IEEE Power & Energy Magazine - May/June 2021 - 26

The latter can be achieved by increasing their power rating or by applying extensive cooling of their
switching devices. Both options lead to high investment costs.
2) Replace the existing passive protective devices with
numerical protective relays, which provide advanced
protection functionalities, and deploy adaptive protection based on the microgrid state.

island code for PE-interfaced units. Therefore, both the
battery energy storage and wind turbine are disconnected
almost instantly, before the fault is cleared. After the fault
clearance, the diesel generator remains the only operating
unit, which cannot take over the total load. As a result, the
frequency drops [Figure 5(b)], causing load shedding or
even blackout. This example illustrates the importance of a
coordinated response of microgrid sources under fault conditions for operation security.

Directional Overcurrent Relays

Protection Schemes for Microgrids

120
110
100
90
80
70
60
50
40
30
20
10
0
1.8

State-of-the-art industrial relays can distinguish the flow
direction of fault currents using directional overcurrent elements. These elements determine the voltage and current
phasors and compare their angles to decide if the fault occurs
forward or backward, serving as a reliable solution against
sympathetic tripping. Directional overcurrent relay elements
can be used to detect the direction of positive, negative, or
zero-sequence currents.

Wind Turbine Voltage
Fault Ride-Through Curve
Diesel and Battery Energy Storage Voltage

51
Frequency

50.5
50
Frequency (Hz)

Voltage (%)

As demonstrated in Table 1, there are mainly two options to
deal with low short circuit currents in islanded microgrids.
1) Increase the short circuit current levels. This can be
done by installing a source of high short circuit current, e.g., a flywheel, to satisfy the operating settings
of fuses and overcurrent relays or by increasing
the fault current capability of PE-interfaced units.

49.5
49

Fault

48.5
48
47.5

Frequency
Collapse
Fault
Clearance

DER
Disconnection

47
2

2.2

2.4
Time (s)

2.6

2.8

1.8

(a)

2.2

2.4
2.6
Time (s)

2.8

(b)

figure 5. (a) Voltage sags for operating units during a three-phase short circuit and Greek noninterconnected island systems fault ride-through requirements and (b) the microgrid frequency response.

table 1. A comparison of the approaches to microgrid protection.
Approach

Advantages

Disadvantages

* Fewer modifications required in the
protection system
* Experience since flywheel and
synchronous condensers have already
been used for many years

* Increased investment costs
* Reduced flexibility on future
microgrid upgrades

* Lower investment costs
* Increased flexibility (configurable
protection devices)

* Reliability concerns
* Higher complexity
* Requires a retrofit of the existing
protection schemes

Increase short circuit current levels
* Install source of high short circuit
current (e.g., flywheel)
* Oversize PE generation

Advanced protection schemes
* Deploy digital relays in the system
* Introduce adaptive protection
techniques

ieee power & energy magazine

may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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