Abstract: The XST module is a drive module manufactured by Siemens specifically for controlling control valves. It has advantages such as fast action, precise control, comprehensive protection functions, and the ability to achieve contactless bidirectional smooth adjustment. It simplifies the functional design and cost of the electric part of the control valve. Therefore, it has been widely used in closed-loop control circuits. This paper analyzes in detail the circuit working principle of each unit of the XST module and its application shortcomings, which has certain guiding reference value for closed-loop regulation and hardware maintenance of closed-loop circuits.
Keywords: XST module; regulating valve; closed-loop control circuit
Chinese Library Classification Number: TH Document Identifier Code: Article Number:
The principle and debugging of the closed loop module
HOU Yao, Ling Nong, Su ben-xin, Zhu Feng
(China National Neuclear Corporation Jiang Su Neuclear Power Corporation Jiang Su Neuclear Power Corporation, Lian Yun Gang Of Jiang SU 222042, China)
Abstract: The XST module of Siemens is specially used as the drivinge module of regulating valves. The advantages of XST module contain fast action, accurate control, complete protection function, the function of realizing contactless two-way smooth adjustment etc. The XST module has contracted the regulating valve electric part of the function design and cost. Therefore, The XST module has been widely used in the closed loop control circuit. This paper will analyze circuit working principle and the insufficiency of each unit of XST module, which have some certain reference significance to guide the hardware maintenance of closed loop control and the closed loop circuit.
Keywords : XST module regulating valves closed loop control circuit
1 Introduction
In the closed-loop drive circuit of Tianwan Nuclear Power Plant, the control drive module is centrally installed in the XST cabinet, which can centrally control regulating valves with power of 2.2KW, 5.5KW and 7.5KW. Its main circuit, built with thyristor elements, has the advantages of fast and accurate action, full protection functions, and the ability to achieve contactless bidirectional smooth adjustment. In particular, it simplifies the functional design and cost of the electric part of the regulating valve. Therefore, it has been widely used in closed-loop control circuits.
As the core component of the closed-loop drive circuit, understanding the working principle of the drive module and its characteristics in conjunction with the valve is of certain guiding significance for the closed-loop regulation and hardware maintenance of the closed-loop circuit.
2. Working principle of the drive module
2.1 Working principle of the main circuit
The main circuit consists of two parts: the main operating circuit and the protection circuit of the main circuit.
2.1.1 Working principle of the main circuit
Figure 1 Main working circuit of the driving module
The main function of the main working circuit is to control the opening, de-energizing, and closing directions of the regulating valve by changing the phase sequence of the power supply without contact using thyristors. The main working circuit consists of four bidirectional thyristors, which are used in pairs in phases A and C of the three-phase AC transmission line (A, B, C) for the asynchronous motor, as shown in Figure 1.
When the motor rotates forward, thyristors A and D are simultaneously turned on under the triggering of the gate circuit. The correspondence between the three-phase power supply and the three-phase phase sequence of the asynchronous motor stator is: L1(A)—U, L2(B)—V, L3(C)—W. When the motor rotates in reverse, thyristors B and C are turned on under the triggering of the gate circuit. At this time, the correspondence between the three-phase power supply and the three-phase phase sequence of the asynchronous motor stator is: L1(A)—W, L2(B)—V, L3(C)—U. Therefore, the forward and reverse rotation control of the asynchronous motor is achieved by exchanging the phase sequence of phases A and C of the three-phase power supply, that is, bidirectional control of the regulating valve is achieved. When all gate trigger circuits have no output, the main working circuit will rely on the negative voltage difference generated by the power supply on the thyristors to achieve natural current interruption. The correspondence between the output waveform of the trigger circuit and the output waveform of the thyristors is shown in Figure 2.
Figure 2. Correspondence between the output waveform of the trigger circuit and the output waveform of the thyristor
It should be noted that the width of the trigger pulses of thyristors A, D and B, C is fixed, and the length of the trigger time is controlled by external logic. Thyristors A, D and B, C are mutually interlocked in the trigger circuit. That is, if thyristor A and D are triggered, the trigger circuit of thyristor B and C is interlocked. Only in this way can the coordination of forward and reverse control of asynchronous motor be achieved.
2.1.2 Principle of Thyristor Protection Circuit
When the valve reaches the target position, the control system interrupts the control command, de-energizes phases A and C of the asynchronous motor power supply, and simultaneously applies a DC electrical brake to the motor (at this time, the thyristors in phases A and C of the main circuit are used as rectifier diodes, and the phase A and C circuits act as rectifier circuits to generate the DC voltage required for DC braking) (the electrical brake duration depends on the brake logic b and the setting of S7) and applies a mechanical brake to the valve, enabling the valve to stop quickly and thus achieving precise control. Since the motor windings are inductive, at the moment of brake application, part of the motor's kinetic energy is converted into heat energy due to friction during the brake application process, while the other part is stored in the coil windings. This will cause a sudden and sharp increase in the coil current. To prevent this current from reverse-current breakdown of the thyristors, an RC circuit is used as the thyristor protection circuit, as shown in Figure 3.
Figure 3. Thyristor RC protection circuit
From a voltage perspective, the R and C values of the RC circuit in Figure 2.3 are designed based on the maximum possible current in the stator winding to limit the large current in the stator winding and protect the thyristors.
2.2 Working principle of the trigger circuit
The trigger circuit consists of a timing circuit, a pulse generation circuit, and an internal interlock circuit. The pulse generation circuit defines the waveform and width of the trigger pulse, and the timing circuit defines the trigger duration based on the control commands given by the external logic (TXP). The internal interlock circuit triggers the thyristors in response to external and reverse commands, while simultaneously blocking trigger circuits that should not be triggered. The brake-holding logic is also interlocked with the trigger circuit to correctly determine whether the trigger circuit is operating, thus enabling timely brake application. The schematic diagram of the trigger circuit is shown in Figure 4.
Figure 4. Trigger circuit schematic diagram
2.3 Working principle of the control circuit
The control circuit mainly performs the following two functions: firstly, it can realize the forward and reverse control of the motor according to the external logic (TXP); secondly, the forward and reverse control circuits of the motor should be mutually interlocked. The schematic diagram of the control circuit is shown in Figure 5.
Figure 5. Schematic diagram of forward and reverse rotation control circuit
2.5 Working principle of XST cabinet protection circuit
The protection circuits in the XST cabinet are distributed throughout the internal electrical circuits. The ultimate manifestation of these protection circuits is the tripping of the upstream switch or the tripping of the branch switch powering the drive module. The branch switch's inward protection functions include short-circuit, grounding, and overload protection for the main operating circuit; fault monitoring of signal voltage; motor temperature (PTC: thermistor) monitoring; voltage monitoring of the pulse generation circuit; and detection of the brake output signal and trigger pulse. When a fault occurs, the fault signal triggers a photoelectric trigger, which then triggers the fast trip coil of the XST cabinet module power supply, cutting off the power supply to the module.
2.5.1 Short circuit, grounding, and overload protection of the main circuit
When detection unit a detects a short circuit, grounding, or overload in the main circuit, the detection unit will trigger the photoelectric switch, which in turn triggers the trip coil of the module power supply to cut off the power supply to the module, as shown in Figure 6.
Figure 6. Schematic diagram of short circuit, grounding, and overload protection of the main circuit.
2.5.2 Working Principle of Signal Voltage Fault Monitoring
The working principle of signal voltage fault monitoring is similar to that of short circuit, grounding and overload protection. When a signal voltage fault signal occurs, the detection unit will trigger the photoelectric switch, and then the photoelectric switch will trigger the trip coil of the module power supply to cut off the power supply to the module.
2.5.3 Motor Temperature Protection
Figure 7. Valve temperature protection principle diagram
When special reasons (such as motor operating with a single phase loss, or valve jamming due to high medium concentration, causing the motor's operating current to exceed its rated current but be less than the overcurrent detection device's operating current, and running for an extended period) lead to severe motor overheating, thermal protection is needed to prevent thermal damage. The working principle of motor thermal protection is as follows: A thermistor (PTC) is installed in the motor's temperature-sensitive area. If an abnormal motor temperature occurs, the thermistor's resistance will increase rapidly with rising temperature. When the resistance reaches a certain value (greater than or equal to 2.7 kΩ), the temperature monitoring circuit will be considered an open circuit. The "open circuit trip unit" will then generate a fault signal. This fault signal serves two purposes: firstly, it triggers the trip coil of the module's power switch, de-energizing the module; secondly, it can be used as a temperature alarm signal for the host computer. The valve temperature protection principle diagram is shown in Figure 7.
2.5.2 Three-phase voltage imbalance detection circuit for power supply
A three-phase unbalanced relay is connected in parallel at the three-phase power input terminal of the XST cabinet. In order to protect the motor of the drive module and regulating valve from damage when the three-phase grid voltage is unbalanced, when there is an imbalance in the three-phase grid voltage, the three-phase unbalanced relay will be activated. This will cause one normally closed contact of the unbalanced relay between the signal voltage of the drive module and the signal fault monitoring relay (when the module is working normally, the coil is energized and the normally closed contact is in the open state). The fault detection unit of the main circuit will detect that there is a fault signal in the trigger circuit unit, and finally trigger the trip coil of the module input power supply through the photoelectric trigger to cut off the power supply to the module.
3. Debugging of the drive module
3.1 Selection of Brake Time (S7 Selector)
Most control valves on site do not have mechanical brake devices, therefore, setting the electrical DC brake time is crucial for the rapid and stable control of the valve. The principle of the electrical brake is as follows: When the external controller stops sending control pulses to the XST module, the A and C phase thyristor circuits in the drive module become rectifier circuits. A DC pulsating voltage is applied to the stator winding of the control valve's asynchronous motor. The duration of this DC voltage application is selected by the S7 selector in the drive module. This voltage generates a DC pulsating magnetic field in the air gap between the stator and rotor windings. If the rotor is rotating, a DC pulsating current is generated in the rotor winding. This DC pulsating current causes the rotating rotor winding to exert a reverse force in the pulsating magnetic field until the rotor winding stops rotating, i.e., the control valve is braked. If the electrical brake time is set too long, when the control valve needs to be adjusted in the opposite direction to its original opening, a brake effect will also occur, hindering the closed-loop control. After repeated tests on the valves on site, the optimal braking time was finally determined to be 40ms, that is, the time selector S7 selects mode 4.
3.2 Selection of Alarm and Protection Signals (S1 Selector)
The S1 selector is a selector for alarm and protection signals of the module. It has four sets of jumpers. The first set selects the temperature protection signal for the regulating valve motor; the second set selects the alarm and trip signal for the module voltage fault; the third set selects the interlock signal for the voltage fault signal on the internal pulse circuit of the drive module, meaning that when a voltage fault signal occurs in the drive module, it will trip regardless of whether its internal pulse circuit is working properly; the fourth set selects the interlock signal for the voltage fault signal on the internal pulse circuit of the drive module, meaning that when there is a voltage fault in the XST cabinet or a fault in the internal pulse circuit, the power auxiliary contact of the module will activate, and the module power protection will trip. During the initial commissioning phase, to meet the actual needs of protecting the regulating valve motor and drive module, the first, second, and third sets of contacts of S1 are closed. For regulating valves without temperature protection, the temperature protection terminal is short-circuited to reduce the resistance between the terminals and avoid tripping.
3.3 Deactivation of trip protection function
Towards the end of the commissioning phase, it was discovered that many drive modules frequently adjusted valves during operation. Analysis revealed that this was due to factors such as the high concentration of the medium being regulated by the valve or poor valve starting characteristics, leading to excessive starting current in the valve motor and triggering the protection trip. Once the valve returned to normal operation, the current returned to normal. To address this issue, we disabled the power trip protection function for some critical valves (those requiring continuous operation).
4. Summary of problems encountered during debugging
4.1 The power switch cannot be closed.
(1) Check the temperature protection resistor of the valve motor. Disconnect the wiring of the thermistor (PTC) on the power cable terminal block of the XST cabinet and measure whether its value is less than 1.65. If the resistance value exceeds this range, it may be due to poor contact of the thermistor wiring or the thermistor being burned out. If some valves do not have temperature protection, the two terminals of the thermistor wiring on the power cable output terminal block can be shorted with a jumper wire.
(2) Check the V-phase insulation of the stator winding of the valve motor. Since the B-phase of the motor input power supply is directly connected to the V-phase of the motor, if the V-phase (power supply B-phase) of the stator winding of the motor is grounded or its insulation is damaged, it will cause an overcurrent phenomenon, which will cause the power switch to trip.
(3) Check whether the three-phase unbalance relay on the three-phase AC power input side of the XST cabinet has an alarm. If an alarm occurs, it means that the power grid has experienced a three-phase imbalance phenomenon, and the three-phase unbalance relay needs to be reset.
(4) If the thyristors between different phase power supplies are broken down, resulting in a short circuit between A and C, the circuit cannot be closed.
4.2 The power supply to the branch switch is normal, but the power trips when the module starts.
(1) Check the insulation of the U and W (power supply A and C phases) winding coils of the valve motor.
(2) Check if the valve is stuck.
4.3 Uneven output voltage of the drive module
If there are no alarms after the drive module is powered on, and the power switch does not trip, but the output power voltage of the module is 398V, 198V, and 198V respectively after the module starts, this indicates that one of the thyristors A and C is short-circuited, and the module needs to be replaced.
4.4 Reasons and solutions for all modules having no power in the XST cabinet
The cause of this situation is the activation of the unbalanced relay in the XST cabinet. Resetting the unbalanced relay will resolve the issue.
4.5 Main Transformer Switching
During the switching of the main transformers of Units 1 and 2, a sudden and rapid voltage transition caused a low voltage situation on the bus, resulting in an overcurrent trip of the drive module and the closed-loop circuit deactivating. Initially, the proposed solution was to add a time-delay relay to the trip circuit to avoid the overcurrent period during switching. However, considering that the existing time-delay relays had not undergone nuclear-level certification, Siemens modified the overcurrent protection function in the ROM inside the drive module, increasing the overcurrent protection time and resolving the issue of the closed-loop circuit deactivating during main transformer switching.
5. Conclusion
Following on-site commissioning of the drive modules within the XST cabinets at the Tianwan Nuclear Power Plant, all XST cabinets are now operating normally and stably. The common problems and solutions encountered during commissioning provide valuable reference for the commissioning and maintenance of the unit's closed-loop regulation and closed-loop circuit drive amplification units.
6 References
1. Leng Zengqiang, Fundamentals of Power Electronics, Southeast University Press, 1997, 5
2 Wu Tianming, Xie Xiaozhu, Peng Bin, *MATLAB Power System Design and Analysis*, National Defense Electric Power Press, 2004, 1
