Chapter 3: MSDG Interconnection Requirements (Part 2)
3.6 Protection Requirements (continued)
3.6.2 Interconnection protection scheme
The Interconnection Protection Scheme must provide protection against short circuit, earth fault and overloading conditions. This scheme shall consist of the following protection functions:
- Multi steps instantaneous/time delayed and IDMT overcurrent (50/51)
- Multi steps instantaneous/time delayed and IDMT earth fault (50N/51N)
- Neutral Voltage Displacement (59N)
The protection relay for CB1 (see Annex 7) must comprise of multi-stages for both overcurrent and earth fault:
- The I>/Io> (low set for overcurrent and earth fault) elements must provide a selection of inverse definite minimum time (IDMT) curves, or be settable to a fixed (definite) time delay.
- The I>>/Io>> (high set for overcurrent and earth fault) elements must provide both time delayed and instantaneous functions.
The characteristics for these protection functions are detailed in Annex 7 (section 7.7 of this Grid Code). The protections 50/51 and 50N/51N shall act on the interconnection circuit breaker of the CEB Interconnection facilities (CEB MSDG Circuit Breaker (CB1)) and the protection 59N shall act on all client's outgoing circuit breakers of the Interconnection facilities.
The settings for the protection functions 50/51 and 50N/51N shall be determined by the applicant, through a proper protection study, and submitted to CEB for information.
The setting for the neutral voltage displacement is to be calculated as follows:
Table 4: NVD trip settings
| Parameter | Symbol | Trip setting | Clearance |
|---|---|---|---|
| Neutral Voltage Displacement (59N) | NVD | Subject to clearance time, trip signal to be issued when the neutral point voltage displacement on the 22 kV side exceeds 10% of 22kV/√3 (1,270 V). | 5.5 s |
CEB reserves the right to set updated settings for the NVD relay based on network parameters.
Under normal setup, the NVD relay shall be driven from PTs on the client side of the MV switchgear.
Exceptionally, where it is practically not possible to install PTs on the client side, the NVD relay (client side) may be driven by the PT protection core (CEB side), via a set of Test Terminal Block (TTB) on the CEB side of the switchgear.
Protection settings shall be adjusted so that the protections closer to the fault act first and the second level protections only act if the short circuit condition persists. Thus, the time settings shall be adjusted in coordination with and to the satisfaction of CEB.
Note: The protection requirement for the step-up transformer shall be determined by the applicant.
Under certain circumstances, if in case discrimination of setting is not achieved and for MSDG installations having synchronous and/or induction machines, CEB reserves the right to request the client to install directional protection relays which shall act on CB1.
Directional Over-Current and Earth-fault Relay
The relay shall be numerical type with appropriate current and voltage inputs to achieve directional protection.
The relay shall provide at least three independent stages for overcurrent (I>, I>>, I>>>) and earth fault (Io>, Io>>, Io>>>).
The time/current characteristics associated with the low stage elements (I> and Io>) must provide a selection of inverse definite time (IDMT) curves and definite time delay (DT).
The high set elements (I>>, I>>>, Io>> and Io>>>) must be settable to definite time and instantaneous.
Directional Control shall be determined by Voltage Polarization Principle for both Over-Current and Earth Fault and should be settable for both forward and reverse direction of the fault.
3.6.3 Anti-islanding protection
The MSDG shall not supply power to the CEB's network during any outages of the system. The MSDG shall cease to energise the CEB's network within 0.5 seconds of the formation of an island. The following protection functions and settings are required:
- Over and under frequency (functions 81O and 81U)
- Three-phase under-voltage and overvoltage (functions 27 and 59)
- Loss of mains protection: Rate of Change Of Frequency (ROCOF) and/or Voltage vector shift (VVS)
Table 5: Anti-islanding protection trip settings
| Parameter | Symbol | Trip setting* | Clearance |
|---|---|---|---|
| Overvoltage (27) | U>> | Vφ-φ + 9% | 0.2 s |
| Overvoltage (27) | U> | Vφ-φ + 6.0% | 1.5 s |
| Undervoltage (59) | U< | Vφ-φ – 10% | 3.0 s |
| Overfrequency (81O) | f> | 52 Hz | 0.5 s |
| Underfrequency (81U) | f< | 47 Hz | 3 s |
| Loss of mains | LoM | 2.5 Hz/s (ROCOF) 10 degrees (Vector Shift) |
0.5 s 0.5 s |
NB: Vφ-φ is the nominal line-to-line voltage at generator terminals
* The above trip settings are indicative and may be subject to change upon request of the CEB for safe interconnection to the network.
The anti-islanding protection shall act on CB4 (MSDG circuit breaker) as shown in the typical switchgear and protection arrangement in Annex 8 (section 7.8 of this Grid Code).
In case inverters are used, the anti-islanding protection of the inverters may be acceptable if the inverters satisfy the standards required by CEB and set forth in the Grid Code.
For MSDG facilities of capacity equal to or greater than 1 MW, inter-tripping facility using fiber optic cables or wireless communication will be required. This is detailed further in section 3.6.4.
3.6.4 Inter-tripping protection for MSDG of capacity equal to or greater than 1 MW
The inter-tripping scheme shall be designed and pre-wired such that tripping of the interconnecting feeder circuit breaker in the CEB 22 kV substation results in the tripping of CB1 (see Annex 8, in section 7.8 of this Grid Code). The tripping of CEB's 22 kV circuit breaker shall be a tripping due to protective relay action at CEB 22 kV substation level. Manual opening and tripping due to protective relay of CB1 shall not cause tripping of corresponding circuit breaker at CEB 22 kV substation. However, the above scheme shall be wired but disabled initially.
So as to harmonise MSDG installations with existing solar PV farms, the following indicative tripping scheme shall be implemented for MSDG installations employing solar PV systems:
- During the day, upon tripping of the 22 kV circuit breaker at the respective CEB substation on fault, CEB System Control operator shall open CB1 remotely. CB1 intertrips CB2 (refer to typical schematic diagram in Annex 8). Upon supply restoration, CEB System Control operator shall reclose CB1 remotely and liaise with the contact person at the MSDG site to reclose CB2 locally.
- During the night, upon tripping of the 22 kV circuit breaker at the respective CEB substation on fault, CEB System Control operator shall not open CB1 as there is no PV generation at night, and hence no fault contribution due to the PV installation.
- In case of abnormal setup (MSDG shifted to another feeder), CEB System Control shall adopt the same philosophy as above.
For MSDG using RE technologies other than solar PV systems, generation occurs throughout the day and night. Hence, upon tripping of the 22 kV circuit breaker at the respective CEB substation on fault, CEB System Control operator shall open CB1 remotely, irrespective of the time of occurrence of the fault.
CEB reserves the right to amend the above indicative procedure depending on operational constraints and RE technologies employed.
Reliable communication is required between CEB System Control Centre and the MSDG site.
Communication scheme shall be set as per section 3.11.
3.6.5 Protection against relay malfunction
The watchdog function of the protection relay protection must issue an alarm and trip the circuit breaker on which the protection relay normally acts in case there is a malfunction.
For MSDG of capacity equal to or greater than 200 kW, this alarm signal, if required by CEB, shall be transmitted to the interconnecting CEB substation via the fibre optic channel or wireless communication.
3.6.6 Protection Settings: Grading and Discrimination
For MSDG of capacity above 200 kW, the Applicant shall submit to CEB appropriate settings for grading and discrimination of the interconnecting protection (22 kV circuit breaker, CEB side) with the upstream CEB substation protection.
The applicant shall also submit to CEB the fault contribution (both single phase to earth and three phase) on 22 kV side from the generating plant.
3.7 Additional Protection and Safety requirements
The protection requirements set forth in the previous sections are mandatory on all MSDG irrespective of the generation technology used. In addition to mandatory safety interlocks as per IEC 62271‑200, for metal-enclosed MV switchgear, appropriate interlocking mechanism shall be incorporated between the circuit breakers on the CEB and Client side as a measure of protection against an incorrect sequence of manoeuvres by operating personnel. This interlocking mechanism shall prevent the option of mechanically closing CB1 onto a live busbar on Client Side via a mechanical interlocking system between CB1 and CB2. However CEB may request additional interlocking and protection systems for safety reasons.
The MSDG Owner shall be required to demonstrate the incorporation of the above safety interlocking mechanism both at design and implementation/commissioning stages.
In case the MSDG contains synchronous and/or induction machines, additional measures listed below are required:
- A dead-bus/live-line check synchronism relay shall be provided to prevent remote/electrical closure of CB1 as long as the MSDG-side 22 kV busbar is energised.
- Check Synchronizing shall be provided on all generator circuit breakers and any other circuit breakers (including low voltage circuit breakers), unless interlocked, that are capable of connecting the MSDG plant to the CEB's network.
3.8 Re-connection
Following a protection initiated disconnection, the MSDG is to remain disconnected from the network until the voltage and frequency at the supply terminals has remained within the nominal limits for at least 3 minutes. Automatic reconnection is only allowed when disconnection was due to operating parameters being outside the normal operating range stated in Table 1, not if disconnection was caused by malfunctioning of any devices within the MSDG installation.