1 Introduction to photovoltaic power station energy storage system
With the development of the power industry and the large-scale access to
new energy, the power transmission and distribution system is faced with the
requirements of improving system reliability and stability, improving power
quality, and preventing power outages, and energy storage is the best solution.
This project intends to use the latest technology research on energy storage
systems to propose energy storage system configurations and energy management
systems suitable for the safe and stable operation of microgrid systems, to
achieve safe and stable operation of the power grid, and to promote relevant
research results in similar photovoltaic power stations.
The main functions of the energy storage system in the microgrid system
include the following aspects:
(1) Ensure system stability. In the photovoltaic power station system,
there is a big difference between the photovoltaic output power curve and the
load curve, and both have unpredictable fluctuation characteristics. The energy
storage and buffering of the energy storage system allows the system to still
operate in a stable state even when the load fluctuates rapidly. Stable output
level.
(2) Energy backup. The energy storage system can play a backup and
transitional role when photovoltaic power generation cannot operate normally.
For example, when the battery array cannot generate electricity at night or on
rainy days, the energy storage system can play a backup and transitional role.
The amount of its energy storage capacity Depends on load requirements.
(3) Improve power quality and reliability. Energy storage systems can also
prevent grid fluctuations caused by voltage spikes, voltage drops and other
external interference on the load from having a major impact on the system.
Adopting enough energy storage systems can ensure the quality and reliability of
power output.
(4) Daily energy storage. When the solar irradiance is strong and the load
is light, excess solar energy is stored and the solar energy is fully
absorbed.
It can be seen that the energy storage system is crucial to the stable
operation of photovoltaic power plants. Energy storage systems not only ensure
the stability and reliability of the system, but are also an effective way to
solve dynamic power quality problems such as voltage pulses, inrush currents,
voltage drops and instantaneous power supply interruptions. In addition, the
energy storage system accounts for a considerable proportion of the overall
investment in the power station. The reasonable selection and daily management
of the energy storage system capacity also have a decisive impact on the overall
economics of the system, so it must be analyzed in depth and made
rationally.
The specific target of this project is the Ali Photovoltaic Power Station.
The power station is located in Shiquanhe Town, where the Ali Prefecture
Administrative Office is located in the Tibet Autonomous Region. The
administrative division is under the jurisdiction of Gar County, Ali Prefecture,
Tibet, with an altitude of 4250 to 4300m. External transportation is only
connected by highways. It is 1,752km away from Lhasa City and 1,334km away from
Kashgar, Xinjiang. The transportation is inconvenient.
As the first large-scale microgrid photovoltaic power generation project in
Tibet, Ali Photovoltaic Power Station has an important demonstration role in
Tibet and even the country. It will provide valuable reference for the
construction of photovoltaic power stations in other microgrid areas across the
country. It will also have a certain impact on the economy of minority areas.
promotion effect.
2 project technical solutions
2.1 Overall technical overview of the project
Photovoltaic power generation system is a power generation system that uses
the photovoltaic effect of solar cell semiconductor materials to directly
convert solar radiation energy into electrical energy.
When sunlight hits the surface of a solar cell, the solar cell absorbs
light energy and generates photogenerated electron-hole pairs. Under the action
of the built-in electric field of the battery, photo-generated electrons and
hole pairs are separated, and charges of different signs accumulate at both ends
of the battery, which generates photo-generated voltage. If electrodes are drawn
out on both sides of the built-in electric field and a load is connected, the
photogenerated current flows through the load, thereby obtaining power output.
In this way, the sun's light energy is directly converted into practical DC
power through solar cells.
At present, there are three main ways of photovoltaic power generation:
independent hybrid power generation system, grid-connected photovoltaic power
generation system, and photovoltaic micro-grid system.
(1) Independent hybrid power generation system
The independent hybrid power generation system includes battery arrays,
batteries, power conversion and control, as well as diesel generators and other
power generation sources. When there is sufficient electric energy, the energy
from the battery array and other power generation sources is stored in the
battery pack through the charge controller; when there is a shortage of electric
energy, the capacity in the battery is converted into power that meets the
user's needs through the discharge controller and the electric energy conversion
device. Diesel generators serve as cold backup and are used to power loads
during emergencies.
The independent hybrid power generation system is currently the main form
of power supply in remote areas. The technology development has been very
mature, and the scale ranges from street light systems of tens of W to
independent hybrid power stations of hundreds of kW. The technology of inverter
and battery charge and discharge controller has also been industrialized, and a
series of products with power levels ranging from tens of W to tens of kW have
been formed.
(2) Grid-connected photovoltaic power generation system
The grid-connected photovoltaic power generation system mainly includes a
low-voltage grid-connected photovoltaic power generation system and a
high-voltage grid-connected power generation system. The system consists of a
battery array and a grid-connected inverter. At present, there are mature
products for low-voltage and high-voltage grid-connected inverters. The maximum
single-machine capacity of the low-voltage grid-connected photovoltaic power
generation system inverter is 500kW, while the maximum single-machine capacity
of the high-voltage grid-connected photovoltaic power generation system inverter
is 1MW. The grid-connected inverter is a current source that follows the
frequency and voltage changes of the grid. The power factor is 1 or the command
regulation is supported by the grid. It cannot generate electricity alone. The
capacity is limited in the grid, and the output power is determined by the
photovoltaic input.
(3) Photovoltaic microgrid system
Photovoltaic microgrid systems can operate in parallel with other power
sources or grids. The system includes a battery array, conventional
grid-connected inverter, energy storage unit, bidirectional converter, diesel
generator, etc.
Diesel generators and bidirectional converters (adjustable frequency and
voltage) are networked individually or jointly. Conventional photovoltaic
grid-connected bidirectional converters (maximum tens of kW for a single unit)
can be operated in parallel via communication lines and perform microgrid energy
management at the same time.
At present, the mature technology of this system in Germany, Japan and
other countries is 100-300kW system, and the distributed multi-energy
complementary power generation microgrid system is a current research hotspot.
It is still in the research stage in China.
In the photovoltaic microgrid system, the photovoltaic power station can be
operated in parallel with the hydraulic turbine generator set and diesel
generator. The microgrid energy management system ensures the coordinated
operation of photovoltaic power stations and hydraulic turbines to transmit
power to the grid. The photovoltaic microgrid system can meet the needs of the
Shiquanhe power grid in Tibet.
After the power station is completed and put into operation, it will form a
water/light/diesel microgrid system with the existing 4×1600kW hydroelectric
generators and 4×2500kW diesel generators of the Shiquanhe Power Grid.
Considering that the weather in Tibet changes rapidly, it has a great impact on
the output of photovoltaic power stations. Through the collection and analysis
of Yangbajing operation data, due to sudden weather changes, the maximum output
of the photovoltaic power station will drop to about 35% of the rated output.
The following is an analysis of the installed capacity of 10MWp (because after
the photovoltaic power station is completed, the system still lacks power,
consider that the battery configuration scale can only cope with 1 to 2 sudden
changes per day).
Energy storage systems account for a considerable proportion of the overall
investment in power stations. Reasonable selection of energy storage system
capacity, equipment selection, determination of main technical parameters,
operation management, etc. play a decisive role in the safety, stability and
economy of energy storage systems. impact, so it must be analyzed in depth and
chosen rationally. The working principle of the energy storage system is shown
in Figure 1. (Above the AC bus of the power grid are the photovoltaic power
station battery arrays and conventional grid-connected inverters, below are the
batteries and bidirectional inverters, on the left are hydropower units and
diesel generator units, and on the right are the load terminals.)
Consider that under full power conditions, due to weather changes, the
output suddenly drops to 35% of the rated value, and the load fluctuation that
needs to be borne by the diesel generator or hydropower station is 65% of 10MWp,
that is, 6.5MW (see Figure 2 for details). Consider this Both the Shiquanhe
Hydropower Station and the system's diesel generators are on cold standby and
cannot provide spinning reserve capacity. Therefore, the load of 6.5MW needs to
be supplemented by the energy storage system. Without low-frequency load
shedding, a certain margin should be considered and analyzed according to the
load of 7MW.
Consider that the energy storage system is responsible for the electric
energy output during the start-up period of the hydro-generator. The energy
storage system starts to output when the output of the photovoltaic power
station drops to 35% until the hydro-generator is fully loaded to meet the load
demand of 7000kW. Since the hydro-generator changes from shutdown to It takes
about 6 minutes to reach full load, and the energy storage system needs to be
able to continuously output 7000kW of energy for 10 minutes. Taking into account
the worst working conditions, the energy storage system needs to be discharged
twice continuously without charging, and considering the inconvenient
transportation of the project location, it is not suitable to perform frequent
maintenance and replacement of energy storage components, making the solution
difficult for the storage system. Energy system configuration capacity and
operating life have put forward higher requirements.
2.2 Energy storage system planning
At present, global power energy storage technologies mainly include three
categories: physical energy storage, chemical energy storage and electromagnetic
energy storage.
The most mature solution in physical energy storage is pumped hydro
storage, with an energy conversion efficiency of about 75%. It is mainly used
for peak shaving and valley filling, frequency modulation and equalization of
the power system. The construction of pumped storage power stations has high
requirements on local topography, hydrology, etc. For the Shiquan River area,
the construction period, cost and difficulty are relatively long, and it cannot
meet the requirements of coordinated operation with photovoltaic power stations
in the short term.
Another type of physical energy storage is flywheel energy storage, which
has long life and no pollution, but has low energy density and is not suitable
for use as a large-scale energy storage system alone.
The current development of electromagnetic energy storage is more
constrained by cost, such as superconducting electromagnetic energy storage,
etc. The cost is high and the technology is not mature enough, so it is not
worthy of large-scale promotion.
Chemical energy storage is currently a relatively mature solution for this
project. Chemical energy storage mainly includes sodium-sulfur battery energy
storage, flow battery energy storage, lithium iron phosphate battery energy
storage, lead-acid battery energy storage and supercapacitors.
Sodium-sulfur batteries have the advantages of high energy density and high
charging efficiency. However, because they need to work at high temperatures,
they have certain safety risks, and the production process is complex.
Currently, the patent rights are mainly in the hands of Japanese companies, and
the cost is relatively high.
Liquid flow alumina batteries have the advantages of high energy density
and a discharge depth of up to 100%. However, since the positive and negative
electrolytes are prone to cross-contamination and have a great impact on the
environment, some problems still need to be solved before they can be promoted
on a large scale.
Supercapacitor energy storage is generally used as a fast-response energy
storage system. Due to low energy density and high unit cost, it is not suitable
for overall large-scale energy storage system configuration, but can be used as
a supplement to large-scale energy storage systems.
Lead-acid batteries are currently the most mature energy storage system
solution, with the advantages of mature technology, low cost, and the ability to
build large-scale energy storage systems. However, it has high operating
temperature requirements, low energy storage density, low discharge depth
(conventional discharge depth should not exceed 30%, and special applications
should not exceed 50%), and limited number of charge and discharge times, which
restricts its use in large-scale storage. Energy systems, especially
applications in microgrid systems in western China where the climate is harsh
and transportation is inconvenient. The acid mist generated during the
production process of lead-acid batteries also pollutes the environment and is
not conducive to environmental protection requirements.
Lithium iron phosphate battery is a type of battery that has developed
rapidly in recent years. It is favored by everyone because of its
characteristics of high energy density, long cycle life, large discharge depth
and large discharge current. At present, companies such as BYD first use it in
the energy storage system of electric vehicles, and gradually promote it to
large-scale energy storage systems in power systems. The discharge depth of
lithium iron phosphate batteries can reach more than 80% during normal
operation, and the number of charge and discharge times after being grouped can
also reach more than 1,500 times. It is very suitable for systems that require
frequent charging and discharging. However, lithium iron phosphate batteries
have high requirements for charge and discharge system control, which also
restricts their development to a certain extent.
According to the actual situation of this project, if lead-acid batteries
are selected as energy storage components, a 7000kVAh single battery must be
configured based on the analysis of 40% discharge depth and appropriate margin.
If you choose a lithium iron phosphate battery as the energy storage component,
and the discharge depth is calculated as 80%, you only need to configure a
3500kVAh lithium iron phosphate battery. Considering that the current price
ratio of lithium iron phosphate batteries and lead-acid batteries is about 2:1,
the initial investment is considerable. . However, considering the operating
mode requirement of deep discharge once or twice a day, and considering the cost
of maintenance and battery replacement, the advantages of lithium iron phosphate
batteries are more obvious. This project recommends using lithium iron phosphate
batteries to construct the energy storage system.
2.3 Determining the operation mode of energy storage system
According to the requirements of this project for the energy storage
system, when the output of the photovoltaic power station decreases, the energy
storage system should be able to output enough electrical energy and support the
system voltage. Currently, the inverters used for energy storage systems are
current source bidirectional inverters. This type of inverter can only simulate
the same voltage waveform based on the system voltage and output current, but
cannot support the system voltage. The voltage source bidirectional inverter
currently has the disadvantages of too small single capacity and inability to
operate in parallel. It can only meet the use of small-scale power stations and
cannot be applied to large-scale microgrid projects in the Ali region.
Due to the urgent requirements of the project construction period, and
under the objective conditions that the current technical bottleneck of
large-capacity parallel voltage source bidirectional inverters has not yet been
overcome, current source bidirectional inverters can only be used as a backup
plan in advance. When the output of the photovoltaic power station decreases, ,
the Shiquanhe Hydropower Station provides voltage support, and the energy
storage system only serves as a power source to provide electrical energy. After
the large-capacity parallel voltage source bidirectional inverter technology
matures, technical transformation will be carried out to ensure the stable
operation of the Shiquanhe power grid. At the same time, the dispatch center
will also need to be transformed to meet the requirements of simultaneous
dispatch of multiple power modes.
3Conclusion
With the rapid development of the new energy industry, new energy sources
such as wind power and photovoltaics account for an increasing proportion of the
power system. Due to the uncertainty and unschedulability of new energy power
generation, it has brought certain consequences to the stability of the power
system. hidden dangers. Countries around the world have put forward requirements
for the proportion of new energy power generation capacity in the power grid,
which generally stipulates that it should not exceed 10% to 15% of the entire
power grid capacity. Since areas with good wind and light resources in my
country happen to be areas with relatively weak power grids, the development of
new energy in these areas has encountered technical bottlenecks. Research on
large-scale energy storage systems has proposed a practical solution to the
problems of new energy and grid stability, and will also play a key role in the
construction of future smart grids. With the development of various types of
energy storage systems, the future power grid will surely be more
environmentally friendly, stable and reliable.
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