Reactive power and Voltage control
• With the increased use of PV and wind systems on the transmission network, the
industry is moving towards the ability to provide reactive power capability.
• Some PV inverters have the capability to absorb or inject reactive power, if
needed, provided that current and terminal voltage ratings are not exceeded.
• Reactive power capability from the inverter, to the extent that is available, varies
as a function of terminal voltage
Capability curve of a VSC
• There are mainly three factors that limit the capability
• First one is the maximum current through the IGBT:s.
This will give rise to a maximum MVA circle in the
power plane where maximum current and actual AC
voltage is multiplied. If the AC voltage decreases so will
also the MVA capability.
• The second limit is the maximum DC voltage level.
The reactive power is mainly dependent on the voltage
difference between the AC voltage that the VSC can
generate from the DC voltage and the grid AC voltage.
• The third limit is the maximum DC power
Grid Voltage dependent reactive power generation
•
In general, to inject real and reactive power command-there are two
methods to generate the reference current-through Voltage control and
through Power Control.
• In Voltage oriented control, real and
Real power control by Vdc control
reactive power controls are
decoupled.
• Hence d and q components of
current can be independently
controlled to regulate real and
reactive power respectively
Reactive power control by Vac control
PLL Dynamics
⚫
In grid connected mode the system dynamics are mostly dictated by
grid which are very slow in nature because of huge inertia
⚫
But in islanded system Power dynamics are mostly because of DG
system or inverter system.
⚫
Although in islanded steady state the PLL works well but during
dynamic conditions PLL may fail to track the phase and frequency
which may lead to increased power oscillations in the system.
⚫
To avoid this the design of PLL should be fast, robust and should be
tested for all possible dynamic conditions.
Ride through capabilities
• Wind and solar generating units connected at all voltage levels to the grid, shall be
remain connected to the grid when voltage at the interconnection point on any or
all phases rise above the values given below
• Wind and solar generating units connected to the grid, shall be remain connected
to the grid when voltage at the interconnection point on any or all phases dips up
to level depicted below
Single phase Grid connected PV
• Challenges
• Oscillating active power at twice the supply frequency,
• Tuning of PI controller for sinusoidal signals,
Grid Synchronisation in single phase
⚫
In 3-phase system PLL can be used for synchronizing inverter with the grid system
⚫
But in single phase system normal dq-based PLL fails to track phase and frequency
⚫
So synchronization in a single phase domain is challenging issue
⚫
Synchronization can be achieved by using special techniques like
◦ unit template(UT) based synchronization,
◦ second order generalized integrator(SOGI) based synchronization.
❑ Islanding
➢ Islanding is the condition in which a distributed generator (DG) continues to power a location even though
external electrical grid power is no longer present.
➢ Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent
automatic re-connection of devices.
➢ Additionally, without strict frequency control, the balance between load and generation in the islanded circuit can be
violated, thereby leading to abnormal frequencies and voltages.
➢ For those reasons, distributed generators must detect islanding and immediately disconnect from the circuit; this is
referred to as anti-islanding.
➢ Intentional and unintentional islanding.
Islanding scenario for DG.
➢ Need to go for islanding
The reasons for islanding are given as follows.
• Safety concerns: if an island forms, repair crews may be faced with unexpected live wires
• End-user equipment damage: customer equipment could theoretically be damaged if operating parameters differ greatly from the norm. In
this case, the utility is liable for the damage.
• Ending the failure: Reclosing the circuit onto an active island may cause problems with the utility's equipment, or cause automatic reclosing
systems to fail to notice the problem.
• The voltage and frequency may not be maintained within a standard permissible level.
Reference :1 "IEEE Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems," in IEEE Std 1547.4-2011 , vol., no., pp.1-54, 20 July 2011, doi: 10.1109/IEEESTD.2011.5960751.
2. A. Hussain, C. -H. Kim and A. Mehdi, "A Comprehensive Review of Intelligent Islanding Schemes and Feature Selection Techniques for Distributed Generation System," in IEEE Access, vol. 9, pp. 146603-146624, 2021, doi:
10.1109/ACCESS.2021.3123382.
Islanding detection
⚫
IEEE 1547-2003 standard stipulates a maximum delay of 2 seconds
for detection of an unintentional island and all DGs ceasing to
energize the distribution system
❑Islanding Detection
➢
The main philosophy of detecting an islanding situation is to monitor the DG output parameters and/or system parameters and decide
whether or not an islanding situation has occurred.
➢
Islanding detection techniques can be divided into remote and local techniques.
1. Remote Techniques: Remote islanding detection techniques are based on communication between utilities and DGs. Although these
techniques may have better reliability than local techniques, they are expensive to implement and hence uneconomical. Some of the remote
islanding detection techniques are as follows:
a.
Power line signaling scheme.
b.
Transfer trip scheme.
In a Power Line Signaling scheme a signal generator at the transmission system
continuously broadcasts a signal to the distribution feeders using the power line as the
signal path. DGs are equipped with signal receivers. If the receiver does not sense the
signal (caused by the opening of breakers between the transmission and distribution
systems), there is an island condition .Figure shows a power line signaling scheme.
Fig. Remote islanding detection scenario.
➢Transfer trip scheme: The basic idea of transfer trip scheme is to monitor the status of all the circuit breakers and reclosers that could
island a distribution system. Supervisory Control and Data Acquisition (SCADA) systems can be used for that . This method requires a
better interaction between the utility and DGs and this often increases the costs for both the utility and DG owners.
Islanding detection
Some drawbacks without islanding detection are as follows:
⚫ Line worker safety can be threatened by DG sources feeding a system after
primary sources have been opened and tagged out.
⚫ The voltage and frequency may not be maintained within a standard permissible
level.
⚫ The islanded system may be inadequately grounded by the DG interconnection.
⚫ Instantaneous reclosing could result in out of phase reclosing of DG. As a result
of which large mechanical torques and currents are created that can damage the
generators or prime movers.
⚫ Due to these reasons, it is very important to detect the islanding quickly and
accurately
Islanding Detection cont..
2.Local detection techniques:- It is based on the measurement of system parameters at the DG site, like voltage, frequency, etc.
➢
Some of the remote islanding detection techniques are as follows:
a.
Passive detection techniques
b.
Active detection techniques
c.
Hybrid detection techniques
a)
Passive detection techniques
▪
Passive methods work on measuring system parameters such as variations in voltage, frequency, harmonic distortion, etc. These parameters vary
greatly when the system is islanded.
▪
Passive techniques are fast and they don’t introduce disturbance in the system but they have a large non detectable zone (NDZ) where they fail to
detect the islanding condition. Some of the Passive techniques are
i.
Rate of change of output power.
ii.
Rate of change of output frequency,
iii.
Change of impedance
•
The passive plans are realized simply and cost-effectively; however,
they suffer a large non-detection zone.
(NDZ) is the situations wherein the Islanding Detection Method (IDM) fails to identify islanding.
General schematic of passive methods.
➢
Rate of change of Power
• This method consists of monitoring the output power (active and/or reactive) at the PCC. If the variation rate is higher than
a value, the method considers that islanding has occurred.
• In the case of islanding, dq/dt or dp/dt value of system will increase i.e. dq/dt or dp/dt value is greater than the prescribed
value, which indicates the detection of islanding.
• The technique used will continuously monitor the rate of change of active or/and reactive power where DG is connected
whenever any disturbance occurs at the distribution side. With the comparison of post and pre islanding the decision is
taken.
•
𝑑𝑞
𝑑𝑞
>
𝑑𝑡 𝑝𝑜𝑠𝑡
𝑑𝑡 𝑝𝑟𝑒
•
𝑑𝑝
𝑑𝑝
>
𝑑𝑡 𝑝𝑜𝑠𝑡
𝑑𝑡 𝑝𝑟𝑒
In the figures The islanding occurs at t=10s
M. Arshad, M. Sufyan, M. M. Aman, S. R. S. Raihan and N. A. Rahim, "Islanding detection technique
based on rate of change of reactive power (dq/dt)," 5th IET International Conference on Clean Energy
and Technology (CEAT2018), 2018, pp. 1-7, doi: 10.1049/cp.2018.1350.
➢
Rate of change of output frequency
• The performance of the ROCOF passive method is based on the measurement of frequency at
PCC .
•
The frequency measurement is done through Phase locked loop.
• In the cases when the grid system is lost, a change in DG’s loading is happening and its
instantaneous output frequency is changing called as ROCOF.
• On grid supply disconnection, the power imbalance causes transients in the islanded system
leading to a gradual change in frequency. The rate of change of frequency is measured over a
few cycles and After a preset time delay, a trip signal is sent to the DG .
• The expression for the ROCOF is
𝑑𝑓
𝑓 𝑛 −𝑓𝑑 (𝑛−1)
(n)= 𝑑
where fd is frequency deviation.
𝑑𝑡
∆𝑇
ROCOFD- Rate of change of frequency deviation.
∆𝑇: −samplinng time
•
P. Kumar, V. Kumar and R. Pratap, "Rate of Change of Frequency Deviation Based Unintentional Islanding Detection for Microgrids," 2018 International Conference on Control,
Power, Communication and Computing Technologies (ICCPCCT), 2018, pp. 90-95, doi: 10.1109/ICCPCCT.2018.8574297.
Passive detection techniques considering change in impedance.
SI-Superimposed impedance
CSOSI-cumulative sum of superimposed impedance
where n indicates the length of the window, which is obtained
from the sampling frequency
•
The threshold setting entirely depends on sampling interval, window size, and
the capacity of different DGs, network topology, and the system parameters.
This is obtained by simulating numerous islanding and non-islanding cases
B. K. Choudhury and P. Jena, "Superimposed Impedance Based Passive Islanding Detection Scheme
for DC Microgrids," in IEEE Journal of Emerging and Selected Topics in Power Electronics, doi:
10.1109/JESTPE.2021.3076459.
Islanding Detection cont..
b)Active detection techniques
• With active methods, islanding can be detected even under the perfect match of generation and load, which is not possible in case of the passive
detection schemes.
• Active methods directly interact with the power system operation by introducing perturbations.
• The idea of an active detection method is that this small perturbation will result in a significant change in system parameters when the DG is
islanded, whereas the change will be negligible when the DG is connected to the grid. Some of the Active techniques are.
i.
Impedance measurement method
ii.
Phase (or frequency) shift methods
iii.
Reactive power export error detection.
Example :-Injection of current disturbance on D axis.
Drawback of active method
• Injected disturbance may create power quality issue in power system.
Thus need to go for hybrid method.
Where I*td:- is the reference value,
Idist:- is injected disturbance values.
1.P. Mahat, Zhe Chen and B. Bak-Jensen, "Review of islanding detection methods for distributed generation," 2008 Third International Conference on Electric Utility Deregulation and
Restructuring and Power Technologies, 2008, pp. 2743-2748, doi: 10.1109/DRPT.2008.4523877.
2.G. Hernandez-Gonzalez and R. Iravani, "Current injection for active islanding detection of electronically-interfaced distributed resources," in IEEE Transactions on Power Delivery, vol. 21, no.
3, pp. 1698-1705, July 2006, doi: 10.1109/TPWRD.2006.876980.
c) Hybrid detection schemes Hybrid methods employ both the
active and passive detection techniques.
•
The active technique is implemented only when the
islanding is suspected by the passive technique.
•
Some of the hybrid techniques are discussed as follows:
i.
Technique based on positive feedback (PF) and voltage
imbalance (VU):
ii.
Technique based on voltage and reactive power shift.
General scheme of hybrid method.
Operational principle of
hybrid method
|∆Vd|
Block diagram of the control circuit for Q axis current injection.
[1]M. Dietmannsberger and D. Schulz, "Impacts of Low-Voltage Distribution Grid Codes on Ancillary Services and Anti-Islanding Detection of Inverter-Based Generation,"
in IEEE Transactions on Energy Conversion, vol. 31, no. 4, pp. 1287-1294, Dec. 2016, doi: 10.1109/TEC.2016.2589461.
[2]Cebollero, J.A.; Cañete, D.; Martín-Arroyo, S.; García-Gracia, M.; Leite, H. A Survey of Islanding Detection Methods for Microgrids and Assessment of Non-Detection
Zones in Comparison with Grid Codes. Energies 2022, 15, 460. https://doi.org/10.3390/en15020460
(difficult to observe change)
Block diagram of the frequency variation
detection Method.
Where,
AROCOF:-mean of absolute
rate of change of frequency.
ROCOF:- rate of change of frequency.
Qf:- Quality factor
IIS:- islanding identification signal.
S. Murugesan and V. Murali, "Decentralized Unintentional Islanding Identification for
Converter-Interfaced Multiple DGs," in IEEE Transactions on Industrial Informatics, vol.
17, no. 7, pp. 4512-4520, July 2021, doi: 10.1109/TII.2020.3020073.
Responses of (a) frequency, (b) ROCOF, (c) AROCOF mean,
and (d) IIS, for exactly matched power condition of various Qf .
Islanding Detection
Techniques
Advantages
Disadvantages
Examples
1 Remote Techniques
Highly reliable
Expensive to implement especially for small
systems.
• Power line signaling scheme
• Power line signaling scheme
2 Passive Techniques
• Short detection time
• Do not perturb the system
• Accurate when there is a large
mismatch in generation and demand in
the islanded system
• Difficult to detect islanding when the load and
generation in the islanded system closely match
• Special care has to be taken while setting the
thresholds
• If the setting is too aggressive then it could
result in nuisance
• Rate of change of output power
scheme
• Rate of change of frequency scheme
• Rate of change of frequency over
power scheme
• Change of impedance scheme
• Voltage unbalance scheme
• Harmonic distortion scheme
3 Active Techniques
• Can detect islanding even in a perfect
match between generation and demand
in the islanded system (Small NDZ)
• Introduce perturbation in the system
• Detection time is slow as a result of extra time
needed to see the system response for
perturbation
• Perturbation often degrades the power quantity
and if significant enough, it may degrade the
system stability even when connected to the grid
• Reactive power export error detection
scheme
• Impedance measurement scheme
• Phase (or frequency) shift schemes
(like SMS, AFD, AFDPF and ALPS)
4 Hybrid Techniques
• Have small NDZ. • Perturbation is
introduced only when islanding is
suspected.
• Islanding detection time is prolonged as both
passive and active technique is implemented
• Technique based on positive
feedback and voltage imbalance
• Technique based on voltage and
reactive power shift
P. Mahat, Zhe Chen and B. Bak-Jensen, "Review of islanding detection methods for distributed generation," 2008 Third International Conference on
Electric Utility Deregulation and Restructuring and Power Technologies, 2008, pp. 2743-2748, doi: 10.1109/DRPT.2008.4523877.
Off grid mode/isolated mode
❑Operates as VOLTAGE CONTROLLED SOURCE.
❑Need to share the P and Q through varying the Terminal voltage V and
Frequency.
DROOP CONTROL
• The system equilibrium is temporarily disrupted following a
change in load.
• For multiple generators as shown, a power sharing control is
needed so that the generators do not fight among
themselves.
• The terminal characteristics of each energy source is
drastically different.
P1, Q1
P2, Q2
• Isochronous inverters can also participate in frequency
regulation (PV with battery) while the PV can keep operating at
its MPP.
➢ Basic difference between synchronous generator based
droop and inverter based droop—
• The output frequency of any DC/AC inverter is fixed unlike
the synchronous gen. where the increase in PL causes the
freq. to decay at a rate determined by the rotor inertia.
• Equations for synchronous gen. based and inverter based
droop control –
𝜔𝑛𝑙 − 𝜔𝑓𝑙
𝑅 % =
× 100
𝜔0
∆𝑓 𝐻𝑧
𝑅=
ൗ𝑀𝑊
∆𝑃
∆𝑃2 𝑅1
=
∆𝑃1 𝑅2
𝜔∗
𝑉𝑑
= 𝜔𝑛𝑙 − 𝐾𝑓 ∆𝑃
∗
= 𝑉𝑛𝑙 − 𝐾𝑣 ∆𝑄
PL, QL
Schematic Diagram of AC Microgrid
Droop characteristics for power sharing (cont.)
• Droop characteristics is shown in Fig. (a) and Fig. (b)
• Reference signal calculation is shown in Fig. (c) and Fig. (d)
Droop characteristics for power sharing (cont.)
• Limitations of P-f and Q-V droop
• Due to less X/R ratio in Microgrids, the decoupling between P and Q is absent
• Because of this power sharing is hampered while using P-f and Q-V droop
• A better transient response is obtained with higher droop coefficients but this is
achieved at the cost of small signal stability. The small signal stability is deteriorated
with higher droop coefficients
Active Power (KW)
Reactive Power (KVAR)
Droop characteristics for power sharing (cont.)
Modified droop characteristics
• Modified droop characteristics in the
steady state under low X/R ratio [1]:
• 𝑓𝑟𝑒𝑓 = 𝑓0 − 𝐾𝑓 ∆𝑃cosϕ + 𝐾𝑣 ∆𝑄 sinϕ
• 𝑉𝑟𝑒𝑓 = 𝑉0 − 𝐾𝑓 ∆𝑃sinϕ − 𝐾𝑣 ∆𝑄 cosϕ
(a)
(b)
Droop control under (a) high X/R ratio (b) low
X/R ratio
[1] J. Peng et al., “Droop-controlled inverters as educational control design project,” IEEE Trans. Power Syst., in early access.
PV-Diesel Generator Microgrid System Operation
No requirement
• for detection of islanding explicitly,
• measurement of local load level,
• Look up table or info about PV array characteristic to decide the
voltage reference.
PV-Diesel Generator Microgrid System Operation
No requirement
• for detection of islanding explicitly,
• measurement of local load level,
• Look up table or info about PV array characteristic to decide the voltage reference.