Practice for power system protection

Code of practice for power system protection

01 The entire wiring of circuitry for indications, alarms, metering and protection should be permanent wiring.
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.

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