Download Towards Optimal Energy Store-Carry-and-Deliver for

Voltage Regulation in Distribution
Systems with
Distributed Generation (DG)
Presented by: Hao Liang
Broadband Communications
Research (BBCR) Lab
Smart Grid Research Group
2012.11.7
Outline
• Introduction
• The Voltage Rise Problem
• Optimization Based Voltage Regulation
• Estimation Based Voltage Regulation
• Combined Measurement and Estimation Based Voltage Regulation
• Remote Terminal Unit (RTU) Coordination Based Voltage Regulation
• Discussions
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Introduction
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• Distributed Generation (DG) Integration
• In the recent years, distributed generation (DG) has experienced significant
growth all over the world
• Potentially powered by renewable energy sources such as wind and solar, DG
units are able to supply the electricity demands in an more economical and
environmentally-friendly way as compared with the conventional centralized
generators
• However, the adoption of DG units (which are intermittent in nature) poses
new challenges on distribution system engineering
• One of the most challenging issues is the voltage regulation, which is aimed at
keeping the voltage level of the distribution system within a certain range
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• Smart Grid Communications
• WAN: Fiber optics, microwave, cellular (e.g., GPRS, 3G, HSPA+, and LTE)
• HAN: ZigBee, WiFi, power line communications (PLC)
• NAN: ZigBee, WiFi
Focus of This Talk
Picture: http://www.fluidmesh.com
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The Voltage Rise Problem
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• Causes of Voltage Rise
• Model of a 3-bus feeder (no DG)
• The voltage (V2) at a remote point on a feeder can be approximately
calculated as
or
in per unit
V1 – Source voltage
R + jX – Impedance of the line between the source and the remote point
P and Q – Active and reactive power flows, respectively
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• Causes of Voltage Rise (Cont’d)
• Model of a 3-bus feeder (with DG)
• If the DG unit injects active power into the system (i.e., the DG unit is working
at unity power factor), we have
• If the output of the DG unit is large, (P − PG) becomes negative. As a result,
V2 can be larger than V1, which causes the voltage rise problem
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• Possible Solutions for Voltage Rise Problem
• Network Reinforcement – The network reinforcement method aims at
reducing R to mitigate the voltage rise effect. However, the cost is relatively
high for upgrading the feeders
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• Possible Solutions for Voltage Rise Problem (Cont’d)
• Generation Curtailment – When the load demand is low, some of the
generation capacity (PG) can be curtailed to reduce the voltage rise, at the
cost of a reduction in the profit of DG. The significance of the cost is heavily
dependent on the electricity price and may be less when the electricity price
is low as the load demand is low
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• Possible Solutions for Voltage Rise Problem (Cont’d)
• Reactive Power Compensation – The reactive power compensation method
requires the generator to absorb a certain amount of reactive power (Q).
However, this method is not very efficient (especially for cable feeders) since
the X/R ratio in distribution systems is typically much smaller than that in
transmission systems. At the same time, this method results in higher losses
in distribution systems
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• Possible Solutions for Voltage Rise Problem (Cont’d)
• Voltage Regulator – The voltage rise effect can also be addressed by
decreasing the source voltage V1. Remote terminal units (RTUs) are deployed
at strategic locations across the network for voltage and power flow
measurements. The measured information is transmitted to the voltage
regulator at the primary transformer (via certain communication networks in
the context of Smart Grid) for voltage control
Cost of Communications?
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Optimization Based Voltage Regulation
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• Basic Idea
• A linear programming (LP) based formulation of the optimal power flow (OPF)
is considered. The objective is to minimize the annual active generation
curtailment cost, while satisfying voltage and thermal constraints. The
decision variables are the generation curtailment, reactive compensation, and
coordinated voltage regulation (area voltage control) using OLTC
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• Problem Formulation
Minimize the annual active generation
curtailment cost
Active power injection
Reactive power injection
Load flows of the branch ij
Limits
Tap setting of the tap-changer k
Reactive power curtailment may be
correlated with the active power curtailment
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• Discussions
• The main drawback of the optimization based voltage regulation scheme is
that it can hardly be implemented in real time as it is not possible to read the
information of all nodes in the distribution system. How to reduce the number
of measurements is the main issue to be addressed
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Estimation Based Voltage Regulation
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• Basic Idea
• A reference feeder without DG unit is needed
• The key technique is the estimation of the output of generator which is
connected at a remote point on the feeder
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• The Voltage Regulation Strategy
• An additional current measurement IFG is used. The ratio EST represents the
load share between feeders with embedded generation to those without
generators, given by
where ITL is the summation of transformer currents. The factor EST is
calculated before the connection of the DG unit or when the output of the DG
unit is zero
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• The Voltage Regulation Strategy (Cont’d)
• Then, during the operation of the DG unit, the generation output can be
estimated, given by
• Based on the value of IG , the voltage rise at the connection point of the DG
can be calculated as
This value corresponds to the necessary voltage reduction at the substation in
order to bring the voltage level at the point of DG connection within statutory
limits
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• Discussions
• This scheme does not take advantage of the instantaneous measurements
obtained from a communication network, which is expected to be deployed in
the next generation power grid (also referred to as the smart grid)
• This scheme requires a reference feeder without DG
• In addition, the scheme is used to solve the voltage rise problem, while the
low voltage point (below the statutory limit) can hardly be detected
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Combined Measurement and Estimation Based
Voltage Regulation
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• Basic Idea
• Investigate both real-time measurements and statistical estimation
• Specifically, based on the information of real-time measurements and load
profile, the voltage magnitude at each network node (supplied by the primary
substation) can be estimated with certain accuracy
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• Voltage Estimation
• A weighted least squares method is used, where the state variables are
defined with respect to the node voltage magnitudes and relative phase
angles, given by
where xj represents the state variables, Ns is the number of state variables, zi
denotes the ith measurement, Nm is the number of measurements, fi is a
function relating ith measurement to state variables, and σi is the variance of
the ith measurement
• Three types of function fi is used, which relates the state variables Vi, Vk, θi,
and θk to the measurements Pinj, Qinj, and Vi, given by
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• Voltage Estimation (Cont’d)
Power flow equations, which
can be solved by using the
Newton-Raphson method
• For state estimation on a distribution network, we have Nm ≪ Ns. However,
the necessary condition for a weighted least squares state estimation
algorithm to have a unique solution is Nm ≥ Ns
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• Voltage Estimation (Cont’d)
Power flow equations, which
can be solved by using the
Newton-Raphson method
• Therefore, the pseudo measurements which are derived from offline data, can
be used to provide unmeasured quantities, so that Nm ≥ Ns
• In this work, the unmeasured quantities are Pinj and Qinj at load nodes and so
pseudo measurements are used for these. The measurements are assumed to
be independent normal random variables, with variance
being greater for
pseudo measurements than for real-time measurements
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• Discussions
• The combined measurement and estimation based voltage regulation scheme
is very complicated which requires the solution of non-linear equations. How
to reduce the complexity and apply the scheme in real time is still an open
issue
• Moreover, the performance of the scheme heavily depends on the load
models. How to improve the modeling accuracy based on load profiles is
interesting and requires future research
• In addition, the voltage controller used in this work is not optimized, which
potentially degrades the efficiency of voltage regulation
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RTU Coordination Based Voltage Regulation
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• Voltage Profile Estimation
• RTUs are deployed at the DG and capacitor connecting buses
• Theorem 1. For the voltage profile of a feeder, maximum voltage can happen
only at the DG connecting buses, capacitors connecting buses, and the
substation bus, provided that the R/X ratio of the feeder is constant along the
Directly Measurable
whole feeder
• Theorem 2. There exists a minimum voltage point in between two DG
connecting buses if and only if, for both DGs, the voltage of the DG
neighboring bus, in the direction of the other DG, is less than the voltage of
the DG bus
Need to be Estimated
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• Voltage Profile Estimation (Cont’d)
• Assume the load between two DG units is concentrated halfway between
them. The minimum voltages calculated by DG1 and DG2 are, respectively,
given by
• In order to get a better estimation of the minimum voltage, we take the
average:
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• Voltage Profile Estimation (Cont’d)
• In order to achieve voltage regulation, each RTU measures the following
information
1) The voltage of its element bus
2) Active and reactive power flow in lines connected to its element bus
3) The voltages of the immediate neighbor buses of its element bus
• Based on the measurements, each RTU can perform the following calculation
1) Measure a maximum voltage point
2) Check one part of the condition for the possibility of the existence of a
minimum voltage point between its element and any neighbor element
3) Estimate the value of the minimum voltage point on each side of its
element, if it exists
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• Voltage Profile Estimation (Cont’d)
• A communication network is established for the RTUs to pass the calculated
information to the voltage regulator
• Data passing starts from the farthest RTU towards the voltage regulator
• Along the data passing path, each upstream RTU updates the maximum and
minimum voltages based on the information received from its downstream
RTU
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• Voltage Regulator Operation
• When the information with respect to the maximum and minimum voltages
are received by the voltage regulator, the following operations are performed:
1) If the absolute maximum voltage is greater than maximum permissible
voltage, then the voltage regulator will decrease the current tap position till
the maximum voltage of the feeder is within the permissible range
2) If the minimum voltage of the feeder is below the minimum permissible
voltage, then the voltage regulator will increase the tap position to bring the
minimum voltage into the permissible range
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• Discussions
• The RTU based voltage regulation can efficiently utilizes the instant
measurements at the generation buses. However, how to integrates the
estimates of the loads to improve the voltage regulation performance is still
an open issue
• The scheme is not optimized for certain objective functions
• The reliability of the communication network?
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Thank you!
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