Introduction
In this article, we focus on how to Set Siemens 7SR1003 relay parameters for protection of a 3 MVA, 33/0.433 KV Transformer using the example of a 33kV Substation with SEIMENS Switchgear.
7SR1003 Relay basically known as Nondirectional O/C Relay provides phase overcurrent (50/51) and earth fault (50N/51N) protection.
Protection Function Packages :
Standard version – included in all models
46BC Broken Conductor/Load Unbalance
46NPS Negative Phase Sequence Overcurrent
49 Thermal overload
50 Instantaneous Phase Fault Overcurrent
50BF Circuit Breaker fail
50G/N Instantaneous Earth Fault
50SEF Instantaneous Sensitive Earth Fault Overcurrent
51 Timed Delayed Phase Fault Overcurrent
51 G/N Timed Delayed Earth Fault
51SEF Time-Delayed Sensitive Earth Fault
74T/CCS Trip/Close Circuit Supervision
81HBL2 2nd Harmonic block/Inrush restraint
86 Hand Reset Contacts
51C Cold Load Pickup
Programmable Logic
81THD Total Harmonic Distortion Supervision
Siemens 7SR10 Relay
1. Understanding Relay Settings
Before setting up a relay, it’s essential to understand the key parameters:
- CT Primary Current: The primary current of the Current Transformer (CT) determines the system’s load current.
- CT Secondary Current: Typically 1A or 5A, this is the current on the CT secondary side.
- Full Load Current (Ir): Represents the maximum permissible current under normal operating conditions.
- Overcurrent Protection Settings: Defines the threshold and time delay for tripping during overcurrent scenarios.
2. Calculating the Running Load
Based on the attached data:
- CT Primary Current (In) = 300 A
- CT Secondary Current = 1 A
- Transformer FLC (Ir) = 52.49 A
- CT Secondary Current = 52.49 / 300 = 0.175 A
3. Recommended Settings for the 7SR1003 Relay
- Here are the key settings and their significance:
Phase Overcurrent (50/51 Elements)
1. Instantaneous Overcurrent Protection (50) :
Two instantaneous overcurrent elements are provided in the 7SR10 relay.
50-1, 50-2
Each instantaneous element (50-n) has independent settings. 50-n Setting for pick-up current and
50-n Delay follower time delay.
In transformer protection, the instantaneous overcurrent (OC) relay is set at a higher threshold than other relays. This prevents it from tripping unnecessarily when the transformer is first energized, as inrush currents can be quite large. However, if a severe internal fault occurs on the primary side, the relay will still trip quickly. This setup ensures proper coordination, avoiding false alarms while keeping the system safe.
How to Set Siemens 7SR1003 Relay
Setting:
- Transformer FLC (Ir) = 52.49 A
Set at 5 to 8 times FLC, i.e., 250 – 400 A (Primary) or 0.83 – 1.33 A (Relay)
Gn 50‐1 Element : Enabled
- Gn 50‐1 Setting: Set to 0.8 x In [0.8 x 300 = 240 A]
Gn 50‐1 Delay : 0.05 Sec
Gn 50‐2 Element: Enabled
Gn 50‐2 Setting : 1.67xIn (=500A)
Gn 50‐2 delay : 0 sec
2.Time Delayed Overcurrent Protection (51):
Two time-delayed overcurrent elements are provided in the 7SR10 Overcurrent and Earth Fault relay : 51-1, 51-2
51-n Setting sets the pick-up current level.
- An inverse definite minimum time (IDMT) characteristic is selected from IEC, ANSI curves using 51-n Char. A time multiplier is applied to the characteristic curves using the
51-n Time Mult setting. - Alternatively, a definite time lag delay (DTL) can be chosen using 51-n Char.
Alternatively, a definite time lag (DTL) is selected the time multiplier is not applied and the 51-n Delay (DTL)setting is used instead.
Setting :
51-1 (Inverse Time Overcurrent – Low Set): 120-150% of FLC, i.e., 60 – 80 A (Primary) or 0.2 – 0.27 A (Relay)
Gn 51‐1 Element : Enabled
- Gn 51‐1 Setting: Set to 0.2 xIn [0.2 x 300=60 A] for low fault levels and sensitive protection.
- Gn 51‐1 Char: IEC‐NI (It refers to the normal inverse curve type)
Gn 51‐1 Time multiplier (IEC/ANSI): 0.1 TMS
Gn 51‐2 Element: Enabled
- Gn 51‐2 Setting: Set to 0.4 xIn [0.4 x 300=120 A]
- Gn 51‐2 Char: DTL (itrefers to the definite time lag curve type)
Gn 51‐2 Time multiplier (IEC/ANSI): 0.1 S
Derived Earth fault (50N/51N Elements)
The earth current is derived by calculating the sum of the measured line currents. The elements measure the fundamental frequency RMS current.
1) Instantaneous Derived Earth Fault Protection (50N):
Two instantaneous derived earth fault elements are provided in the 7SR10 Overcurrent and Earth Fault relay. 50N-1, 50N-2 Each instantaneous element has independent settings for pick-up current 50N-n Setting and a follower time delay 50N-n Delay. The instantaneous elements have transient free operation.
2) Time Delayed Derived Earth Fault Protection (51N):
Two time-delayed derived earth fault elements are provided in the 7SR10 Overcurrent and Earth Fault relay.
51N-1, 51N-2
51N-n Setting sets the pick-up current level.
A number of shaped characteristics are provided.
- An inverse definite minimum time (IDMT) characteristic is selected from IEC and ANSI curves using 51N-n Char. A time multiplier is applied to the characteristic curves using the 51N-n Time Mult setting.
- Alternatively, a definite time lag delay (DTL) can be chosen using 51N-n
Char. When definite time lag (DTL) is selected, the time multiplier is not applied and the 51N-n Delay (DTL) setting is used instead.
An earth fault protection system with a “51N” first stage and a second stage works like a two-layer safety net for ground faults. The first stage is highly sensitive and quickly trips the circuit breaker for smaller faults, stopping them before they become a bigger problem. If a larger, more severe fault occurs—one that the first stage might not fully catch—the second stage kicks in with a higher current threshold, acting as a backup to ensure the system stays protected.
For Earth Fault (EF) settings, we usually set them at 10-30% of FLC since earth faults typically have lower fault currents.
Recommended E/F Settings (Primary Side)
50N-1 (Instantaneous Earth Fault): 10-20% of FLC, i.e., 5 – 10 A (Primary) or 0.017 – 0.033 A (Relay)
50N-2 (High Set Earth Fault): 30-40% of FLC, i.e., 15 – 20 A (Primary) or 0.05 – 0.067 A (Relay)
51N-1 (Inverse Time Earth Fault – Low Set): 5-10% of FLC, i.e., 2.5 – 5 A (Primary) or 0.008 – 0.017 A (Relay)
51N-2 (Inverse Time Earth Fault – High Set): 10-20% of FLC, i.e., 5 – 10 A (Primary) or 0.017 – 0.033 A (Relay)
Time Delay: 0.2 – 1 sec, depending on grading with downstream relays.
Setting :
Gn 51N‐1 Element : Enabled
Gn 51N‐1 Setting :0.05 x In (15A)
Gn 51N‐1 Char : IEC NI
Gn 51N‐1 Time multiplier (IEC/ANSI) : 0.08 TMS
Gn 51N‐1 Minimum operating time : 0 sec
Gn 51N‐1 Follower DTL: 0 sec
Gn 51N‐2 Element: Enabled
Gn 51N‐2 Setting: 0.05 xIn(15A)
Gn 51N‐2 Char: DTL
Gn 51N‐2 Delay (DTL): 0 sec
Gn 51N‐2 Minimum operating time: 0 sec
Gn 51N‐2 Follower DTL: 0 sec
Summary of Phase Overcurrent (50/51) Settings
Usage Context
- When to Use 50 (Instantaneous):
For detecting and clearing severe faults quickly without intentional delay. Recommended only for very high fault levels (e.g., Gn 50-2 at 1.6 In). - When to Use 51 (Time Overcurrent):
For gradual overcurrent conditions with time-delayed tripping, ensuring coordination with upstream and downstream devices (e.g., Gn 51-1 with TMS of 0.1).
SEF (Sensitive Earth Fault) Protection
A Sensitive Earth Fault (SEF) protection system kicks in when a tiny earth fault happens in an electrical circuit—something regular protection might miss. It’s mainly used in high-resistance grounded systems where fault currents are too low for standard detection. The system works by using a Core Balance Current Transformer (CBCT) to monitor the combined current across all phases. If it detects even a small imbalance, it triggers an alarm or shuts down the circuit to prevent further issues.
Recommended Settings
SEF is used for high-resistance grounded systems where earth fault currents are very low (typically below 1A primary).
- SEF First Stage (Alarm Only) (51N-SEF-1): 0.5 – 1% of FLC, i.e., 0.3 – 0.5 A (Primary) or 0.001 – 0.002 A (Relay)
- Time Delay: 1 – 3 seconds
- SEF Second Stage (Trip) (51N-SEF-2): 1 – 2% of FLC, i.e., 0.5 – 1 A (Primary) or 0.002 – 0.003 A (Relay)
- Time Delay: 0.5 – 2 seconds
Remarks:
The recommended settings prioritize simplified earth fault protection and improved stability by disabling SEF elements. If SEF is essential, coordination with downstream devices should be reviewed to retain these elements.
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