shivam.pandey
Member
- Location
- Chula Vista
- Occupation
- Solar design engineer
Do we need to connect grounding transformer to LV side for each solar transformer?
We have two 2000kVA XFMR connected in loop
Utility requirement from service manual-
Three phase transformers on the load side of Main Disconnecting Means should be equipped
with suitable protective devices to prevent the Loss of Phase condition.
Customers must carefully evaluate the operational characteristics of transformers located on
the load side of Main Disconnecting Means. Many step up / step down transformer
manufacturers use a standard three-legged core design. On a grounded-wye system, the Wye
/ Wye-grounded construction reduces the likelihood of ferroresonance and provides a ground
source. However, under the loss of phase condition (unbalanced voltage) or unbalanced load,
this can create zero sequence voltage (zero sequence flux in the core). In the three-legged
construction, there is no path for this flux to circulate within the core. Thus, the flux jumps
through the tank and air gap. Flux through the transformer tank (enclosure) creates localized
eddy current heating so much as to discolor / overheat the unit and possibly leading to failure.
It is highly recommended to incorporate one of the following transformer options to help
mitigate the conditions:
1. One side of the transformation being Delta connected.
2. A delta tertiary connection associated with the Wye / Wye Configuration.
3. Three individual single-phase transformers connected to form a three-phase bank or
triplex construction.
4. A four- or five-legged core.
5. Shell type core construction.
6. A three-legged transformer with primary protective devices that sense loss of voltage
magnitude and unbalance. When excessive voltage unbalance is detected, all
primary phases need to be disconnected simultaneously.
If this voltage transformation is associated with a DER installation (solar / energy storage /
wind) that utilizes inverters, a Wye / Wye configuration with a Delta connected tertiary
winding may prevent the inverter from sensing a Loss of Phase event on a three-phase
system. The consequence of not detecting the Loss of Phase condition would be a
violation of Standard IEEE 1547. Compliance with IEEE 1547 is required by Ameren
Illinois and Ameren Missouri for all customer with DER interconnections.
DER utility requirement-
For 5 kV and 15 kV class distribution systems, Ameren Illinois circuits most often employ 4‐wire
configurations with multiple neutral grounding connections. To mitigate potential ground fault
overvoltage (GFOV) conditions, new RLG customer interconnection transformers may be
required to act as a ground source, as determined by Ameren Illinois during the interconnection
study process.
RLG systems required to act as ground sources should be designed for a coefficient of
grounding of 0.8 or less without the consideration of connected loads for ground faults on the
Ameren Illinois feeder when temporarily islanded with the RLG. The interconnection
transformer configuration may be grounded‐wye on the high voltage side and grounded‐wye
on the low voltage side with a supplemental grounding transformer, or grounded‐wye with a
neutral impedance on the high voltage side and delta on the low voltage side.
Three‐phase, three‐legged, core form transformers should not be used because internal
magnetic coupling can disguise the loss of an incoming phase and prevent the proper shut‐
down of inverter‐connected generation. This type of core form is more prone to ferro‐
resonance during open phase conditions and grounded‐wye / grounded‐wye winding
configurations typically produce excessive tank wall heating during ground faults due to zero‐
sequence currents circulating in the tank.
We have two 2000kVA XFMR connected in loop
Utility requirement from service manual-
Three phase transformers on the load side of Main Disconnecting Means should be equipped
with suitable protective devices to prevent the Loss of Phase condition.
Customers must carefully evaluate the operational characteristics of transformers located on
the load side of Main Disconnecting Means. Many step up / step down transformer
manufacturers use a standard three-legged core design. On a grounded-wye system, the Wye
/ Wye-grounded construction reduces the likelihood of ferroresonance and provides a ground
source. However, under the loss of phase condition (unbalanced voltage) or unbalanced load,
this can create zero sequence voltage (zero sequence flux in the core). In the three-legged
construction, there is no path for this flux to circulate within the core. Thus, the flux jumps
through the tank and air gap. Flux through the transformer tank (enclosure) creates localized
eddy current heating so much as to discolor / overheat the unit and possibly leading to failure.
It is highly recommended to incorporate one of the following transformer options to help
mitigate the conditions:
1. One side of the transformation being Delta connected.
2. A delta tertiary connection associated with the Wye / Wye Configuration.
3. Three individual single-phase transformers connected to form a three-phase bank or
triplex construction.
4. A four- or five-legged core.
5. Shell type core construction.
6. A three-legged transformer with primary protective devices that sense loss of voltage
magnitude and unbalance. When excessive voltage unbalance is detected, all
primary phases need to be disconnected simultaneously.
If this voltage transformation is associated with a DER installation (solar / energy storage /
wind) that utilizes inverters, a Wye / Wye configuration with a Delta connected tertiary
winding may prevent the inverter from sensing a Loss of Phase event on a three-phase
system. The consequence of not detecting the Loss of Phase condition would be a
violation of Standard IEEE 1547. Compliance with IEEE 1547 is required by Ameren
Illinois and Ameren Missouri for all customer with DER interconnections.
DER utility requirement-
For 5 kV and 15 kV class distribution systems, Ameren Illinois circuits most often employ 4‐wire
configurations with multiple neutral grounding connections. To mitigate potential ground fault
overvoltage (GFOV) conditions, new RLG customer interconnection transformers may be
required to act as a ground source, as determined by Ameren Illinois during the interconnection
study process.
RLG systems required to act as ground sources should be designed for a coefficient of
grounding of 0.8 or less without the consideration of connected loads for ground faults on the
Ameren Illinois feeder when temporarily islanded with the RLG. The interconnection
transformer configuration may be grounded‐wye on the high voltage side and grounded‐wye
on the low voltage side with a supplemental grounding transformer, or grounded‐wye with a
neutral impedance on the high voltage side and delta on the low voltage side.
Three‐phase, three‐legged, core form transformers should not be used because internal
magnetic coupling can disguise the loss of an incoming phase and prevent the proper shut‐
down of inverter‐connected generation. This type of core form is more prone to ferro‐
resonance during open phase conditions and grounded‐wye / grounded‐wye winding
configurations typically produce excessive tank wall heating during ground faults due to zero‐
sequence currents circulating in the tank.
