A rough overview of topics related to grounding in AC Microgrids

1. AC Microgrid
Grounding

2. Grounding
Connection of the system to ground.
Purpose:
1. Controlling the voltage with respect to
earth.
2. Fault detection
3. Protection from overvoltage and
overcurrent.
Types:
Solid Grounding
Impedance Grounding

3. Ungrounded
● Stray Capacitance Charging Current ~ 1% of 3ph fault current
● Over voltage on unfaulted lines -> Line to line voltages
● Transient overvoltage - 3 pu
Advantages:
● Possibility of service during L-G faults
● Low investment
Disadvantages
● Excessive transient voltage

4. Solidly grounded
● No intentional impedance
● I(LG)/I(3ph) ~ 60%
● Overvoltage < 1.5 pu
Application
● Single phase load supply during LG faults in LV system
● In MV and HV system with BIL (Basic insulation level) below LL voltage

5. Resistance grounded
High resistance Grounding - Similar to ungrounded
● Transient overvoltage - 2.73 pu
● Low fault current
Low resistance Grounding - Similar to Solidly grounded
● Limits LG fault current to 25% of 3ph fault current
● Transient overvoltage < 2.5 pu

6. Reactance grounded
Low reactance:
● LG fault current ratio between 25% to 100%
● Limits the transient overvoltage to 2.3 pu
● Requires fast fault clearing
High reactance
● LG fault current ratio is between 5% to 25%
● High transient overvoltages >2.3 pu

7. Ground fault neutralizer
● Designed to resonate with Stray Capacitance
● LG fault current is small and in phase with LG fault voltage

8. Grounding in Microgrids

9. Different types of DG’s!
Synchronous
generators
High short-circuit
current that may
reach 10 pu of the
rated current
Example: small
hydro, solar
thermal etc.
Asynchronous
generators
The currents
decay rapidly and
may go below the
rated current, as
the asynch.
generator does
not have an
independent
excitation system.
Eg: SCIG, WRIG
Doubly fed asynch.
generator
Usually has a
crowbar scheme for
protection of the
rotor circuit. The
rotor is short
circuited and the
DFIG behaves like
the conventional
asynchronous
generator, with a
large initial current
that quickly decays.
Inverter based
systems
Very low (about 2-5
pu) short circuit
current.
Inverter control
ensures that this
current is quickly
reduced to below
the withstand level
of the power
electronic devices

10. Challenges in Microgrid Protection
Backfeed
Changing System
Hierarchy
During a fault, and it’s
subsequent islanding,
microgrid loses the
grounding of the grid
Fault current depends
upon the grounding of
the sources
Sympathetic
Tripping
False Tripping
The tripping of an
unfaulted region of any
power system based upon
the fluctuations caused
due a fault in another
region
Blinding
Decreasing sensitivity of
protection devices
Typical protection devices
estimate the location of
fault by measuring system
positive impedance
Backfeed disturbs these
estimations, effectively
“blinding” it

11. Transformer topology

12. Transformer topologies impact
LV side grounded
A fault in the MV side
would not be fed with any
zero sequence current by
the source
Leads to sympathetic
tripping
Both sides grounded
A fault on the LV side may
be fed with a large inrush
current by the grid, and
the source
MV side grounded
The transformer acts as a
source of zero sequence
current, disrupting relay
coordination

13. Microgrid Grounding Configurations
Three-Wire Ungrounded Microgrid
● Do not suffer from disruptive LG fault
currents
● Designed to tolerate LG fault-induced
overvoltages
● Locating LG faults in an ungrounded
system is extremely difficult
● Possibility of undetected LG faults evolving
to LL faults
Three-Wire Uni-grounded Microgrid
● MG connected to a three-wire uni-
grounded utility system
● May not suffer from the fault location and
overvoltage issues associated with the
ungrounded configuration (only in grid
connected mode)

14. Four-Wire Multigrounded Microgrid
● Improved voltage transients, safety
● Optimized size of OV protective devices.
● Suffer from ground potential rise, large LG
fault currents, and ground fault protection
coordination issues.
● LG faults must be detected, located, and
isolated as quick as possible
Microgrid Grounding Configurations
Four-Wire Unigrounded Microgrid
● Does not become ungrounded while
islanded
● The neutral conductor continues to provide
grounding support to the microgrid
● Microgrid may experience neutral voltage
rise and large voltage imbalance if the
neutral is not solidly grounded or if the
feeder is long

15. Bibliography
Mohammadi, J., Ajaei, F.B. and Stevens, G., 2018. Grounding the AC Microgrid. IEEE
Transactions on Industry Applications, 55(1), pp.98-105.
IEEE 142-2007 - IEEE Recommended Practice for Grounding of Industrial and
Commercial Power Systems