Modernization of the electrostatic filter management system

DESCRIPTION OF THE PROBLEM AND THE CLIENT

The filtration of solid particles in the exhaust gases of solid fossil fuel power plants is the most crucial factor in meeting the high ecological criteria prevailing today in most countries. This is especially important for power plants that use fuels of lower energy value and a large proportion of ash and other solid particles during most of their working life, which is a group that certainly includes Kosovo’s “A” thermal power plant. The elimination of solid particles from flue gases and their disposal is one of the leading problems of this plant, especially if one considers the very dense population in the vicinity of the power plant and the negative impact of large amounts of ash on people’s health and life.

WHAT DID YOU WANT TO ACHIEVE?

Bearing in mind the wear and tear of the existing equipment and the outdated technology, it was concluded that the electrofilter plant of TE Kosovo “A” requires a thorough revitalization, so that the power plant can function according to future legal frameworks and to reduce as much as possible the impact of exhaust gases on the population in the vicinity of the power plant.

In addition to the mentioned (environmental) reasons, an additional motive for the revitalization of the control system is closed-loop operation, that is, the possibility that the consumption of the electrostatic precipitator, which is not negligible, is regulated depending on the number of solid particles in the exhaust gas, i.e., the quality of the spent fuel.

TECHNICAL DESCRIPTION OF THE REALIZED SYSTEM

Bearing in mind all the above, the SIEMENS SIFUPIC-F power supply-control module for electrostatic filters with the following essential characteristics was selected:

• a wide range of output voltage and current control of T/R (transformer/rectifier) ​​units realized by controllable SCR thyristors;
• possibility to limit output sizes;
• the possibility of remote control from the central control room using the SCADA interface;
• short-term and long-term archiving of process data.

The mechanical part of the electro filter consists of 9 chambers of a square scheme, as shown in Figure 3.

Each chamber is further divided into sections by long flat metal plates (mantle), between which electrodes are fixed with weights. The technology is standard for this type of electrostatic precipitator: a high rectified voltage must be brought between the electrodes and the mantle, which creates a large electric field in a relatively narrow area between them. When that voltage reaches a critical point (Corona Onset Voltage), a breakthrough occurs; that is, the gas flowing through the chambers is ionized and becomes electrically conductive. As a result of this phenomenon, many free charge carriers (electrons, positive and negative ions) appear in the gas, which, under the influence of the electric field, accelerate towards the collecting electrodes, dragging dust and other solid particles with them.

The electrical part consisted of 9 cabinet fields with associated high-voltage transformers. The system was controlled manually; that is, the operator monitored the output values ​​(voltage of the secondary and current of the primary circuit of the transformer) and manually set the operating point. Fields 1-4 and 6-9 were performed using technology with SCRs, while field five was performed using an even older technology of primary current regulation using intermediate inductance. The high-voltage transformers fields 1-4 and 6-9 were fully compatible with the SIFUPIC-F system, while in field 5, it was necessary to intervene in the energy circuit of the T/R set.

All fields had to be adapted to the new system in such a way as to retain all the existing energy components (high-voltage transformers and associated switching equipment) and to use the existing thyristor modules.

The principle of operation of the electrical part of the system is as follows: the SIFUPIC-F microcontroller reduces the firing angle of the thyristor in the primary circuit by a slight upward ramp. As the tripping angle decreases, the primary current and secondary voltage increase towards the set operating point (Ip=125A, Is=800mA, Us=40kV). The secondary voltage is measured using the HV resistor installed on the transformer, and the primary current is measured using a current-measuring transformer. SIFUPIC measures the secondary current. Suppose there is a breakdown inside the filter, which SIFUPIC registers via the measuring resistor in the feedback loop. In that case, the thyristor trigger angle is automatically increased to reduce the voltage on the secondary and protect the high-voltage equipment. SIFUPIC goes into regulation again when the situation stabilizes, according to the previously mentioned algorithm.

Since the thyristors that regulate the primary current cannot be turned off by an impulse to the control electrode (Gate Turn-off Thyristor), in the event of a breakdown, it is essential to provide a mechanism for limiting the primary current. Namely, when a breakthrough occurs, the control algorithm can react by increasing the trigger angle only in the next half-cycle. Predicting the inductance in the primary circuit is essential to protect the energy components from long-term overload or destruction.

In addition to the previously stated requirements for adjustment, Figure 4 shows the required principle diagram of the energy part of the system.

Fields 1-4 and 6-9 are fully compatible with the new system regarding the energy part. The thyristor modules were in good condition, and the high-voltage transformer met all the requirements. Field 5, however, was not designed in thyristor technology, so a new thyristor module was installed. In addition, the associated transformer was not equipped with an inductance to limit the primary current. This inductance is crucial for equipment protection, and it was necessary to dimension and install it.

The rest of the control and communication equipment is installed and wired according to the principle diagram in Picture 2.

SUPERVISORY-MANAGEMENT SYSTEM

In addition to being directly from the plant, SIFUPIC-F microcontrollers can be controlled remotely using a personal computer equipped with the appropriate WINPIC software. Figure 5 shows the topology of the monitoring and control system.

It can be seen from the picture that optical cables interconnect the SIFUPIC-F control modules, and one of them is equipped with an additional card that enables communication between the modules themselves and communication with the computer. The connection to the computer is physically made through a PROFIBUS connection, and the OLM (Optical Link Module) is responsible for adequately functioning the connection between the PROFIBUS and the optical part of the network.

The WINPIC software enables full functionality and flexibility when remotely managing the de-ashing process. The operator does not need to be in the plant in the automatic operating mode. In addition, using the WINPIC software, it is also possible to:

• Turn on and off all cells simultaneously;
• Monitor any number of process variables in real time;
• Quickly and easily change parameters and operating points;
• Short-term and long-term archiving of process data;
• Fully automate all the above actions.

The installed PC also enables, in the event of a problem with the operation of the electro filter or control modules, an adequately trained person can log on to the system via the Internet and assist operators in solving problems or repairing malfunctions.

Pictures 6 and 7 show the layout of the WINPIC software user interface.

In the presented situation, the system works in the so-called open circuit. Namely, the operating point of power transformers is adjusted according to various factors: the boiler’s power, the type and amount of fuel, and the assessment of the proportion of solid particles in the flue gas. The operator’s and accompanying staff’s experience in these situations is the most important factor for the correct operation of the electrostatic precipitator.

To take full advantage of all the technical possibilities the SIFUPIC-F control system provides, a system for measuring the amount of solid particles in the exhaust gases should be installed. This way, full feedback would be achieved and the system would work in the so-called closed loop, eliminating the need for operator intervention, except in emergencies. In addition to a higher level of automation, in this way, significantly greater savings are achieved than in open-loop operation due to the possibility of regulating the power of each individual cell depending on the exhaust gas parameters.