Operating and Reset Characteristic of Protection 81

What the functional logic describes

Clause 4 of IEC 60255-181 describes what a frequency protection does from the moment it receives the signal until it delivers an output. Its Figure 2 is a simplified block diagram with four parts: inputs, measuring element, time characteristic and outputs. The standard requires the manufacturer to provide the block diagram of its specific implementation (clause 4.1).

The inputs are the measured voltages and binary signals. A block, for example, can arrive by contact, by a GOOSE message or from another internal function of the relay (clause 4.3). The standard clarifies that the way the signal is received matters little, as long as it meets the operating requirements.

Operating characteristic

The relationship between operating time and the characteristic quantity is called the characteristic curve. The manufacturer must declare it with an equation (preferred) or a graph (clause 4.4.1.1).

IEC 60255-181 specifies a single form: the independent time characteristic, also called definite time. It is described with two setting values:

• the characteristic quantity setting value, Gs, which is the threshold;
• the operate delay, t_delay, which is an intentional wait.

If there is no intentional delay, the function is called instantaneous.

In 81R the setting can be defined for positive rate only, negative only or both, according to the user, and the manufacturer must declare it. The positive and negative thresholds can differ and be handled with two ROCOF elements.

The time accuracy is guaranteed within the effective range, but the function must operate over the whole operating range (clause 5.2).

Reset characteristic

Clause 4.4.2 says that, in general, there is no intentional delay on reset. The function returns to its idle state when:

• in underfrequency, the frequency rises above Gs plus the reset hysteresis;
• in overfrequency, the frequency falls below Gs minus the hysteresis;
• in ROCOF, the absolute value of df/dt falls below the reset ratio multiplied by the threshold, or below the threshold minus the hysteresis.

The standard defines the two quantities as follows:

• Reset hysteresis: absolute value of the difference between the reset value and the start value.
• Reset ratio: quotient between the reset value and the start value.

In 81R the reset ratio can be accompanied by a minimum absolute hysteresis, useful with low thresholds. In that case, the reset value is the greater of the two. As an example of format, the standard cites a ratio of 98 % with a minimum hysteresis of 40 mHz/s.

For frequency functions only the reset time is declared. It is how long the start signal takes to drop out when the quantity returns to normal. Normally there is no intentional reset time. Figure 6 of the standard distinguishes both times.

Output signals

Clause 4.6 defines two main outputs:

• Start (or pick-up): output of the measuring and threshold elements, with no intentional delay. The manufacturer must specify, as far as needed to apply and test the function, its logic and the associated settings. If the device has no start signal, it must say how to carry out the tests that depend on it.
• Operate (or trip): output after the intentional delay is completed. In an instantaneous element it can coincide with the start.

Other outputs, such as alarms or outputs to other functions, must appear in the logic diagram (4.6.4). They can be contacts, communication messages or internal connections (4.6.1).

The total time has a precise definition (clauses 3.8 and 3.9): the operating time is the sum of the start time and the set operate delay. According to clause 5.6, some devices include the internal measuring time within the delay and not as an addition; the manufacturer must declare how its implementation handles it.

What the block diagram must show

The standard does not stop at asking for a diagram: it says what it must contain. According to clauses 4.1, 4.3 and 4.6.4, the diagram of the manufacturer’s implementation has to show:

• the analog inputs that feed the measurement;
• the binary inputs that influence the function, for example a block, and where they come from (contact, communication message or internal logic);
• the measuring element, the threshold comparator and the timer;
• the start and operate outputs and any additional output, with the way they are generated.

An additional text can clarify the intended use of each signal. If the diagram does not show a binary input that does affect the function, the documentation is incomplete with respect to the standard.

What you should verify

• That the manufacturer’s block diagram shows all the binary inputs and the outputs, including blocks.
• That the relay provides a usable start signal. Without it, timing tests are done on the operate output with zero delay.
• Which reset value your device uses. A reset very close to the threshold can produce intermittent trips on a bus with unstable frequency.
• How the internal measuring time is added to the delay in the specific device.

These points are confirmed with a secondary test. At Tiza Group we perform secondary injection on frequency protections, in Colombia or elsewhere on request. If you need an engineer to review the logic and settings of your relays, see our remote relay configuration.

Related guides

• IEC 60255-181: frequency protection requirements
• Frequency protection 81U/81O: what it is and how it works
• Ranges and accuracy of a frequency protection
• Start and reset time of the 81 protection
• Hysteresis and reset ratio of the 81 protection

References

• IEC 60255-181:2019, clauses 3.8 to 3.13, 4.1, 4.3, 4.4.1, 4.4.2, 4.6 and 5.6, Figures 1 to 6.
• IEC 60255-1, general requirements for measuring relays and protection equipment.
• IEC 61850-7-4, logical nodes PTUF, PTOF and PFRC.

Frequently asked questions