Published On: Sun, Dec 5th, 2021

Illustrated “alphabet” of soft starters for everyone – Part 2

Illustrated “alphabet” of soft starters for everyone – Part 2

Author: Radoje Jankovic

8. Medium voltage soft starters, graphic and schematic symbols

Here we will show almost all graphic and schematic symbols used in everyday practice of practical application of medium voltage soft starters that comply with IEC standards and some are in accordance with ANSI standards since both organizations cooperate with each other.

I have tried to adopt some logical order for clarity and better understanding of practical technical documentation in the practical use of medium voltage soft starters. I started with the basic electrical measuring instruments that are in use today, whether they are analog or electronic. Some of the electronic instruments include all the analogs listed here and even more. For many colleagues of lower and higher level of education, they are more difficult to understand, as well as for colleagues with many years of practical experience in designing and performing installation work, testing and finally technical maintenance of this equipment during its service life.

Voltage levels for medium voltage soft starters range from 1kVAC to over 35 kVAC. This series of voltages includes all levels of medium voltages used in practice around the world in a variety of applications of modern electric motor drives in all branches of economy and industry.

Most important of all is that I tried not to make technical mistakes, although to some colleagues, I am sure, a lot of things will seem unusual, but here I did not invent or think of anything, but I used many of my technical practical documentation, various documentation and factory literature world manufacturers of this, modern electrical equipment.

Fig.8. 1. Standard graphic symbols of electrical measuring instruments of the main and most common electrical measuring quantities.

Fig. 8. 2. Three-line schematic symbol of manual switch-disconnector with earthing blades (a) (main isolator / earth switch disconnector) and single-line symbol (b). It is shown here as a supply in the output field and is also used as a supply in the supply field of a medium voltage distribution.

Fig. 8. 3. Three-line (a) and single-line (b) graphic symbol for a standard earthing switch, ie. three-pole earth switch.

Fig. 8. 4. Single-line schematic representation of a medium voltage circuit breaker in the output field (feeder) and interlocking main circuit breaker with the earth switch, e.g. in front of the electric motor.

Fig. 8. 5. Pull-out (withdavable) power switch in three-line (a) and single-line (b) in the power circuit diagrams. (c) other form of withdravable contacts.

Fig. 8. 6. Pull-out (withdavable) power contactor (contactor power switch) in three-line (a) and single-line circuits (b) in the power circuit diagrams. (c) other form of pull-out contacts.

Fig. 8. 7. Three-line (a) and single-line surge protection circuit with metal oxide varisotors (MOV).

Fig. 8. 8. Cathodic surge arrester, three-line (a) and single-line circuit (b).

Fig. 8. 9. Three-line (a) and single-line (b) power circuit diagram for voltage sensors in all three phases of medium voltage devices.

Fig. 8.10. Pull-out (withdavable) main power contactor with fuses on the input side in three-line and single-line power circuit diagrams. Another form of retractable contacts.

Fig. 8.11. Pull-out (withdravable) contactor power switch with output side fuses in three-line (s) and single-line power circuit diagrams. Another form of retractable contacts.

Fig. 8.12. Typical three-line circuit (a1, a2) and single-line (b1, b2) circuit diagram of current transformers in the power circuit diagrams, in all three phases and output-outgoing feeders With the input-input cubicles, everything is the same, only the primary and secondary connection terminals change their places.

Fig. 8.13. Three-line (a) and single-line (b) circuit diagram of a toroidal current transformer for ground fault control (ground fault current transformer). All three phases go through the secondary transformer together. In practice, it is also called a sensor current transformer. It is used to measure the fault current of a three-phase electric motor power supply circuit.

Fig. 8.14. Three-line schematic symbol of a medium-voltage three-phase pull-out soft starter.

Fig. 8.15. Medium voltage soft starter with pull-out cassette thyristor units A1.1, A1.2 and A1.3 housed in a common housing.

Fig. 8.16. Single-line medium-voltage circuit diagram of a single cable outlet field with a pull-out power switch Q1 that provides load interruption, short-circuit protection and complete disconnection of the high-voltage outlet when disconnected.

On the cable side, in front of the cable terminals, there is an earth fault switch Q2 which is interlocked with the power switch so that it can only be closed when the power switch Q1 is in the draw-out position.

Fig. 8. 17. Single-line high-voltage cable feeder cubicle circuit with pull-out power contactor Q1 and integrated F1 power fuses in all three phases. The contactor provides interruption – disconnection of the terminal under load and complete disconnection of the terminal from the voltage when the contactor is in the draw-out position. Fuses F1 provide short-circuit terminal protection.

On the cable side, in front of the cable terminals, there is an earth fault switch Q2 which is interlocked with the power switch so that it can only be closed when the power switch Q1 is in the extended position.

Fig. 8.18. SF6 gas insulated rotary disconnector for high voltages. The SF6 power disconnector has three physical operating positions.

In the “ON” position, the disconnector switching on the load. In the “OFF” position, completely disconnect the terminal from the high-voltage busbars. In the “EARTH” position, the disconnector blades ground the terminal blocks.

F1 fuses provide short-circuit protection. A fixed power switch can be used instead of fuse F1.

Fig. 8.19. A typical single-line circuit diagram of a high-voltage field (feeder) of output or supply type. Q1 is a pull-out power switch, CT1-3 are current transformers in all three phases, Q2 is a disconnector-switch for grounding the feeder and the supply PT1 is a voltage measuring transformer with pull-out fuses.

Fig. 8.20. Practical single-line schemes of some medium voltage fields, feeders.

a) supply cell (cubicle),

b) measuring cell (cubicle),

c) feeder cell (cubicle).

Fig. 8.21. A typical single-line scheme of one of the many types of supply fields (incoming)  in medium voltage switchboards. As a rule, the first cell is on the left side of the switcboard. The busbars are in the upper part of the distribution switchboard.

XQ connection terminals are usually bus-type (rectangular cross-section)  with holes for connecting the required number of single-core conductors per phase. Input-power cables are today mostly single-core type and can be in 1, 2, 3, 4 or more per one phase. The same applies to earthing PE conductors with the difference that they are smaller and often of the same cross-section as phase conductors. (Example in the picture: 3×50 + 3×25 / 3, 12/20 kV)

Fig. 8.22. Typical, one of many types of single-line circuits of the output (outgoing) cell power in medium voltage swtchboards. This cell is the final one in the distribution switchboard. The busbars are rail type in the upper part of the switchboard cabinet.

Depending on the load of the terminals, cables are almost always single-core today, and there can be 1, 2, 3, 4 or more of them per phase. Terminals X are usually of the busbar type with holes for connecting the required number of single-core conductors per phase. The same applies to earthing PE conductors with the difference that they are smaller and often of the same cross section as phase conductors. (Example in the picture: 3×50 + 3×25 / 3, 12/20 kV).

Fig. 8.23. A typical single-line circuit of a medium-voltage power circuit is performed by the cells for powering the cell (field) of a medium-voltage soft starter.

Fig. 8.24. A typical single-line cell scheme of a medium-voltage soft starter. More or less all cells for medium voltage soft starters are the same and, as a rule, typed. Of course, there are small differences from manufacturer to manufacturer. Typical voltages are used for electric motor drives in our country; 6 kV, 10 kV and 20 kV. Depending on local standards, the world works for voltages of 2.2 kV to 14.5 kV or more.

Fig. 8.25. A typical circuit of a power circuit with two anti-parallel power thyristors connected to a snuber circuit in a medium-voltage three-phase soft starter. RC snuber circuit, R2 graduation resistors, V1, V2, V3, V4 thyristors.

The snuber circuit ensures the stability of the thyristor control and the level of overvoltage protection.

In medium voltage soft starters, resistors for graduation (voltage distribution) are connected in parallel to each thyristor. The resistors are wirewound and of adequate rated power.

Fig. 8. 26. A typical single-phase thyristor array electronic circuit for medium voltage soft starters. Each medium voltage soft starter has three such series that are in separate housings, modules, cassettes and are of the withdrawable type, which enables easy installation and also replacement in case of failure. Here, each power section of one soft starter has 18 thyristors, 6 thyristors per phase. These starters are called cassette type soft starters.

Fig. 8. 27. A typical three-line circuit diagram of a three-phase medium voltage soft starter. Each phase is equipped with one string with three groups of thyristors connected in parallel, two in an antiparallel connection. Each phase group of thyristors is an independent-modular-cassette unit and each can be replaced in case of failure or need.

Fig. 8. 28. A typical three-line circuit diagram of a three-phase medium voltage soft starter. Same as the previous one, with the difference that in this starter unit, current transformers CT1, CT2, CT3 are installed in each thyristor cassette on the input side. In front of the thyristor are three leads that are connected to connection terminals B1, B2, B3 on the input side of the contactor for external bypassing the thyristors.

Fig. 8. 29. A typical three-line circuit diagram of a three-phase medium voltage soft starter. Same as the previous one, with the difference that in this starter unit, current transformers CT1, CT2, CT3 are installed in each thyristor cassette on the output side. Behind the thyristor are three leads that are connected to the connection terminals B1, B2, B3 on the output side for the external bypassing the thyristors.

9. Block diagrams of medium voltage soft starters

Here I have processed block diagrams of medium voltage (high voltage) soft starters in such a way that according to them every practitioner and more educated electrical expert can easily draw a single-line and three-line scheme of any electric motor drive with medium voltage soft starters. As we know, medium voltage levels range from 1000 VAC to over 35000 VAC and are all covered by IEC and ANSI standards.

Fig. 9.1. General block diagram of a medium voltage soft starter for electric motor drives. The main energy elements of each medium voltage are: busbars, switching devices, voltage transformers, protection devices semiconductor fast fuses, line contactor, cassette type soft starter, contactor for external bridged thyristor phase circuits, current transformers, disconnector / earthing switch in front of the motor.

Fig. 9.2. Block diagram of the direct connection of a medium voltage soft starter for starting all types of medium voltage three-phase asynchronous motors. Only two electrical devices were used in this configuration; line contactor and fast semiconductor fuse for protection of thyristor phase branches. This is the simplest connection configuration without bypassing device, contactor or circuit breaker.

Fig. 9.3. Block diagram of the connection of a medium-voltage soft starter for starting medium-voltage three-phase asynchronous motors without bypassing devices for thyristor phase branches only with a line contactor and high-speed semiconductor fuses. Here the thyristor branches of the soft starter are connected in phase windings of the electric motor (in-side-delta connection).

Fig. 9.4. Block diagram of control of three-phase cage asynchronous electric motor using medium voltage soft starters of cassette type. Only the main mains contactor and the external bypassing contactor are used. The input terminals of this contactor 1, 3 and 5 are connected to the bypasing terminals on the input side of the soft starter and the output terminals of bypassing contactor 2, 4 and 6 are connected to the row terminals in the lower part of the cabinet to which the motor is connected.

Fig. 9.5. Block diagram, medium voltage soft starter used to control a three-phase cage-type  asynchronous motor with external bypass of thyristor phase units. Connection of phase thyristor branches in series with motor windings connected in a triangular connection. Only three electrical devices were used; line contactor, external baypass contactor and fast semiconductor fuses on the motor side. Here that the output terminals 2, 4 and 6 of the contactor are connected behind the fuse because the fast fuses protect the thyristor phase units. 

Fig. 9. 6. Block diagram of a medium voltage soft starter for controlling a MV asynchronous electric motor. A line power switch is used and the same one for external bypassing of soft starters that are interconnected with a mechanical lock. 

Fig. 9. 7. Block diagram for starting an induction motor with a wound rotor using a medium voltage soft starter. The system is equipped with a main contactor, a contactor for external bypassing of phase thyristors, a soft starter of the cassette type, a rotor resistor and a contactor for shortcircuiting the rotor resistor after starting the motor.

Fig. 9. 8. One-line power circuit diagram for one medium voltage soft starter (10 kV). The soft starter is of the cassette type.

Depending on the nominal voltage, each thyristor phase cassette (block) contains a certain number of antiparallel connected thyristor pairs and series with the corresponding inverse peak voltage.

So,

for voltages 3.3 kV / 4.16 kV is two pairs of parallel thyristors in series and 13000 V,

for voltages 5 kV / 5.5 kV / 6 kV / 6.6 kV there are 3 pairs of parallel thyristors in series and 18000 V;

for a voltage of 10 kV is 4 or 5 pairs of parallel thyristors in series and 26000 V,

for 11 kV is 5 pairs of parallel thyristors in series and 32500 V and

for 13.8 kV is 6 pairs of parallel thyristors in series and 39000 V peak voltage.

This and similar medium voltage soft starters are used to drive large compressors, fans, water pumps, crushers, mixers, belt conveyors, as well as other machinery and are widely used in industries such as; metallurgy, mining, port plants, mechanical engineering, electric power, etc.

Fig. 9. 9. Three-line power circuit diagram for one medium voltage, 10 kV, cassette type soft starter. Each phase (phase cassette) consists of 5 (2 in antiparallel) groups of antiparallel connected thyristors. 

Fig.9.10. Single-line power circuit, medium voltage, for starting a three-phase high-voltage electric motor using a medium-voltage soft starter composed of 3 thyristor cassettes, as a rule, pull-out type. 

Q1 – earth switch,

F1 – fast acting semiconductor fuses, providing short circuit protection,

PT1- Measuring voltage transformer, 

K1 – Line contactor,

K2 – Bypass contoractor, external,

A1 – Medium voltage soft starter,

CT1 – Measuring current transformer, 2 or 3,

M1 – Motor.

Fig. 9. 11. Single-line power circuit diagram, medium voltage, for starting and controlling three-phase asynchronous motor using medium voltage soft starter consisting of 3 thyristor cassettes pull-out type with the appropriate number of anti-parallel thyristors in series (the number can be 1 – 10 or more). Rated voltage 10 – 20 kV or more.

U1 – Overvoltage protection,

Q01 – Earthing switch,

PT1 – Measuring voltage transformer,

Q1 – Line circuit breaker,

Q2 – Bypass circuit breaker – switch for external starter bypass,

CT1 – Measuring current transformers 2 or 3,

Q02 – Earting switch motor side.

Literature:

1. Various personal documentation of autor.

2. Various manufacturing literature of leading manufacturers such as: ABB, Siemens, Danfos, Aucom…

About the Author

- Radoje - Rade Jankovic Electrician, Electrical Technician, Electrical Engineer, PhD, Ecologist, Environmentalist, Designer, Educator, Investigator... Today PROFESSIONAL TECHNICAL WRITER AND DOCUMENTATOR Massive practical experience in almost all fields of electrical engineering over 40 years. My International education group in Facebook: EEEW - ELECTRICAL & ELECTRONIC EDUCATION WORLD where published more than 8000 small or bigger articles, lessons, technical advice, projects, technical calculations, test questions with and without answers, illustrated test questions with and without answers; all my original works and few thousands of may original photos from practice, drawings, circuit diagrams, environmental lessons and examples from everyday practice etc., etc.

Displaying 2 Comments
Have Your Say
  1. I delight in, cause I found exactly what I was taking a look for. You have ended my 4 day long hunt! God Bless you man. Have a great day. Bye

  2. Well I truly enjoyed studying it. This article offered by you is very effective for good planning.

Leave a comment

XHTML: You can use these html tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Illustrated “alphabet” of soft starters for everyone – Part 2