in islanded (off-grid) and grid-connected ac
Different control approaches are available for driving the
power converter in specific modes of operations. The
system controller can be designed to address a wider
range of single- and three-phase converter applications,
including at least one of following control modes of
a) grid-forming stand-alone (UPS mode)
b) grid-supporting (islanded mode and grid-connected)
c) grid-feeding (grid-connected)
d) grid-loading (charge mode)
The integration of the aforementioned control modes into
the system controller enables versatile power converter
operation in both islanded and grid-connected ac
a) In grid-forming mode, the controller is designed to
emulate the converter into an ideal ac voltage-controlled
source. A typical practical example is a line-interactive
uninterruptible power supplies application, due to its high
fidelity voltage and frequency regulation. Power sharing
by parallel multi-converters can be achieved through the
implementation of accurate synchronization between the
converters [22]. The advantage resides in the fact that the
ac bus voltage and frequency are well regulated due to
low-output impedance. Through communication, the
multi-converter system easily controls the main transfer
switch for stand-alone UPS (back-up) and grid connected
modes. The major disadvantage is the need of hardware
communication between converters, and more attractive
solutions have been proposed to avoid the communication
signals [23].
b) In grid-supportive mode, the controller is designed to
emulate the converter into combined voltage- and
current-controlled source with typical applications in
microgrids. The controller's main purpose is to participate
in ac voltage and frequency regulation by controlling the
active power (P) and reactive power (Q) delivered from
the converter. The power sharing control between the
paralleled converters within a microgrid system is
implemented using droop and virtual output voltage
controllers [24] - [31]. Low-voltage microgrid-support
has been proposed in the form of voltage-based droop
using P/V-droop characteristic to control the active power
depending on the grid voltage variation range [32].
Control strategies for seamless transfer between islanded
and grid connected modes of operation have been
proposed in [33] - [35].
c) In grid-feeding mode, the controller is designed to
emulate the converter into an ideal current-controlled
source, with a wide applicability in utility-grid interactive
distributed generation. Grid-feeding based converters
must meet stringent requirements to comply with
standards and certifications (e.g. IEEE1547, CSA107.101, DIN EN 50438, VDE-AR-N 4105, AS4777,
JEAC9701).
The primary low-level controller for current regulation is
designed with high bandwidth and performance, to ensure
fast and stable control [36]. The higher-level control
layers are designed to access the current reference loop in
order to perform slower tasks (e.g. maximum power point
tracking, temperature compensated P/Q reference setpoints). Transmission system operators (TSOs) require
access of communication with grid-feeding converters
P/Q set-points for flexible power dispatch. Solutions for
the integration of renewable energy sources, PV and
wind, with battery energy storage, have been investigated
with energy management with multiple energy storage
systems [38] - [40]. However, the integration of PV and
battery EES for grid-support controls with extended
capabilities, beyond GC standards, have not been
investigated. The specific GC requirement that converters
cease operating beyond a certain frequency limit is
implemented, in the existing methods, by suddenly
disconnecting from the grid during power curtailing,
which induces a local perturbance in the grid network.
Furthermore, this effect could be amplified in cases
where there is high renewable PV/wind penetration in the
grid network. In contrast, the proposed controller
transitions the power flow from grid-feeding to force
charge, using a grid-loading mode for ac network power
d) Grid-loading charge mode represents the new
operational state used for the proposed control strategy.
The controller commands the EES converter to transition
from operating in grid-feeding, to operating in force
charge grid-loading. The transition is implemented with
adjustable ramps (mP, mr) for overall power
smoothing/balancing, which results in improvements to
grid stability.
The typical application for the proposed controller relates
to variable renewable PV/wind generation, which
markedly affects the stability of the operation of utilitygrid systems (e.g. Hawaiian Islands). The main
contributions of this work are listed below and
conceptually represented in Fig. 1(b).
1) Extended P(f) utility-grid support strategy: the
converters capacity to actively generate/receive power in
response to frequency variation. The controller features
an ESS converter that operates in grid-feeding mode,
while outside of the permissible GC frequency-voltage
limits, commands the transition to force charge gridloading in an attempt to gain stability through frequencyassist. The transition between grid-feeding and gridloading is achieved using an adjustable power gradient,
for smooth network power balancing.
2) Combined P(f) - Q(v) grid support strategy: the
converters active/reactive power manipulation for
frequency- and voltage-assist in response to signs of grid
instability. The P(f) - Q(v) control references follow preconfigured force charge/discharge set-points, for
operation in the direction of frequency and voltage
correction errors induced by variable PV/wind
3) The P-Q decoupled set-points are calculated from the
pre-configured values of SoCmin, power gradient, energy
shifting for TOU, voltage and frequency limits for
immediate grid frequency- and voltage-assist in a
network of DGs to achieve grid stabilization.
The P(f) and Q(v) grid support strategies are
implemented and integrated within a bi-directional EES
converter with the power exchange occurring between dc
EES and ac grid port. The proposed strategies can be
implemented in EES converters topologies used in both
renewable energy storage and electric vehicles
applications, due to the commonality of the ac grid and
dc battery port. The proposed controller with ancillary
supportive features is applicable for both low-voltage
(mainly resistive, / 1) and medium/high-voltage
(inductive dominant, / 1) grids. The advantages of
the proposed controller reside in the flexibility of the
controller features (e.g. configuration and set-points for
frequency- and voltage assist with P(f) - Q(v) strategies
and energy management, Fig. 1(b)) and in the power
balancing with applications in both utility-grid and
microgrid systems.
Section II presents the control architecture and describes
the implementation of P(f) - Q(v) beyond standard grid
support requirements. Section III presents simulations
and experimental verification of the proposed P(f) and
Q(v) grid support strategies. The conclusions are
provided in section IV.
GRID SUPPORT IN GRID-CONNECTED CONVERTERS
USING P(F) AND Q(V) STRATEGIES
EES converters will have a potentially great impact on
renewable support, in energy smoothing/shifting, fossil
generation support (e.g. diesel generators), contingency
reserve, and power outage mitigation. An example of a
PV smoothing application, is the use of EES converters
designed with grid support features, in conjunction with
the active and reactive power controls.
In this work, the EES converter monitors the grid
frequency-voltage deviations for the active and reactive
power flow manipulation in grid-feeding/loading mode,
by using the following two proposed methods: P(f) and
combined P(f) - Q(v) grid support strategies.
A) P(f) strategy: active power control in
response to line frequency variation
The grid power over-production results in line frequency
increases, where the distributed generators cease to
operate over a specified limit due to standard
requirements. Instead, the proposed solution supports the
ac network grid by reversing the active power flow of the
EES converter by loading the grid at upward frequency
variation. The ac network distributed loading has a
stabilizing effect on the overall power generation and
consumption. The focus of this work is on the grid
support control strategy, implemented at the local level of
the distributed power generator (EES converter), while
the power balancing effect on consumption vs. generation
within the ac network, is not quantified. To further
enhance grid support under frequency response, this
paper proposes a solution that includes increasing active
power generation/receiving for under/over frequency
operation with automatic upward and downward ramp
rate control. The recent standards for power curtailment
are conceptually represented in Fig. 1 (a), indicating two
operating regions. The proposed strategy is depicted in
Fig. 1 (b), extending the operating modes well beyond
existing standards.
Fig. 1 (a) shows the standard requirement for curtailing a
converters active power under grid frequency variation.
If the grid frequency increases above fHstart limit, the
converter must reduce its momentary output power Pm by
a specified gradient (e.g. mP=40% of rated power per
If the grid frequency is outside of [fmin, fHstop], the
converter must cease to generate power, as per GC
requirements. The existing methods meet the grid
requirement through grid-disconnection during the power
curtailment, causing potential perturbance to the grid
network. This undesirable effect could be amplified in
applications with high PV/wind grid network penetration.
. In contrast, the controller changes the operation from
grid-feeding to charge grid-loading mode.
The transition is implemented using adjustable ramps
(mP, mr) for overall power smoothing/balancing, which
results in improvements to grid stability and, more
importantly, in numerous renewable DGs within the ac
In comparison, Fig. 1 (b) illustrates the proposed P(f)
strategy, which provides the ability to charge and
discharge the EES element for grid support purposes
(frequency-assist with power balancing), and which has
the following modes of operation:
Normal operation P(SoC)-generation, in which the
converter delivers active power from the state of charge
(SoC) availability, under a controlled energy discharge
from the EES element. While the converter operates at
the nominal line frequency (fn), the momentary active
power (Pm) is supplied to the grid at a controlled SoC
level. This specific mode of operation is usually used for
load shaving, where the stored energy is supplied back to
the grid during times when the price rate of electricity is
higher in energy shifting for TOU.
P(f)-curtail operation mode, where the active power
reduces under frequency increase. In this case, the
controller is designed with two modes of configuration
based on the frequency range:
a) Immediate transition to grid-loading mode. The
controller detects a rate of change in frequency increase
(f>fn and |f/t|>0) and commands force charge with
adjustable gradient mr in power receiving mode. This
specific frequency-assist configuration provides fast and
pro-active assistance in a network of DGs for grid
stabilization purposes.
b) Standard P(f) curtailment followed by grid-loading
mode. This configuration is used when priority is given to
power generation, and presents a reactive response to grid
stabilization. The EES converter operates in generation
mode, performs energy shifting for TOU or exports
potential extra power received by the battery from
another device. As the grid line frequency increase over a
configurable threshold (fHstart), the grid-connected
converter enters P(f)-curtail mode by linearly decreasing
the active power according to a configurable gradient
(mP). According to the current standards [2] - [6], the
converter must stop operating (cease power generation)
when the frequency exceeds the frequency limit fHstop.
When the line frequency returns below fHrec, the gridconnected converter recovers and starts supplying power
to the grid with a time constant linear increase (%P/dt).
These measures are imposed to avoid the 50.2Hz problem
(e.g., reported as occurring in the low voltage distributed
network in Germany), which can potentially lead to major
grid disruption.
P(f)-receiving operation, in which the converter forces
its operation mode to charge for grid-loading (active
rectifier) mode beyond fHstop. The controller addresses the
EES converter's operation outside of tolerable standard
limits, since the GCs restrict the power generation only
(cease export power). In this proposed strategy, the
converters become a load for the grid by importing power
(Pr) into the EES dc port. After the entry moment in gridloading mode, the converter linearly increases active
power receiving to the maximum limit Pr. If the line
frequency starts to recover, the converter linearly
decreases its receiving power and exits the rectification
mode, while the frequency decreases below frstop, as
illustrated in Fig. 1 (b). There are two immediate benefits
to this suggested grid support strategy. First, the ac
network loading contributes to a natural grid frequency
recovery, compensating for the excess in power
production. Second, during the line frequency increase,
the energy is stored in the EES element and later used
(energy shifting for TOU) when the grid frequency
recovers at the nominal frequency (fn), or when the
frequency decrease below a critical value (fLstart).
P(SoC)-generation increases by drawing from the SoC
power reserve as a result of the line frequency reduction.
Like the over-frequency (e.g. > 50.2Hz) problem, the
under-frequency (e.g. < 49.5Hz) problem represents a
major risk to the stability of the ac network, and can lead
to disruptions in the power supply. In the proposed
strategy, the controller is designed with the following
modes of configuration:
Fig. 2. Single phase EES converter controller block diagram example: P(f)-generate/receive and combined P(f)-Q(v) strategies for ac network power
1) P(f) active power generation (increase or curtail) and receive (grid loading with EES charge mode) arbitration with grid line frequency variation,
2) Q(v) reactive power generation under line voltage variation, Eq (3), (4).
0 if f f min
Pn Pm
( f f Lstop ) if f Lstop f f Lstart
Pn f
p ( f , Pm , mP , SoC )
Lstart f Lstop
Pm if f Lstart f f Hstart
Pm 1 mP ( f f Hstart ) if f Hstart f f Hstop
Pr ( SoC ) if f f Hstop
c) Immediate grid-support frequency-assist mode. The
controller detects a rate of change in frequency decrease
(f<fn and |f/t|<0) and commands force discharge, with
increased power generation.
d) Standard P(SoC) generating mode. The converter
supports the grid by increasing the level of power
generated from the SoC reserve when the frequency falls
below fLstart (force discharge). It should be noted that if
the SoC reserve falls below a critical level (e.g.
SoCmin=25%), the priority is changed and the EES
element enters in conservation mode, where the converter
ceases to operate, regardless of the status of the ac
The P(f) strategy illustrated in Fig. 1 (b) is programmed
in response to line frequency variation and is described as
per relationship (1).
Fig. 2 illustrates the single phase converter control
diagram for P(f)-generate/receive and combined P(f) Q(v) strategies. The P(f)-generation strategy is achieved
with the converter operating in current-controlled source
inverter mode. The ac-current reference Iac* is function of
the dc voltage-loop controller GgV, EES state of charge
(SoC) and ac power-loop controller GgP. The output of
the selection function Fg is a normalized value of voltage,
SoC or ac power, which controls the magnitude of the
sine-wave generator Im. The sine wave reference I ac
generated by a look-up table with the amplitude scale
reference Im (function of EES voltage regulation, active
power and SoC) and phase angle v (function of the
desired reactive power generation):
I ac* (t ) I m sin[(0 k AI (t )) t v ]
I m f (Vdc , P, SoC )
The grid-connected converter current reference is
designed with an islanding search sequence [7] at the
PLL level, where the anti-islanding algorithm (kAI)
detects the grid faults. The current controller GgI output
signal is further processed through the modulator in order
to control the gate of the single-phase, two-level, fullbridge converter. The grid support function FgGS selects
the converter's mode of operation, P(f)-generate or
receive, depending on the frequency variation obtained
from equation (1). The P(f)-receiving strategy is
represented by charge (active-rectifier) mode of
operation, where the converter has three control loops.
The inner power factor control loop (Gr) commands the
input ac current to track the ac grid voltage (Vac) with
near unity power factor. Two low-bandwidth outer
control loops are implemented for dc voltage regulation
(GrV) and dc current regulation (GrI), for the EES
charging phase. The P(f) strategy has an automatic power
generation/receive response to the frequency variation,
and does not require an extra communication protocol
between distributed EES converters and the transmission
system operator of the main ac network generator. The
medium of communication within the ac network is the
frequency and voltage variation [41].
B) Combined P(f)and Q(v) strategy for
enhanced grid support
The imbalance between power production and
consumption within an ac power generation network can
lead to grid frequency-voltage instability with the
potential to disrupt power delivery. The P(f) strategy
forces the EES converter to generate/receive active
power, when the grid frequency varies from its nominal
value. The grid support P(f) capability of distributed ESS
converters is enhanced with reactive power
generate/receive Q(v), at grid voltage deviation. The
proposed grid support capability uses the P(f) strategy
combined with a reactive power Q(v) controller (local
voltage-assist mode).
When the ac grid network operates at a lower value
voltage than the nominal (e.g. Vn=230Vrms) of ac grid
network, the grid support Q(v) controller uses relation
(3), which is applicable in a case where the converter is
controlled with power factor lead.
Relation (4) is used by the grid support Q(v) strategy at
higher than nominal ac voltage, in a case where the
converter is controlled with power factor lag.
The combined P(f) and Q(v) strategies provide a control
option for active and reactive power grid support during
deviations of both line frequency and voltage from the
nominal values (local frequency- and voltage-assist). Fig.
3 shows a P(f)-Q(v) grid support flowchart with the states
of ESS converter operation.
Fig. 3. P(f)-Q(v) grid support flowchart diagram.
In Fig. 4 is shown a typical dc-centric power system used
in commercial and residential applications. The system
consists of an EES battery (similar to line-interactive
uninterruptible power supply) connected to a gridconnected bidirectional converter and an ac load port.
The battery lifetime is function of SoH and is dependent
on charge/discharge rates, SoC swing operation ranges,
DoD depth of discharge, number of cycles and operating
temperature. Longer battery life can be achieved in the
following two cases:
As an alternative, an optional dc-dc PV converter is
installed to increase the battery lifespan and the
amount of renewable power generated. As can be
seen in Fig. 4 (b), the battery bank is conditioned
with a three-stage charge profile (bulk, absorption
and float), while the PV energy surplus is exported to
the ac load and grid port.
When the PV converter is not part of the system, the
active power set-point with SoC depth of discharge is
limited to a minimum value (SoCmin), and if the
critical level is reached, then the grid support is
abandoned, based on a pre-configured priority to
preserve EES. The complete EES re-charge process
is programmed during the off-peak tariff (e.g. night
0 if 0 v VL min VacHyst
qmax if VL min VacHyst v VL min
v VL min
qPFlead (v)
if VL min v VL
max VL VL min
VL VH
0 if VL v
if VH v VH max
qPFlag (v) max VH max VH
qmax if VH max v VH max VacHyst
0 if v VH max VacHyst
Fig. 4. Grid-connected EES power system for energy management and
P-Q control strategies application.
a) Typical dc-centric architecture: the battery EES can be charged and
discharged (1) through bidirectional grid-connected converter.
Optionally, for better battery SoC/SoH maintenance, the battery can be
charged (2) through unidirectional dc-dc PV converter.
b) Daily energy harvesting during a mixed sunny and overcast. Data
collection: September 2015, Greater Vancouver area, Canada.
The variability of PV/wind power generation influences
the grid system's stability. The controller can be
configured using external forecast communication
'heads-up' data of PV/wind ramp events.
Additionally, the EES converter's controller is designed
with automatic pre-configured control settings and
monitoring for frequency- and voltage-assist, time
shifting for TOU and power smoothing.
The power balancing exchange with the grid is
performed using the battery EES, which is a vital system
component, and careful attention must be paid to battery
sizing to ensure the maximum performance and
longevity of the EES system. The battery EES system
sizing is based on the scheduled energy shifting for
TOU, maximum allowable DoD, SoC swing range,
frequency of charge/discharge rates and operation
The metrics of dc to ac system conversion efficiency
) are also considered in system sizing in order to
calculate the effective usable energy from EES to ac
port, e.g. Eac [ kWh ] E EES [ kWh ] DoD[%] conv [%] .
As shown in Fig. 4, the system controller can be
designed to drive the EES converter operation in gridforming (stand-alone UPS mode under grid fault), grid
supporting (microgrid), grid-feeding (utility-grid) and
grid-loading charge mode.
Fig. 5. Experimental setup for P(f) - Q(v) frequency- and voltage-assist
control strategies evaluation.
It is possible for the controller to operated in multiple
modes while using the same converter hardware, thanks
to the presence of a battery energy storage system and
the use of a four-quadrant selection converter topology.
The ESS converter designed and equipped with P(f) and
Q(v) features, when distributed within an ac network,
provides a balanced power generation/consumption ratio,
to help stabilize the grid.
The performance of the grid support strategy presented
in this paper was tested beyond the standard requirements
by implementing and testing the strategy on a two-level,
single-phase four-quadrant, 6-kVA, 230V/50Hz converter,
48V-10kWh energy storage system (Fig. 5). The 6-kVA
EES converter, equipped with P(f) and Q(v) controllers,
monitors the variation of the grid frequency-voltage from
the nominal values and the generate/receive
active/reactive power for grid support purpose.
Fig. 6 (a) shows the EES converters response at
frequency upward deviation: the active power curtails
with unity power factor and automatically reverses its
direction by receiving the power towards EES port, i.e.
loading the grid (force charge mode for frequency-assist).
The deviation of frequency and voltage below from their
nominal values (50Hz and 230Vrms) is shown in Fig. 6
(b). The converter operates with increased active power
drawn from the SoC reserve of EES port in order to
support the grid under low frequency operation. While
the grid voltage magnitude reduces to 210Vrms, the
combined P(f) - Q(v) strategy uses equations (1) - (4).
The converter operates with increased active power and
power factor lead in order to stabilize grid frequency and
voltage (frequency- and voltage-assist).
Fig. 6. Simulation results using P(f) and Q(v) grid support strategies.
(a) Test performed with P(f)-controller enabled during upward frequency variation.
P(f) configuration: fHstart=50.2Hz, fHstop=51.5Hz, fHrec=50.05Hz.
(b) Test performed with P(SoC) reserve active power and reactive power Q(v) during downward frequency variation (50 to 47.5Hz) and voltage
reduction (230 to 210Vrms).
Y-axis: C1- Vdc, battery dc voltage 5V/div, C2 Idc, battery current 50A/div, C3 - Va, AC grid voltage 200V/div, converter ac current 20A/div, C4 f,
grid line frequency, 2Hz/div. X-axis: 100ms/div.
A) Experimental results using P(f) grid support
Fig. 7. Experimental results with P(f) control strategy performed during
the following test conditions:
-P(f) curtail operation mode: upward frequency-voltage variation (1Hz/s
and 8Vrms/s) from 50Hz to 52.5Hz and 230 to 250Vrms.
-P(f) receive operation mode: downward frequency-voltage variation
(0.25Hz/s and 2Vrms/s) from 52.5Hz to 50Hz and 250 to 230Vrms.
-P(SoC) generation: normal operation under nominal grid conditions:
50Hz/230Vrms.
- Voltage and frequency settings for protection under unintentional
islanding (configurable, based on country-specific standard): (205 262)Vrms, (47.5 - 51.5)Hz.
P(f) configuration: fHstart=50.2Hz, fHstop=51.5Hz, fHrec=50.05Hz,
mP=(40%Pn)/Hz.
Y-axis: C1- Vdc, battery dc voltage 10V/div, C2 Idc, 200A/div, C3 Va, AC grid voltage 200V/div, C4 converter ac current, 50A/div. Xaxis: 2s/div.
Fig. 7 demonstrates the P(f) grid support
strategys performance during frequency-voltage
variation. At the upward frequency variation, the EES
converter curtails the power until the frequency exceeds
the set-point of 51.5Hz. While the grid frequency
continues to increase (up to 52.5Hz), the controller
changes the operation mode to P(f) receive, by loading
the grid. The controllers change in operation occurs at
the high frequency limit, 51.5Hz, since the converter
must only cease generating power to the grid, while
receiving/loading power from the grid is permitted by the
GCs. The converter performs a two-stage charge,
receiving active power and maintaining the dc regulation
voltage from bulk to absorption charge phase. Finally, the
grid frequency and voltage returns to the nominal values
(50Hz, 230Vrms) and the EES converter returns to P(SoC)
mode, actively generating power (discharge) from the
available SoC available energy reserve.
Fig. 8 shows the experimental results with P(f) strategy
enabled, and with a higher degree of detail: the converter
detects the grid frequency upward change from nominal
50Hz to 52.5Hz (Vac=230Vrms, Iac=15Arms, unity power
Using the control strategy shown in Fig. 1, the converter
linearly decreases the current using the P(f)-generate
strategy. While the frequency is above fHstart, the
converter starts to load the grid by receiving current using
the P(f)-receive strategy. In this mode of operation, the
grid energy is converted and stored in the EES element
(48V lead-acid battery system, 10kWh).
B) Experimental results using P(f)-Q(v) grid
Fig. 9 (a) shows the converters operation while both
frequency (50 to 52.5Hz) and voltage (230 to 250Vrms)
are ramping upward. The Q(v) strategy becomes active
and linearly delivers reactive power (Q>0) with power
factor lag for grid support purpose. Fig. 9 (b) shows the
converters power flow reversal, at higher grid frequency
operation (f > 51.5Hz). In this case, the energy is stored
in the EES element for later use, while the grid is loaded
in an attempt to balance the ac networks power
generation and consumption.
Fig. 10 (a) shows the converters operation
under a fast downward frequency ramp from nominal to
48Hz: the P(f) strategys fast response is reflected in the
ac current (Iac, Fig. 10) waveform. Fig. 10 (b)
demonstrates the combined P(SoC) - Q(v) grid support
strategy at low frequency and voltage operation (48Hz
and 210Vrms). In this case, at low frequency operation
(48Hz), the converter supplies active power from the SoC
reserve. At the same time, under low voltage (210Vrms)
operation, the converter receives reactive power (Q < 0)
with power factor lead, for grid support purposes in
frequency- and voltage-assist modes.
Fig. 8. Experimental results performed with P(f) control strategy enabled during upward frequency variation (1Hz/s) from 50Hz to 52.5Hz at unity
power factor and nominal grid voltage (Vn=230Vrms).
(a) Test performed with P(f)-curtail.
(b) Test performed with P(f)-receive.
P(f) configuration: fHstart=50.2Hz, fHstop=51.5Hz, fHrec=50.05Hz, mP=(40%Pn)/Hz.
Voltage and frequency settings for protection under unintentional islanding (configurable, based on country-specific standard): (205 - 262)Vrms, (47.5
- 51.5)Hz.
Y-axis: C1- Vdc, battery dc voltage 10V/div, C2 Idc, 100A/div, C3 - Vac, AC grid voltage 350V/div, C4 converter ac current, 20A/div. X-axis:
1s/div and 10ms/div.
Fig. 9. Experimental results obtained through P(f)-Q(v) strategy test evaluation with frequency and voltage variation.
a) Test performed with combined P(f) and Q(f) strategies at voltage-frequency variation: 230 to 250Vrms (8Vrms/s) and 50 to 52.5Hz (1Hz/s).
b) Test performed with P(f)-receive strategy at voltage-frequency variation: 230 to 250Vrms (8Vrms/s) and f>51.5Hz (1Hz/s).
P(f) configuration: fmin=47Hz, fLstop=47.5Hz, fLstart=49.5Hz, fn=50Hz, fHrec=50.05Hz, fHstart=50.2Hz, fHstop=51.5Hz, mP=(40%Pn)/Hz.
Q(v) configuration: VLmin=210Vrms , VacHyst=5Vrms , VHmax=250Vrms , qmax=60%.
Y-axis: C1- Vdc, battery dc voltage 10V/div, C2 Idc, 200A/div, C3 - Va, AC grid voltage 350V/div, C4 converter ac current, 20A/div. X-axis:
Fig. 10. Experimental results performed with P(f)-Q(v) strategy test evaluation with frequency and voltage variation.
a) Test performed with P(SoC) strategy at fast frequency (20Hz/s) downward ramp, 50 to 48Hz.
b) Test performed with combined P(SoC) and Q(v) strategies under voltage-frequency variation: 230 to 240Vrms (8Vrms/s) and f < 48Hz.
P(f) configuration: fmin=47Hz, fLstop=47.5Hz, fLstart=49.5Hz, fn=50Hz, fHrec=50.05Hz, fHstart=50.2Hz, fHstop=51.5Hz, mP=40%Pn/Hz.
The EES converters that use P(f) - Q(v) strategy
are able to smooth the frequency and voltage
intermittencies caused by the effects of weather or other
conditions on distributed PV generators. The proposed
concept can be implemented in the single and three-phase
distributed EES converters, with no extra communication
protocols needed in the transmission system operator of
the ac network.
The utility grid code requirements for the
distributed power generators are permanently subject to
change from different regulatory agencies, in order to
ensure the stability and quality of power delivery from
distributed generators. This paper has presented a flexible
grid support architecture strategy, where the proposed
P(f) and Q(v) strategies are easily implemented within
EES converters for active/reactive power generation
(grid-feeding) and receiving (grid-loading). The P(f) Q(v) grid support strategy implemented in local EES
distributed converters, monitors the grid frequency and
voltage. The frequency-voltage deviation from the
nominal values is an indicator of the imbalance between
power generation and consumption within the ac grid.
During upward and downward frequency-voltage
variations, the presented strategies control power
generation and receiving in real-time, providing
automatic grid support. The proposed controller is
designed with ancillary supportive frequency- and
voltage-assist features for network stability and energy
management. The active/reactive power flow control is
performed using an adjustable gradient to make the
transitions between grid-feeding and charge grid-loading.
The proposed strategy is primarily intended for grid
networks with a high penetration PV/wind, where the
mechanism described is designed to achieve grid stability
with power balancing, beyond GC standards.
The effectiveness of the proposed grid support
strategies has been demonstrated through simulation and
experimental results, beyond the standard requirements.
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Emanuel Serban (M99-SM09) received the
B.Sc. and M.Sc. degrees in electrical
engineering from University Politehnica of
Timisoara, Romania in 1994 and 1995,
respectively. In 1997 he joined Xantrex
Technology Inc. where he developed several
power electronics platforms for industry and
renewable backup applications.
Since 2009, he has been with the Solar
Business at Schneider Electric, Vancouver,
B.C. Canada, where he is Research & Development Chief Engineer,
Power Electronics Design, responsible for hybrid distributed power
systems, renewable multi-level converters architecture and platform
design. He developed single-phase and three-phase converter platforms
for Residential and Commercial solar and electrical energy storage
He has been working toward the Ph.D. degree in Electrical Engineering
and Computing Science at University of British Columbia, Vancouver,
Canada since 2013. His main fields of interest are in power electronics
modeling and control, analysis and design of power converters for
renewable, storage and distributed energy systems.
Martin Ordonez (S02M09) was born in
Neuquen, Argentina. He received the Ing.
degree in electronics engineering from the
National Technological University, Cordoba,
Argentina, in 2003, and the M.Eng. and Ph.D.
degrees in electrical engineering from the
(MUN), St. Johns, NL, Canada, in 2006 and
2332 Main Mall - Kaiser Building, Room: 3044
Phone: +1 (604) 521-9329, +1 (604) 500-9703.
Email: emaserban@ieee.org
Phone: +1 (604) 827-1423
Cosmin Pondiche
Burnaby, BC, V5G 4M1
Phone: +1 (778)-320-3149
Email: cpondiche@yahoo.ca
He is currently the Canada Research Chair in Power Converters for
Renewable Energy Systems and Associate Professor with the
Department of Electrical and Computer Engineering, University of
British Columbia, Vancouver, BC, Canada. He is also the holder of the
Fred Kaiser Professorship on Power Conversion and Sustainability at
UBC. He was an adjunct Professor with Simon Fraser University,
Burnaby, BC, Canada, and MUN. His industrial experience in power
conversion includes research and development at Xantrex Technology
Inc./Elgar Electronics Corp. (now AMETEK Programmable Power in
San Diego, California), Deep-Ing Electronica de Potencia (Rosario,
Argentina), and TRV Dispositivos (Cordoba, Argentina). With the
support of industrial funds and the Natural Sciences and Engineering
Research Council, he has contributed to more than 90 publications and
R&D reports.
Dr. Ordonez is an Associate Editor of the IEEE TRANSACTIONS ON
POWER ELECTRONICS, a Guest Editor for IEEE JOURNAL OF
EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS,
serves on several IEEE committees, and reviews widely for IEEE/IET
journals and international conferences. He was awarded the David
Dunsiger Award for Excellence in the Faculty of Engineering and
Applied Science (2009) and the Chancellors Graduate Award/Birks
Graduate Medal (2006), and became a Fellow of the School of Graduate
Studies, MUN.
Cosmin Pondiche (M'10) received the B.Sc.
and M.Sc. degrees in Electrical Engineering
from University Politehnica of Timisoara,
Romania in 1994 and 1995, respectively. In
1999 he joined Alpha Technologies Inc.,
Vancouver, Canada and his research and
development were in advanced switch mode
power electronics for UPS and renewable
products. Since 2010, he has been with the
Renewable Energies Business at Schneider
Electric, Vancouver, B.C., Canada where he is a Senior Power
Electronics Design/Research Engineer. His current research interests are
in power electronics modeling and control, analysis and design of power
converters for renewable, storage and distributed energy systems.
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