Chapter 3: MSDG Interconnection Requirements and Safety Aspects

3.1 Interconnection Facility Characteristics

The interconnection facility required for the MSDG facility is described hereunder:

• The MSDG facility is connected to CEB's 22 kV network through a switchgear panel and a step-up interconnection transformer.
• The "CEB Interconnection Facilities" which shall be the facilities required to interconnect the Generation Facility to the CEB 22kV Distribution System located on the CEB side of the Point of Common Coupling(PCC) / Point of Delivery,(A), as shown in Annex 7.
• The "MSDG Interconnection Facilities" which shall be the facilities required to interconnect the Generation Facility to the CEB System located on the Generation Facility side of the Point of Common Coupling (PCC) / Point of Delivery (A) as shown in Annex 7.

3.2 22 kV System Parameters

The MSDG shall have the following design parameters. The MSDG has to function and protect itself within the following range of the voltages, currents and frequencies existing on the CEB grid.

Under normal system conditions:

*The applicant should contact CEB to determine what type of system earthing is being used on the line to which his plant is proposed to be connected and adopt the system earthing type communicated by CEB.

CEB is responsible for maintaining those values inside the fixed limits using all normal means available to CEB.

After a single incident or under emergency conditions, frequency and voltage may go out of normal limits but still inside operational acceptable values. Generators must be able to operate within the following limits:

After a single incident or under emergency conditions:

3.3 MSDG High Voltage Switchgear

The switchgears shall be arranged as illustrated in Annex 7 (see section 7.5-7 of this Grid Code), consisting of CEB interconnection facilities and MSDG interconnection facilities demarcated by a metallic barrier.

The applicant shall construct, install, test and commission the complete 22kV switchboard as per the default scheme shown in Annex 7 (section 7.7 of this Grid Code), i.e. the CEB as well as the Client side. CEB will take ownership of its side after the guarantee period. Alternative connection and switchgear schemes are possible if so approved by the CEB for every particular case. Switchgear with outgoing 22kV fuses on client side shall be replaced by outgoing 22kV Circuit Breakers by the Client. All other equipment (such as PT, CT, etc. as well as appropriate spares) which are not compliant with the current HT Metering Specifications as in Annex 7, shall be replaced by the Client to ensure compliance with the MSDG Grid Code and the HT Metering Specifications. The cost of all the above modifications in the 22 kV switchgear, be it on the CEB side or on the client side, shall be borne by the client.

The switchgear shall have the following characteristic:

The detailed technical characteristics and responsibilities of the 22kV switchgear should comply with the current HT Metering specifications, as given in Annex 7 (section 7.7 of this Grid Code). The 22kV switchgear shall be approved by the CEB prior to ordering.

For existing 22kV switchgear, all modifications required on CEB side of the interconnection facility for accommodating an MSDG installation, shall be performed by CEB at the expense of the MSDG owner.

3.4 MSDG Interconnection transformer

The MSDG interconnection transformer shall be of vector group Dyn11 (Delta on High Voltage Side and Star on MSDG Side). The delta winding on the CEB side ensures that:

1. The performance and sensitivity of the earth fault protection scheme at CEB substation are not affected;
2. Triplen harmonics from the MSDG do no reach CEB's network
3. The MSDG is provided some isolation from voltage sags due to single-line-to-ground faults, allowing it to better ride through voltage sags.

Alternative transformer vector groups may be used subject to CEB approval.

The detailed specifications of the interconnection transformer are given in Annex 7 (section 7.7 of this Grid Code, Schedule A5). The transformers shall be approved by the CEB prior to ordering.

3.5 Earthing Arrangement

The earthing arrangement of the generating plant must be designed to ensure compatibility with the earthing system of the CEB distribution network. The actual earthing arrangements will also be dependent on the number of generating units in use and the generators system configuration and method of operation. The system earth connection shall have adequate electrical and mechanical capability for the duty.

In the event that the Applicant is allowed to operate independently and isolated from the CEB's system, the Applicant shall ensure that the electrical and protection systems of the facility are designed to support such mode of operation. Safety of personnel and integrity of equipment shall be guaranteed at all times during both parallel and isolated modes of operation.

Earthing systems shall be designed, installed, tested and maintained according to BS 7354 (Code of Practice for Design of high voltage open terminal stations) and BS 7430 (Code of Practice for Protective Earthing of electrical installations). Steps must be taken to prevent the appearance of hazardous step and touch potential when earth faults occur on the 22 kV network. The 22 kV earth electrodes and low voltage earth electrodes shall be adequately separated to prevent dangerous earth potentials being transferred to the low voltage network.

3.6 Protection Requirements

3.6.1 General requirements

The coordination and selectivity of the protection system must be safeguarded even with the entrance of new generation into the system. To satisfy this condition, the protections to be installed are listed in the following sections and the settings of those protections shall be proposed by the promoter and accepted by CEB.

In case of short circuits in the MSDG side, the MSDG shall adjust its protections in such a way that they will avoid unnecessary trips in the CEB's side of the interconnection facilities and at the same time avoid that the incident propagates to the general system.

In case of incidents of origin external to the MSDG system, such as short circuits in the distribution system, voltage or frequency excursions, MSDG shall give priority to the network protections to solve the incidence and act accordingly with the coordination and selectivity principles of the protections system.