KACO XP500, XP550-HV User Manual

KACO new energy only accepts the warranty for medium-voltage transformers that have been installed

following guideline provided in this application note.

Design Guide of the Medium Voltage
Transformers for KACO Central Inverter

This document describes the requirements of medium-voltage transformers that are connected to

KACO central inverter.

12-NPD-01-3dm-170c

Model

Rated Power

Rated AC Voltage

Protection Class

Type

XP100-HV

100kW

3*380V(±10%)

IP21, Indoor

Transformer

XP200-HV

200kW

3*380V(±10%)

IP21, Indoor

Transformer

XP250-HV

250kW

3*380V(±10%)

IP21, Indoor

Transformer

XP200-HV-TL

200 kW

3*290V(±10%)

IP21, Indoor

Transformerless

XP250-HV-TL

250 kW

3*290V(±10%)

IP21, Indoor

Transformerless

XP350-HV-TL

350 kW

3*290V(±10%)

IP21, Indoor

Transformerless

XP500-HV-TL

500 kW

3*370V(±10%)

IP21, Indoor

Transformerless

XP550-HV-TL

550 kW

3*370V(±10%)

IP21, Indoor

Transformerless

XP500-OD-TL

500kW

3*370V(±10%)

IP54, Outdoor

Transformerless

XP550-OD-TL

550kW

3*370V(±10%)

IP54, Outdoor

Transformerless

blueplanet 333 TL3 OD

333kW

3*370V(±10%)

IP54, Outdoor

Transformerless

blueplanet 1000 TL3 OD

1000kW

3*370V(±10%)

IP54, Outdoor

Transformerless

1. LIST OF TH E CENTRAL INVERTERS OF KACO

Table. 1 List of central Inverters of KACO

Table 1 shows the list of central inverter models of KACO, which can be connected to medium voltage

transformer.

KACO’s solar inverters are classified into 2 types : transformer type and transformerless type.

Unlike transformer inverters, the transformerless inverters don’t have transformers inside, so their

medium voltage transformers must be designed according to the guide when they need to be connected

with external medium voltage transformers.

2/14

Conductor voltage
regarding ground

V

0

Central Inverter

MV Transformer

2. TECHN ICAL PROPERTIES

Medium voltage transformer that is connected with transformerless solar inverter must comply with

following technical specifications:

1. The transformer must be suitable for PWM (Pulse Width Modulation) Inverter. The transformer

should be designed such that its magnetic flux is not saturated even if 1% of DC current flow on its

low voltage winding. Also the transformer should be designed and tested in accordance with

ANSI/IEEE, NEMA, and Department of Energy standards.

2. The transformer must be designed considering the voltages that arise during pulsed operation of

the inverter. The voltages can reach a magnitude of maximum ±1400 V reference to ground. The

rms‑value of the voltages reference to ground is maximum 700V. (See Fig.2)

Fig. 1 KACO central inverter with Double-winding Transformer

Fig. 2 Conductor voltage to ground and line-to-line voltage

3/14

3. The transformer must be designed for voltages on its low‑voltage windings that can exhibit a

voltage gradient dV/dt of up to 500V/µs reference to the ground. The line‑to‑line voltages must be

sinusoidal.

4. A shield winding that is grounded to the tank is necessary between the low‑voltage windings and

the high voltage windings. This shield plate must be designed to protect against the heat due to

eddy current by the flux of both the low voltage winding and the high voltage winding. This serves

as an additional dV/dt filter.

5. In ambient temperatures off up to 50 °C the transformer must have a current load capability of

110 %.

Further information can be found in the following documents:

‒ KACO central inverter data sheet

‒ Power derating of KACO central inverter

6. During thermal rating, the load curve and the ambient conditions at the respective installation site

should be taken into account.

7. KACO new energy recommends to use a transformer with a tap changer on the high‑voltage side

that enables an alignment to the voltage level of the medium‑voltage grid. Our recommendation

is 5 taps with 2.5% resolution.

8. The country-specific grid frequency should be taken into consideration.

9. The country specific standards valid should be taken into consideration.

4/14

Central Inverter

MV Transformer

3. REQUIREMENTS FOR MEDIUM-VOLTAGE TRANSFORMERS, THAT ARE CONNECTE D

TO 1 CENTRAL INVERTER

The transformer shown in Fig.3 is a double-winding transformer. Its low-voltage side is connected to KACO

central inverter of which rated output voltage is 370V, and its high voltage side is for connection to the

medium-voltage grid in Europe which is 20kV typically. However, other high voltages are also available: 10

kV, 15 kV, 22 kV, 25 kV, 27 kV, 30 kV, 34.5kV or 35 kV etc.

Fig. 3 KACO central inverter with Double-winding Transformer

This transformer must comply with the following technical specifications:

1. Equivalent series impedance between low voltage and high voltage winding:

The equivalent series impedance Z(%) of the transformer must be 6 %. Impedance voltage

tolerance limits of 5.4 % ~ 6.6 % must be maintained. This value can be determined when the high-

voltage winding is short-circuited and the voltage on the other low‑voltage winding is increased

until the nominal current flows. (see Fig.4).

Fig.4 Equivalent circuit with short circuited high voltage winding

5/14

ZH(1%) ZLa(5%)

2. Equivalent series impedance Z(%) of the double-winding transformer:

To summarize of the contents in Article 2, the equivalent series impedance Z(%) of the double-

winding transformer can be shown as followed.

ZL is the equivalent series impedance of low voltage winding, and ZH is the equivalent series

impedance of high voltage winding.

Fig. 5 Equivalent series impedance Z(%) of double-winding transformer

3. No neutral point is required on the low voltage side. Nevertheless, if a neutral point shows up on

the low‑voltage side, this neutral point must not be either connected or grounded.

4. Double-winding transformers with varying windings each on the high‑voltage side and the

low‑voltage side can be used. For example YNd11, YNd5, YNd1 or Dy11, Dy5, Dy1 with an

ungrounded neutral point on the low‑voltage side (see Fig.6)

Fig. 6 Diagram of double-winding transformer

6/14

Central Inverter

Central Inverter

MV Transformer

4. REQUIREMENTS FOR MEDI UM-VOLTAGE TRANSFORMERS, THAT ARE CONNECTED

TO 2 CENTRAL INVERTERS

The transformer shown in Fig.7, is a dual stacked (four-winding) transformer. Its low-voltage side is

connected to KACO central inverter of which rated output voltage is 370V, and its high voltage side is for

connection to the medium-voltage grid in Europe which is 20kV typically. However, other high voltages

are also available: 10 kV, 15 kV, 22 kV, 25 kV, 27 kV, 30 kV, 34.5kV or 35 kV etc.

Fig. 7 KACO central inverter with dual stacked (four-winding) transformer

This transformer must comply with the following technical specifications:

1. Equivalent series impedance between low voltage and high voltage winding:

The equivalent series impedance Z(%) of the transformer must, in relation to every inverter, be 6 %

in each case. Impedance voltage tolerance limits of 5.4 % ~ 6.6 % must be maintained. This value

can be determined when the high-voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time another low

voltage windings are idle (see Fig.8).

7/14

Y

Y

V

sc

I

sc

∆

∆

Fig.8 Equivalent circuit with short circuited high voltage winding

2. Equivalent series impedance between both low voltage winding:

The equivalent series impedance Z(%) between both low‑voltage windings must be 10 %. The

tolerance limits of this impedance voltage of 9 % ~ 11 % must be maintained. This value can be

determined when one of the low‑voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time the

high‑voltage windings are idle (see Fig.9).

Fig.9 Equivalent circuit with short circuited low voltage winding

3. Equivalent series impedance Z(%) of the dual stacked (four-winding) transformer:

To summarize of the contents in Article 1 and Article 2, the equivalent series impedance Z(%) of

the dual stacked (four-winding) transformer can be shown as followed.

ZL is the equivalent series impedance of low voltage winding, and ZH is the equivalent series

impedance of high voltage winding.

8/14

ZH(1%)

ZLa(5%)

ZLb(5%)

Fig. 10 Equivalent series impedance Z(%) of dual stacked (four-winding) transformer

4. No neutral point is required on the low voltage side. Nevertheless, if a neutral point shows up on

the low‑voltage side, this neutral point must not be either connected or grounded.

5. dual stacked (four-winding) transformers with varying windings each on the high‑voltage side and

the low‑voltage side can be used. For example YNd11d11, YNd5d5, YNd1d1 or Dy11y11, Dy5y5,

Dy1y1 with an ungrounded neutral point on the low‑voltage side (see Fig.11).

Fig. 11 Diagram of dual stacked (four-winding) transformer

9/14

Central Inverter

Central Inverter

MV Transformer

Central Inverter

5. REQUIREMENTS FOR MEDIUM-VOLTAGE TRANSFORMERS, THAT ARE CONNECTED

TO 3 CENTRAL INVERTERS

The transformer shown in Fig.12 is a section-winding transformer.(Refer to the appendix) Its low-voltage

side is connected to KACO central inverter of which rated output voltage is 370V, and its high voltage side

is for connection to the medium-voltage grid in Europe which is 20kV typically. However, other high

voltages are also available: 10 kV, 15 kV, 22 kV, 25 kV, 27 kV, 30 kV, 34.5kV or 35 kV etc.

Fig. 12 KACO central inverter with section-winding transformer

This transformer must comply with the following technical specifications:

1. Equivalent series impedance between low voltage and high voltage winding:

The equivalent series impedance Z(%) of the transformer must, in relation to every inverter, be 6 %

in each case. Impedance voltage tolerance limits of 5.4 % ~ 6.6 % must be maintained. This value

can be determined when the high-voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time another low

voltage windings are idle (see Fig.13).

10/14

Y

Y

Y

V

sc

I

sc

∆

∆

∆

Fig.13 Equivalent circuit with short circuited high voltage winding

2. Equivalent series impedance between both low voltage winding:

The equivalent series impedance Z(%) between both low‑voltage windings must be 10 %. The

tolerance limits of this impedance voltage of 9 % ~ 11 % must be maintained. This value can be

determined when one of the low‑voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time the

high‑voltage windings are idle (see Fig.14).

Fig.14 Equivalent circuit with short circuited low voltage winding

3. Equivalent series impedance Z(%) of the section -winding transformer:

To summarize of the contents in Article 1 and Article 2, the equivalent series impedance Z(%) of

the section -winding transformer can be shown as followed.

ZL is the equivalent series impedance of low voltage winding, and ZH is the equivalent series

impedance of high voltage winding.

11/14

ZH(1%)

ZLa(5%)

ZLb(5%)

ZLc(5%)

Fig. 15 Equivalent series impedance Z(%) of section-winding transformer

4. No neutral point is required on the low voltage side. Nevertheless, if a neutral point shows up on

the low‑voltage side, this neutral point must not be either connected or grounded.

5. Section-winding transformers with varying windings each on the high‑voltage side and the

low‑voltage side can be used. For example YNd11d11d11, YNd5d5d5, YNd1d1d1 or Dy11y11y11,

Dy5y5y5, Dy1y1y1 with an ungrounded neutral point on the low‑voltage side (see Fig.16).

Fig. 16 Diagram of section-winding transformer

12/14

Central Inverter

Central Inverter

MV Transformer

Central Inverter

Central Inverter

6. REQUIREMENTS FOR MEDI UM-VOLTAGE TRANSFORMERS, THAT ARE CONNECTED

TO 4 CENTRAL INVERTERS

The transformer shown in Fig.12 is a section-winding transformer.(Refer to the appendix) Its low-voltage

side is connected to KACO central inverter of which rated output voltage is 370V, and its high voltage side

is for connection to the medium-voltage grid in Europe which is 20kV typically. However, other high

voltages are also available: 10 kV, 15 kV, 22 kV, 25 kV, 27 kV, 30 kV, 34.5kV or 35 kV etc.

Fig. 17 KACO central inverter with section-winding transformer

This transformer must comply with the following technical specifications:

1. Equivalent series impedance between low voltage and high voltage winding:

The equivalent series impedance Z(%) of the transformer must, in relation to every inverter, be 6 %

in each case. Impedance voltage tolerance limits of 5.4 % ~ 6.6 % must be maintained. This value

can be determined when the high-voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time another low

voltage windings are idle (see Fig.18).

13/14

Y

Y

Y

V

sc

I

sc

∆

∆

∆

Y

∆

Fig.18 Equivalent circuit with short circuited high voltage winding

2. Equivalent series impedance between both low voltage winding:

The equivalent series impedance Z(%) between both low‑voltage windings must be 10 %. The

tolerance limits of this impedance voltage of 9 % ~ 11 % must be maintained. This value can be

determined when one of the low‑voltage winding is short-circuited and the voltage on the other

low‑voltage winding is increased until the nominal current flows. At the same time the

high‑voltage windings are idle (see Fig.19).

Fig.19 Equivalent circuit with short circuited low voltage winding

3. Equivalent series impedance Z(%) of the section -winding transformer:

To summarize of the contents in Article 1 and Article 2, the equivalent series impedance Z(%) of

the section -winding transformer can be shown as followed.

ZL is the equivalent series impedance of low voltage winding, and ZH is the equivalent series

impedance of high voltage winding.

14/14

ZH(1%)

ZLa(5%)

ZLb(5%)

ZLc(5%)

ZLd(5%)

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

1W

1V

1U

3W

3V

3U

1W

1V

1U

1W

1V

1U

3W

3U

3V

1W

1U

1V

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

2W

2V

2U

2W

2U

2V

2

4

6

5

3

1

2

4

6

5

3

1

2

4

6

5

3

1

4W

4V

4U

4W

4U

4V

Fig. 20 Equivalent series impedance Z(%) of section-winding transformer

4. No neutral point is required on the low voltage side. Nevertheless, if a neutral point shows up on

the low‑voltage side, this neutral point must not be either connected or grounded.

5. Section-winding transformers with varying windings each on the high‑voltage side and the

low‑voltage side can be used. For example YNd11d11d11d11, YNd5d5d5d5, YNd1d1d1d1 or

Dy11y11y11y11, Dy5y5y5y5, Dy1y1y1y1 with an ungrounded neutral point on the low‑voltage side

(see Fig.21).

Fig. 21 Diagram of section-winding transformer

15/14

Core

LVW1 LVW2 LVW3

HVW1 HVW2 HVW3

Core

LVW1 LVW2

HVW1 HVW2

LVW: Low Voltage winding
HVW: High Voltage winding

Case1. Dual stacked Case2. Triple stacked

Core

Low Voltage winding1

Low Voltage winding2

High Voltage winding

Core

Low Voltage winding1

Low Voltage winding2

High Voltage winding

Case1. LLH Case2. LHL

APPENDIX

 Winding technology

Medium voltage transformer that is connected with transformerless solar inverter must be

designed with section winding transformer.

 Section winding transformer

 Multi-layer winding transformer

16/14

Core

Low Voltage winding1

Low Voltage winding2

Low Voltage winding3

High Voltage winding

Case3. LLLH

17/14

B-1701~1716, 2ND Woolim Lions Valley, 146-8, Sangdaewon-dong, Jungwon-gu, Seongnam-si, Gyeonggi-

do, South Korea

TEL [phone] FAX [phone] www.kaco-newenergy.kr

12-NPD-01-3dm-170c

Revision

number

Description

Name

(Date)

“0”

 Initial version

Joon.Kim

(20-Oct-2014)

“a”

 Revision by adding the model (blueplanet 1000 TL3 OD )

Joon.Kim

(12-Jan-2015)

“b”

 Revision by adding the model (blueplanet 333 TL3 OD )

Joon.Kim

(13-Feb-2015)

“c”

 Revision by adding appendix
 Revision by changing terms(Multi winding  Section winding, Four-

winding transformer --> Dual stacked (four-winding) transformer)

 Revision by adding chapter 6.

Yeji.Jang

(10-Mar-2015)

Written

Checked

Approved

디지털 서명
자Yeji.jang

DN: cn=Yeji.jang,
o=KACO-newenergy,
ou=System,
email=Yeji.jang@kac
o-newenergy.kr,
c=KR
날짜: 2015.03.10
21:24:41 +09'00'

Nick
.choi

디지털 서명
자Nick.choi

DN: cn=Nick.choi,
o=KACO,
ou=system,
email=nick.choi@ka
co-newenergy.kr,
c=KR
날짜: 2015.03.11
09:37:09 +09'00'

REVISION HISTO RY

19/14