ABB UNITOL 1020 Compact Digital Excitation Regulator AVR Replacement and Troubleshooting



ABB UNITOL 1020 Compact Digital Excitation Regulator AVR Replacement and Troubleshooting

In the operation and maintenance practice of power plants and industrial self owned power plants, the reliability of the excitation system directly determines the voltage stability, reactive power distribution capability, and fault ride through level of the entire unit of synchronous generators. Similar to the well-known scenarios of "Woodward shutdown module replacement" or "Honeywell safety system troubleshooting" among engineers, a large number of old units are still using analog or early digital automatic voltage regulators (AVRs). These devices face common challenges such as aging and drift of components, shortage of spare parts, and lack of fault diagnosis methods. When the excitation system experiences "no-load voltage instability" or "reactive power oscillation after grid connection", without complete diagnostic data support, on-site investigation often relies on extensive methods such as "changing boards and testing parameters", which is time-consuming and carries the risk of misjudgment.

As a compact digital AVR designed for small and medium-sized synchronous motors (with a maximum excitation current of 20A), ABB UNITOL 1020's core value lies not only in replacing the old phase controlled thyristor scheme with IGBT chopper technology, but also in integrating complete fault recording, event recording, and limiter status visualization functions into a device weighing only 3.8kg. For engineers planning to replace outdated excitation systems, understanding the troubleshooting logic of UNITROL 1020 is essentially establishing a "data-driven" debugging methodology.

Key judgment before replacement: Should your old system be upgraded?

Before deciding to replace the existing AVR with UNITOL 1020, engineers need to clarify several hard boundary conditions. The continuous excitation current of UNITOL 1020 is 20A at 55 ° C and the derating is 10A at 70 ° C. If the original system excitation current is in the range of 25A to 40A for a long time, the combination scheme of UNITOL 1020 with UNITOL 1000-PM40 power module should be considered instead of direct single machine replacement.

The compatibility of power supply methods is the first "pit" in replacement engineering. The power part of UNITOL 1020 supports AC (0-250V AC, single-phase or three-phase, 40-600Hz) or DC (0-300V DC) power supply, which means it can adapt to both self shunt supply scenarios and old systems powered by permanent magnet generators (PMGs) or auxiliary windings. However, special attention should be paid: when using self parallel excitation or auxiliary power supply, an excitation transformer must be configured, and the transformer capacity must not exceed 10kVA; If a DC battery is used for power supply, a current limiting resistor must be connected in series to limit the average surge current within 10ms to below 200A. Otherwise, the charging shock of the DC capacitor inside the equipment may directly damage the power device.

The phase compensation of the voltage measurement circuit is the most easily overlooked link in the replacement process. If the old system is equipped with a step-up transformer (usually Yd1 or Yd11 wiring) between the generator and the grid, there is a fixed 30 ° phase difference between the measured voltage UNET and UM. UNITROL 1020 provides the "Angle Offset" parameter (± 180 ° adjustable) in the software, which can directly compensate for this phase difference. But for the Synchrocheck function, if the compensation setting is incorrect, a huge surge current will be generated at the moment of grid connection. Therefore, after the replacement is completed, it is necessary to confirm through the Sync Diagram of CMT1000 that the synchronization table pointer is at the "12 o'clock" position and the phase difference is 0 °.

The core logic of troubleshooting: from "symptoms" to "data link"

The fault diagnosis of UNITROL 1020 should not start from guessing, but from real-time data of panel LED and CMT1000. The three LEDs on the front panel of the device form the first layer of diagnostic interface: the green LED (D800) is always on, indicating that the DSP and MCU have been powered on, and flashing indicates that the target software is running; The yellow LED (D802) lights up to indicate that the excitation has been activated (Excitation ON), and flashing indicates that the limiter is in operation; The constant red LED (D804) indicates an alarm or trip, while flashing indicates specific issues such as startup failure, parameter loading failure, or excitation output being locked.

Scenario 1: No pressure building after unit startup

This is the most common malfunction after replacement. The troubleshooting path should follow the order of "power supply → circuit → setting". Firstly, confirm that both the auxiliary power supply UAUX (terminals 11/13/15) and the power supply UPWR (terminals 12/14/16) are supplying power normally, and check if the corresponding protective circuit breaker has tripped. Secondly, check if the Field Circuit Breaker (FCB) is closed. If the FCB is not closed, the excitation current cannot form a circuit. Finally, check the "Field Flashing" parameter: if Off Level is set to 0%, the excitation function is disabled and the unit cannot automatically build voltage when the residual voltage is insufficient. It should be noted that the current design value of the excitation flicker circuit should be 10% to 15% of the no-load excitation current. If it is too high, it will cause overvoltage, and if it is too low, it will not be able to establish the initial voltage.

Scenario 2: Voltage instability during no-load operation

If the unit operates without load at rated speed and there are periodic or random fluctuations in the terminal voltage, the UM, Ie, and PWM signals should be captured first through the CMT1000 oscilloscope. If the PWM output fluctuates within a stable range but UM oscillates with it, it usually indicates a PID parameter problem: a high proportional gain Vp can cause overshoot oscillation, and a short integration time Ta can cause integration saturation. If there are irregular jumps in the PWM output itself, it is necessary to check the measurement circuit: loose wiring of the voltage transformer (PT), poor terminal contact, or external interference may all cause measurement distortion. In addition, the "Droop" parameter Kq of UNITROL 1020 should not have any effect when the machine is unloaded. If Kq is mistakenly set to a large negative value, it may also cause voltage fluctuations.

Scenario 3: Reactive power oscillations or reversals after grid connection

When the unit is connected to the power grid and experiences periodic fluctuations in reactive power, or even "reverse regulation" (increasing excitation current actually leads to a decrease in reactive power), the primary suspect is the polarity of the current measurement circuit. UNITROL 1020 measures the generator current through the IM2 terminal (terminal 1/2). If the P1/P2 direction of the CT is reversed or the S1/S2 polarity is reversed, the reactive power direction calculated by the regulator will be opposite to the actual direction, causing negative feedback to become positive feedback. Troubleshooting method: Observe the PQ Diagram of CMT1000 in grid connected state. If the actual output reactive power of the generator is positive (delayed phase), but the working point on the PQ diagram falls in the negative Q region, it can be confirmed that the CT polarity is incorrect.

Scenario 4: Frequent actions of the limiter result in the inability to adjust the operating point

If the operator attempts to increase the excitation current but the reactive power of the unit no longer increases, the limiter indicator on the panel should be checked. The UNITROL 1020 is equipped with multiple layers of protection, including V/Hz limit, Ie maximum/minimum limit, PQ limit, UM maximum/minimum limit, etc. The "Combined Limit" status code of CMT1000 can accurately locate the currently activated limiter. For example, a status code of 4 indicates the action of the maximum excitation current limiter, and a status code of 5 indicates the action of the maximum terminal voltage limiter. When replacing the old system, special attention should be paid to the fact that the limit curve of the old system may be set through hardware potentiometers, and the limit values of UNITOL 1020 need to be configured point by point in the software (such as the Q limit values corresponding to the 5 power points of PQ limit). If the parameter settings are improper, the limiter may malfunction near the normal operating point.

From 'usable' to 'easy to use': the deep diagnostic value of CMT1000

The CMT1000 software that comes with the UNITROL 1020 is worth much more than just parameter configuration. For engineers in the "search solution" stage, the following three functions are key to troubleshooting hidden faults.

The transient recorder can capture the voltage difference and closing command of the last three grid connected processes. If the impulse current is too high during grid connection, comparing the "lead angle" with the actual closing time can determine whether the mechanical closing time of the circuit breaker matches the "Total CB Closing Time" parameter. If the actual closing time is more than 20ms slower than the set value, the phase angle difference at the moment of closing will significantly increase.

The Event Monitor records all state changes that occur within the device (mode switching, limiter start stop, alarm triggering, parameter modification) in the form of timestamps. When the fault is sporadic or intermittent, event recording is the only means of restoring the fault sequence. For example, if the unit suddenly trips during operation, by examining the last few events before the trip, it is possible to quickly distinguish whether it is an "external trip command" or an "internal protection action".

The data logger supports recording up to 12 signals at a sampling rate of 10ms (7 fixed signals+5 customizable), with a sampling depth of up to 2000 points per signal. For scenarios that require dynamic performance verification after replacement, data recording can be triggered at the moment of grid connection, and the response curves of PWM, Ie, UM, and P can be replayed and analyzed afterwards to quantitatively evaluate whether the PID parameters have reached the optimal level.

Advanced configuration: Dual channel and power system stabilizer

For power plants with extremely high reliability requirements, UNITOL 1020 supports the construction of a Double Channel redundant system through the CAN bus. Two channels are configured as Main and Redundancy respectively, exchanging set values and status information through the CAN bus. When the main channel triggers a trip or communication failure, the backup channel can take over control within milliseconds. It should be noted that the dual channel function has compatibility limitations with the UNITOL 1000-PM40 power module: the CAN communication priority of PM40 is higher than that of dual channel communication. If there are both PM40 and dual channel slave stations on the same CAN bus, DCH data transmission will be automatically blocked. Therefore, if the system requires both 40A excitation current and dual channel redundancy, a "dual AVR+dual PM40" architecture must be adopted, or state interaction between channels must be achieved through hard wired I/O.

For units connected to the power grid, power system stabilizers (PSS) are a key additional function for suppressing low-frequency oscillations. The PSS of UNITROL 1020 complies with the IEEE 2A/2B standard. It uses two forward channels, frequency injection and power injection, and after DC isolation, lead lag correction, and amplitude limiting, it is superimposed on the output of the voltage regulator. When debugging PSS, CMT1000 provides 16 test points (TP1-TP16) and the "Common Step" function, which can simultaneously apply the same set value step to multiple parallel units to test the damping effect of PSS on site.