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.
KACO new energy only accepts the warranty for medium-voltage transformers that have been installed
following guideline provided in this application note.
12-NPD-01-3dm-170c
1. LIST OF THE CENTRAL INVERTERS OF KACO
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
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
2. TECHNICAL 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)
Central Inverter
MV Transformer
V0
Conductor voltage
regarding ground
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
3. REQUIREMENTS FOR MEDIUM-VOLTAGE TRANSFORMERS, THAT ARE CONNECTED
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.
Central Inverter
MV Transformer
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 highvoltage 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
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 doublewinding 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.
ZH(1%)
ZLa(5%)
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
4. REQUIREMENTS FOR MEDIUM-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.
Central Inverter
Central Inverter
MV Transformer
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
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).
Isc
Vsc
Y
∆
Y
∆
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
ZLa(5%)
ZH(1%)
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
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.
Central Inverter
Central Inverter
MV Transformer
Central Inverter
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
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).
Isc
Vsc
Y
∆
Y
∆
Y
∆
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
ZLa(5%)
ZH(1%)
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
6. REQUIREMENTS FOR MEDIUM-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.
Central Inverter
Central Inverter
Central Inverter
MV Transformer
Central Inverter
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
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).
Isc
Vsc
Y
∆
Y
∆
Y
∆
Y
∆
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
ZLa(5%)
ZLb(5%)
ZH(1%)
ZLc(5%)
ZLd(5%)
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).
1V
1U
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
4U
3W
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
6
4
2
1
3
5
1W
1W
4W
4V
4U
4W
3U
3V
2W
3U
3W
4V
1U
1V
2U
2V
2U
2W
3V
1V
1W
1U
1U
1W
2V
1V
Fig. 21 Diagram of section-winding transformer
15/14
APPENDIX

Winding technology
Medium voltage transformer that is connected with transformerless solar inverter must be
designed with section winding transformer.

Section winding transformer
Case1. Dual stacked
Case2. Triple stacked
HVW1
HVW2
Core
LVW3
HVW3
HVW2
LVW2
HVW1
LVW2
LVW1
LVW1
Core
LVW: Low Voltage winding
HVW: High Voltage winding

Multi-layer winding transformer
Case1. LLH
Case2. LHL
Low Voltage winding2
High Voltage winding
Low Voltage winding1
Core
High Voltage winding
Low Voltage winding2
Low Voltage winding1
Core
16/14
Case3. LLLH
High Voltage winding
Low Voltage winding3
Low Voltage winding2
Low Voltage winding1
Core
17/14
B-1701~1716, 2ND Woolim Lions Valley, 146-8, Sangdaewon-dong, Jungwon-gu, Seongnam-si, Gyeonggido, South Korea
TEL +82-31-8018-2700
FAX +82-31-8018-2738
12-NPD-01-3dm-170c
www.kaco-newenergy.kr
REVISION HISTORY
Revision
Name
Description
number
“0”
(Date)

Joon.Kim
Initial version
(20-Oct-2014)
“a”

Joon.Kim
Revision by adding the model (blueplanet 1000 TL3 OD )
(12-Jan-2015)
“b”

Joon.Kim
Revision by adding the model (blueplanet 333 TL3 OD )
(13-Feb-2015)
“c”

Revision by adding appendix

Revision by changing terms(Multi winding  Section winding, Fourwinding transformer --> Dual stacked (four-winding) transformer)

Yeji.Jang
(10-Mar-2015)
Revision by adding chapter 6.
Written
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'
Checked
Approved
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'
19/14