Thyristor Gate Control or Firing Circuit Design

 

Consideration must be given to the following points when designing gate control circuits.

1. The gate signal should be removed after the thyristor has been turned on. A continuous gate signal will increase the power loss in the gate junction.
2. No gate signal should be applied when the thyristor is reversed biased. If a gate signal is applied under these conditions, the thyristor may fail due to an increased leakage current.
3. The width of the gate pulse must be greater than the time required for the anode current to rise to the holding current. In practice, the gate pulse width is made wider than the turn-on time of the thyristor.

In thyristor converters high ac voltages exists between anode and cathode of the thyristor, while low voltage level pulses are placed between gate and cathode. Isolation is necessary between the gate-cathode circuit and the anode-cathode Circuit so that unwanted short-circuits between devices are avoided..

Gate Drive Circuits

Two basic types of drive circuits are 1) dc-coupled drive circuits and 2) Electrically isolated (optocoupler or pulse transformer drive circuits.

In the case of optocoupler isolation, the low voltage gate drive circuit is optically isolated from the high voltage anode-cathode circuit as shown in figure below.

Gate Drive Circuits (Typical Example for Thyristor)

Summary

1. A thyristor is a latching device and it can be turned on with a small gate pulse, typically 100 μs . They are of various types
2. Thyristors are generally off by line commutation due to the natural behavior of the input line supply.
3. During the turn-off process, thyristors must be subjected to a reverse voltage for a certain minimum time known the turn-off.
4. Due to the junction capacitances and turn-on limit, thyristors must be protected from high di/dt and dv/dt failures.
5. Appropriate gate drive circuit together with appropriate means of isolation between power circuit and gate circuits is also necessary for firing the thyristyors.

previous Device Protection with Thyristors

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Design Consideration of Power Electronic Equipment

Power electronic equipment design can be divided into four main areas:

1. Design of power circuits
2. Protection of power devices
3. Determination of control strategy
4. Design of logic and gating circuits

Peripheral Effects

Power converters operate on the basis of switching power semiconductor devices on and off. These switching actions of converters introduce current and voltage harmonics into:

1. The supply system
2. The output of converters

The problems caused by these harmonics are:

1. Distortion of the output voltage
2. Distortion of the supply voltage
3. Interference with communication and signaling circuits
4. Reduction of input power factor

The following methods can be used to solve or reduce harmonic problems caused by power converters:

1. Use of input and output filters on power converters
2. Choice of control strategy used, 3. Grounded shielding.

The role of transformers in power electronics

1. It provides for isolation of high and low voltage circuits, such as between control circuits, power circuits and sensor circuits.
2. It matches the output voltage of a converter to the requirements of a load, by taking advantage of the turns-ratio. Often the power device voltage rating and the fixed ac supply voltage forces a transistor to be used.
3. It allows operation of a converter at a suitable duty cycle (i.e., not too small) when the converter is required to develop a low output voltage.
4. Use of a high frequency transformer often reduces the overall size of the converter and allows operation at a convenient duty cycle.

Transformer relationships:

previous Classification of Converters

DCS system for large thermal power plant

In the past 20 years, according to global economic development, especially in the economy of the process of rapid industrial growth, demand for the process industry, rapid growth. Instrumentation and automation for digital progressive, intelligent, network, integration and transition countries, the instruments on the ground, but in 4 ~ 20mADC classical instruments, standard HART instrument: content, counter fieldbus 05:04:01 amplifier, while the system is DCS, PLC IPC: SLP (including single controller, paperless recorder, digital instrumentation) 5:3:1.5:0.5 this stage of the process industry and indeed the middle of Great HART, FF, field equipment and plants with PA and apply the ratio of the higher DCS. This paper focuses on large thermal plant DCS, where the trend is still focused on developing automation systems and related technologies, and some ideas.

The development trend of automation of central

Battlefield last instrument standards engineering phase of the practice have been trained by the coexistence of multiple buses. FF Initiative Foundation Fieldbus Technology in the DCS system, and said: After certified the interoperability of DCS FF is the host. There are currently 11 companies in 20 main-control system senses that most countries DCS. This idea was tested in the works of the practice and the consensus at home and abroad have been in power projects with a capacity of fieldbus.

and information technology (IT) merger. In accordance with COTS (Commercial Off-the-shelf technology Commercial off the) principle, with Fast Ethernet (including TCP / IP, etc.) than the factory system, the backbone of the infrastructure.

Technology functional safety in the ongoing process of industrial automation projects is an integral part of a Safety Instrumented System (SIS) and industry process safety integrity level certification system (SIL) is taking shape. the integration of SIS and DCS or consistency of the trend has materialized.

EDDL Device Description Language technology and system integration and formed the basis for this equipment management system (AMS) is an integral part of basic automation system has, and extends the field of management include buildings, equipment, information platforms, intelligent system management equipment, machinery and management systems, equipment, systems performance testing. The current system of asset management in the world has more than 1,000 types of applications, business and management full life cycle asset management strategy project is emerging.

Integration of measurement and control technology, the integration of electronic control units in the staged implementation is united, has ERP / MES / PCS three-level structure of a consensus, based on the progressive realization of the resulting automation and seamless integration of information management company. Motor Control Center (MCC) and the business of electricity distribution and use of intelligence in the core module based on fieldbus technology and gradually with temperature, pressure integrated flow systems and other instruments signals.

The properties of thermal power plants with DCS

Large thermal plant DCS to remove the overall trend was not in the personality:

Requirements of the turbine, boiler and safety systems, such as complexity, fuel, water and bodies connected with the ashes, like the electric energy by the high demands of the times produced more leading to the plant DCS has a feedback loop control, flow control, hybrid control and other complex control functions, led to the requirements of the various implementing agencies of the processing power and control fully advanced feature complex (APC).

Since the sequence of events in the records as a plaintiff and FSSS can be combined to the multi-system, SOE with a time stamp of digital input devices and related functions will be available to complete.