Research on Distribution System Planning Theory
under the Trend of Load-supply Integration
Li Jingru, Huang Ping, and Cui Kai
State Power Economic Research Institute (SPERI)
Abstract—In this paper, a new distribution system
planning theory under the trend of load-supply
integration is explored. First, this paper analyzes
the development trends and characteristics of
future distribution system, and points out that the
integration of load-supply will be the most
important feature of future distribution system.
The feature blurred the boundaries between the
load and the power supply, makes energy
complementary strengths fuller, power system
operation more flexible, electricity balance more
timely. The feature also has a major impact on
distribution
system
planning.
Boundary
conditions of planning become less clear and
more uncertainty, optional schemes varied. This
paper analyzes the basic theory of traditional
distribution network planning methodology and
inadaptability in the situation of integration of
load-supply. Further, this paper proposes a new
construction idea of theoretical system of
distribution system planning under the situation of
load-supply integration from the five areas of
objective, requirements, core, key points and
process. Distribution system planning objective
will change from single, short-term to
multidimensional, long-term. The requirements of
long-term system reliability, economy and asset
life-cycle management will be higher. Planning
accuracy requirements will further increase and
be closer to the operational requirements.
Accurate planning will establish load models of
the
load-supply
integration,
carry
out
characterization analysis and distribution network
system calculation to search limit states and weak
links of system operation. Core of distribution
system planning is using divided-period
probabilistic production simulation of power
system based on synchronization of load and
supply to replace the traditional method of power
balance. Simulation results contain system
operation reliability analysis of different types of
typical periods and operational state, and iterative
optimization with all distributed power supply,
micro-grid, active load of planning area and
external power exchange. Overall operating cost
index obtained from optimization analysis supply
the basis for scheme comparison and selection.
Planning key is to improve the network
adaptability, to achieve the maximum range of
plugging and playing of distributed power, energy
storing device and various control units. Planning
process shall include repeated iterative
optimization of source, load, transmission system
and distribution system.
Index
Terms—
load-supply
Integration,
distribution system planning, probabilistic
production simulation, distributed power
I. INTRODUCTION
In order to promote consumption of new energy,
new technologies, such as energy storage
technology, active load response technology, and
distributed energy-centered micro-grid technology,
have been constantly developed. From the
perspective of load, not only electrical vehicles
and other loads that can switch to power
generation mode at any time emerge, but also
highly effective interaction & control technology
realizes online demand-side response (i.e. active
load response) and facilitates grid dispatching; on
the other hand, with the energy storage and
micro-grid technologies, power sources can be
switched to load at any time as , making the
boundary between load and power source
gradually indefinite, and leading to profound
change of the power distribution network.
Traditionally, the power distribution system was
planned centering on consumers' demand, with
the power balance as benchmark, and by selecting
the best one from multiple schemes through
techno-economic comparison, which, however,
does no longer meet the operation requirements of
the overall distribution system with the change of
loads and power sources. To address this problem,
the author has explored the new way for planning
of
the distribution system from theoretical
perspective.
II. BASIC CONCEPT OF LOAD-SUPPLY
INTEGRATION
The intermittence and fluctuation of new energy
sources, such as wind power and solar power,
poses a great challenge to the power system
requiring real-time dynamic balance. To better
accommodate such clean energy, the source side,
load side and the power grid itself all present a
tendency of integration of power source and load.
As for power source, the development of
distributed power sources accelerates the booming
of energy storage technology. Energy storage has
the characteristics of both power source and load.
As for load, electric vehicles and similar loads are
mainly power loads but can be switched to the
power generation mode at any time; meanwhile,
the management technologies currently available
on the demand side can realize active load
response, which can reduce load demand in real
time, and can be regarded as participation of
load-type power sources in power dispatching. As
for power grid, distribution network has always
been considered as a load of transmission network;
however, with the penetration of distributed
power sources, two-way power flow will occur
between distribution network and transmission
network, in which case, the distribution network
is both a load and source for the transmission
network. This will occur between micro-grid and
distribution network. Such sources or loads which
can be switched to loads or power sources at any
time in the power grid are called load-supply
integration.
supply-demand relation from the transmission
network to distribution network and to consumers
from top to bottom as shown in Fig. 1. The
planning of distribution network is mainly based
on the increasing trend of loads during the
planned period and basic status of the current
network to determine the best scheme, minimize
the construction and operation costs of
distribution network on the condition that power
supply for increasing load in a safe and reliable
manner can be ensured.
Transmission System
Distribution
System
Power Supply
Load
Fig.1: Model of traditional distribution system
Transmission System
Traditional Power
Supply
Active Load
Distribution Power
Supply
Distribution
System
Distribution Power
Storage
Traditional Load
Microgrid
III. TREND AND IMPACT OF LOAD-SUPPLY
INTEGRATION
A. Trend Analysis
Due to uncertainty and surge of output of new
energy source, a large number of flexible, quick
and adjustable sources or loads need to be added,
and this rapid change highlights the characteristic
of load-supply integration. In comparison with
pumped storage, new integrated source-load
system can achieve ±100% adjustment in just a
few minutes and even several seconds, allowing
for even more flexible and quick adjustment,
making it the best tool for balance of new energy.
In the long run, the new energy sector will grow
at a faster pace. In 2014, the installed capacity of
new energy across China accounted for 9% of the
total installed capacity, and this proportion will
reach to 41% by 2020 and 62% by 2050[1]. With
the development of new energy, real-time balance
of power system will definitely require a higher
percentage of integrated load-supply, so as to
ensure consumption of new energy and safety of
power system.
B. Impact on Planning of Distribution Network
We have long been used to the one-way
Fig.2: Model of future distribution system
Load-supply integration blurs the boundary
between load and power source, strengthens
complementary advantages of multiple energy
sources, allowing for higher flexibility of
operation of power system and higher timeliness
in power balance; the model of future distribution
system is shown in Fig.2. With respect to
planning, boundary condition becomes indefinite
and even more uncertain, and more options are
available. The traditional planning method
centering on consumers' demand, with the power
balance as benchmark, and by selecting the best
one
from
multiple
schemes
through
techno-economic comparison does no longer meet
the operation requirements of the overall
distribution system.
IV. CONSTRUCTION OF NEW
THEORETICAL SYSTEM FOR
DISTRIBUTION SYSTEM PLANNING
This paper proposes the idea for constructing the
new theoretical system for distribution system
planning under the trend of load-supply
integration from the aspects of the objective,
requirement, key, core and process.
A. Planning Objective: Supply-Demand Balance
Load-supply integration requires consideration of
the roles of power source, load and grid from an
overall view, to meet power demands, satisfy
requirements for integration and consumption of
various types of clean energy sources; the overall
distribution system shall be planned in new way
to reduce waste of wind and solar energy.
Therefore, the traditional demand-guided
planning
will
be
transformed
to
supply-demand-guided planning, and in addition
to minimum cost, the planning shall also aim to
achieve maximum emission reduction and other
long-term objectives, so as to ensure clean and
sustainable development of power system.
B. Planning Requirements: Precision
Traditional distribution network planning is
mainly oriented to common loads and maximum
operation mode. However, load-supply integration
means that traditional load nodes become nodes
of both load and source features. For nodes and
related lines, the maximum load will not only
occur at peak load, but also at the time of
maximum power generation. For the power
system, the planning must consider the
uncertainty of operation, and take into account the
limit states of operation and control, as well as
operation characteristics and probabilities of
occurrence of various loads and power sources.
P/kW
P/kW
Pmax
24
24
T/h
T/h
Pmax
Node daily load curve in
traditional distribution system
Node daily load curve under the
trend of load-supply integration
Fig.3: Comparison of node daily load curve
under the trend of load-supply integration
The core of precision planning is establishment of
load-supply integration model by considering
operation characteristics of active load,
dispatchable load, dispatchable distributed
sources, energy storage equipment, as well as
many electric & electronic devices and control
units, based on which, calculation and analysis of
distribution network system shall be carried out to
find out limit operation state and weak links of the
system, put forward method for classified load
power supply mode and emergency response plan,
propose feasible schemes for dispatching
supply-type load, distributed sources, energy
storage equipment and control equipment, to
guide dispatching, operation and engineering
construction.
C. Planning Core: Divided-period probabilistic
production simulation based on synchronization
of load and supply
Currently, power balance is calculated using
deterministic methods by accumulating loads and
sources separately. Load operation condition is
only expressed in typical daily maximum load
curve, while power operation condition is only
expressed in routine power output plan; in the
case of high percentage of hydropower, energy
balance is calculated respectively for flood season
and dry season[2]; as the percentage of new
energy is not high, it’s not included in balance
calculation.
With the development of new energy, power
balance must consider output of wind power, solar
power, hydropower, and their combinations. Wind
and solar power may reach the limit in a season or
even in hours, and shall be calculated by typical
periods during probabilistic production simulation
to adapt to intermittent characteristic of various
types of distributed energy sources and new
energy sources. Therefore, power balance in
future distribution network planning will be based
on load-supply types and features, apply
probability analysis, consider the characteristics
of output of distributed sources, micro-grid and
active load in the same supply areas by typical
periods of time, conduct divided-period
probabilistic production simulation based on
synchronization of load and supply, and calculate
loss-of-load probability (LOLP), expected energy
not supplied (EENS), voltage quality and other
supply indices[3]. In the calculation process for
the power supply area, consideration shall be
given to power exchange with upper-level grid or
external lines, so as to derive economic indexes
such as system operation cost to provide a basis in
the selection of power balance solutions.
1. Calculation framework
Key steps for divided-period probabilistic
production simulation based on synchronization
of load and supply are stated hereinafter. It is
known from literature [4] that, in probabilistic
production simulation, equivalent continuous load
curve is the result of continuous corrections of
original continuous load curve with the impact of
stochastic failure of generator unit assumed as
equivalent load. Corrected load curve
of
generator unit i is as follows:
𝑓 (𝑖) (𝑥) = 𝑝𝑖 𝑓 (𝑖−1) (𝑥) + 𝑞𝑖 𝑓 (𝑖−1) (𝑥 − 𝐶𝑖 )
(1)
Where, 𝑓 (𝑖−1) (𝑥) =corrected load curve of
generator unit i-1; 𝑞𝑖 =forced shutdown ratio of
generator
i;
𝑝𝑖 = 1 − 𝑞𝑖 ,
𝐶𝑖 =capacity
of
generator i; 𝑓 (𝑖−1) (𝑥 − 𝐶𝑖 )=a curve formed by
parallel movement 𝐶𝑖 of corrected load curve of
generator unit i-1 to the right.
Consider the output of distributed power source in
a given period of time. Assume that in the period
of 𝑡𝑗 , no deviation occurs with the probability of
various types of power sources, generator i is a
distributed generator unit, its continuous output
probability function is expressed as 𝐶𝑗,𝑖 (𝑥), then
(𝑖)
the corrected load curve 𝑓𝑗 (𝑥) of generator i is
simulation provides the basis for energy storage
operation planning; Assuming that the total
amount of distributed energy storage in power
supply area is under control, without considering
the probability of full loss, and at a given period
of 𝑡 𝑗 , power from energy storage is arranged as
𝑆𝑗 (generation is positive, load is negative),
energy storage efficiency is μ, then the power
balance of energy storage can be expressed as
follows:
∑ 𝑆𝑗 ≥0 𝑆𝑗 𝑡𝑗 + 𝜇 ∙ ∑ 𝑆𝑗 <0 𝑆𝑗 𝑡𝑗 = 0
(5)
After arrangement of all generator units in the
system, assuming that the equivalent continuous
load
as follows:
(𝑖)
(𝑖−1)
(𝑥) ∗ 𝐶𝑗,𝑖 (𝑥)
𝑓𝑗 (𝑥) = 𝑓𝑗
(𝑛)
(2)
In the overall study period of T, system LOLP and
EENS are as follows, respectively:
(𝑛)
LOLP =
∑𝐽1 𝑓𝑗
(𝐶Σ )∙𝑡𝑗
(3)
𝑇
𝐶 +𝑥𝑗,𝑚𝑎𝑥
EENS = ∑𝐽1 𝑡𝑗 ∫𝐶 Σ
Σ
(𝑛)
𝑓𝑗
(𝑥)𝑑𝑥
(4)
where, T = 𝑡1 +. . +𝑡𝐽
Key points for realizing divided-period
probabilistic production simulation based on
synchronization of load and supply are that, (1)
standard probability model is established for
distributed sources such as wind and
photo-voltaic power as well as micro-grid, active
load and external line, and uniform and standard
model is used for calculation of power balance in
distribution network planning, so as to ensure
verifiability and universality of calculation results;
and (2) standard simulating operation mode
matching actual situation is constructed, i.e.
loading sequence of various types of power
sources. Renewable energy loading sequence is
largely dependent on local structure of resources
and operation mode of power system, with which
a complete universal principles shall be
established for distribution simulative operation
mode.
2. Power Balance with Energy Storage
Considered
Under the trend of load-supply integration, energy
storage is an important factor in the calculation of
power balance. Scientific arrangement for energy
storage instead of energy storage itself is the key
to optimizing system operation. Unreasonable
arrangement of energy storage may cause energy
storage device to absorb active power when the
power system is lack of active power, and
generate electricity while the system generates
sufficient power. Subsection stochastic production
𝑓𝑗
curve
formed
by
energy
storage
is
(𝑥 − 𝑆𝑗 ), then it can be derived that:
(𝑛)
LOLP𝑗 = 𝑓𝑗
(𝐶Σ − 𝑆𝑗 )
𝐶 +𝑥𝑗,𝑚𝑎𝑥
EENS𝑗 = 𝑡𝑗 ∫𝐶 Σ
Σ
(𝑛)
𝑓𝑗
(𝑥 − 𝑆𝑗 )𝑑𝑥
(6)
(7)
With minimum loss of electric energy as
optimization objective, the following function is
derived:
min ∑𝐽𝑗=1 EENS𝑗
(8)
D. Key Points of Planning: Improve Network
Adaptability
Under the trend of load-supply integration,
distribution system will also need to connect to
unplanned distributed sources, energy storage
devices, and other devices, and therefore, the
planning of distribution system shall mainly aim
to improve its adaptability, so as to maximize
plug-and-play applications of distributed power
sources, energy storage equipment and various
control units in distribution system. Therefore,
power network calculation and check shall
consider the impact on the power network due to
increase of a large number of power sources or
load changes in short period. In the future, such
impact will be more common, its importance and
hazard shall be the same as that of various
traditional power failures, and thus, simulation
model and standard algorithm shall be established
for such impact, and included in electric
calculation and check, and its safety boundary
shall be mainly thermal stability limit of line and
overload limit of transformer. Of course, it is
impossible to realize integration without absolute
capacity limit. Future distribution network check
will specify the margin for integration of various
types of sources and loads at various nodes in
typical mode, and give the operation control plan
in case of exceeding the margin. Moreover,
network construction time sequence will be
proposed by taking economy as a key factor.
E. Planning Process: Iterative Optimization of
Source,
Load,
Transmission
System
&
Source planning
Routine source
planning
Load forecast
Distributed source
planning
divided-period probabilistic production
simulation based on synchronization of
load and supply
Power balance
simulation
Meet balance
requirement or
not
Y
Optimization
calculation for
electric energy
balance
Distribution System
Routine source
forecast
Active load
scheduling
Current status of
distribution
network
Current status of
transmission
network
Energy storage
planning
Transmission
network planning
Analysis of
combined schemes;
transmission
scheme;
N
+
energy storage
scheme
+
Comparison and
selection among
schemes
active load scheme
Arrangement of
energy storage
operation mode
Network
framework
planning
Electric calculation &
analysis (Safety & stability,
reactive power, reliability
calculation etc.)
Investment analysis
(construction
scheme, time
sequence)
Fig.4: Distribution system planning process under the trend of load-supply integration
Under the trend of load-supply integration,
balance is finished, proceed to network
distribution network planning process shall
framework planning, safety & stability calculation,
include repeated iterative optimization of source,
reactive
power
check
and
engineering
load, transmission system and distribution system,
construction planning.
as shown in Fig. 4. Compared with traditional
IV. SUGGESTIONS
planning, distributed power source planning is
With the development of new energy, a large
included in power source planning, and active
number of distributed sources are integrated to
load forecast is included in load forecast,
various levels of power grids, making the power
implying higher randomness on power source side
system more complex and the relations between
and controllability on load side. Calculation
power source, load and power grid closer. To
inputs of power balance include power source,
ensure safety of power system, uniform planning
load, current power grid and energy storage
and operation of various power sources and loads
planning data. After stochastic production
of various levels and types shall be made instead
simulation calculation of the power supply area,
of separate planning and operation. In view of the
loss-of-load probability and operation cost are
current situation, the following suggestions are
obtained. If balance requirements are not met,
given:
outsourced power, energy storage and active load
(1) Accumulation of information regarding
etc. shall be considered in combination,
operation properties and economic aspects of
comparison and selection among schemes, with
various integrated load-supply is far from
the results fed back to upper-level power grid and
sufficient, and study on the operation mode in
energy storage planning. After unified
combination with distribution system is still in
coordination of upper-level grid, power supply
pilot stage, thus forward-looking study shall be
plan and cost shall be fed back, the supply area
sped up in aspects of large-scale operation mode,
will include the feedback results in stochastic
properties and models of integrated load and
production simulation. After calculation of power
supply.
(2) The planning of distribution system will
involve comparison and selection among a lot of
candidate schemes, as well as repeated iterative
optimization of load-supply network; this paper
only makes a preliminary discussion, while
further study is required in aspects of theory and
methodology, and more efforts are required to
develop new calculation tools of distribution
system, so as to adapt to future planning and
operation requirements.
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Li Jingru was born in Shanxi, China.
She received the Bachelor's degree in
Tsinghua University and the master’s
degree in China Electric Power Research
Institute, Beijing, China, in 1992 and
1995, respectively.
She is currently an assistant dean, senior
engineer of State Power Economic Research Institute. She is
also the secretary-general of gird planning and design
technical standard work group of State Grid Corporation of
China. Now her research interests include distribution system
planning, distributed power planning and relative algorithm
research.
Huang Ping was born in Jiangxi, China.
He received the Bachelor's degree and the
master’s degree in Tsinghua University,
Beijing, China, in 2005 and 2007,
respectively.
He is currently a senior engineer of State Power Economic
Research Institute. He has long been engaged in power system
planning and simulation research. Now his interests include
distribution system planning and calculation.