Class Notes
Overview
What is a power grid and how to distinguish between power grids.
Master the basic concepts.
1.2 Wiring mode and voltage level of power system The
voltage level is tested every year, some basic concepts.
There is a picture, and some related parameters are counted as other parameters. Long term short term
Neutral point of operationQuite important,
neutral point is puzzled, why can you continue to work?
Why is the neutral point grounded through the arc suppression coil?
Why is the neutral point grounded through a small resistance?
The neutral point is directly grounded.
What to do if the shell of TN-C system is charged.
TN-S system,
TN-CS system, and mains power are these three systems.
TN system. The neutral line is grounded. What is the function: ppt three
TT system
IT system
Chapter 2 Factory Power Load and Its Calculation
What is the purpose of power load calculation? Five blanks (tested every year)
Chapter 3 Short Circuit Current Calculation
(Question about ten minutes)
Short circuit consequences, calculation purposes, what
are the forms and what are the solutions
? Features of short circuit forms
Asymmetrical short circuits are transformed into symmetrical short circuits, why only three-phase short circuits are calculated (understand other three-phase short circuits), symmetrical component method
What is an infinite power system.
How to understand the full current and how to calculate the calculation of the
three-phase short-circuit current.
Ohm method and standard (standard unit value?) method, both have
some formulas to remember (or derive)
short-circuit current calculations with transformers We must consider the transformation ratio
. What is the transformation ratio?
Chapter 4 Factory Substation and Its Primary System
What is the primary device, the primary circuit is what
the primary equipment which
the secondary circuit is it
high and low voltage primary device
what function is overloaded, overload Why should function (design temperature than the actual temperature is not met, charge unevenness)
of the transformer overload ability,
The scope of application of internal and external bridges, or double busbar system is tested every year.
Power line selection:
Chapter 5 Relay Protection?
A good understanding of several kinds of protection can be tested
Summary of knowledge points
Chapter 1 Overview
1. What is the concept of grid/grid?
The power lines at all levels of voltage in the power system and their associated substations are called power grids or power grids
. 2. How to distinguish/classify power grids?
Supply voltage
- Regional power grid: R>50km U>35kV
- Local power grid: 20<R<50km U<35kV
- Terminal power grid: R<20km U<35kV
3. How to connect the power system?
Two systems and three types:
single-wire two-wire system (you can consider the double bus system to understand)
radial trunk type ring type
4. Voltage level?Calculation of voltage level and line rated voltage
Voltage rank order: 220 380 660 3k 6k 10k 25k 110k 220k 500k 1000k
- Rated voltage of power grid (power line): the basic basis for determining the rated voltage of various power equipment
- Rated voltage of electrical equipment: the same as the rated voltage of the grid at the same level
- Generator rated voltage: 5% higher than the rated voltage of the grid at the same level
- Rated voltage of power transformer (primary side, secondary side)
Primary side: connected to the generator, the same as the rated voltage of the generator; not connected to the generator, the same as the rated voltage of the connected grid
Secondary side: if the line is longer, then 10% higher than the rated voltage of the connected grid, if not longer, the rated voltage of the secondary side is 5% higher than the rated voltage of the connected grid
5. Neutral point operation mode
- The neutral point is not grounded
- The neutral point is grounded through the arc suppression coil
- The neutral point is grounded through a small resistance
- Neutral point is directly grounded
6. The neutral point is not grounded, and a certain phase is grounded, why can it continue to work?
Because the line voltage of the line has not changed regardless of its phase or magnitude, the three-phase electrical equipment can still operate normally.
However, a system with a single-phase ground fault is not allowed to operate for a long time, so as to avoid the formation of a two-phase ground short circuit when a ground fault occurs in one phase, which will enlarge the fault.
7. Why is the neutral point grounded through the arc suppression coil?
In order to prevent intermittent arcing at the grounding point when single-phase grounding, causing resonance overvoltage, when the single-phase grounding capacitor current is greater than a certain value, the neutral point of the power system must adopt the operation mode of arc suppression coil grounding.
The arc suppression coil is actually an adjustable iron core inductance coil with very small resistance and large inductance.
The current flowing through the grounding point is the sum of the capacitor current and the arc suppression coil current, and the two can compensate each other. The compensated current can be less than the minimum current for arcing, so there will be no resonance overvoltage.
8. Why is the neutral point grounded through a small resistance?
Why is it grounded directly?
9. Why is the neutral point directly grounded?
When a single-phase ground fault occurs, a single-phase short circuit can be formed through the grounded neutral point, and the voltage relative to the ground does not increase.
In addition, the short-circuit current is much larger than the normal load current, so when a single-phase short-circuit occurs, the protection circuit should act as a trip to remove the short-circuit fault and restore normal operation in other parts of the system.
10. How to ground the three-phase four-wire power distribution system?
- TN system
- TT system
- IT system
11. How many types of TN system?
TN system: the neutral point is directly grounded, and all exposed conductive parts of the equipment are connected to PE or PEN lines
- TN-C system: suitable for balanced load, the shell is live when the load is unbalanced (N line and PE line are combined)
- TN-S system: suitable for unbalanced load, the shell will not be charged when the load is unbalanced
- TN-CS system: the front section is suitable for unbalanced load, the back section is suitable for balanced load
12. What should I do if the TN-C system shell is electrified?
The shell of the electrical equipment is directly grounded to form a protective ground, namely TT
13. What is the role of the neutral line (N line) and the protection line of the three-phase four-wire power supply and distribution system?
Line N:
- Connect single-phase equipment to transmit single-phase current
- Transmission of unbalanced current in a three-phase system
- Reduce load neutral point potential shift
Protection line: protection of personal safety
14. What is the TT system?
The neutral point of the system is directly grounded, that is, working grounding. The
shell of electrical equipment is connected to the earth, that is, protective grounding.
15. What is an IT system?
The neutral point of the transformer is not grounded or grounded through a large resistance, and the shell of the electrical equipment is directly grounded.
When a single-phase ground fault occurs in the IT system, three-phase equipment and single-phase equipment with wiring voltage can still operate normally.
It is widely used in places that require high continuous power supply and are inflammable and explosive, especially in mines, underground and other places.
16. What are the characteristics of the different grounding methods of the three-phase four-wire power distribution system?
Chapter One Supplement
1. How is the factory power supply?
- Medium-sized factories: The power input voltage is 6~10kV, and the electric energy is first concentrated by the high-voltage power distribution station, and then the electric energy is distributed by the high-voltage power distribution line to the substation of each workshop, or the high-voltage power distribution line is directly supplied to the high-voltage electrical equipment
- Large factories: and some medium-sized factories with incoming line voltage above 35kV, generally have to go through two steps, and some only go through one step
- Small factories: The required capacity is generally not more than 1000kV·A or slightly more, so usually only one step-down substation is set up to reduce 6-10kV to the voltage required by low-voltage electrical equipment.
2. What is the choice of factory power supply and distribution voltage?
High voltage 6-10kV, preferably 10kV; low voltage 220V/380V
Chapter 2 Factory Power Load and Its Calculation
1. What is the calculated load? It is calculated
through the statistical calculation of the load, and the load value of each component in the power supply system is selected according to the allowable heating conditions, which is called the calculated load
. 2. The purpose of the load calculation? (Provided by face change)
- Choose the wire cross-sectional area of the power supply system
- Determine transformer capacity
- Formulate measures to improve power factor
- Selection and setting of protection equipment
- Verify power supply voltage quality
3. What is a load curve?
A graph (or curve) that characterizes the change of electric load over time
4. The physical quantity related to the load curve?Required definition
- Annual maximum load and annual maximum load utilization time
- Average load and load factor
Annual maximum load: Pmax is, the maximum working load shift throughout the year (the maximum load work shift not occasional, but the year with at least 2 to 3 times) the maximum power consumption of the half-hour average power, Also marked as P30.
Annual maximum load utilization time: Tmax is an imaginary time. During this time, the electric energy consumed by the continuous operation of the electric load at the annual maximum load is exactly equal to the actual electric energy consumed by the electric load throughout the year: Tmax = Wa/Pmax Wa The
annual maximum load utilization hour of the actual electric energy consumed in a year is an important parameter reflecting the characteristics of the electric load, which is obviously related to the actual shift system of the factory.
Average load: the power
load factor averaged by the electric load in a certain time t : also known as Load factor is the ratio of average load to annual maximum load
5. What is the need coefficient method? (Determination of calculation load of three-phase electrical equipment group)
6. What is the binomial coefficient method?
Chapter 3 Short Circuit Current and Its Calculation
1. The consequence of short circuit and the purpose of calculating short circuit current?
- 1. A large electric power and a high temperature rise will be generated when a short circuit occurs, which may cause malfunction and damage components or equipment
- 2. The voltage will suddenly change during short circuit, which will affect the normal operation of electrical equipment
- 3. A power failure will be caused when a short circuit occurs. The closer the short circuit point is to the power supply, the greater the power failure range.
- 4. A severe short circuit will cause the engine group to lose synchronization and de-load, affecting the stability of the power system
- 5. Asymmetrical short circuit (single-phase & two-phase short circuit) can interfere with communication lines
2. How to solve the short circuit problem?
- Try to eliminate all the reasons that may cause short-circuit current, such as insulation damage, overvoltage breakdown, and misoperation by staff, etc.
- Take protective measures and select based on short-circuit current
- Select electrical equipment by calculating short-circuit current to have sufficient dynamic and thermal stability
3. What are the characteristics of the short circuit form? ? ?
Short-circuit form: single-phase, two-phase and three-phase
, divided into short-circuit to ground and short-circuit between phases.
Features:
(1) Single-phase short-circuit can only occur when the neutral point is grounded.
(2) Single-phase short-circuit and two-phase short-circuit are asymmetrical short-circuits.
(3) Symmetrical short circuit when three-phase short circuit
:
(1) It is not necessary to calculate single-phase short circuit, because the neutral point of the high-voltage power supply system is not grounded, single-phase short circuit is impossible.
(2) Three-phase short-circuit current is greater than two-phase short-circuit current. It is more convenient to calculate the three-phase short-circuit current.
(3) The two-phase short-circuit current can be vectorized into positive sequence, negative sequence, and zero sequence through the symmetrical component method , and then the three-phase short-circuit current of each sequence is calculated, and finally the total current is calculated. san
4. Asymmetrical short circuit is transformed into symmetrical short circuit. Why is only three-phase short circuit calculated?
Under normal circumstances, especially in the factory power supply system far away from the power supply, the short-circuit current of the three-phase short circuit is the largest, so the damage caused is the most serious. In order to make the electrical equipment in the power system work reliably under the most severe short-circuit conditions, the short-circuit current calculation for selecting and verifying electrical equipment is mainly based on three-phase short-circuit calculation.
In addition, asymmetric short-circuit can also be decomposed into symmetrical positive sequence, negative sequence and zero sequence components according to the symmetrical component method, and then analyzed and calculated according to the symmetrical quantity.
(Understand the other three-phase solution easily), symmetrical component method
5. What is an infinite power system?
Infinite power supply
A power system whose power supply capacity is much larger than that of the user power supply system.
Its characteristics are: when the load of the user's power supply system changes or even a short circuit occurs, the voltage on the feeder bus of the power system substation can basically remain unchanged.
6. How to understand the full current and how to calculate it? ? ?
Short-circuit full current = periodic component + non-periodic component = steady state component + transient component = zero state response + zero input response = general solution of homogeneous equation + a special solution
7. How to calculate the three-phase short-circuit current? ? ?p54
Ohm method, standard unit value method
The basic principle of the symmetrical component method?
Chapter 4 Factory Substation and Primary System
1. What is a substation?
Receive electrical energy from the power system, transform it, and then distribute it
2. What is a distribution station ?
Receive electrical energy from the power system, and then directly distribute power
3. What is a primary circuit and primary equipment?
Primary circuit: The circuit in the substation that is responsible for the task of transmitting and distributing electric energy, also called the primary circuit or main circuit, main wiring.
Primary equipment: all electrical equipment in the primary circuit.
4. What is the classification of the primary equipment?
- Conversion equipment: transformer, CT, PT
- Control equipment: various high and low voltage switches
- Protective equipment: circuit breakers, fuses, lightning arresters
5. What is a secondary circuit and secondary equipment?
Secondary circuit: A circuit used to control, indicate, measure and protect the operation of primary equipment. Or called secondary wiring, secondary wiring** (control finger measurement and protection) **
Secondary equipment: all electrical equipment in the secondary circuit
6. What are the high and low voltage primary equipment?
High-voltage primary equipment: (1) high-voltage fuse; (2) high-voltage isolating switch (3) high-voltage circuit breaker; (4) high-voltage load switch (5) high-voltage switch cabinet.
Low-voltage primary equipment: (1) Low-voltage fuse; (2) Low-voltage knife switch (3) Low-voltage circuit breaker; (4) Low-voltage knife-fuse switch and load switch (5) Low-voltage power distribution panel.
7. What is overload capacity?
If the transformer works for a long time, if it is overloaded, the insulation will age due to the influence of temperature, and the mechanical strength will decrease. If it encounters lightning, it will break and cause a short circuit.
8. Why is the transformer allowed to have normal overload?
- The specified ambient temperature of the transformer is 40℃, but the actual highest temperature in my country is 35℃, and the average temperature is 20℃
- Uneven load, discontinuous full load
9. The basic requirements for the main wiring of the substation?
(Answer the key words)
- Reliability: meet the reliability requirements of different levels of load power supply
- Flexibility: Simple operation, flexible operation, convenient operation
- Economical: low investment and low operating cost
- Promising: there is room for development
10. What is the scope of application of internal and external bridge wiring?
Internal bridge wiring:
- Applicable to the situation of long lines, many failures and maintenance opportunities
- The load is relatively balanced, without the need for frequent roll-out and switching of transformers
External bridge wiring:
- Suitable for short circuit, less failure and maintenance opportunities
- Unbalanced load, the transformer needs to be rolled out and switched on frequently
11. What is the busbar system?
"Bus bar" is a three-phase conductor in which the in and out lines of a transformer or generator are connected in parallel to form the same group of three-phase conductors. It plays the role of electric energy collection and distribution, that is, the "bus bar".
12. Classification of busbar system?
Single busbar and double busbar, segmented and unsegmented
13. What are the characteristics and functions of the unsegmented double busbar system? (Bus disconnect switch circuit breaker)
- Take turns overhauling the busbar without stopping the normal operation of the substation
- Repair any bus isolation switch to disconnect this circuit
- For repairing any circuit breaker, the circuit breaker between the spare bus and the bus can be used to replace the function of the repaired circuit breaker of the bus
14. The selection principle of power lines?
- fever
- Voltage loss
- Economic current density
- Mechanical strength
15. General experience in power line selection?
- Due to the large composite current of the low-voltage power line, first select the cross-sectional area of the power line according to the heating conditions, and then check the voltage loss and mechanical strength
- Because of the high voltage level requirements of low-voltage lighting lines, first select the cross-sectional area according to the voltage conditions, and then verify the heating conditions and mechanical strength
- Select the cross-sectional area of the high-voltage line according to the economic current density, and then check the others.
16. What is open standby and secret standby?
Illuminated standby: In normal operation, one is working and the other is fully standby (not working).
Dark standby: two working at the same time, if one fails, the load of the other doubles
Chapter 5 Overcurrent Protection of Factory Power Supply System
How to reflect the selectivity of inverse time overcurrent protection?
Average load and load factor