［ Distributed Power Supply ］

Energy Storage System

Keywords

Energy storage system, Renewable energy, Grid stabilization, BCP

Abstract

To realize a low-carbon society, wide introductions of renewable energy resources like solar and wind power are advancing worldwide. In Japan, there areconcerns regarding negative inﬂuences, such as disturbance in the balance ofpower demand and supply and output ﬂuctuations and frequency changes inpower grids caused by the extensive introduction of renewable energy resources.
Because of the aforementioned, in some areas in Japan, the inter-connection ofrenewable energy to power grids is restricted. In addition, because of deregulation of the power retail market and successive legislation for the unbundling ofpower generation, the transmission and distribution from a power utility ﬁrm
became effective in April 2016. There is expected to be a change in relationshipbetween energy demand and supply. The increase of renewable energy resources
is an important target for the national energy policy in Japan and the stabilization of grid systems is a pressing technical challenge of our time. As an effective
tool in order to solve this issue, energy storage systems are getting increasedattention.

1 Preface
In an effort to reduce unusual climatic change
occurring worldwide, various actions have beentaken at home and abroad to reduce CO2 emissions. In the ﬁeld of power generation, a large volume of CO2 is generated from fossil fuels and therate of thermal power generation must be suppressed. For this reason, wider introductions ofrenewable energy resources like solar and wind
power are observed worldwide.
In Japan, the Feed-in-Tariff (FIT) Law was put
into effect force in July 2012. Supported by this law,
the total amount of renewable energy resources has
remarkably increased.
Since the Great East Japan Earthquake
caused a large-scale disaster in March 2011, mostnuclear power plants in Japan ceased operation
despite providing stable power before the earthquake. This caused Japanese Government toreview its energy policy. Securing energy resourcesin the event of a disaster became a particularly
important issue for households and communities.
Solar and wind power have been widely introduced in Japan as renewable energy resources.

34Takahiro Murai

MEIDEN REVIEW Series No.169 2017 No.1

They have generated power which is changed
greatly by variations in sunlight or wind velocity,
thus causing voltage rise, frequency deviation, andan imbalance in demand and supply. These factors
are the source of concern because they can cause
adverse inﬂuences upon stable operation of powersystems. Thus, veriﬁcation research programs forstable management of power grids had been carried out in various places. As a solution to this technical challenge, the use of energy storage systems
is getting increased attention and is nearing thephase of practical implementation.
This paper introduces some examples of application of the Power Conversion System (PCS), akey component of the energy storage system. Inaddition, the paper also introduces the energy storage systems delivered to overseas countries for
power system stabilization.

2 PCS for Energy Storage
2.1 Features( 1) A single PCS unit 250kVA can offer a combination of total capacity (a maximum of eight units:
2MVA in total) and parallel generation. According tothe scale of each facility, an optimal capacity can bedetermined.
( 2 ) In addition to the conventional function of loadleveling, this model offers new functions such as a
ﬂuctuation relaxing function for renewable energy,system voltage regulation, and a frequency ﬂuctuaTable 1 Specifications of PCS Equipment
The PCS products are manufactured in accordance with both
Japanese and overseas standards.
Rating and performance
A0: 100% continuous

Inverter control system

Self-cooled voltage typecurrent controlled PWM
inverter

Insulation type

Power frequency insulationtransformer type

Conversion efficiency

95% or above

Environ- Cooling system
Forced-air cooled
ment
Ambient tempera- 0〜40℃
ture
15〜85%

Quake durability

Horizontal: 1.0G, Vertical: 0.5G

Dimensions

W1600 × H1950 × D700mm

Mass2000kg

Applicable standard

JEC2440, JEC2433
EN50178, EN61000-6-2, -4
EN50160, IEC60146-1-1

Specifications for grid-connected operation

AC

Specifications for islanding operation

Relative humidity

2.2 Equipment Speciﬁcations and CircuitConﬁguration
Table 1 shows speciﬁcations of the PCS equipment and Fig. 1 shows the circuit conﬁguration ofthe PCS (including main control unit). The PCS is3P3W, AC 400V class
50Hz or 60HzNo. of phases

3-phase 3-wire

Rated voltage

400V class

Permissible
voltage regulation
range

Rated voltage −12〜10%

Rated frequency

50Hz or 60Hz

Permissible
frequency deviation range

Rated frequency ±6%

Effective power
control range−100% (charge)〜0〜＋100%(discharge)

Reactive power
control range−100% (lagging)〜0〜＋100%(leading)

INVPower control
accuracy±1%

CHP

F×2 VT
CT
Interconnection
transformer
250kVAHCTDCLDC

Voltage range

240〜600V

HCTAC

No. of phases

3-phase 3-wire

Rated voltage

400V class

Rated frequency

50Hz or 60Hz

Rated load powerfactor

0.9〜1.0 lagging

Voltage control
accuracy

Within ±3%

Frequency control Within ±0.1%
accuracy
Voltage harmonics 3% or below
content rate
Voltage range

Main control unit
Voltage atincoming point

ACL

Current harmonics Total 5% Max., each order 3%
content rateor less

DCPCS

AC switchM

240〜600V

Individual PCS, Control circuit

Major factors

Type of rating

IGBT unit

Item

Rated capacity of equipment 250kVA〜2MVA

tion relaxing function.
( 3 ) A function of an islanding operation is provided.
In place of emergency generators, this model canbe applied to the Business Continuity Plan (BCP).
( 4 ) A bidirectional chopper is installed on the PCS
battery side so that equipment can operate in awide voltage range. Various types of storage batteries such as a lithium-ion battery, lead battery,lithium-ion capacitor, redox ﬂow battery, and Durathon
battery (zebra battery) can be used as DC powersources. The most effective batteries can be selected per the required application.
( 5 ) This model conforms to the recent EU Directive
such as Low Voltage Directive (EN50178) and EMC
Directive (EN61000-6-2, EN61000-6-4). In addition,
it can also conform to Japanese standards.

Control
opticalcable

PCS General, Control
circuit (with system
protective function)

Operatorpanel

PLC (Sequencer)

HubM

DC switch
DC240～600V
Storagebatteries

Fig. 1

PLC: Programmable
Logic Controller

Circuit Configuration of PCS (Including MainController)

A single unit of the main controller can accommodate a maximum of eight PCS units connected in parallel.

MEIDEN REVIEW Series No.169 2017 No.1

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composed mainly of AC and DC switches, a ﬁlter,an Insulated Gas Bipolar Transistor (IGBT) Unit ofthe inverter and chopper type, and the main control
unit (for PCS control, monitoring sequencer, operator panel, and auxiliary devices.)

2.3 Major Functions
The basic operation mode of the PCS is introduced below. Table 2 shows control functions andfunctional outlines to be covered by the PCS.

2.3.1 System Interconnection Charge-Discharge Operation
This is an operation mode intended to makecharges and discharge in interconnection with theutility system. When the right of control is “local,” anarbitrary charge or discharge action can be taken
per the power setup value indicated on the operator
panel. When the right of control is “remote,” an arbitrary charge or discharge action may be taken
according to the power dispatch value given from aTable 2 Control Functions and Functional Outlines
The PCS has a variety of functions such as system protection
function, grid stabilization function, and battery control function.
Control function

Functional outlines

System intercon- This function is used to make a gate block for
nection protecinverters in the case of a system error andtion
the AC switch is tripped for their parallel off
from the system. Available factors for protection are AC overvoltage, AC undervoltage,frequency rise, frequency drop, and isolated
operation detection (passive and active).
Signals of external relay contacts are also
dealt with.
Fault RideThrough (FRT )In order to avoid simultaneous and unnecessary PCS parallel off in the case of system
voltage error, this function is used to continue
operation at the specified voltage drop rate
and within the specified time range.

Interconnection
inrush current
prevention

In order to prevent device deterioration andsystem disturbance due to inrush current of
an interconnected transformer during systeminterconnection, the PCS output voltage is
synchronized with the system in advance andsystem interconnection is then accomplished.

Renewableenergy tidal flow
deviation relief

This function is used to smooth the generated
power from renewable energy by solar or wind
power generation. Output current from renewable energy is detected, charge-discharge
control of storage batteries is carried out, andchanges in tidal flow are smoothed.

Load deviation
relief

In order to smooth the load side tidal flow
deviation, the load side current is detected
and charge-discharge control is performed
for storage batteries.

Battery refresh
operation

This operation mode accompanies CV (constant voltage) charging in order to adjust theState Of Charge (SOC) of storage batteries.
This function is also expressed as a battery
refresh operation or reset charge and discharge.

Power distribuIn order to relieve the differential component
tion controlof battery SOC in the parallel system,
according to SOC charge-discharge command values arecontrolled to accomplish adequate power
dispersion to each PCS.

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MEIDEN REVIEW Series No.169 2017 No.1

host system. If the power dispatch value is not given,
the standby mode is assumed by taking an action ofa gate block for inverters so that switching loss iseliminated.

2.3.2 Islanding Operation
This is an operation mode to feed power to theload at the constant voltage and frequency by using
storage batteries if the power source of the utility
system is lost due to a power outage or systemerror. When a maximum of eight PCS units, 250kW
per unit, are used for a parallel operation, an islanding
operation is possible at the maximum of 2MVA.
These PCS units can be used as an alternative
power source like an emergency generator. They canstart up in a matter of a few seconds. This feature issuperior to that of an emergency generator.

3 Utilization of Energy Storage
System and Practical Application
The purposes of using the energy storage system are classiﬁed into three categories: peak shaving, countermeasures against renewable energy
output deviation and excess power, and system stabilization.
At overseas markets, the demands for storage
batteries are visibly increasing. Some Independent
System Operators (ISO) and such grid system management companies in foreign countries are beginning to take measures for securing conditions forusing storage batteries based on their wider renewable energy sources in the future. Japan has a government policy to increase the capacity of inter-connecting renewable energy resources by installingenergy storage systems with renewable energy
resources.
A considerable difference of the energy storage system (from other distributed energy resources) is that it can offer negative power generation (so-called charging). Since excess power canbe absorbed in the energy storage system, the Load
Frequency Control (LFC) for conventional power
generation (thermal and hydraulic) can be replacedby an energy storage system. As described above,the capability of demand and supply adjustment
can be deemed as the greatest advantage of theenergy storage system.
Active use of this system as a backup power
resource can be utilized for the application of theBCP. Other cases are for the support of community
activities, etc.

3P3W, AC20kV, 50Hz

＊C＊C

3P4W, AC400V, 50Hz

Low-voltage substation facility

Special high voltage substation
facilities (accommodated in acontainer)
Special high voltage
incoming panel
ACB

Incoming measurement dataMain controller

Powerquality
measurement

V0＞
V＞
V＜F＞
F＜PCS General・Control circuit

Power quality
measurement
serverWeb client
Monitorcontrol server

＊B
HubDI

Special high voltage
transformer panel

Battery
controlsourceTo batterybank 1～8

Control Auxiliarysource source

To host system
SCADA

Coolingfansource C

3P3W, AC400V, 50Hz

M

Power quality
measurement

WhMultimeter

Interconnection
transformer
250kVA

ACL

Control INV
source＊B

IGBT unit

84MT

＊A

AC switch

Battery Control Air
control source conditioner
sourceLighting

Control and auxiliary source
3P4W, AC400V, 50Hz

Interlock

HubPanel
insidecontrol

Communicationterminal for batteries＊AM

M

Sequencer

Incoming and auxiliary source panel

3P3W, AC400V, 50Hz

Special high voltage
incoming transformer
2000kVA

84BT

Unit
sourcePCS General・Measuring circuit

I＞
I0＞ DG
67N

VCB

Operator panel

ControlsourceCHP

DCL

Individual PCS, Measurement circuit
Individual PCS, Measurement circuit

3PDS

IRC61850 communication terminal (Gateway)

PCS auxiliary source
Feeder panel 1
UPS

UPS

PCS control
sourceBatterycontrol
sourceM

DC switch
PCS1

Incoming and auxiliarysource panel

Battery 1
Battery
capacity 250kW
Battery system 1 (accommodated in container)

Fig. 2Battery system 2～8 (accommodated in container)

Example of System Configuration

This system is composed of substation facilities, a PCS, an incoming and auxiliary source panel, control and monitoring equipment, andsubsidiary components. Operation and control command signals are sent from the host SCADA.

4 Case Study of an Energy Storage
System Application
We built an energy storage system in theEuropean market for grid stabilization.

4.1 System Conﬁguration
Fig. 2 shows an example of a system conﬁguration. This system is composed mainly of special-high voltage (over 20kV substation facilities,eight 250kVA PCS units connected in parallel, incoming and auxiliary source panel, control and monitoring circuits, and an outdoor container. Since thissystem is intended for shipment for the European
market, the respective components and module
units are CE marking-compliant.
Since this system is to be managed by powerreceiving and distribution service providers, it isdesigned for connection with the Supervisory
Control and Data Acquisition (SCADA) equipmentfrom which this system is controlled and monitored.

A speciﬁc communication protocol is adopted basedon IEC61850 that is an international standard. Thissystem can assure a high-speed response of about
300ms to meet the SCADA control command.
This system also offers a variety of functions
such as emergency operation and islanding operation in the event of a power outage in the utility gridand automatic system shutdown.

4.2 Grid Stabilization Function
This system is generally connected with theterminals of unstable power systems where many
renewable energy resources like solar and wind
power are connected. In this situation, it is expectedthat this system will stabilize the grid by suppressing
voltage and frequency ﬂuctuations. Table 3 shows
the grid system stabilization functions of this system.

4.3 Remote Control and Monitoring Function
This system has a remote control and monitoring function in its remote control server. It, thereMEIDEN REVIEW Series No.169 2017 No.1

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Table 3 Grid Stabilization Functions
This system is provided with a variety of functions for grid stabilization. According to system conditions, a system manager
makes arbitrary ON/OFF setting in regard to system functions.
Function

Application

Voltage imbalancecompensation

In order to relieve voltage imbalance at thepower incoming point, negative-phasesequence voltage is extracted from thedetected voltage and the intensity ofnegative-phase sequence current isdetermined. When this function is madeeffective, autonomous control is carriedout on the system side.

Momentary voltage When a voltage dip occurs in the FRT
dip compensation domain, the system discharges effective
power at a maximum value of possiblepower. When this function is made effective, autonomous control is carried out on
the system side.
Harmonics compensation

In order to relieve voltage harmonics at thepower incoming point, harmonic component is extracted from the detected voltage
and the intensity of the 5th and 7th harmonic
current is determined. When this function
is made effective, autonomous control iscarried out on the system side.

Voltage regulation

When the detected voltage value at thepower incoming point deviates from thedead band, reactive power output isautonomously generated so that the voltage
settles within the band. This function ofregulation conforms to the computed
formula set up in advance for the system.

Frequency regulation

When the detected frequency value at thepower incoming point deviates from thedead band, effective power output isautonomously generated so that the frequency settles within the band. This function of regulation conforms to the computed
formula set up in advance for the system.

Load leveling
These functions are controlled by the P and
Demand and supply Q control commands from the SCADA.
adjustment
According to these commands, the system
Power factor
performs a charge and discharge so thatcompensation
the system is stabilized.
Control of preference for eachfunction

When two or more functions are madeeffective, the control logic incorporated in
the system determines the order of preference for each function in order to avoid
functional interference and inadequate
system operation.

fore, works as a control server playing the role of amachine standing by for the SCADA. It also has auser interface (Web GUI) to cope with control command equivalent to the SCADA and system supervision toward the remote control clients (Web client).
Fig. 3 shows an example of the Web client screen.
These remote control and monitoring functions
are used at the time of failure in the SCADA system,
communication stacking, or maintenance. Since thesystem has various functions equivalent to theSCADA, it offers equivalent advantages and systemredundancy.

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Fig. 3

Example of Web Client Screen

The Web client is provided with functions of the systemʼs real-timedata display and performing monitor and control equivalent to
the SCADA.

Various logging data and trend graphs savedin the server can be picked up from remote places
via the web clients. By analyzing the logging data,
the cause of system problems can be grasped at an
early opportunity. Thus, systemʼs maintainability isexponentially improved during system management.

5 Postscript
For the purposes of system stabilization andBCP, the energy storage system is now in the phaseof commercialization. It is expected to be widely
introduced soon. There are still some issues regarding wider acceptance such as further improvementof system management efﬁciency and cost reduction.
Japanʼs energy-related policy is now in a timeof great change in the deregulation of power retailing and the separation of power generation andtransmission. Consequently, the market of energy
storage systems is anticipated to grow in an accelerated manner. Drawing on our engineering resources and expertise cultivated through theseexperiences, we will positively promote productdevelopment of PCSs that are key components ofenergy storage systems. We will continue to offer
high-value added systems.
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.