Chapter 3 (Part 2) - Power Quality & Protection
3.4 Power Quality
The MSDG facilities and equipment shall not cause excessive voltage excursions nor cause the voltage to drop below or rise above the range maintained by CEB. The MSDG facility and equipment shall not introduce excessive distortion to the sinusoidal voltage or current waves.
3.4.1 Limitation of DC injection
The MSDG shall not inject a DC current greater than 0.25 % of the rated AC output current per phase.
3.4.2 Limitation of voltage flicker induced by the MSDG
The MSDG installation shall not cause abnormal flicker beyond the limits defined by the "Maximum Borderline of Irritation Curve" specified in the IEEE 1453.
3.4.3 Harmonics
The total harmonic distortion will depend on the injected harmonic current and the system impedance seen from the PCC.
The MSDG 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 MSDG system electrical output at the PCC should comply with IEEE 519 (Recommended Practice and Requirements for Harmonic Control in Electric Power Systems) which will be used to define the acceptable distortion levels for MSDG installations connected to the utility. The IEEE 519 recommendation is to be applied at the PCC, not to downstream equipment.
Table 4: Recommended Harmonic Voltage Limits
| Bus voltage V at PCC | Individual harmonic (%) | Total harmonic distortion THD (%) |
|---|---|---|
| V ≤ 1.0 kV | 5.0 | 8.0 |
| 1 kV < V ≤ 69 kV | 3.0 | 5.0 |
Table 5: Maximum Harmonic Current Distortion
| ISC/IL | Individual harmonic order (odd harmonics) | TDD | ||||
|---|---|---|---|---|---|---|
| 3 ≤ h < 11 | 11 ≤ h < 17 | 17 ≤ h < 23 | 23 ≤ h < 35 | 35 ≤ h ≤ 50 | ||
| < 20 | 4.0 | 2.0 | 1.5 | 0.6 | 0.3 | 5.0 |
| 20 < 50 | 7.0 | 3.5 | 2.5 | 1.0 | 0.5 | 8.0 |
| 50 < 100 | 10.0 | 4.5 | 4.0 | 1.5 | 0.7 | 12.0 |
| 100 < 1000 | 12.0 | 5.5 | 5.0 | 2.0 | 1.0 | 15.0 |
| > 1000 | 15.0 | 7.0 | 6.0 | 2.5 | 1.4 | 20.0 |
3.4.4 Surge Withstand Capability
The interconnection system shall have a surge withstand capability, both oscillatory and fast transient, in accordance with IEC 62305-3 and IEC 62305-4, 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.
3.4.5 Voltage Unbalance
The total voltage unbalance in the grid should be smaller than 2%, where the unbalance, Uunbalance, is defined as the maximum deviation from the average of the three-phase voltages, Ua, Ub and Uc, divided by average of the three-phase voltages.
The contribution from the MSDG installation may not cause an increase of the voltage unbalance of more than 1.3%.
3.4.6 Voltage Step Change
The process of starting a medium scale distributed generation (MSDG) 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. 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.
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.
3.5 Power Factor
The power factor of the MSDG at normal operating conditions across the statutory range of nominal voltage shall be between 0.95 leading and 0.95 lagging.
3.6 Maintenance
3.6.1 Generation Forecast
Generators under 200 kW do not need to do Generation Forecast in order to communicate it to CEB.
3.6.2 Generation Maintenance
Generators under 200 kW do not need to communicate the maintenance plans to CEB, for its approval.
3.6.3 Network Maintenance
The MSDG owner shall disconnect its MSDG system during the maintenance of the network by CEB.
For preventive maintenance and corrective action, no compensation will be applied for the loss of generation.
CEB will communicate their maintenance plans alike the general clients.
3.7 Safety, Isolation and Switching
3.7.1 Rules for working on low voltage grid (LV)
The safety of the personnel working on the network should comply with the "CEB T&D Safety Rules".
The "CEB T&D Safety Rules" are in compliance with the Occupational Safety and Health Act 2005 or any subsequent amendment(s) in connection with this present Act.
According to the CEB safety rules based on Occupational Safety and Health Act 2005, the following rules must be respected before working on a LV grid:
- The system must be made DEAD, 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 MSDG 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 switches or breakers. These switches 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 not be possible for persons other than the CEB authorized personnel, which can be assured by keeping the keys to lockable switches. Alternatively the CEB's authorized personnel will remove and keep fuses while its personnel are working on lines.
- In all circumstances the switches, which must be manually operated, must be capable of being secured in the 'OFF' isolating position. The switches must be located at an easily accessible position in the producer's installation.
- The Busbar system, referred to as the Joint Use Facility, shall be accessible to the CEB on a 24 Hours basis for switching and isolation operations by CEB personnel.
- All MSDG installations should be labelled with proper signage at appropriate locations.
- The number and sequence of procedures shall be affixed in the Joint Use Facility and shall be followed at all times to ensure that correct switching operation during both the earthing of the busbar system and isolated generation setup.
- The CEB will maintain an updated register of all MSDG installations with precise addresses, connecting points and relevant transformers.
3.7.2 Safety Concerns
For any work or activity in the connection point between distributed generation and the CEB network, all agents involved will follow the "CEB T&D Safety Rules".
The "CEB Safety Rules" are based on the Occupational Safety and Health Act 2005. In addition to the requirements stipulated in the "CEB Safety Rules", the MSDG owner shall observe the following safety concerns:
- Persons must be warned that the installation includes any MSDG so that precautions can be taken to avoid the risk of electric shock. Both the mains supply and the electric generator must be securely isolated before electrical work is performed on any part of the installation. Adequate labelling must be available to warn that the installation includes 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 shall be taken to prevent the generator from generating electric energy or even generating voltage, 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.
- The manufacturer or supplier of the MSDG is required to certify compliance with the Electrical Equipment Safety Regulations and the Electromagnetic Compatibility Regulations. The MSDG will be CE marked or tested by equivalent accredited testing agencies to confirm this. This should ensure that the MSDG is satisfactory in a domestic installation in terms of the power factor, generation of harmonics and voltage disturbances arising from starting current and synchronisation.
- The maintenance works at the MSDG installation shall be carried out in such a way that all safety procedures recommended by the manufacturer or supplier are followed.
- CEB personnel must be warned of the safety procedures pertaining to switching operation applicable to the MSDG. These procedures must clearly be displayed and visible at the MSDG site.