Voltage regulating relay 90 (ATCC): tap changer control

What the 90 regulator is

In IEEE C37.2, number 90 is the regulating device: it keeps a quantity (voltage, current, power, speed, frequency or temperature) at a value or between limits. In substations, the 90 par excellence is the automatic voltage regulator of the on-load tap changer (OLTC): the controller that decides when to raise or lower a tap to hold the voltage of the bus or of the load point.

It is not protection in the strict sense, because it does not clear faults. But it lives in the same panel, shares instrument transformers with the protection, and its malfunction has physical consequences: voltages outside limits for customers and premature wear of the tap changer diverter switch. It must not be confused with the generator automatic voltage regulator (AVR), which acts on the excitation.

What the 90 looks after:

• The service voltage, within the band required by the utility or the industrial process.
• The tap changer, by avoiding unnecessary operations and by blocking commands under conditions in which the diverter switch must not operate.

Where it is applied

It does not apply at low voltage: distribution transformers generally have a de-energized tap changer, which is moved with the transformer de-energized.

How it measures

The regulator needs three inputs:

• Voltage on the regulated side, from a voltage transformer (VT). Many regulators measure a single voltage, phase-to-phase or phase-to-neutral.
• Current on the same side, from a current transformer (CT), for line drop compensation, overcurrent blocking and paralleling logic.
• Tap position and motor drive status: whether a change is in progress, whether it has finished, whether it has reached an end position.

Its outputs are the raise and lower commands to the tap changer motor drive. It has no trip output.

The characteristic

It is drawn on a voltage versus time plane:

• Set point: the voltage to be held.
• Bandwidth: a margin above and below the set point. While the voltage is inside, the regulator does nothing.
• Time delay: when the voltage leaves the band, the regulator waits before issuing a command. With a definite time delay the delay is fixed; with an inverse time delay it is shorter the larger the deviation. Many regulators use a second, shorter time for the following steps.
• Step: each tap change moves the voltage by a fixed amount. If the voltage returns to the band, the timer resets.

Line drop compensation (LDC). With the measured current and the R and X of the line up to the load center, the regulator estimates the voltage at that remote point and regulates that voltage instead of the bus voltage. Some regulators offer a simpler variant that raises the set point in proportion to the current.

Blocking. Commands are blocked by:

• undervoltage, when the measurement is lost or the network is faulted;
• overcurrent, so the diverter switch is not operated with high currents;
• end of travel, at the highest or lowest tap;
• motor drive failure, when a change does not finish within the expected time.

On a severe overvoltage many regulators order a lower command with a short delay.

Setting criteria

• Set point: the voltage required by the utility or the process, measured at the point to be controlled.
• Bandwidth: larger than the voltage of one tap step, with margin. If it is narrower, the tap changer hunts between two taps.
• Time delay: long enough to ignore brief network variations and not so long that it leaves the voltage outside limits. With regulators in cascade (transmission, distribution, line) the one closest to the source acts first and the following ones wait longer.
• LDC: R and X of the line to the load center, in the phase reference the manufacturer requires.
• Blocking: undervoltage and overcurrent thresholds according to the diverter switch breaking capacity given by the tap changer manufacturer.

The values come from the network study and from the transformer and tap changer data.

Interaction with other functions

• Paralleling of transformers: two independent regulators in parallel diverge and create circulating current. They are coordinated by master-follower, circulating current or power factor, as explained in parallel transformers with tap changers.
• Differential 87T: each tap changes the transformer ratio and an unbalance differential current appears. The transformer differential 87T is set taking the whole tap range into account.
• Tap changer protection: the flow relay of the tap changer compartment and the mechanical protection 63 and 26 trip the transformer; the regulator does not replace them.
• Trip and lockout 86: if the transformer trips, the regulator stays blocked until reset.
• Local and remote control: the local, remote or automatic selector defines who controls the tap changer, as shown in control levels and the local-remote switch.
• Reactive compensation and plants: capacitor banks, reactors and the control of solar plants also move the voltage. Their times are coordinated so they do not compete.

How each manufacturer implements it

There are dedicated regulators and transformer relays that include the function:

• Dedicated regulators: the Maschinenfabrik Reinhausen TAPCON, the SEL-2414, the Beckwith M-2001 and line regulator controls. We describe them in voltage regulators: TAPCON, SEL-2414 and others.
• Relays and bay controllers: the Hitachi Energy Relion 670 series and other transformer IEDs include automatic tap changer control, with paralleling over communications.

The practical differences are the shape of the inverse time delay, how LDC is entered (R and X, or Z), the reverse power flow modes and how the tap position and motor drive status are read.

IEC 61850 logical node

IEC 61850-7-4 splits the function into several nodes:

• ATCC, automatic tap changer control: the regulator logic, with its raise and lower blocking limits, local or remote mode and raise and lower commands.
• AVCO, voltage control: the node that represents the voltage controller, whatever the control method.
• YLTC, the tap changer as equipment: tap position and change in progress.

Measurements come from the instrument transformer nodes (TVTR and TCTR). In paralleling schemes, the regulators exchange position, current and commands by GOOSE. There is no chain to PTRC, because the 90 does not trip.

How it is tested

In general, with secondary injection and the motor drive isolated, the following are verified:

• The set point, the bandwidth and the reset.
• The definite or inverse time delay and the time of the following steps.
• Undervoltage, overcurrent, end-of-travel and motor drive failure blocking.
• LDC, injecting current at different angles.
• The reading of the tap position and the commands through the control system.
• The paralleling logic, if present.

At commissioning it is completed with the actual motor drive and the network voltage.

Typical mistakes

• Bandwidth narrower than the step: the tap changer hunts between two taps.
• Wrong phase reference in the LDC: the regulator measures a phase-to-phase voltage and the current of another phase without the angle correction the manufacturer requires, and compensates wrongly.
• LDC active with reverse power flow on a network with generation, which raises the voltage when it should lower it.
• Tap position wrongly scaled or wired: the regulator thinks it has reached the end, or does not notice that it has.
• No overcurrent blocking: the diverter switch operates with high currents.
• Regulators in cascade with the same delay, which chase each other.
• Parallel transformers without paralleling logic, with permanent circulating current.

We configure your voltage regulator

Does your tap changer operate too often, or does the voltage leave the band? We review the regulator records and settings remotely, worldwide: set point, bandwidth, times, compensation, blocking and paralleling, on any make. Tell us the make and model through remote relay configuration or contact us.

Related guides

• Parallel transformers with tap changers
• Transformer differential 87T
• Automatic capacitor bank control

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (device 90).
• IEC 60214-1, Tap-changers – Part 1: Performance requirements and test methods.
• IEEE C57.131, Standard Requirements for Tap Changers.
• IEEE C57.15, Standard Requirements, Terminology, and Test Code for Step-Voltage Regulators: line regulators and line drop compensation.
• IEC 61850-7-4, Compatible logical node classes and data object classes: ATCC, AVCO and YLTC nodes.

Frequently asked questions