Chapter 3 - Grid Code Requirements and Safety Aspects

1.0 Design Parameters

The SSDG shall be connected to the 230/400 V system and operated within the parameters as listed in Table 1 below. The SSDG has to be functioning and protect itself within the range of the voltages, currents and frequencies existing in the CEB's grid.

Description	Range
Voltage	230/400 V ± 6 %
Short Circuit Characteristics	(1 sec) 18 kA, (50 Hz)
Nominal Frequency	50 Hz
Statutory Frequency Deviation	50 Hz ±1.5 %
Operating Frequency Range	47 Hz – 52 Hz

Table 1: Normal operating parameters of the CEB's Low Voltage grid

The CEB LV grid is designed as a TT system.

Note: The above values are mandatory for all SSDGs. Same values and provisions apply in the case that the SSDG has a battery set.

2.0 Protection Requirements

The coordination and selectivity of the protection system must be safeguarded even with the entrance of new generation into the system. To guarantee this requirement, the protections to be installed are listed in the following chapters and the settings of those protections shall conform at minimum to the requirements of the Grid Code.

In case of short circuits in the generator's side, the SSDG shall adjust its protections in such a way that will avoid unnecessary trips and at the same time avoiding that the incident propagates to the CEB LV network.

In case of incidents originated external to generator's system, like short circuits in the distribution system, fluctuations of frequency or voltage, Generators will give priority to the network protections to clear the incidence and act accordingly with the coordination and selectivity principles of the protections system.

2.1 Availability of Protection

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

The SSDG shall be protected against:

Overload
Short circuit within the SSDG
Earth faults in the LV grid close vicinity of the SSDG
Over Current
Abnormal Voltages (Table 2)
Abnormal Frequencies (Table 2)
Lightning
Loss of Mains

2.2 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.

2.3 Protection Flagging, Indication 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 identification of which devices caused a particular trip.

Any failure of the applicant's tripping supplies, protection apparatus and circuit breaker trip coils shall be supervised within the applicant's installation, and the applicant shall be responsible for prompt action to be taken to remedy such failure.

2.4 Trip Settings

The basic trip settings must comply with the values stated in Table 2.

Parameter	Symbol	Trip Setting	Clearance Time
Over Voltage (a)	U>>	230 V + 10 %	0.2 s
Over Voltage	U>	230 V + 6 %	1.5 s
Under Voltage	U	230 V – 6 %	1.5 s
Over Frequency (b)	f>	50 Hz + 2 %	0.5 s
Under Frequency	f	50 Hz - 6 %	0.5 s
Loss of Mains	df/dt Vector shift	2.5 Hz / s 10 degrees	0.5 s

Table 2: Default interface protection settings.

NOTE: Voltage and frequency is referenced to the Supply Terminals.

(a) If the SSDG can generate higher voltage than the trip setting, the step 2 over voltage is required.

(b) The trip setting for over frequency is set lower than the maximum operating frequency defined in Table 1 in order to avoid contribution of the SSDG to rising frequency.

2.5 Network Islanding

The applicant shall not supply power to the CEB's network during any outages of the system. The SSDG 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 SSDG shall be disconnected from the CEB's network within 0.5 seconds of the formation of an island as shown in Table 2.

2.6 Re-connection

Following a protection initiated disconnection, the SSDG 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 SSDG installation.

2.7 Synchronizing AC generators

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

2.8 Earthing Requirements

Earthing shall be according to IEC 60364-5-55.

For systems capable of operating in isolated generation protection by automatic disconnection of supply shall not rely upon the connection to the earthed point of the utility supply system.

When a SSDG is operating in parallel with the CEB's network, there shall be no direct connection between the cogenerator 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 d.c. source or d.c. generator could be earthed provided the inverter separates the a.c. and d.c. 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 d.c. side.

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

Warning: Notice that "CONDUCTORS MAY REMAIN LIVE WHEN ISOLATOR IS OPEN" shall be conspicuously displayed at the installation.

3.0 Power Quality

3.1 Limitation of DC Injection

The SSDG should not inject a DC current greater than the largest value of 20 mA and 0.25 % of the rated AC output current per phase.

3.2 Limitation of Voltage Flicker induced by the SSDG

The SSDG installation shall not cause abnormal flicker beyond the limits defined by the "Maximum Borderline of Irritation Curve" specified in the IEEE 519-1992.

3.3 Harmonics

Based on IEEE 519, the Total Harmonic Distortion (THD) Voltage shall not exceed 5.0% of the fundamental on 400 V when measured at the point of common coupling (PCC).

The total harmonic distortion will depend on the injected harmonic current and the system impedance seen from the PCC. However, in order to facilitate the fulfilment of the requirements by e.g. inverter manufacturers, the voltage distortion limits have been translated into a similar requirement on current distortion.

The SSDG system output should have low current-distortion levels to ensure that no adverse effects are caused to other equipment connected to the utility system. The SSDG system electrical output at the PCC should comply with Clause 10 of IEEE Std. 519-1992 and should be used to define the acceptable distortion levels for PV systems connected to a utility. The key requirements of this clause are summarized in the following:

Total Harmonic Current Distortion (Total Demand Distortion, TDD) shall be less than 5% of the fundamental frequency current at rated current output.
Each individual harmonic shall be limited to the percentages listed in Table 3. The limits in Table 3 are a percentage of the fundamental frequency current at rated current output.
Even harmonics in these ranges shall be

Odd Harmonics	Maximum Harmonic Current Distortion
3^rd - 9^th	4.0%
11^th - 15^th	2.0%
17^th - 21^st	1.5%
23^rd - 33^rd	0.6%
Above the 33^rd	0.3%

Table 3: Distortion Limits as recommended in IEEE Std. 519-1992 for six-pulse converters

3.4 Surge Withstand Capability

The interconnection system shall have a surge withstand capability, both oscillatory and fast transient, in accordance with IEC 62305-3, the test levels of 1.5 kV. The design of control systems shall meet or exceed the surge withstand capability requirements of IEEE C37.90.

3.5 Voltage and Current Unbalance

The connection of unbalanced loads and generation to the distribution network can result in unbalanced currents and voltages. Generators that use 3-phase generators or inverters which inject balanced currents into the distribution network do not increase levels of voltage imbalance in the network. In fact, embedded generators which use 3-phase induction generators can actually reduce voltage imbalance.

The total voltage unbalance in the grid should be smaller than 2%, where the unbalance, U_unbalance, is defined as the maximum deviation from the average of the three-phase voltages, U_a, U_b and U_c, divided by average of the three-phase voltages.

U_unbalance = Max(U_a, U_b, U_c) − U_avg(a,b,c) U_avg(a,b,c) × 100%

The contribution from one installation may not cause an increase of the voltage unbalance of more than 1.3%.

When considering three phase units, the contribution to the voltage unbalance can be described as:

U_unbalance = √3 × I_{neg seq load} × U_line S_sc

I_{neg seq load} = √3 × U_unbalance(%) × U_line S_sc

Where:

S_sc is the Three phase short circuit power
I_{neg seq load} is Negative sequence of component loads
U_line is the line voltage
U_unbalance is the voltage unbalance

If nothing else is stated the S_sc shall be 2.5 MVA. The demand on voltage unbalance on a three phase load can be translated into a demand on the maximum negative sequence current.

I_{max neg seq load} = √3 × 1.3% × 400 2.5 = 3.6 A

3.6 Voltage Step Change

The process of starting an SSDG can sometimes cause step changes in voltage levels in the distribution network. These step changes are caused by inrush currents, which may occur when transformers or induction generators are energised from the network. Synchronous generators do not give rise to inrush currents themselves, but their generator transformers may do so if they are energised from the network. Step voltage changes will also occur whenever a loaded generator is suddenly disconnected from the network due to faults or other occurrences.

Step voltage changes caused by the connection and disconnection of generating plants at the distribution level, should not exceed ±3% for infrequent planned switching events or outages and ±6% for unplanned outages such as faults.

If the connection of the SSDG to the grid does not exceed the following values in Table 4 it is expected to stay within the above mentioned voltage levels.

Connection	Inrush current
Single phase	19 A
Three phase	30 A

Table 4: Maximum inrush current

Where induction generators are used, as in fixed speed wind turbines, they shall be fitted with soft starters. These devices limit inrush currents to roughly the same level as the normal rated current. This reduces the magnitude of the step voltage changes which occur on starting.

4.0 Power Factor

The power factor of the SSDG at normal operating conditions across the statutory range of nominal voltage shall be between 0.95 leading and 0.95 lagging.

5.0 Network Maintenance

The Preventive and Corrective maintenance of the feeder where the SSDG is connected may interrupt the SSDG's generation. No compensation shall be applicable for the loss of generation. CEB will communicate their maintenance plans following the same media as for the rest of the network customers.

6.0 Safety, Isolation and Switching

6.1 Rules for working on Low Voltage (LV) grid

According to the CEB Safety Rules based on Occupational Safety and Health Act 2005, the following rules, amongst others, must be respected before working on a LV grid:

The system must be Isolated from all possible sources of supply, all switches must be locked in visibly open positions, the system must be tested on the site of work, and the system must be short-circuited and Earthed.
The SSDG shall have a local means of isolation that disconnects all live conductors including the neutral. The producer shall not energize a de-energized CEB's Power circuit.
Switches shall be installed to disable the automatic or manual closing of the interconnecting switch or breaker. This switch shall be accessible to the CEB's personnel to obtain the necessary safety requirements when the CEB's personnel is working on associated equipment or lines. While the CEB's personnel is working on the grid, the operation of switches shall be restricted to the CEB only. This can be assured by keeping the keys of lockable switches in safe custody. Alternatively the CEB's personnel will remove and keep fuses while working on lines.
In all circumstances the switch, which must be manual, must be capable of being secured in the "OFF" isolating position. The switch must be located at an easily accessible position in the producer's installation.
The visible switch shall be visibly marked. Also all transformers that carry SSDG installations on the LV side shall be visibly marked. Additionally the CEB will maintain an updated register of all SSDGs with precise addresses, connecting points and relevant transformers.

6.2 Safety Concerns

The SSDG owner shall observe the following safety concerns:

Persons must be warned that the installation includes a SSDG so that safety precautions should be taken to avoid the risk of electric shock/electrocution. Both the mains supply and the micro-generator must be securely isolated before electrical work is performed on any part of the installation. Adequate labelling must be placed to warn that the installation is connected to another source of energy.
Photovoltaic (PV) cells will produce an output whenever they are exposed to light, and wind turbines are likely to produce an output whenever they are turning. Additional precautions such as covering the PV cells or restraining the turbine from turning will be necessary when working on those parts of the circuit close to the source of energy and upstream of the means of isolation.

To guarantee this isolation, the generator operator shall follow the supplier instructions or propose any other means to guarantee it.
The manufacturer or supplier of the SSDG is required to certify compliance with the Electrical Equipment Safety Regulations and the Electromagnetic Compatibility Regulations. The SSDG will be CE marked or tested by equivalent accredited testing agencies to confirm this. This should ensure that the SSDG is satisfactory in a domestic installation in terms of the power factor, generation of harmonics and voltage disturbances arising from starting current and synchronization.

6.3 Electromagnetic Emission / Immunity

The SSDG shall comply with the requirements of the EMC Directive and in particular the product family emission standards.

7.0 Metering

In order to calculate the export or import of the applicant, a bidirectional meter (Import/Export Meter) measuring both the imported and exported energy shall be installed.

A second meter (Production Meter) measuring the gross energy production of the SSDG shall be installed.

The Import/Export Meter and the Production Meter shall be installed next to each other and be easily accessible to CEB personnel.

Note: Greenfield installation shall not be entertained in this project.

8.0 Testing, Commissioning and Maintenance

Testing and commissioning of SSDGs will be done in the presence of the CEB. The applicant shall notify the CEB in advance with a testing and commissioning plan. The applicant shall keep written records of test results and protection settings. The applicant shall regularly maintain their protection systems in accordance with good electrical industry practice.

9.0 Standards and Regulations

All electrical apparatus, materials and wiring supplied shall comply with the Electricity Act, the Central Electricity Board Act, Electricity Regulations, this code and relevant international standards including but not limited to IEC and IEEE standards for PV modules, inverters, grid-connected systems, power quality, and general engineering standards.

Note: All specifications shall be according to the latest edition of the standards. The applicant may be required to produce certificates conforming to the above standards for their SSDG installation upon CEB's request.