The 2×135 MW units of the Xinxiang Technical Renovation Project are 440 t/h circulating fluidized bed (CFB) units. The DCS adopts the SIMATIC PCS-7 system from SIEMENS. Most of the inlet and outlet damper actuators for fans on the boiler side and the actuators for the unit bypass system are SIEMENS SIPOS5 actuators, with a total of 20 sets.
These 20 SIPOS5 actuators are intelligent integrated regulating actuators. Both input and output signals are standard 4–20 mA signals. The actuators feature excellent performance and satisfactory operation. However, problems of incorrect valve position display occurred at the interface with DCS.
1 Problems at the Interface with DCS
During the initial commissioning of the actuators, local and remote operation of the actuators was normal, and the valve position feedback was displayed correctly on the DCS screen.With the trial operation of the units, problems gradually emerged. For example, the valve position feedback could be displayed within a small opening range, but the DCS failed to correctly display the feedback over the full stroke.
That is:
The DCS output could control the local actuator over the full stroke.
The valve position feedback displayed on the actuator itself was consistent with the DCS output command.
The feedback current measured by a multimeter was balanced with the command.
When the valve position feedback reached 30%–40% or above, the DCS could not display it correctly.
2 Troubleshooting the Causes of the Interface Problem
Similar problems had occurred on some actuators during commissioning of other units, and were solved by installing photoelectric isolators at the valve position feedback terminals.To identify the cause, the following steps were taken:
2.1
First, the DCS feedback loop was disconnected from the DCS. The output was manually operated from 0% to 50% and then to 100%. The feedback display on the DCS screen still showed a wire break.The actuator feedback was measured with a multimeter, and the result was consistent with the DCS command.This indicates that disconnection of the feedback loop does not affect actuator operation.
2.2
A photoelectric isolator was installed at the actuator valve position feedback.After manual operation, the valve position feedback over the full stroke was displayed correctly on DCS.
2.3
The DCS hardware manual was reviewed.The DCS analog input (AI) module model is 6ES7331-7KF02-0AB0.The input channels are optically isolated from the DCS control system bus and control power supply.The allowable DC voltage between the input terminal and DCS MANA (analog ground) is 2.5 V, and the allowable common-mode DC voltage for input channels is also 2.5 V.
2.4
The actuator manual was reviewed.The negative terminal of the DCS current output signal is not connected to the actuator MANA, while the negative terminal of the actuator current feedback is connected to the actuator MANA.Actual measurements confirmed consistency with the manual.Since the negative terminal of the DCS current output is connected to DCS MANA, a potential difference may exist between the feedback current negative terminal and DCS MANA, which may affect correct DCS sampling.
2.5
The voltage between the actuator feedback terminals and DCS MANA, as well as the voltage between feedback terminals, was measured with a multimeter.The results showed that the voltage between the actuator feedback current and DCS MANA exceeded the allowable range of the module.
2.6
The negative terminal of the DCS valve position command was short‑circuited to the actuator MANA, and no photoelectric isolator was used for valve position feedback.After manual operation, the valve position feedback over the full stroke was displayed correctly on DCS.
From the above tests, it was preliminarily concluded that:Voltage mismatch between the actuator feedback signal and the DCS module input caused the DCS to fail in correctly displaying the valve position feedback.
3 Measurement, Data Analysis and Comparison
To further verify the cause, voltages between the feedback signals of multiple actuators and DCS MANA were measured.Identical measurements were carried out:
after installing photoelectric isolators on the actuator feedback;
after short‑circuiting the actuator command negative terminal to actuator MANA.
A large amount of data was obtained.Analysis and comparison of the data confirmed the above judgment.Table 1 shows the voltage comparison of the low‑pressure bypass actuator before and after installing the photoelectric isolator, and after short‑circuiting the command negative terminal.
4 Conclusions
4.1
In the 2×135 MW units of the Xinxiang Technical Renovation Project, the SIPOS5 actuators could display valve position feedback within a small opening range, but the DCS could not correctly display the feedback over the full stroke.
Through measurement of the voltage between feedback signals of multiple actuators and DCS MANA, tests with photoelectric isolators, and tests with short‑circuiting the command negative terminal to the actuator analog ground, together with data analysis and comparison, the root cause was identified:Voltage mismatch between the actuator feedback signal and the DCS analog input module caused incorrect valve position display on DCS.
4.2
Through analysis and comparison, the final solution adopted was to short‑circuit the negative terminal of the actuator command to the actuator analog ground.
This method:
Saved equipment investment for 20 photoelectric isolators;
Avoided secondary wiring for installing isolators;
Accelerated the unit trial operation.
4.3
Since the implementation of this improved measure, all actuators have been operating well, and the above problems have not recurred.
