Chapter 3 - MSDG Interconnection Requirements and Safety Aspects

3.1 Interconnection Facility Characteristics

The facility shall have the following characteristics:

• The MSDG facility is connected to the CEB's 22 kV network through a dedicated 22/0.415 kV transformer.
• Metering is to be performed on the Low Voltage (LV) side.

3.2 Interconnection Facility Design 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. The CEB LV grid is designed as a TT system.

Table 1: Design parameters under normal conditions

CEB is responsible to maintain those values 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:

Table 2: Design parameters under incident and emergency conditions

3.3 Protection Requirements

3.3.1 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 chapters 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 originated 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.

The protection system shall provide protection against fault occurring on both the CEB's network and the MSDG facility. The protection system is to provide protection against short circuit, earth faults and overloading conditions and also prevent islanding operation of the CEB distribution feeder.

In addition, the applicant must provide any additional protection functions necessary to adequately protect all equipment and personnel. The settings of the additional protection systems must be appropriately graded to prevent unnecessary trips during disturbances that affect voltage and frequency on the CEB system. Any modifications in the protection settings carried out by the MSDG promoter shall be communicated to CEB.

3.3.2 Availability of protection

The applicant shall ensure that all equipment is protected and that all elements of the protection, including associated inter-tripping, are operational at all times. Unavailability of the protection will require the MSDG plant to be taken out of service.

The MSDG shall be protected against:

1. Overload
2. Short circuit within the MSDG
3. Earth faults in the close vicinity of the MSDG
4. Over Current
5. Abnormal voltages (Table 3)
6. Abnormal frequencies (Table 3)
7. Lightning
8. Loss of mains

3.3.3 DC Functions of Protection Apparatus

All protection apparatus functions shall operate down to a level of 50% of the nominal DC supply voltage of the DC system, or the system must be able to safely disconnect and shutdown when operation conditions are outside the nominal operating DC voltage specified in the DC system specifications.

3.3.4 Protection Flagging, Indications and Alarms

All protective devices supplied to satisfy the CEB's requirements shall be equipped with operation indicators. Such indicators shall be sufficient to enable the determination of which devices caused a particular trip.

3.3.5 Trip settings

The trip settings must comply with the values stated in Table 3.

Table 3: Default interface protection settings

Figure 1: Typical layout for any MSDG of capacity greater than 50 kW and not exceeding 200 kW

3.3.6 Network Islanding

The applicant shall not supply power to the CEB's network during any outages of the system. The MSDG may only be operated during such outages to supply the applicant's own load (isolated generation) with a visibly open tie to the CEB's network. The MSDG shall cease to energise the CEB's network within 0.5 seconds of the formation of an island as shown in Table 3.

3.3.7 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 been restored within the nominal limits for at least 3 minutes. Reconnection is only allowed when disconnection was due to operating parameters being outside the normal operating range stated in Table 3, not if disconnection was caused by malfunctioning of any devices within the MSDG installation.

3.3.8 Synchronising AC generators

The MSDG shall provide and install automatic synchronizing. Check Synchronizing shall be provided on all generator circuit breakers and any other circuit breakers, unless interlocked, that are capable of connecting the MSDG plant to the CEB's network. Check Synchronising Interlocks shall include a feature such that circuit breaker closure via the Check Synchronising Interlock is not possible if the permissive closing contact is closed prior to the circuit breaker close signal being generated.

3.3.9 Earthing requirements

Earthing shall be according to IEC 60364-5-54.

For systems capable of operating in isolated generation, the neutral point of the a.c. generator must not be earthed when operating in parallel with CEB's network. When the MSDG operates in isolation, the generator neutral-to-earth connection must be closed. The operation of the neutral-to-earth connection shall be carried out by an inter-locking system.

When a MSDG is operating in parallel with the CEB's network, there shall be no direct connection between the co-generator winding (or pole of the primary energy source in the case of a PV array or Fuel Cells) and the CEB's earth terminal.

The winding of an a.c. generator must not be earthed. Note that a DC source or DC generator could be earthed provided the inverter separates the AC and DC sides by at least the equivalent of a safety isolating transformer. However, consideration would then need to be given to the avoidance of corrosion on the DC side.

At the CEB's grid TT earthing system is adopted. The neutral and earth conductors must be kept separate throughout the installation, with the final earth terminal connected to a local earth electrode.

The Busbar System, referred to as the Joint Use Facility in Figure 1, shall be equipped with visible lockable earthing facility, with appropriate labelling, padlock and isolation procedure.