Busbars, where high short-circuit currents flow, can saturate current transformers during external

short-circuits, possibly undermining protection system efficiency. It's a condition that you must
to considerate when still parametrizing your busbar differential protection.

Poor busbar differential protection operation can disconnect transmission lines and transformers,
jeopardizing system stability and causing power interruptions for consumers.

But in this brief guide, I'm not here to delve into theoretical concepts from textbooks related to
busbar differential protection. Instead, let's embark on a practical journey exploring the B90
busbar protection.

A Practical Example: Rock On!

For those seeking practical insights beyond theory, consider applying the GE B90 busbar
protection IED.

The differential protection employed by the B90 includes two slopes and two inflection points,
as depicted in the following figure.

Figure 1 - Differential Busbar Curve B90 (GE) [1]

The directional unit in the IED algorithm is active within the low differential current region
(Region 1) and operates continuously. In the high differential current region (Region 2), where
CT saturation is detected, the B90 IED also operates.

Pretty cool, don't you think?

The principle of directional detection is relative, meaning there is no need for polarization by a
voltage signal.

The criteria are presented as follows:

1. I. When all measured currents flow in the same direction, the fault is declared as internal.
2. II. When at least one current flows in the opposite direction to the sum of the other
currents considered, the fault is deemed external.

The directional algorithm is implemented in two stages:

The directional algorithm is implemented in two stages:

The initial assessment detects the fault current by comparing magnitudes, establishing it as the
reference current. This reference current isn't regarded as a load current since, in many cases, it
flows out of the busbar due to loading. A secondary evaluation is conducted using a fraction of
the restraining current.

The highest-magnitude incoming current in the protected zone is used as the restraining signal.

This is essential information, especially for those analyzing the function's operability using short-
circuit simulation software.

I'll repeat for clarity:

"The highest-magnitude incoming current in the protected zone is used as the restraining signal.

If CT saturation is detected, the B90 can apply algorithms to test the value and direction of
currents. If CT saturation is not detected, the principle of differential protection alone can cause
the element to operate.

The saturation detector is an integral part of the busbar differential element. It has no specific
settings but uses some of the parameters configured in the Bus Zone Differential settings.

Alright, I've got this now... But how about adjustments during field
parametrization?

The restricted busbar differential function features a dual-slope operational characteristic (see the
figure below), operating in conjunction with saturation detection and directional comparison
principles.
Figure 2 - Differential Busbar Curve B90 (GE) [1]

The B90 busbar differential protection has, let's say, four main settings that study engineers need
to accurately estimate through calculations and analysis.
Figure 2 - Differential Busbar 87B (B90)

Pickup: This setting defines the minimum differential current required for the operation of the
differential element.

The Pickup value should be adjusted with the aim of preventing undesired tripping due to
differential currents caused by measurement errors when the load is very low.

The current is defined as the highest value of the adjusted primary current, considering all the
bays connected to the busbar.

The Pickup value should be higher than the maximum load current and lower than the short-
circuit current.

To ensure a safety margin in the setting, the following inequality will be adopted:
where IL is equal to the maximum busbar load current, and IF is the minimum short-circuit
current in the busbar.

The coolest adjustment comes next...

Low Slope: The first slope of the curve. It covers errors proportional to the current, mainly due
to CT (Current Transformer) transformation errors.

This setting defines the percentage polarization for restraining currents, from zero to the lowest
pickup point. This configuration determines the relay's sensitivity to low current internal faults.

The chosen value needs to be high enough to accommodate the spurious differential current
resulting from CTs operating inaccurately in their linear mode, i.e., under load conditions and
during distant external faults.

A practical calculation equation can be applied, with IDiff representing the differential and
restraining current for minimum short-circuit scenarios and maximum CT reading errors.

Low Bpnt: transition point to the first slope of the curve provided to specify the linear operating
limit of CTs under the most unfavorable conditions, such as high residual magnetism in magnetic
cores or multiple automatic reclosure operations.

The methodology adopted by the manual considers the maximum current related to the worst-
case residual magnetism (assuming only symmetrical saturation will occur) to be equal to.
where (1-k) corresponds to the residual magnetism component, and Vsat is the saturation voltage
of each of the CTs. The voltage Vsat is the saturation voltage of the CT corresponding to that
bay.

In per unit values (input for the IED parameter), you should calculate this parameter for all the
CTs of the bay and use the following inequality to adjust the value.

It's easy, there's no mistake! For example, if you analyze it with 80% residual magnetism, k =
0.20.

This calculation is performed for all the CTs connected to the bays of the bus protection zones.
These values must be converted into primary values and subsequently into per unit (pu) values.

Remember that the L90 bus protection considers the base current as the highest value among the
primary current ratings of all CTs connected to the bays.

High Slope: curve slope 2. Lower or higher values can be configured to ensure safety and
reliability.

This setting typically receives standard values, and its adjustment validation depends on the
analysis through routines and curve plotting using algorithms embedded in MATLAB or Python.
Keep calm, we are on the last adjustment!

High Bpnt: transition point between the first and second slope of the curve provided to specify
the operating limits of the CTs without significant saturation. It provides greater stability in the
range of higher currents that could potentially lead the CTs to saturation.

As an external fault can occur in any of the connected circuits, threatening the saturation of any
of the CTs, this parameter must be defined as the limit of linear operation that disregards both
residual magnetism and the effect of the DC component.

Here, the same reasoning as the Low Bpnt setting is used, but Ipumax is calculated for the
condition of (1-k) equal to 1 (no residual magnetism).

The criteria assigned for adjusting this parameter is described in the B90 IED manual and states
that:

Because they have the lowest primary current, ensuring operation without saturation, CTs with a
lower Ipumax value are more exposed to possible saturation.

During an external fault in the circuit, these current transformers carry the fault current
contributed by potentially all the remaining circuits.

The fault current is greater than any contributing current, and therefore, the current from these
CTs becomes the limiting signal for the differentially polarized characteristic for external faults.

Phew, we've reached the end!