Danfoss vacon nx Design guide

vacon nx

®

ac drives

design guide

hybridization

vacon • 1

TABLE OF CONTENTS

Document ID:DPD01887

Revision release date: 24.10.2016

1. BASICS ........................................................................................................................2

1.1 Power or energy storage....................................................................................................... 3

1.2 Battery current dimensioning............................................................................................... 5

2. BASIC TOPOLOGIES FOR CONNECTION....................................................................... 6

3. SPECIAL CHARACTERISTICS AFFECTING THE SELECTION ......................................... 8

3.1 Voltage window...................................................................................................................... 8

3.2 Galvanic isolation requirement........................................................................................... 11

3.3 Balance or maintenance charge......................................................................................... 14

3.4 System control principles ................................................................................................... 15

4. CHOOSING A CORRECT TOPOLOGY............................................................................ 17

4.1 Allowed topology configurations......................................................................................... 18

5. BASIC VARIANTS....................................................................................................... 19

5.1 Direct to DC ......................................................................................................................... 19

5.1.1 Control structure .................................................................................................. 20

5.2 DC to DC .............................................................................................................................. 22

5.2.1 Filter...................................................................................................................... 22

5.2.2 Control Structure.................................................................................................. 34

6. PRODUCT CONFIGURATION EXAMPLES.................................................................... 36

6.1 Scope of delivery ................................................................................................................. 36

6.1.1 Direct to DC........................................................................................................... 36

6.1.2 DC to DC ................................................................................................................ 37

6.2 Example configurations ...................................................................................................... 39

6.2.1 DC/DC for supply interruptions ............................................................................ 39

6.2.2 Direct DC for Grid Support.................................................................................... 40

7. SIZING OF THE SYSTEM AND PRODUCT .................................................................... 41

7.1 Direct to DC ......................................................................................................................... 41

7.2 DC/DC .................................................................................................................................. 42

8. INFORMATION TO ACQUIRE FROM CUSTOMERS ....................................................... 47

NOTE! You can download the English and French product manuals with applicable safety,
warning and caution information from

http://drives.danfoss.com/knowledge-center/technical-documentation/.

REMARQUE Vous pouvez télécharger les versions anglaise et française des manuels produit
contenant l’ensemble des informations de sécurité, avertissements et mises en garde
applicables sur le site http://drives.danfoss.com/knowledge-center/technical-documentation/
.

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vacon • 2 BASICS

Energy Production

kW

Average Power

t

Charging

Discharging

1. BASICS

The basic idea is always to achieve energy and/or power management of Common Point of Coupling.
Typical use cases are

• time shift for production

• peak load shaving for distribution

• smoothen load for average energy

• backup power or black out start

• grid support

Average Power

kW

Process Power
Grid Power

Charging

Discharging

t

Figure 1. Power balancing

1

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BASICS vacon • 3

Time [h]

Power: Energy

MW: MWh

4:1 3:1 2:1 1:1

1:1 1:4

1:2 1:3 1:4

Time [h] Time [h]

Power [MW]

Power [MW]

Power [MW]

Power Applications Energy Applications

4:1

1.1 Power or energy storage

It is important to distinguish the system’s "nature", that is, whether it is a power application or an
energy application. Another relevant thing to note is the dynamic requirements of the application.

Determining the application:

• Energy vs. power (kW/kWh ratio)

• Dynamic requirements:
o Grid support functions (Harmonics, FRT)
o Bulk energy time shift

Figure 2. Power vs. energy

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1

vacon • 4 BASICS

BA C D E

F

G

100s

1,000s

1,000

100

10

1

0.1

0.01
10 100 1,000 10.000

10,000s

10s

1s

0.1s

Energy density in Wh/kg

Power density in W/kg

# Reference # Reference

A Batteries E Li-ion
B Pb F Double layer capacitors
C NiCd G Electrolytic capacitors
DNiMH

Figure 3. Comparision of battery systems

Table 1. Comparision of battery systems

Battery type

Lead acid battery 30-50 150-300 300-1,000/3-5

Nickel-metal hybride

battery

Lithium-ion battery 90-150 500 -> 2,000 >2,000/5-10

Spercaps (double

layer capac.)

Energy density

Wh/kg

60-80 200-300 >1,000/>5

3-5 2,000-10,000 1,000,000/unlimited

Power density W/kg

Service life in

cycles/years

1

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BASICS vacon • 5

© Electricity storage association

Discharge Time [hr]

100

PHS

A

B

C

10

Li-ion

1

Ni-MH

0.1

0.01

0.001

0.0001

0.001 0.01 0.1 1 10 100 1000 10,000

VR

Zn-Br

FW

EDLC

Rated Power [MW]

Na-S

CAES

L/A

Ni-CD

Na-S

# Reference # Reference

A Energy management

Dicharge timeBBridging power

CPower quality
CAES Compressed air Ni-Cd Nickel-cadmium
EDLC Dbl-layer capacitors Ni-MH Nickel-metal hybride

FW Flywheels PSH Pumped hydro
L/A Lead-acid VR Vanadium redox

Li-ion Lithium-ion Zn-Br Zinc-bromine

Na-S Sodium-sulfur

Figure 4. System ratings

1.2 Battery current dimensioning

In a battery, the nominal current is denoted with C. For example, a 10Ah 1C current would be 10A.
In some cases, the below rated currents are marked as 0.5C = C5. In that case, for example a 10Ah
rated current used with a 1A current would mean 0.1C or C1. In the same example, 2C would mean
20A.

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1

vacon • 6 BASIC TOPOLOGIES FOR CONNECTION

Filter

Filter

Filter

Filter

Filter

2. BASIC TOPOLOGIES FOR CONNECTION

The basic connections are divided into multible possibilities.

Table 2. Basic connections

Use case Topolo gy Pros Cons

• No competitive
"technology" when
DC-grid connec-

Common DC energy
storage connection

Energy storage to AC­grid with combination of
DC/DC converter + grid
converter

Filter

tion needed

• Different storage
voltage/techno­logy adaptations

• Different storage
voltage/techno­logy adaptations

•Expansion easy

• Battery stack
replacing due to
ageing

•Large number of
components

• Lack of efficiency

•Size

Energy storage directly to
AC-grid with grid
converter

Energy storage close to
load and AC-grid with
DC/DC converter con­nected between DC-link
and storage

• Small number of
components

• Efficiency

•Size

• Power vs. energy
dimensioning is
independent from
each other

•Load power/
energy support
close the con­sumption

• Different storage
voltage/techno­logy adaptations

•Expansion easy

• Battery stack
replacing due to
ageing

• Expansion difficult

• Battery stack
replacing due to
ageing

•Large number of
components

•Size

2

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BASIC TOPOLOGIES FOR CONNECTION vacon • 7

Filter

•Load power/

Energy storage close to
load and AC-grid with
direct DC-link connection

energy support
close the
consumption

• Large number of
components

• Efficiency

•Size

• Power vs. energy
dimensioning is
independent from
each other

•Voltage window
limiting the scope
only in range of
400 Vac using DC
range 600-1100
Vdc

• System expansion
later with
additional
batteries difficult

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2

vacon • 8 SPECIAL CHARACTERISTICS AFFECTING THE

120

100

80

0

20 40 60 80 100

U

DC

[%]

SOC [%]

Charging of batteryDischarging of battery

1C 2C 3C 6C 9C

3. SPECIAL CHARACTERISTICS AFFECTING THE SELECTION

Different chemistry causes different behavior in cell voltage as a function of charge/discharge and
SOC (State of Charge). This creates "voltage window" requirement similar to the solar inverter.

Galvanic isolation requirement is different from many industrial drive application. This is due to the
fact that the battery system should not be predisposed for common mode voltage.

For the Battery Management System (BMS) to be able to reset the SOC calculation, it is necessary
to charge the battery to 100% SOC. This ensures that BMS is able to calculate SOC accurately and
maintain the battery in safe operating area. For this, a balance charger or a maintenance charger is
needed in some cases.

3.1 Voltage window

For both the DC/DC converter and the GTC (Grid Tie Converter) the first dimensioning question
comes from energy storage (battery) voltage dimensioning. It is important to define the “voltage
window" for empty and full battery cell voltage. Depending on battery chemistry the ratio can be full/
empty = 1,2… 2… (meaning, for example, full being 1000 Vdc, and empty being from 800 Vdc to 500
Vdc) and for super capacitors even bigger. Especially for GTC this is a limiting factor. The limitations
come from minimum tolerable DC-link voltage to maintain controllable grid voltage and from
maximum allowed voltage to maintain within design criterion of the hardware.

The behavior of voltage stretch in a battery can be illustrated with a spring being pulled or pushed.

3

Figure 5. Spring analogy of the battery voltage change

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SPECIAL CHARACTERISTICS AFFECTING THE SELECTION vacon • 9

U

[%]

DC

120

Charging of batteryDischarging of battery

100

80

0

1C 2C 3C 6C 9C

20 40 60 80 100

SOC [%]

Figure 6. Battery voltage change as a function of State Of Charge (SOC)

The voltage window is important also from the process dynamics point of view. If we expect the
battery system to take energy (either discharge or charge), we create change in voltage of the
battery. The voltage controller needs to be capable to change the actual voltage of the battery in a
controlled way from full to empty value or from empty to full value. For example, if the battery is
wanted to be discharged in 30 s - 300V voltage window from 1000 Vdc - 700 Vdc it means roughly 10
V/s voltage change of rate. This is huge difference in comparison to for example case where
discharge time is longer, say 30 min resulting in 0,2 V/s. This way the SOC (State of Charge) behavior
is observed.

Below is a case where same sized of DC-power units are charged/discharged from the battery.

Figure 7. Battery string number effect on voltage change using the spring analogy

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3

vacon • 10 SPECIAL CHARACTERISTICS AFFECTING THE

123456 12345 12 1

120

100

80

120

100

80

8

6

4

2

0

-2

-4

-6

-8

0.2

0.25

0.2

0.35

0.4

0.6

0.65

0.7

0.75

0.8

0

0.1

0.2

0.7

0.5

0.4

0.3

0.8

0.9

1

0.6

tt t

increasing charge current

increasing load current

I [C-rating]U

DC

[%] U

DC

[%]

The difference in the cases is that the battery size in energy is changed from 6 strings in parallel to
one string in parallel. This will lead in higher C-rates in the battery having smaller amount of strings
when the same amount of power is taken out of each battery setup (current going from 1C --> 6C).
The effect is visible in higher stretch of voltage levels needed in controlling the battery.

Figure 8. Number of batteries

Figure 9. Battery sizing effect on voltage change during equal power changes

The spring analogy works also when thinking of parallelizing of batteries (springs). The more you
have batteries (springs) in parallel, the less you need to use voltage stretch to gain the same
response.

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3

SPECIAL CHARACTERISTICS AFFECTING THE SELECTION vacon • 11

3.2 Galvanic isolation requirement

The pulse width modulation (PWM) produces common mode voltage. Because every phase (a, b and
c) can be connected only either to positive DC-bus (+U
output voltages is always unequal to zero. The common mode voltage (CM-voltage) U
calculated as average of output voltages:

Table 3 presents all possible common mode voltages produced by different switching states. Used
reference point is in the middle of the DC-link.

Table 3. Common mode voltage as function of modulation sequence

Switching vector a b c

U

1

U

2

U

3

U

4

U

5

U

6

U

7

U

8

+--

++-

-+-

-++

--+

+-+

+++

---

/2) or to negative DC-bus (-Udc/2), sum of

dc

can be

cm

U

cm

-U

/6

dc

U

/6

dc

-U

/6

dc

U

/6

dc

-U

/6

dc

U

/6

dc

U

/2

dc

-U

/2

dc

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3

vacon • 12 SPECIAL CHARACTERISTICS AFFECTING THE

600

400

40 40.10 40.20 40.30 40.40 40.50

Time [ms]

40.60 40.70 40.80 40.90 41

200

0

-200

-400

-600

UDC/6 UDC/2 CM voltage

CM voltageU [V] Common mode

# Curve info max min rms

......

----

U

DC

U

/6

DC

171 171 171

512 512 512

/2

___ CM voltage 512 -512 264

Figure 10. Simulated CM-voltage, Udc=1025V, fsw=5kHz.

Because of the common mode DC-link starts to jump compared to ground. Main frequency for this
jumping is switching frequency but also higher frequencies will be present. As an example, a typical
measured DC+ to ground voltage can be seen in Figure 11. A rule of thumb is that with a typical DC­link voltage 1025V, the voltage spikes will be about 1.5kV.

3

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SPECIAL CHARACTERISTICS AFFECTING THE SELECTION vacon • 13

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

-1000

-500

0

500

1000

1500

Sampled waveform

time [s]

voltage [V] voltage [V]

time [s]

-600

-400

-200

0

200

400

600

800

1000

1200

Sampled waveform

Figure 11. DC+ to ground voltage. On the left U

= 1200 V, on the right 800 V.

dc

The battery system does not withstand unfiltered common mode voltage. Because PWM modulation
is a CM voltage source, the DC side of the energy storage system must be stabilized. This means
that there must be a flexible element in electrical system that is able to take this common mode
voltage fluctuation. This element is now a transformer star point (instead of a motor stator star
point) that shall not be grounded.

DC-link

AFE

(Active Front End)

LCL-filter

Transformer

CM

In the grid side filter, if LCL is used, the grounded capacitors cannot be kept connected to ground. If
transformer inductance is bigger or at least the same as proposed grid side inductance, it is
possible to use only an LC filter (sine) to avoid additional voltage drop in the grid side choke.

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Figure 12. Transformer must be isolated from ground.

3

vacon • 14 SPECIAL CHARACTERISTICS AFFECTING THE

-L1

-L2.1

-L2.2

-L2.3

-C1

-C2

V1

W1

HF

-C3

HF

no HF

U2 U1

V2

W2

-R1

-C1.1 -C1.2 -C4.1-C4.2

-R2

-C2.1 -C2.2 -C5.1

-R3

-C3.1 -C3.2 -C6.1

-R4

-R5

-C5.2

-R6

-C6.2

Figure 13. LCL ground capacitor must be disconnected

3.3 Balance or maintenance charge

The maximum voltage of the battery is needed only when charging the battery at the fullest level.
Current in that voltage is small. However, the time during which this voltage prevails can be
theoretically infinite if the battery is continuously kept 100% full (which is not advisable because of
the aging of the battery). When the charging is finished and even only little load is given to the
battery, the voltage decreases rapidly.

It is necessary (after a certain time or a number of battery charge/discharge cycles) to "reset the
trip meter" of the Battery Management System. Otherwise the state of charge calculations can
become misleading and result in poor behavior or even in exceeding the safe operation limits. The
only good way to "reset the trip meter" is to charge the battery to the full state where the Battery
Management System can safely tune its SOC value back to 100%.

Every cell must be charged extremely slowly so that the current of each cell goes as low as possible
(the cell reaches its full voltage). For a big battery system that has many cells in parallel and in
serial this is done from the same DC+ and DC- connections with the same Udc control. Do not start
to dismantle batteries to charge them individually. Because of the differences in cell level (for
example SOC, impedance) this means that some of the cells fill up sooner than others.

To avoid overcharging, the natural passive balancing of the battery system is needed. However, this
is a slow process and that is why the balancing charge needs to be slow with an accurately
controlled small current. It is difficult to say how accurate and small the current needs to be, but
the rule of thumb is that 0.01C is needed. If the device is not able to provide accurately such a
current, it is necessary to add a balance charger to the system. The battery manufacturer can also
be consulted about balance chargers.

3

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