Reverse power protection 32 (PDOP): how it works

Short answer

Reverse power protection (ANSI 32) measures active power at a point and operates when it flows in the opposite direction to the expected one for a set time. On generators it detects motoring, when the machine stops delivering power and draws it from the grid, which damages the turbine or prime mover. At interconnections it prevents exporting energy when that is not allowed.

What protection 32 (reverse power) is

Function 32 calculates three-phase active power from voltages and currents and compares it with a threshold in a defined direction. When power flows in the direction opposite to normal and exceeds the threshold for the set time, the relay alarms or trips.

The 32 family also includes maximum and minimum forward power (overpower and underpower). This article focuses on reverse power, which is the most common use.

ANSI/IEEE C37.2: device 32, directional power relay.
IEC 61850: logical node PDOP (directional overpower), set in the reverse direction. For underpower there is the node PDUP.
IEC 60617: no symbol is given here because it varies between drawings and manufacturers.

Where it is applied

Synchronous generators: protection against motoring. If the prime mover loses its energy source (steam, water, gas or fuel) with the breaker closed, the generator keeps running as a synchronous motor and draws power from the grid. More context in the synchronous generator and its protections.
Standby and cogeneration plants in parallel with the grid.
Interconnection of self-generation and solar plants that are not allowed to export: 32, or a directional power function, opens or curtails the plant when it detects export.
Industry with its own generation, to separate from the grid when there is unauthorized flow toward the utility, or the other way round, depending on the agreed scheme.
Interconnections between systems where the direction of flow must be controlled.

How it works

The relay calculates:

P = Va·Ia·cos φa + Vb·Ib·cos φb + Vc·Ic·cos φc

or the equivalent expression with positive sequence, depending on the manufacturer. It defines a reference direction (for example, power leaving the generator toward the bus) and operates when power in the opposite direction exceeds the threshold.

Details that matter:

Sensitivity: motoring power is usually a small fraction of rated power, and it depends heavily on the type of prime mover. A steam or hydro turbine can draw very little power (from tenths of a percent to a few percent of rating); a diesel engine, around 25 %, and a single-shaft gas turbine can exceed 50 %, because it drives the compressor. That is why on steam and hydro units 32 is set far below rated power.
Angle accuracy: with such low power and a power factor close to zero, a small angle error in the current transformers changes the sign of the measured power. Some relays allow the angle error to be compensated.
Time delay: one or two definite-time stages. A long stage for the steady condition and, in some schemes, a short stage supervised by the position of the turbine valves.
Sequential tripping: on steam units it is common practice to trip the turbine first and let 32 open the breaker once it confirms reverse power. This prevents the machine from overspeeding on trapped steam.

Setting criteria

Motoring power declared by the prime mover manufacturer or measured in tests. The threshold is set below that value with margin.
Accuracy class and angle error of the CTs and VTs, and their reference in the relay. This is critical with low thresholds. Check the current transformer connections before blaming the setting.
Time delay long enough to ride through power swings and synchronization without tripping.
Sequential tripping and its logic with the turbine, according to the plant philosophy.
At interconnections: the limit and time required by the utility or the interconnection agreement, and coordination with the plant control so that it curtails before the protection trips.

Final values come from the coordination study and the machine manufacturer’s data.

How each manufacturer implements it

Siemens SIPROTEC (7UM and multifunction relays) with DIGSI: reverse power function with stages and supervision by valve position.
SEL (for example SEL-300G, SEL-700G and feeder relays) with AcSELerator QuickSet: directional power elements, per phase or three-phase, with the direction defined in the setting.
Relion: Hitachi Energy (REG670) and ABB (REG615), both with PCM600: directional, reverse and underpower blocks, with adjustable characteristic angle.
GE Vernova (Multilin G60, 889 and others) with EnerVista: sensitive directional power elements with characteristic angle.
MiCOM P34x (Schneider Electric Easergy MiCOM and GE Vernova MiCOM Agile) or Easergy, with Easergy Studio or MiCOM S1 Agile: power stages with direction selection and CT angle compensation in some models.

The most common differences: whether the threshold is in per unit, in primary or secondary watts; how the reference direction is defined; whether power is three-phase or per phase; and whether angle error compensation exists.

Why it trips or blocks

Legitimate trips:

Loss of the prime mover: fuel failure, valves closing, turbine trip.
Sequential tripping after a planned shutdown.
At an interconnection: real export because load dropped and the plant kept generating.

Misoperations or failure to operate:

Reversed polarity of a CT or VT, or wrong reference direction. It is the classic commissioning error.
CT angle error with very low thresholds.
Short time delay that trips during synchronization or swings.
CT or VT ratio entered wrong, which changes the calculated power.
Loss of potential, which leaves the function without a measurement.

The first step is to compare the power measured by the relay with that of another meter at the same point and with a known load. If the signs do not match, the problem is wiring or configuration.

How it is tested

With secondary injection and a test set such as the Omicron CMC 356, voltages and currents are injected with a controlled angle. In general the test verifies:

The threshold in the reverse direction, at several angles.
That it does not operate with power in the normal direction.
The time of each stage and the sequential tripping logic.
Angle compensation, if available.

During commissioning it is completed with a check under real load: the generator delivering power and, if the process allows it, a controlled motoring test. The generator must also have its loss of field protection 40, which is tested in the same campaign.

Did your relay trip or block on this function and it is not clear why? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
IEEE C37.102, Guide for AC Generator Protection.
IEEE C37.95, Guide for Protective Relaying of Utility-Consumer Interconnections.
IEC 61850-7-4, Compatible logical node classes and data object classes.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why did 32 trip while the generator was being synchronized?

Right after closing, if the generator was slightly slower than the grid or with a lagging angle, it absorbs power for an instant. If the 32 time delay is too short, it trips. Check the delay and the practice of synchronizing with the machine slightly faster than the grid.

Why does 32 measure positive power while the generator is motoring?

Almost always it is the polarity or connection of the current or voltage transformers, or the reference direction configured in the relay. Check the power measured by the relay with a known load and the angle between voltage and current.

What is the difference between 32 and 40 on a generator?

32 monitors active power and detects motoring from loss of the prime mover. 40 monitors impedance or reactive power and detects loss of excitation. They are different failures and the generator needs both.