Resynchronisation or reconnection or transition of microgrid with the utility grid Class-20

P R E S E N T E D B Y :
D r . P r a v a t K u m a r R o u t
S h e e t a l C h a n d a k , S e n i o r R e s e a r c h F e l l o w , C S I R , I n d i a
D e p a r t m e n t O f E l e c t r i c a l & E l e c t r o n i c s E n g i n e e r i n g
S i k s h a ‘ O ’ A n u s a n d h a n ( D e e m e d t o b e U n i v e r s i t y )
B h u b a n e s w a r , O d i s h a , I n d i a
Resynchronisation/Reconnection/
Transition
of Microgrid with the Utility Grid
1

Microgrid
 Microgrid is the main part of future electrical power systems, called
'smart grids’.
 They are expected to be more robust and cost effective than the traditional
approach of centralized grids.
 A number of technical and regulatory issues like: proper power sharing
among DGs in a typical microgrid, load frequency control, synchronization
with utility, and protection in both islanded and grid-connected
modes are to be resolved before the microgrid can become widespread.
 One problem requiring due attention and more interest is the soft
synchronization of a microgrid with utility or other microgrids.
2

Synchronization
 In this context, the synchronization of a microgrid with utility or
other microgrids will be a crucial and commonplace task during power system
operation.
 Synchronization can be defined as the minimization of the variances in
voltage, phase angle, and frequency between the microgrid and the
utility.
 Such synchronization must be achieved before connecting the microgrid into the
power grid. It allows the grid and the synchronized microgrid to work together.
 The IEEE standard states that the reconnection with the restored utility must be
ensured with a delay of about 5 min, such that the steady state voltage and
frequency of the utility attains a stable operation in the recommended range.
3

Continue…
4
 Numerous synchronization methods have been presented over the years to
address issues such as unbalanced condition and frequency variation.
 Several grid codes (e.g., IEEE Std. 1547, IEEE Std. 1588–2008, IEC Std.
1727, IEEE Std. 929–2000) that indicate the limits for frequency, voltage,
harmonics.
 An ideal synchronization approach must:
❖ Competently track the phase angle of the utility grid.
❖ Efficiently detect the frequency variations.
❖ Efficiently eliminate disturbances and high harmonic
components.
❖ Immediately respond to utility grid changes.

Synchronization
 The synchronization requirements depend on different factors:
✓ The type of the application,
✓ Synchronization time, faults,
✓ Transients and failures resistance,
✓ Recovery time
✓ Ability to fulfill automatic synchronizing conditions.
 The synchronization can be considered not only in pure synchronization
terms, but also in terms of active and reactive power regulations, frequency
and voltage variations and fault ride through.
5

Grid synchronization for control of a grid-connected power
converter
6
 Given the multitude of problems concerning grid synchronization, various
estimation methods for phase angle, frequency, and harmonic estimation
have been proposed for synchronization of grid-connected converters.

Requirements of Synchronization Method
 Synchronization methods (SM) must account for the following
features:
 Distortion rejection capability and noise immunity:
 Frequency Adaptivity
 Phase-angle Adaptivity.
 Unbalance Robustness
 Dynamic / Convergence Time
 Structural Simplicity
 Accuracy
 Computational Burden
7

Continue…
8
 Distortion rejection capability and noise immunity:
When harmonic pollution exists the synchronization method should be able to
correctly filter grid-side harmonics in order to track the fundamental cleanly.
 Frequency adaptivity:
While operating at weak grids system frequency might deviate from the nominal
values. If grid frequency is susceptible of suffering from recursive variations, the
selected synchronization method must be able to cope with those changes without
losing synchronism.
 Phase-angle adaptivity:
Some types of voltage-sags might generate sudden phase-jumps. The
synchronization method must be able to correctly detect and ride-through these
jumps.

Continue…
9
 Unbalance robustness:
At distribution level unbalances are often due to the amount of single phase connected loads. At
higher voltage levels, trains, for instance, are single-phase connected loads that generate
temporary unbalances in the grid. Additionally, faults can also be source of transient
unbalances.
 Dynamics/convergence time:
The interest of a fast synchronization method is specially highlighted during transient events such as
voltage sags or swells.
 Structural simplicity:
Including simplicity of design, tuning and implementation.
 Accuracy.
 Computational burden:
Kalman estimators, for example, offer good accuracy and performance but they are usually
computationally heavy.

Quality Criteria for the evaluation of SM
10

 Synchronization criteria:
✓ The synchronization criteria set draws attention to the core of the
synchronization purpose.
✓ It validates properties such as method bandwidth, frequency estimation
accuracy, frequency adaptive operation, high frequency characteristics,
phase-angle jump settling time and overshoot and phase frequency
jump and overshoot.
✓ It focuses on how good the synchronization algorithm is.
11

 Design Criteria:
✓ It take into account features such as application, noise immunity,
single phase utilization, algorithms protection modes, required
additional features (signal filtering etc.), methods of realization
(analog, digital) and THD level of a sinus of estimated phase
angle.
12

 Computation Criteria:
✓ The computation criteria focus on total number of signals and
variables, number of addition/subtractions and
multiplications/scaling operations, transient response, state
variable/integrations, computational load and order of signals
processed in a cascade.
✓ These criteria set purpose is to assess the performance of the
algorithm.
13

 All of those evaluation properties are taken into account in two
cases:
✓ Normal operation.
✓ Operation under distortions.
 The operation under distortions meaning ability to perform under:
 Voltage sags and dips.
 Frequency variations.
 flicker occurrence.
 Harmonics occurrence.
 Short supply interrupts – recovery time.
 Short circuits.
14

Synchronisation Methods
Classification of Synchronization Method15

Continue…
16
 These methods can be classified as frequency domain or time domain.
 The classification can be made by application, for either single-phase
system or three-phase system.
 Further classification narrows these down into open-loop and closed-
loop systems.
 Open-loop systems directly detect the magnitude, phase, and frequency
of the input signal, whereas closed-loop systems adaptively update the
detected parameters through a loop mechanism.

Single-phase synchronization systems
Single-phase open-loop methods:
 Open-loop methods are often based on some type of filtering.
 Their performance depends on their capability of filtering
distorted signals and their adaptability to system changes such as
frequency and phase.
 The main approaches are based on discrete fourier transform
(DFT), weighted-least-square-estimation (WLSE), adaptative
notch filter (ANF), Kalman filtering and artificial neural
networks (ANN).
17

Single-phase closed-loop methods:
 The first single-phase phase-locked loops (PLLs) were presented by
Appleton and Bellescize as early as 1923 and 1932, respectively.
 For the late 1970’s the theoretical foundations of PLLs were well
established but they could only be comprehensively implemented with
the development of integrated circuits (IC) .
18
Over the years, three-phase systems are more popular than single-phase systems
because in single phase systems the tasks of grid fundamental and harmonic
detection are more challenging (only one signal is accessible).

Single-phase closed-loop methods:
 Basic concept of single-phase PLL
A PLL is a device which controls the phase of its output in such a way
that the phase error between the output phase and the reference phase
is minimized.
 Some of the most popular single-phase closed-loop approaches include:
 Enhanced PLL
 Adaptive PLL
 Vector based PLL
19

From the above-mentioned methods,
 Transport-delay, all-pass filters, inverse-park transformation and Hilbert
transformation present frequency dependency. For this reason, they are
not advisable in frequency variable environments.
 For the Kalman method errors due to frequency deviations can be
alleviated either increasing the time constant or adjusting the algorithm
sample time.
 SGI, SOGI and D-filters are based on similar concepts and their main
characteristics are that (i) they filter the voltage signal without delay and
(ii) they are frequency adaptive.
20

Three-phase synchronization systems
Three-phase open-loop SMs:
 Low-pass filtering (LPF) techniques:
 Filtered signals are normalized and passed through a rotation matrix in
order to compensate for the phase lag due to LPFs.
 A lower cut-off frequency guarantees a better filtering of the input signal
at the cost of a slower response.
 The major drawback of the base solution, however, is its dependency
with the grid-frequency (since the phase displacement depends on the
centre frequency) and its sensitivity to voltage unbalance.
 This method is unsuitable for applications where phase-jumps occur.
 Different solutions have been proposed to overcome these
disadvantages: a solution based on two frequency-adaptive sequential
filters and, an approach based on moving-average and predictive filters.
21

Three-phase open-loop SMs:
 Space-vector filter (SVF) techniques:
 Although the basic SVF behaves well face to phase jumps and
harmonic distortion, it introduces a phase-shift when the grid-
frequency varies.
 The extended SVF (eSVF), is designed to be frequency adaptive.
 Nevertheless, it seems that tuning of the added PI-regulator is in
conflict with frequency tracking and voltage harmonics sensitivity.
 Kalman filtering (KF) techniques:
 These synchronization techniques can work in distorted and
unbalanced environments, they can cope with phase jumps and they
are frequency adaptive.
 Their main drawback is their computational burden, their higher
convergence time and the difficulty in selecting the optimal
weighting matrices (process and measure covariance matrices).
22

Three-phase open-loop SMs
 Weighted Least Squares Estimation (WLES) techniques:
 It acts without delay when a sudden voltage sag or unbalance occurs, it
estimates positive and negative sequences separately, and
accommodates grid frequency variations.
 This method can be distinguished from conventional filtering
techniques in its fast transient response.
 However, reports long transient intervals in detecting frequency
changes, computational problems related to least-squares methods and
sensitivity to noise and distortions.
23

Three-phase closed-loop SMs:
 Closed-loop methods operate in a closed-loop structure which
regulates an error signal to zero.
 Probably the most well-known and spread closed-loop
synchronization method is the synchronous rotating frame PLL
(SRF-PLL), which became popular in the late 90’s.
 Similar approaches to the SRF-PLL, and with equivalent
shortcomings, are the pq-PLL, which can be easily interpreted,
thanks to the instantaneous real and imaginary power theory and the
orthogonality-based PLL.
24

Three-phase closed-loop SMs:
 Classical three-phase PLL or SRF-PLL
✓ The three-phase SRF-PLL can be divided in a PD block that is
constituted by a park-transformation, a LF which is often a PI-regulator
and a VCO that is usually an integrator.
25

Filters in SRF-PLL:
 Since the effect of unbalance and harmonic distortion propagates towards the
static- and rotating-frame voltage components, it is possible to damp these
components by an additional filter placed before the LF.
 This filter can be located in the rotating-frame (fig.a) or in the static-frame
(fig.b).
(a) (b)
26

Filters in SRF-PLL:
 LPF-based techniques are simple to implement but they present two
shortcomings:
(a) The narrower the bandwidth of the filter is, the better the immunity to
distortion is at the cost of slower transient times and
(b) It is frequency-dependent introducing variable phase-shifts in the filtered
signals.
 On the contrary to LPFs, resonant filters as such used in do not introduce any phase-
shifts at the resonant frequency and it offers a superior harmonics rejection
capability.
 Notch-filters filter the second harmonic component but their response time is slow
27

Filters in SRF-PLL
 MAF-based techniques demonstrate excellent second-harmonic cancellation and superb
elimination of external AC voltage harmonics but their transient response is unfovarable (due
to the length of the moving window).
✓ The poor phase angle tracking under reduced voltages seems to be solved with an automatic
gain block in the Matlab/Sim -Power Systems blockset.
 Repetitive controller improves the rejection capability of the PI controller by amplifying the
second harmonic.
✓ It works essentially like a bandpass filter in which the odd harmonics are filtered while the even
harmonics are not. This way, the proportional gain of the PI controller is indirectly increased
and thus, the rejection capability. This controller is implemented by means of a DFT algorithm
and is robust against frequency variations and phase-jumps.
28

Filters in SRF-PLL
 The ALOF uses a least-mean-square algorithm that looks like an adaptive linear
neural network (ADALINE) algorithm.
✓ The ALOF shows the characteristics of a band-pass filter at fundamental frequency
and a notch filter at harmonic frequencies and it shows a good tracking accuracy,
dynamic response and immunity to grid voltage disturbances.
 In the static-frame, solutions based on a second-order LPF-based multivariable
filter, a Kalman filter and a ADALINE algorithm have been studied.
✓ The Kalman filter and the ADALINE based filter give similar results, which are
better than the second-order LPF, but ADALINE filter is specially interesting for its
simplicity.
29

 Symmetrical sequence extracting techniques applied to SRF-PLL:
 An interesting solution, other than filtering the looped input (e.g. the quadrature component),
is to extract the positive sequence from the grid-voltage and feeding this sequence to the
classical SRF-PLL.
 This approach allows having a distortion-free input at the PLL that will, in turn, enable tuning
the PLL with a higher bandwidth. Many different alternatives have been proposed for extracting
the instantaneous symmetrical components (ISC) online.
 The following points overview some of them:
o Multiple reference frame based PLL
o Three-phase EPLL.
o Filtered-sequence based PLL.
o Neural-network-based PLL.
o Orthogonal component-based techniques.
o Kalman.
30

Features comparison of the methods
 Three approaches to synchronization have been categorized in:
(i) Active synchronization.
(ii) Passive synchronization.
(iii) Open-transition transfer.
31
Depending on the reference coordinates used the approaches can
be categorized as:
✓ In natural abc coordinates
✓ In stationary alpha-beta ( )coordinates
✓ In rotating d-q coordinates.
 −

Synchronisation
Approach
Approach Complexity and Power
Reliability
Active ✓ Require inbuilt control
mechanism to match the voltage,
frequency and phase angle of
microgrid and power grid.
✓ Require special infrastructure
for communication
✓ Involve complex
controlling techniques.
✓ Maintain the power
reliability.
Passive ✓ Employs synchronisation check
for paralleling.
✓ Does not require special control
mechanism or temporary load
interruption.
✓ Existing simple
synchronization
technologies can be used.
✓ Maintain the power
reliability.
Open transition ✓ Loads and DGs in the island are
de-energised before reconnecting
to the grid.
✓ Simple.
✓ Reduces the power
quality.
32

Synchronisation
Approach
Economy Application
Active ✓Comparatively high capital cost.
✓Depends on the technique
✓Mostly applied for
inverter based systems,
but can be applied in
hybrid systems
Passive ✓Low capital cost compared to
the active synchronisation
✓For synchronous
generator based systems
or hybrid systems having
different types of
Does not require special
control mechanism or
tem- generation
Open transition ✓No capital cost
✓Incur loss of revenue with low
power reliability as it requires
load interruption before
reconnection
✓For inverter based,
synchronous generator
based or hybrid system.
33

Applications of Synchronization
Algorithm
34
The synchronization algorithms are used in variety of power
applications:
✓ In terms of current control: reactive power and harmonics
compensation.
✓ Support for “smart grid” management – fault ride through, carrying
out the connection and disconnection process of network elements,
islanding detection.
✓ Grid monitoring – fault detection by frequency/angle determi-
nation, power factor estimation.
✓ Reactive and active power regulation
✓ Dips and flicker compensation, voltage regulation.
✓ In terms of RES integration in power systems – photovoltaic plants,
wind power plants, wave energy plants etc.
✓ Different kinds of loads integration - AC loads with frequency
converters, DC loads working with DC/AC converters.

Key challenges
 The synchronization schemes presented so far is still based on estimating
the input signal phase whereas the dynamic response during transients is
very sensitive to phase-angle jumps.
 There is still a lack of reports or studies on the use of artificial intelligence
in the synchronization of grid-connected power converters.
 Strategies still lack in triggering the corrective procedures automatically to
maintain power quality. A robust method with advanced features such as
expert systems has been identified as efficiently injecting power into the
grid, with low total harmonic distortion (THD) of the current.
 The biggest concern is how to achieve uninterrupted operation of the RES
in abnormal utility voltage conditions. None of the phase-tracking methods
presented so far can meet the grid code requirement regarding THD,
especially in low power flows.
35

Conclusion
 From literatures, some new methods are found to perform better than
classical PLL yet PLL is still well accepted for its simplicity. Therefore,
many modifications to PLL have been made to enhance and improve its
performance during a weak grid.
 Synchronization methods can be evaluated according to
dynamics/convergence time, accuracy, distortion/ disturbance
rejection, phase-angle adaptivity, frequency adaptivity, unbalance
robustness, noise immunity, structural simplicity, computational
burden and single or three-phase utilization. It is up to users to choose
the method that suits them better.
36

References
 Chandak, S., Bhowmik, P., & Rout, P. K. (2019). Dual-stage cascaded control to
resynchronise an isolated microgrid with the utility. IET Renewable Power Generation.
 Teimourzadeh, S., Aminifar, F., Davarpanah, M., & Shahidehpour, M. (2018). Adaptive
control of microgrid security. IEEE Transactions on Smart Grid, 9(4), 3909-3910.
 Ramezani, M., Li, S., Musavi, F., & Golestan, S. (2019). Seamless transition of
synchronous inverters using synchronizing virtual torque and flux linkage. IEEE
Transactions on Industrial Electronics, 67(1), 319-328.
 Boyra, M., & Thomas, J. L. (2011, August). A review on synchronization methods for grid-
connected three-phase VSC under unbalanced and distorted conditions. In Proceedings
of the 2011 14th European Conference on Power Electronics and Applications (pp. 1-10).
IEEE.
37

Conference
38
 Jaalam, N., Rahim, N. A., Bakar, A. H. A., Tan, C., & Haidar, A. M.
(2016). A comprehensive review of synchronization methods for
grid-connected converters of renewable energy source. Renewable
and Sustainable Energy Reviews, 59, 1471-1481.
 Cho, C., Jeon, J. H., Kim, J. Y., Kwon, S., Park, K., & Kim, S. (2011).
Active synchronizing control of a microgrid. IEEE Transactions on
Power Electronics, 26(12), 3707-3719.
 Das, D., Gurrala, G., & Shenoy, U. J. (2016). Linear quadratic
regulator-based bumpless transfer in microgrids. IEEE
Transactions on Smart Grid, 9(1), 416-425.
 Etemadi, A. H., & Iravani, R. (2017). Supplementary mechanisms
for smooth transition between control modes in a
microgrid. Electric Power Systems Research, 142, 249-257.
 Majumder, R., Ghosh, A., Ledwich, G., & Zare, F. (2009). Power
management and power flow control with back-to-back converters
in a utility connected microgrid. IEEE Transactions on Power
Systems, 25(2), 821-834.

Questions
39
 What are the major criteria for the synchronization approaches?
Or
What are the factors must have in an ideal synchronization approach?
 Mention other applications of synchronization techniques for the
power system?
 Mention all the conventional methods for resynchronization
/reconnection/transition for micro-grid to operate on grid connected or
islanding mode of operation according to the need.

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