FIELD OF THE INVENTION:
The invention relates to a high power thyristor bridge for static excitation equipment.
BACKGROUND AND PRIOR ARTS OF THE INVENTION:
The main function of the excitation system is to provide controlled DC supply to the field of the synchronous machine. For this purpose thyristor based rectifier is employed in the excitation system to convert AC to a controlled DC. Thus Thyristor Bridge plays a major and important role in the excitation system. For higher rating machines field current requirement is very high and is in the range of 5000 to 8000 A. Few bridges of higher rating will be required or more bridges of smaller current rating are to be connected in parallel to achieve the desired current rating. Higher number of bridges increases the size of the equipment and footprint of panels. Hence present development activity it taken up to have a compact thyristor bridge rectifier panel with higher current rating.
In view of the above, certain prior art documents as under are enumerated, details of
which are delineated in subsequent paragraphs under detailed description of the
specification.
US20150002106A1 describes generator excitation apparatus and power conversion
system.
JP5159154B2 describes device for controlling the excitation of the synchronous
machine.
US20160006338A1 describes grid-interconnected power converter.
RU2604874C1 describes synchronous generator automatic voltage regulator.

OBJECTS OF THE INVENTION:
Prime object of the invention is to develop a compact high power thyristor bridge for static excitation equipment providing variable DC current with short time overload capability. Further objective is to have minimum number of bridges in parallel so as to eliminate components required to minimize mis-share in currents and thereby improving the reliability of the equipment.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1: Device and Heat Sink
Fig. 2: Semi-conductor Fuse
Fig. 3: RC Snubber Circuit
Fig. 4: Cubicle Fan
Fig. 5: AC/ DC Isolator Connection
Fig. 6: AC side Off-load Isolator
Fig. 7: DC side Off-load Isolator
Fig. 8: Detailed schematic drawing of Thyristor Bridge
Fig. 9: Clamping arrangement for the power module
Fig. 10: Heat Sink with additional piece welded on top
Fig. 11: Semiconductor Fuse mounted between busbars
Fig. 12: Fuse
Fig. 13: Air flow and Pressure drop requirement for Fan
Fig. 14: Snubber arrangement
Fig. 15: Ducting Arrangement for the Power Module

Fig. 16: Thyristor bridge cubicle without and with ducting arrangement Fig. 17: Air Flow contour during Computational Fluid Dynamics
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION:
High power 3-phase full wave thyristor bridge involves design to select, thyristor device
and heat sink (1), semi-conductor fuses (2), snubber circuit (3) and cooling fan (4).
Also to support, maintenance of the bridge, even when the excitation equipment is in
service, a method of isolation is required.
Thyristor device:
Device / thyristor is the prime component for design for 3-phase thyristor bridge. For
design and development of bridge for high power ratings, the current and voltage level
for which the bridge should be suitable is finalised. Thyristor is selected such that it
meets the high arm current requirement for a bridge of 3000 ADC and highest voltage
rating possible for the excitation field.Considering the high voltages that the device may
experience under various operating conditions of the system, the thyristor selected is of
high voltage rating wherein PIV is of the order of 3600 V with a current rating of 3800
A. Since, the PIV and current requirement is high, device with contact area diameter of
100mm is selected.
Heat sink, device mounting arrangement and cooling:
Very fast evacuation of heat generated at the PN junction of the device is essential to
maintain junction temperature at a safe level for the device. Hence selection of Heat
sink, mounting of the same and finalizing cooling medium become thenext major

activity for a thyristor bridge. The medium of cooling can be air or water. The heat sinks may be air cooled or, water cooled with heat pipe based cooling systems.Heat sink with forced air cooling is used considering simplicity.
When mounting disk devices, it is necessary to know that, such devices must be pressed with a definite force in order to provide appropriate electrical and thermal contact. It is important to provide a uniform mounting force distribution on the contact surface of the device as unequal force distribution leads to destruction of silicon and local overheating causing device failure. Being a 100mm device the mounting force required is in the order of 90kN. Suitable clamping arrangement has been designed to meet the mounting force.
Pressure drop occurs when frictional forces, caused by the resistance to flow, act on the forced air as it flows through the heat sink. Considering this, air flow requirement for the device and heat sink together is finalized. Based on the air flow requirement and pressure drop (13),rating of the cubicle fan (4) is finalised.
Semiconductor fuses:
Semiconductor fuses (2) are used for the short circuit protection of the thyristor devices and must withstand normal current fluctuations without overheating. Theyisolate the fault and prevent further damage to the equipment. The arm side semiconductor fuse is selected for protection of individual device. The fuse is selected to meet the arm current and voltage rating of the bridge finalised. Further, the construction of the fuse determines the bus bar arrangement of the 3-phase thyristor bridge. Square body fuse with appropriate fixing arrangement is finalised.

Formation of bridge circuit:
Both horizontal and vertical arrangements were considered for formation of bridge circuit. The vertical arrangement was safe for assembly / removal of devices for the large dimension of 100 mm and mounting force requirement of 90kN.The tower type mounting arrangement used, provides uniform force distribution and the definite force necessary for goodelectrical and thermal contact, eliminating the consequences of unequal force distribution like destruction of silicon, local overheating and consequent device failure. The cubicle fan:
The other complex design aspect is finalizing the optimum air flow requirement. If the layout of devices with heat sinks is horizontal, provision of a simple duct fitted just above the bridge assembly adequately evacuates heat, cooling heat sinks and devices. When the layout of devices is vertical cooling arrangement becomes involved.Optimum air flow requirement for the device and heat sink together for the complete bridge are worked out. Based on the air flow requirementand pressure drop (13) for the bridge, the cubicle fan (5) of adequate rating is finalised. RC-Snubber circuit:
During thyristor device turn-off, which happens once in every cycle for each device, the load current of the thyristor does not stop to flow at the zero crossing but continues briefly in reverse direction as reverse recovery current.This along with stray inductances causes very dynamic over voltages during turnoff. To protect the thyristors against these dynamic over-voltages, an R-C combination (Snubber circuit) (3) is designed and connected in parallel to the device. RC circuit is so designed to keep the RC discharge current through the device within the specified limit under all circumstances.

Isolator:
When system comprises of two or more bridges, to achieve availability of excitation equipment, even while one or more of them are under maintenance, a provision is required to isolate the desired bridge (5) to be taken up for maintenance. To realise this facility, AC side Isolator (6) and DC side isolator (7) are provided. The bus bar arrangement:
The detailed schematic drawing of the thyristor bridge (8) depicts the detailed electrical connection of various components. The connections from pulse transformer to gate of the device or from shunt to meter are simpler considering that cables are adequate for these connections. The connections to the components like Device, Semi-conductor Fuse, isolators, shunt, current transformers need to be ensured through bus bars. Finalizing the major components, which are bigger in size, carrying large current and which need to be connected through bus bars, the bus bar arrangement is designed. The bus bar arrangement is very involved due to the following:
1. The bus bar size shall be adequate for the rated current carrying capability of the bridge.
2. The routing of the bus bars has to be implemented within the cubicle dimension finalized.
3. The clearances shall be adequate for the voltage rating of the bridge.
4. Each one of the arm fuses shall be mounted so that the micro switch contact is accessible for visual indication and wiring.
5. To the extent possible the orientation of bus bars shall be vertical instead of flat. This is toensure better cooling and minimum pressure drop of air.

6. Busbar arrangement is made such that the fuse can be sandwiched between 2 busbars (11). Also, the fuse microswitch for trip annunciation should be accessible or visible. Considering all of the above a suitable bus bar arrangement was designed. The duct arrangement for effective cooling of heat sink:
After designing the panel for appropriate clampingarrangement of the power module, mounting arrangement of different components, thebusbar arrangement, ducting arrangement (15) was designed to guide the cold air effectively through the heat sinks primarily, keeping provision for cooling other components as well.
After completing the design of complete cubicle, heat losses of various components and temperature at various critical points wereanalysed, to identify the hotspots which can be caused due to restricted air flow on account of arrangement of the different components, bus bar arrangement or ducting arrangement.
The design of cubicle is made such that the cool air enters the cubicle from the cut-outs,provided on the door, with meshes of appropriate protection class. The air enters horizontally through the above cutouts in the door, travels through the duct aligning with heat sink (15) and carries away the heat while passing through theslots ofheatsink. The heatsink slots are so arranged that air entering horizontally through the heat sink travels to the rear of the cubicle and forced draught of the fan blows the air outside (17). To ensure that maximum heat transfer is happening from the heat-sink, air is guided through heatsink, using the ducting arrangement made in front of the power module.

WE CLAIM:
(1) An arrangement for three phase full wave high power converter bridge, having a
current rating up to 2750 A DC and voltage of 1.1 KV comprising,
- Device & Heat sink(1),
- Semiconductor fuse (2)
- AC side off load isolator (6)
- DC side off load isolator (7)
- RC snubber (3)
- Cubicle fan (4)
- Ducting (16)
- Bus bar arrangement(11)
Characterized by high arm current for the bridge, highest voltage rating for the excitation, high peak inverse voltage, effective cooling system.
(2) The arrangement as claimed in claim 1, wherein thyristors (1) for bridge rectifier are selected having a current rating of 2750 A, and voltage of 1.1 KV
(3) The arrangement as claimed in claim 1, wherein the heat sink (1) are provided suitable for the said current rating with the provision of adequate air flow for proper cooling.
(4) The arrangement as claimed in claim 1, wherein semiconductor fuse (2) are
provided for short circuit protection, prevention of semiconductor explosion, damage of
the converter, withstanding normal current fluctuation based on the arm current and
voltage rating of the bridge.

(5) The arrangement as claimed in claim 1, wherein AC off-load isolator (6) is provided for isolation of bridge from AC side for maintenance and overhauling.
(6) The arrangement as claimed in claim 1, wherein DC off-load isolator (7) is provided for isolation of bridge from DC side for maintenance and overhauling.

(7) The arrangement as claimed in claim 1, wherein RC snubber (3) is provided to protect the thyristors from overvoltage in circuit inductance due to reverse current change during turn-off.
(8) The arrangement as claimed in claim 1, wherein cubicle fan (4) is provided to force air draft through front cutouts on the door for circulation of cool air for effective cooling.
(9) The arrangement as claimed in claim 1, wherein ducting (15) is provided for effective circulation of air throughout the system with minimum loss of energy.