［ Introduction of New Products ］

PCS for Lithium Ion Batteries

Keywords

Manabu Teramoto
Takahiro Murai

PCS, Lithium ion battery, BMU, Energy, BCP, Backup power source, Frequency control, FRT

Abstract

Lithium-ion Batteries (LiB) have already been commercialized for vehicle
on-board application and are expected to widely penetrate the market in the future.
They are also noted as devices for application in energy storage systems.
Recently, particularly after the Great East Japan Earthquake in 2011, demands for grid stabilization have increased due to facts such as penetration andexpansion of renewable energy resources, optimization of energy through thepromotion of electriﬁcation, the “no nuke” movement, and readiness for emergency power supply. In this, the use of energy storage systems is expected to be
applied to demand-supply adjustment in the grid system through the energy
storage system, and also frequency control.

1 Preface

2 Energy Storage Batteries

According to various applications, batteries forenergy storage systems, lead batteries, sodium andsulfur batteries, and nickel-hydrogen batteries havebeen used. Recently, Lithium-ion Batteries (LiB) arereceiving high attention as devices for on-board
vehicles application because it features high energy
density and are widely used in cars. These batteries
are also noted as the energy storage batteries and its
research is actively promoted worldwide.
This paper introduces a comparison of characteristics among various energy storage batteries
as well as features our newly developed Power
Conversion System (PCS) for the LiB.

2.1 Comparison of Characteristics among
Various Electric Energy Storage Batteries
In the energy storage system to be introduced
for load leveling with due consideration of demandload curves, it calls for high energy density and highcharge/discharge efﬁciency.
Table 1 shows a comparison of characteristics
among various batteries used for energy storage
systems. Compared with other batteries, the LiB
provides a high energy density. In addition, thecharge/discharge efﬁciency of the battery unit alone
is outstanding compared with others.

2.2 Outline of LiB Speciﬁcations
The LiB is a battery of which the anode is acompound of lithium metal oxide with a major comTable 1 Comparison of Battery Characteristics
This comparison table has been established based on our unique investigation. Compared with other batteries, the LiB offers an outstanding efﬁciency.
Characteristic item

Sodium andsulfur battery

Nickel-hydrogenbattery

LiB

Self-discharge rate (%/day)

0.5

00.7〜1.0

0.3〜0.6

Cycle life

500〜30004500

500〜5000

500〜500040 (167)

100 (786)

60〜90 (125)130 (392〜585)

Charge/discharge efficiency

0.87

0.85

0.6

0.95

Cell voltage

2.0

2.1

1.2

3.8

Energy density ( Wh/kg)
Values in the parentheses (energy densities.

14Lead battery) denote the theoretical

MEIDEN REVIEW Series No.162 2014 No.3

Table 2 Example of LiB Specifications

Table 4 PCS Specifications

An example of LiB speciﬁcations is shown. There are differences
in battery manufacturers.

The inverter capacities are standardized according to the number of PCS parallel connections.

Battery type

Lithium ion battery

Reaction formulaLi1−xMyO2y＋LixC6 ←→ LiMyO2y＋6C

Anode material

Compound of lithium metal oxide

Cathode material

CarbonElectrolyte

Organic solvent

Theoretical energy density 392〜585(wh/kg)Open line voltage/cell ( V )3.6〜3.8

Working temperature ( ℃)

0〜40

Major auxiliary equipment Nil in particular
M: Metalic element (Co, Mn, Ni, etc.)

Table 3 BMU Functions
General BMU functions are shown. While the battery protectivefunction is in action, the PCS also stops the charge/discharge
operation for security.
Item

Contents

Protective function Overcharge, over-discharge, overvoltage,overcurrent, thermal errors, etc.
Status monitoring

Measurement of voltage, current, electricenergy, charge/discharge limit management,
state of charge management, etc.

ponent of cobalt, manganese, or nickel. Its cathodeis carbon and the electrolyte is an organic solvent.
Table 2 shows an example of LiB speciﬁcations.

2.3 Battery Management Unit (BMU)

Item

Rating or performance

Capacity

0.25〜2MVA(250kVA × 8 in parallelconnections)

Inverting system

Voltage type self-excited
PWM current control
systemType of rating

A0 (100% continuous)

AC I/O Rated voltage

400V class

Voltage deviation permissible
range−12〜＋10%Rated frequency

50Hz or 60Hz

Frequency deviation permissi- ±6%
ble range
Phases3-phase, 3-wire

Rated I/O capacity

0.25〜2MVA(250kVA × 8 in parallel
connections)±1%
ConstantActive power
power control±1%
Reactivepower
accuracy

Power control Active power
0〜100%
deviation
Reactive power 0〜100%
range
Current harmonics contentrate

Each order 3%,
total 5% or less

DC I/O Voltage range

240〜420V

Efficiency

95% or above

relays belonging to the power incoming block,inverters, and input ﬁlter which belong to the power
conversion block, and the PCS monitoring and control sequencer, and operator panel which thenreside in the supervisory control block.

Since the LiB uses organic materials that involve risk, the BMU is incorporated for battery safetymanagement. This unit offers a protective function
and a status monitoring function. Cell voltage, current, temperature, and other essential factors inside
the battery are continuously monitored. Table 3shows these functions.

3.2.1 Power Conversion Unit

3 PCS for the LiB

3.3 System Functions

The features of the Meiden PCS developed for
the LiB application are introduced below.

3.1 Unit Speciﬁcations
Table 4 shows PCS speciﬁcations.

3.2 System Conﬁguration
Fig. 1 shows a conﬁguration diagram of a power
storage system. The PCS is composed of a powerincoming switch, interconnection switch, protection

The PCS is used to convert the AC power into
the DC power to charge the LiB. The power stored
in the LiB is inverted into the AC power that is thenfed to the grid system. The PCS is composed of eight
250kVA inverter units connected in parallel. The maximum system capacity is scheduled to be 2MVA.

The outline of the PCS function features isintroduced below.
( 1) Manual operation
At the operator panel for this system, it is possible to perform operation in an arbitrary power setting, such as start-stop and charge-discharge. Also atthe operator panel, equipment status, fault display,and various telemetry values can be monitored.
( 2 ) Remote operation
When the supervisory unit installed in a remotesupervisory room and the PCS are connected withMEIDEN REVIEW Series No.162 2014 No.3

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AC400V classPCSPower incoming switch
Protectionrelay

Auxiliary switch…

× 8 parallels (Max.)

Auxiliary andcontrol source

Interconnection switch

Remotesupervisoryunit
PCS supervisory
control sequencer
Operatorpanel

Interconnection
switchInterconnection transformer
250kVA

Interconnection
transformer
250kVA

Input filter

Input filter× 8 parallels (Max.)

Centralizedcontrol block…

Localcontrolblock

Inverterblock

Localcontrolblock

DC switch

Inverterblock
DC switch

LiBBMU……

BMU× 8 parallels(Max.)

Fig. 1Configuration Diagram of Power Storage System

Eight units of 250kVA inverter systems are connected in parallel. The conﬁguration shown is for a 2MVA system. The number of parallel
connections is changeable arbitrarily from one to eight parallels.

an Ethernet or other like communication line, thesame operation as described above can be performed.
( 3 ) Inrush current preventive function needed at
the time of grid interconnectionWhen the PCS is interconnected with thepower grid system, there may be equipment deterioration due to the effect of inrush current in theinterconnection transformer, or due to system disturbance in some cases. To avoid this, this function
is used to generate a voltage from the PCS side sothat grid-connection can be accomplished smoothly
after being synchronized with the system voltage.
( 4 ) Active power control
According to the power command value, thisfunction controls the active power at end of the AC
terminal in order to be kept constant.
( 5 ) Reactive power control
According to the power command value, thisfunction controls the reactive power at the AC terminal end in order to be kept constant.
( 6 ) Islanding operation function (emergency powersupply)
In the event of a service interruption in the util-16

MEIDEN REVIEW Series No.162 2014 No.3

ity system, as a backup power supply, this function
is used to convert the DC power of the battery system into an AC power (constant voltage and frequency) so that the converted power can be fed tothe load as an emergency power supply.
( 7 ) Frequency control function
This function is used to monitor the AC terminal voltage and frequency, to calculate the amountof active power control comparable to changes infrequency, and to take automatic control in order toadjust the PCS output.
( 8 ) Fault Ride Through (FRT) function
In order to prevent unusual stoppage of thePCS during abnormality in system voltage, thisfunction is used to maintain favorable operation
within the range of the speciﬁed rate and duration ofa voltage drop.
( 9 ) Constant-voltage charging function
For the prevention of battery breakage anddeterioration due to overvoltage at the time of constant-power charging to the LiB, this function isused to change-over the present charging mode toconstant-voltage charging when the speciﬁed voltage (around the charge limit) is attained.

Table 5 Example of Energy Storage System Applications

3.4 Example of Application

The energy storage system can be used for a variety of applications. It can be used for peak-shaving and peak-shifting operation in factories and large buildings, or used as a power supply incases of an emergency. It is applicable to demand-supply control and frequency adjustment in the power grid.

This energy power storage system is used fora variety of applications such parallel grid-connection operation with a renewable energy system oras an emergency power supply operation. Table 5shows an example of energy storage system applications.

Application

Outlined explanation

Load levelingBatteries are charged up during the nighttime
when demand for electricity is low and they discharge in the time zone when power demand
peaked in the daytime. This performance contributes to the reduction of contract power rates.

Emergency
power supply

In the case of a service interruption occurring infacilities, electric power is kept supplied to significant loads. This feature contributes to theBusiness Continuity Plan (BCP).

Regulation
restraintThis feature is effective in avoiding turbulence inthe utility system by relieving voltage regulations
predicted to occur when a PV power source orother renewable energy source is interconnectedwith the power grid.

Peak shift

Batteries are charged in the time zone whenpower generation at the PV generation systemis large, and discharged in the time zone whenpower demand is large. This feature contributes
to the effective utilization of energy.

Demand-supply Based on the plan in regard to power generation
control
by means of generators, utilization of renewableenergy, and demand-supply prediction, measureswith the use of charge/discharge capabilities aretaken for power balancing in the case of deviation from the range planned.
Frequencycontrol

4 Postscript
The LiB energy storage system offers outstanding features in terms of economy, long operational life, and smaller footprint space. Since thesystem is rich in applications, it is expected to be
widely used in the near future. We will continue topromote development of key products that are indispensable for the LiB energy storage systems.
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.

Batteries are discharged when voltage and frequency are lowered in the utility system, and thenare charged when their values are recovered.
This feature is effective in controlling frequency
regulations.

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