High-impedance differential protection: how it works

What high-impedance differential protection covers

It is one way to build differential protection 87, designed for a specific problem: saturation of the current transformer (CT) during an external fault. On a bus with many bays, an external fault on one circuit makes the whole current of the other bays flow through a single CT, which may saturate. With a simple differential, that saturation appears as a false differential current.

The high-impedance scheme protects buses, transformer windings in restricted earth fault, shunt reactors and large motors against internal faults, with fast tripping and no tripping from saturation on external faults.

• ANSI/IEEE C37.2: 87, with the letter of the equipment (87B, 87N, 87M).
• IEC 61850: logical node PDIF.
• IEC 60617: the differential relay symbol; on drawings it is usually shown with the stabilizing resistor and the varistor.

Where it is applied

Measuring principle

The CTs of every boundary of the zone, all of the same ratio, are connected in parallel at a common point. Across the two conductors of that point sits the relay, which is a voltage element, in series with a stabilizing resistor. A varistor, a non-linear resistor that limits the voltage, is installed in parallel.

External fault. The current enters through one CT and leaves through another; it cancels at the common point and the relay sees almost no voltage. If the CT of the faulted bay saturates completely, its magnetizing impedance collapses and it behaves as its winding resistance plus the lead resistance. The healthy CTs push their current through that path, and the voltage at the common point is limited to that current times that resistance. The relay setting is placed above that voltage, so it does not trip.

Internal fault. All currents flow towards the zone and have no return path other than the relay. Since the relay has a high impedance, the voltage rises above the setting and it trips. The varistor clips the voltage so that it does not reach dangerous values.

That is the reason for its stability: it does not depend on algorithms or saturation detectors, but on how current divides in the circuit.

The characteristic

The characteristic is a voltage, not a slope:

• Voltage setting: higher than the maximum voltage that can appear across the relay with an external fault and a saturated CT. That voltage depends on the maximum external fault, on the CT winding resistance and on the resistance of the longest leads.
• Primary sensitivity: the minimum internal fault current that makes the relay operate. It includes the relay current and the magnetizing current of all the CTs at the voltage setting and, if present, the varistor current. That is why more bays mean less sensitivity.
• CT knee-point voltage: must be clearly higher than the voltage setting, so that the healthy CTs can drive the internal fault current to the relay.

Setting criteria

The general criteria, without project values:

• The voltage setting is calculated with the maximum external fault and the worst saturated CT, with a safety margin.
• The CT knee-point voltage is compared with the setting, as recommended by the relay manufacturer.
• The primary sensitivity is checked against the minimum internal fault, especially for ground faults with impedance-grounded neutrals.
• The varistor is selected according to the maximum internal fault voltage.

IEEE C37.234 describes the application to buses and IEEE C37.110 the CT requirements; IEC 61869-2 defines class PX. CT burden is a free tool that helps estimate the loop resistance.

Interaction with other functions

• CT circuit supervision: an open CT in service leaves a load current with no return path and a voltage appears across the relay. A sensitive supervision element is used that alarms and, on buses, shorts the CT bus wires.
• Check zone: on buses, an overall zone that must confirm the trip is sometimes added.
• Lockout relay 86: the trip goes to every bay of the zone through the 86.
• Breaker failure 50BF: started by the zone trip.

How each manufacturer implements it

• Siemens: high-impedance relays and external resistor and varistor units, plus the percentage differential in SIPROTEC.
• SEL: high-impedance elements in bus and transformer relays, with internal or external resistor depending on the model.
• Hitachi Energy and ABB Relion (PCM600): high-impedance differential function in the 670 and 650 series, with resistor and varistor units.
• GE Vernova Multilin: high-impedance relays and elements inside the multifunction relays.
• Schneider MiCOM: high-impedance functions for restricted earth fault and buses.

IEC 61850 logical node

The function is modeled with PDIF, as the percentage differential is. The node publishes the start (Str) and the operate signal (Op); the trip goes through PTRC and reaches the XCBR of each breaker in the zone by GOOSE or by a physical output. CT circuit supervision is usually published as a separate alarm.

How it is tested

In general, with secondary injection and CT tests:

• The operating voltage of the relay and its time.
• The excitation curve and knee point of each CT, compared with the voltage setting. See CT excitation tests.
• The resistance of the CT windings and of the wiring.
• The polarity of every CT with primary injection.
• The primary sensitivity, injecting on the primary side of each bay.

Typical mistakes

• CT of a different ratio or class in one of the bays.
• Setting calculated with the resistance of the shortest leads, not the longest.
• Sensitivity not checked when new bays are added to an existing bus.
• Missing or undersized varistor.
• Using CTs shared with metering or other protection, which alters the circuit.
• Reversed CT polarity, which appears as an internal fault with the first load current.

Do you need to review the setting of a high-impedance differential, or a trip with no apparent fault? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Bus differential 87B
• Restricted earth fault 87N
• Motor differential 87M
• Busbar protection schemes

References

• IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses: high-impedance bus differential.
• IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes: CT requirements.
• IEC 61869-2, Instrument transformers – Part 2: Additional requirements for current transformers: class PX.
• IEC 60255-187-1, Functional requirements for differential protection.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PDIF node.

Frequently asked questions