Code of practice for power system protection
02 The leads should be marked and identified by ferrules near terminals.
03 Every lead should end at a terminal point and no junction by twisting is allowed.
04 The wiring should be by copper leads for C.T secondary for all cores i.e. metering cores as well as protection cores and for PT secondary for protection core.
05 The wiring should be by copper leads 1.07 The copper lead for 1.05 & 1.06 above should be stranded but not single lead type.
06 Aluminum leads can be used for indication, alarms and PT secondary for metering but stranded wires only are to be used. But copper leads are always preferable for these said purposes.
07 The terminations should be lugged by ring shape ‘O’ lugs. ‘U’ shape lugs should be avoided since ‘U’ shape lugs may slip if terminal is loosen.
08 For CT Secondary terminations, two nuts with one spring washer and two flat washers to be compulsorily used.
09 The CT terminal strips should be stud type with nuts and not screw-in-type.
10 Wherever two sets of batteries are available, the primary protection and back-up protection should be from different batteries.
11 Where there is only one battery at an Electrical Power Substation, the primary and back-up protections should be given D.C supply through two individual circuits with independent fuses run from D.C bus.
12 When CBs have two trip coils, both main protection and backup protection will energize both the trip coils.
13 D.C and A.C supplies should not be taken through different cores of the same cable. Totally different cables should be used for DC and AC supplies.
14 Independent D.C cables should be run to each equipment in the yard and looping of D.C supply from one equipment to other is not permitted.
15 The D.C emergency lighting in substation should be through independent cables and not mixed up with protection and other circuitry.
16 Standard color codes for wires in control circuit of different sizes should be as follows,
| PURPOSE | SIZE | COLOR |
|---|---|---|
| Indication, Alarm, trip, close etc | 1.5 mm2 | Gray |
| Red Phase Metering PT Circuit | 1.5 mm2 | Red |
| Yellow Phase Metering PT Circuit | 1.5 mm2 | Yellow |
| Blue Phase Metering PT Circuit | 1.5 mm2 | Blue |
| Red Phase Protection PT Circuit | 2.5 mm2 | Red |
| Yellow Phase Protection PT Circuit | 2.5 mm2 | Yellow |
| Blue Phase Protection PT Circuit | 2.5 mm2 | Blue |
| Red Phase Metering and Protection CT Circuit | 2.5 mm2 | Red |
| Yellow Phase Metering and Protection CT Circuit | 2.5 mm2 | Yellow |
| Blue Phase Metering and Protection CT Circuit | 2.5 mm2 | Blue |
| Phase for auxiliary AC supply | 2.5 mm2 | Red |
| Neutral for auxiliary AC supply | 2.5 mm2 | Black |
| Common star point of CTs | 2.5 mm2 | Black |
| Common star point of Protection PTs | 2.5 mm2 | Black |
| Common star point of Metering PTs | 1.5 mm2 | Black |
| Earthing Connection | 2.5 mm2 | Green |
17 The lead numbers are also standardized as follows so that anyone can easily identify the purpose for which the lead is connected
| Alphabet Series | Purpose | Example |
|---|---|---|
| J Series | D.C Incoming | J1, J2, etc. |
| K Series | Control - Closing, Tripping, etc. | K1, K2, K3 etc. |
| L Series | Alarms, indications and annunciations | L1, L2, L3, etc. |
| M Series | Motor Circuit | M1, M2, etc. |
| E Series | Potential transformer secondaries | E1, E2, E3, etc. |
| H Series | LT A.C Supply | H1, H2, H3, etc.. |
| A Series | C.T secondary for special protection | A1, A2, A3, etc. |
| B Series | Bus bar protection | B1, B2, B3, etc.. |
| C Series | Protection Circuits | C1, C2, C3, etc. |
| D Series | Metering Circuit | D1, D2, D3, etc. |
18 The CT ratios available and adopted with number of cores shall be displayed on each panel as follows: (with underlined position as adopted). 400 - 200 - 100 / 1-1-1
19 Wherever CT cores are not used “SHORTING LOOPS” should be provided in CT secondary terminals and not in marshaling boxes or at panels.
20 The Cable entries in the equipment, marshaling boxes and panels should be through appropriate size of cable glands. No other means are allowed.
21 PT secondaries should have group MOCBs with D.C alarm.
22 Few cells from a battery set should not be used for separate low voltage D.C circuits. Here D.C - D.C converters may be employed for utilizing full D.C voltage of the entire battery as input.
Standard lead numbers used in control circuit of protection of power system
Certain lead numbers are standardized as follows and should be compulsorily adopted with ferrules at terminations of leads.
Main DC Positive supply – J1
Main DC Negative supply – J2
DC Positive bus inside panel – K1
DC Nagetive bus inside panel – K2
Remote Close - K15R
Remote Trip - K5R
Local Close - K15L
Local Trip - K5L
Metering CT secondaries – D11, D31, D51, D71 etc.
Protection CT secondaries – C11, C31, C51, C71 etc.
Special Protection CT secondaries – A11, A31, A51, A71 etc.
PT scondaries - E11, E31, E51, E71 etc.
Main DC Negative supply – J2
DC Positive bus inside panel – K1
DC Nagetive bus inside panel – K2
Remote Close - K15R
Remote Trip - K5R
Local Close - K15L
Local Trip - K5L
Metering CT secondaries – D11, D31, D51, D71 etc.
Protection CT secondaries – C11, C31, C51, C71 etc.
Special Protection CT secondaries – A11, A31, A51, A71 etc.
PT scondaries - E11, E31, E51, E71 etc.
Different relay device number used in protection of power system
| Mark Number | Name of the Device |
|---|---|
| 2 | Time delay relay |
| 3 | Checking or Interlocking relay |
| 21 | Distance relay |
| 25 | Check synchronizing relay |
| 27 | Under voltage relay |
| 30 | Annunciator relay |
| 32 | Directional power (Reverse power) relay |
| 37 | Low forward power relay |
| 40 | Field failure (loss of excitation) relay |
| 46 | Negative phase sequence relay |
| 49 | Machine or Transformer Thermal relay |
| 50 | Instantaneous Over current relay |
| 51 | A.C IDMT Over current relay |
| 52 | Circuit breaker |
| 52a | Circuit breaker Auxiliary switch “Normally open” (‘a’ contact) |
| 52b | Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact) |
| 55 | Power Factor relay |
| 56 | Field Application relay |
| 59 | Overvoltage relay |
| 60 | Voltage or current balance relay |
| 64 | Earth fault relay |
| 67 | Directional relay |
| 68 | Locking relay |
| 74 | Alarm relay |
| 76 | D.C Over current relay |
| 78 | Phase angle measuring or out of step relay |
| 79 | AC Auto reclose relay |
| 80 | Monitoring loss of DC supply |
| 81 | Frequency relay |
| 81U | Under frequency relay |
| 81O | Over frequency relay |
| 83 | Automatic selective control or transfer relay |
| 85 | Carrier or pilot wire receive relay |
| 86 | Tripping Relay |
| 87 | Differential relay |
| 87G | Generator differential relay |
| 87GT | Overall differential relay |
| 87U | UAT differential relay |
| 87NT | Restricted earth fault relay |
| 95 | Trip circuit supervision relay |
| 99 | Over flux relay |
| 186A | Auto reclose lockout relay |
| 186B | Auto reclose lockout relay |
Three Phase Vector Diagram
Power System Protection
Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Concept of three phase vector diagram is very much required for determining fault calculation of electrical power system. Practically almost every electrical power system deals with three phase power.
Diagram on which one or more vectors can be represented is referred as vector diagram. On such diagram, alternating quantities are represented by arrow. The length of the arrow represents the rms value of the alternating quantity. The angular position represents the relative position of a vector (alternating current or voltage) with respect to another vector or to a reference axis. The arrow head represents the direction in which the vector is acting. When an electrical quantity acts away from the source towards the direction of load, the vector represents the quantity is considered as positive vector.
Now let’s have a discussion on some terms related to protective relay
Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated. If the value of actuating quantity is increased, the electro magnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.
Three phase electrical quantities can be represented byvector diagram. The diagram represent three phase quantities is known as three phase vector diagram.
voltages of a three phase system is shown in the figure
Phase-sequenceof the vectors, represent voltages, is shown in the figure by an arrow. The term phase sequence is used to indicate the order in which vectors are placed in relation to one another for counter-clockwise rotation. In the figure here three, phase to neutral voltages, are rotating. That means they would reach their maximum positive values in the sequence first R, then Y and then B. This sequence is referred as positive sequence. This represents the normal healthy condition of system.
Electrical Fault Calculation
Before applying proper electrical protection system, it is necessary to have through knowledge of the conditions of electrical power system during faults. The knowledge of electrical fault condition is required to deploy proper different protective relays in different locations of electrical power system.
Power System Protection
Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Information regarding values of maximum and minimum fault currents, voltages under those faults in magnitude and phase relation with respect to the currents at different parts of power system, to be gathered for proper application of protection relay system in those different parts of the electrical power system. Collecting the information from different parameters of the system is generally kwon aselectrical fault calculation .Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Fault calculation broadly means calculation of fault current in any electrical power system. There are mainly three steps for calculating faults in a system.
1) Choice of impedance rotations.
2) Reduction of complicated electrical power system network to single equivalent impedance.
3) Electrical fault currents and voltages calculation by using symmetrical component theory.
2) Reduction of complicated electrical power system network to single equivalent impedance.
3) Electrical fault currents and voltages calculation by using symmetrical component theory.
Impedance Notation of electrical power system
If we look at any electrical power system, we will find, these are several voltage levels. For example, suppose a typical power system where electrical power is generated at 6.6kv then that 132kv power is transmitted to terminal substation where it is stepped down to 33kv and 11kv levels and this 11kv level may further step down to 0.4kv. Hence from this example it is clear that a same power system network may have different voltage levels. So calculation of fault at any location of the said system becomes much difficult and complicated it try to calculate impedances of different parts of the system according to their voltage level. This difficulty can be avoided if we calculate impedances of different part of the system in reference to a single base value. This technique is called impedance notation of power system. In other wards, before electrical fault calculation, the system parameters, must be referred to
base quantitiesand represented as uniform system of impedance in either ohmic, percentage, or per unit values.
Electrical power and voltage are generally taken as base quantities. In three phase system, three phase power in MVA or KVA is taken as base power and line to line voltage in KV is taken as base voltage. The base impedance of the system can be calculated from these base power and base voltage, as follows,
| Zb = | (KV)2 | ohms |
| KVA |
Per unitan impedance value of any system is nothing but the radio of actual impedance of the system to the base impedance value.
| i.e. Zpu = | Zactual |
| Zb |
Percentage impedancevalue can be calculated by multiplying 100 with per unit value.
Z% = ZpuX100
Again it is sometimes required to convert per unit values referred to new base values for simplifying different electrical fault calculations. In that case,
|
|
The choice of impedance notation depends upon the complicity of the system. Generally base voltage of a system is so chosen that it requires minimum number of transfers.
Suppose, one system as a large number of 132KV over head lines, few numbers of 33KV lines and very few number of 11KV lines. The base voltage of the system can be chosen either as 132KV or 33KV or 11KV, but here the best base voltages 132KV, because it requires minimum number of transfer duringfault calculation.
Network Reduction
After choosing the correct impedance notation, the next step is to reduce network to a single impedance. For this first we have to convert the impedance of all generators, lines, cables, transformer to a common base value. Then we prepare a schematic diagram of electrical power system showing the impedances referred to same base value of all those generators, lines, cables and transformers.
The network then reduced to a common equivalent single impedance by using star/delta transformations. Separate impedance diagrams should be prepared for positive, negative and zero sequence networks.
There phase faults are unique since they are balanced i.e. symmetrical in three phase, and can be calculated from the single phase positive sequence impedance diagram. Therefore three phase faultcurrent is obtained by,
| If = | V |
| Z1 |
Where I f is the total three phase fault current, v is the phase to neutral voltage z 1 is the total positive sequence impedance of the system; assuming that in the calculation, impedances are represented in ohms on a voltage base.
Symmetrical Component Analysis
The above fault calculation is made on assumption of three phase balanced system. The calculation is made for one phase only as the current and voltage conditions are same in all three phases. When actual faults occur in electrical power system, such as phase to earth fault, phase to phase fault and double phase to earth fault, the system becomes unbalanced means, the conditions of voltages and currents in all phases are no longer symmetrical. Such faults are solved by symmetrical component analysis. Generally three phase vector diagram may be replaced by three sets of balanced vectors. One has opposite or negative phase rotation, second has positive phase rotation and last one is co-phasal. That means these vectors sets are described as negative, positive and zero sequence, respectively.
The equation between phase and sequence quantities are,
Er = Er0 + Er1 + Er2
Ey = Ey0 + Ey1 + Ey2 = Er0 + r2Er1 + rEr2
Eb = Eb0 + Eb1 + Eb2 = Er0 + rEr1 + r2Er2
Therefore,
Er = Er0 + Er1 + Er2
Ey = Ey0 + Ey1 + Ey2 = Er0 + r2Er1 + rEr2
Eb = Eb0 + Eb1 + Eb2 = Er0 + rEr1 + r2Er2
Therefore,
| Er0 = | 1 | (Er + Ey + Eb) |
| 3 |
| Er1 = | 1 | (Er + rEy + r2Eb) |
| 3 |
| Er2 = | 1 | (Er + r2Ey + rEb) |
| 3 |
Where all quantities are referred to the reference phase
r.
Similarly a set of equations can be written for sequence currents also. From , voltage and current equations, one can easily determine the sequence impedances of the system. The development ofsymmetrical component analysis depends upon the fact that in balanced system of impedances, sequence currents can give rise only to voltage drops of the same sequence. Once the sequence networks are available, these can be converted to single equivalent impedances.
Let us consider Z1, Z2 and Z0 are the impedances of the system to the flow of positive, negative and zero sequence current respectively.
For earth fault
For earth fault
| Ir0 = Ir1 = Ir2 | E |
| Z0 + Z1 + Z2 |
Phase to phase faults
| Ir1 = | E |
| Z1 + Z2 |
Ir2 = − Ir1
Ir0 = 0
Ir0 = 0
Double phase to earth faults
| Ir1 = | E | |
| Z1 + | Z2Z0 | |
| Z2 + Z0 | ||
| Ir2 = − Ir1 | Z0 |
| Z2 + Z0 |
| Ir0 = − Ir1 | Z2 |
| Z2 + Z0 |
Three phase faults
| Ir1 = | E | , Ir2 = 0, Ir0 = 0 |
| Z1 |
If fault current in any particular branch of the network is required, the same can be calculated after combining the sequence components flowing in that branch. This involves the distribution of sequence components currents as determined by solving the above equations, in their respective network according to their relative impedances. Voltages it any point of the network can also be determine once the sequence component currents and sequence impedance of each branch are known.
sequence Impedance
Positive sequence impedance
The impedance offered by the system to the flow of positive sequence current is called positive sequence impedance .
Negative sequence impedance
The impedance offered by the system to the flow of negative sequence current is called negative sequence impedance .
zero sequence impedance
The impedance offered by the system to the flow of zero sequence current is known as zero sequence impedance .
In previous fault calculation, Z1, Z2 and Z0 are positive, negative and zero sequence impedance respectively. The sequence impedance varies with the type of power system components under consideration:-
.
1) In static and balanced power system components like transformer and lines, the sequence impedance offered by the system are the same for positive and negative sequence currents. In other words, the positive sequence impedance and negative sequence impedance are same for transformers and power lines.
2) But in case of rotating machines the positive and negative sequence impedance are different.
3) The assignment of zero sequence impedance values is a more complex one. This is because the three zero sequence current at any point in a electrical power system, being in phase, do not sum to zero but must return through the neutral and /or earth. In three phase transformer and machine fluxes due to zero sequence components do not sum to zero in the yoke or field system. The impedance very widely depending upon the physical arrangement of the magnetic circuits and windings.
a) The reactance of transmission lines of zero sequence currents can be about 3 to 5 times the positive sequence current, the ligher value being for lines without earth wires. This is because the spacing between the
goand
return(i.e. neutral and/or earth) is so much greater than for positive and negative sequence currents which return (balance) within the three phase conductor groups.
b) The zero sequence reactance of a machine is compounded of leakage and winding reactance, and a small component due to winding balance (depends on winding tritch)
c) The zero sequence reactance of transformers depends both on winding connections and upon construction of core.
