PUTT
Permissive Underreach Transfer Trip
Zone 1 is set to reach short of the far end. When it operates, it trips the local breaker and sends a permissive signal; the remote relay trips instantly if its own overreaching Zone 2 has also picked up.
Engineering
Protection Relays & Protection Couplers
Protection relays detect faults on lines, transformers and busbars and trip the right circuit breaker within milliseconds. On a transmission line, the relay at each end must also know what the relay at the far end sees — that is the job of the protection coupler (tele-protection equipment), which carries trip and permissive commands between substations over PLCC, fibre optic multiplexers or direct fibre.
Protection relays
What each relay protects
ANSI 21
Distance protection
Measures impedance to the fault and trips in stepped zones (Zone 1 / 2 / 3). The main protection for 132 KV and 220 KV lines.
ANSI 87L
Line current differential
Compares currents at both ends of the line over a communication channel. Needs a fast, low-latency fibre path.
ANSI 87T
Transformer differential
Detects internal faults in power transformers by comparing currents on the HV and LV sides.
ANSI 50/51 · 50N/51N
Overcurrent & earth fault
Instantaneous and time-delayed protection for feeders, and back-up protection for lines and transformers.
ANSI 87B
Busbar protection
Isolates only the faulted bus section, keeping the rest of the substation in service.
ANSI 50BF
Breaker failure
When a breaker fails to open, trips the surrounding breakers — locally, and at the remote end through an intertrip.
ANSI 27/59 · 81
Voltage & frequency
Under / over voltage and frequency protection for generating plants at the grid interface.
Protection couplers
Carrying the trip command to the far end
A protection coupler sits between the protection relay and the communication channel. It converts relay contact commands into coded signals, delivers them to the far end within a guaranteed time, and rejects channel noise so that no false trip is issued. Tele-protection performance — transmission time, security and dependability — is specified in IEC 60834-1.
Communication channels
- Analog PLCC
Commands travel as coded carrier signals on the HV line itself — through the PLCC terminal, line matching unit (LMU), coupling capacitor and wave traps.
- Digital — PDH / SDH MUX
Commands ride a 64 kbps or E1 (G.703) channel of the fibre optic multiplexer, or an IEEE C37.94 optical interface, over OPGW.
- Direct fibre
Relay-to-relay over dedicated fibres in the OPGW — the usual path for line current differential (87L).
Command types
- Permissive
Lets the remote relay trip faster when it also sees the fault.
- Blocking
Stops the remote relay from tripping for a fault outside the protected line.
- Direct / intertrip
Trips the remote breaker unconditionally — for breaker failure and transformer or reactor protection.
Tele-protection over PLCC
Tele-protection over fibre (OPGW) and MUX
Schemes
Tele-protection schemes
POTT
Permissive Overreach Transfer Trip
Each end sends a permissive signal when its overreaching zone sees a forward fault. A breaker trips when its own overreaching zone and the received permissive signal coincide — fast clearance for faults anywhere on the line.
DCB
Directional Comparison Blocking
A reverse-looking element sends a blocking signal for faults behind the relay. A breaker trips on a forward fault only if no block is received within a short coordination time.
DTT
Direct Transfer Trip (Intertrip)
The received command trips the remote breaker directly, without a local fault check. Used for breaker failure, transformer and reactor protection — it demands the highest channel security.
87L
Line Current Differential
Current samples from both ends are exchanged continuously over fibre and compared; any difference means an internal fault. Fully selective, and independent of zone settings.
What we deliver
Protection and tele-protection at STEL
Sub Station Automation (SAS) solutions
Complete automation solutions including fiber optic communication systems for tele-protection in 132 KV level substations in solar and wind power plants.
Total analog PLCC communication solutions
Utility approved brands of PLCC panels along with accessories like wave traps, LMUs and FSK modems — total supply and installation for IEC-101 serial data and voice applications.
OPGW / ADSS cable & PDH MUX
Utility approved brands of FOTE PDH MUX panels for IEC-101 serial communication, IEC-104 IP communication, tele-protection and VOIP phone systems, along with OF cable supply and laying.
Protection relays, RTUs and PLCs
Siemens RTUs, protection relays and PLCs, and Schneider MiCOM series RTUs, supplied through our business associates.
Need tele-protection on a new line?
PLCC, PDH MUX over OPGW or direct fibre — we supply, install and test the complete channel with the protection relays.