Basler DECS-400 excitation system

Underexcitation limit (UEL): prevents the generator from entering phase advance operation due to low excitation, resulting in out of step or overheating of the stator end iron core. Users can choose from the built-in five point polyline or custom curve, and support voltage correction function.

Stator current limit (SCL): prevents overcurrent in the stator winding. It has two levels of restrictions: high and low. The high setting allows it to run for a certain period of time, but if it exceeds the time limit, it will be forced to drop to the low setting.

Voltage/Hertz limit: It combines limiting and protection functions to prevent magnetic flux saturation of the generator or main transformer under low frequency or overexcitation conditions.

4. Protection function

The DECS-400 is equipped with over ten independent protection functions, which can serve as a backup for the system's main protection, such as overvoltage/undervoltage, over/under excitation, demagnetization, frequency abnormality, PT disconnection, excitation diode fault monitoring (EDM), etc. Each protection function can be independently enabled and can be associated with a specified relay output.

Power System Stabilizer (PSS)

For large generator sets that require grid connected operation, the DECS-400 can be optionally equipped with a built-in PSS module (Type 1XXX). This PSS complies with the PSS2A model in IEEE 421.5 standard and is a dual input stabilizer based on "acceleration power integration".

1. Working principle

PSS enhances power system damping by suppressing low-frequency oscillations in the range of 0.1 to 5.0 Hz. It abandons the traditional mechanical power signal (which is difficult to measure accurately) and instead uses two signals, generator speed and electrical power, for computation.

Signal processing: After passing through high pass filtering and low-pass filtering, the speed signal is synthesized with the electric power signal (also passing through high pass filtering). Through a simulated "mechanical power" signal, PSS effectively eliminates the torsional vibration component of the shaft system and avoids the need for measuring mechanical power.

Phase compensation and gain: The synthesized signal is passed through a four stage configurable phase lead lag compensation network to compensate for the phase lag introduced by the AVR closed-loop. The compensated signal is passed through the gain stage and output limiting, and finally superimposed onto the voltage reference point of the AVR.

2. Key technical details

Input selection: PSS can choose signal sources based on "speed+power" or only based on "frequency".

Twisted vibration filtering: The software switches (SSW4, SSW5) can enable two independent notch filters (Torsional Filters) to filter out specific torsional vibration modal frequencies of the unit shaft system (by setting Zeta and Wn parameters).

Terminal voltage limitation: When the generator terminal voltage is too high, the terminal voltage limiter of PSS will take priority action, automatically reducing the output upper limit of PSS to prevent overvoltage protection action, reflecting coordination and cooperation with AVR.

Frequency change rate lockout: When the system frequency change rate (ROCOF) exceeds the threshold (such as a large disturbance in the power grid), the PSS output is quickly pulled down and locked for a period of time to prevent PSS from misoperation under non oscillatory conditions.

Engineering debugging and verification based on BESTCOMS

BESTCOMS-DECS400 software is an indispensable debugging tool for DECS-400. This software must be used for setup and verification during the first operation or regular maintenance of the unit.

1. Debugging and verification process

Establish communication: Connect the PC to DECS-400 through the front RS-232 port or remote Ethernet port. It is recommended to record all system parameters (CT/PT ratio, generator rated value, excitation machine time constant, etc.) during the initial debugging stage.

PID tuning: In the "Gain Settings" interface, use the built-in "PID Calculator" to calculate the initial proportional (Kp), integral (Ki), derivative (Kd) gain, and loop gain (Kg) based on the generator's direct axis super transient time constant (T'do) and exciter time constant (Te).

Step response test: In the "Analysis" function, perform RTM step response test. It is recommended to perform a small signal step (such as 5%) in FCR mode initially, observe the response waveform of the magnetic field current, and confirm the stability of the system. Then switch to AVR mode for voltage stepping, adjust PID parameters until the desired response speed and overshoot are achieved.

Limiter verification: sequentially set the constant values of OEL, UEL, and SCL slightly lower than the current operating value to verify the rationality of their action logic and gain parameter (Ki, Kg) settings.

PSS verification: After ensuring that the AVR performance meets the standard, conduct PSS switching tests. Verify whether the PSS provides positive damping torque through step disturbance or small signal frequency response testing.