JSC Energy Advice director, dr. Vytautas Šiožinys
JSC Energy Advice project manager, Mantas Kaminickas
Renewable energy power generation is constantly growing as a result of promotional policies and development of renewable energy harvesting technologies. However, intermittent generation of renewable energy power plants can cause power quality problems and other challenges that must be coped for the successful system integration. JSC Energy Advice collaborate with companies that are facing technical or general type problems related renewable energy power plants integration. Therefore, this article summarizes our accumulated knowledge in the field of technical renewable power plants integration aspects. While majority of these aspects are regulated by international standards, we will also provide a comprehensive list of the relative standards.
Renewable energy power plants can have the significant impact on the power quality. Two of the most important international standards that regulates power quality requirements are EN 50160 „Voltage characteristics of electricity supplied by public distribution networks“ and IEC61000-3-7 „Limits – Assessment of emission limits for the connection of fluctuating installations to MV, HV and EHV power systems“. However, list is not limited to only these two standards. Some counties use international IEC standards, while others have local standards that also provide useful technical details and best practice for renewable power systems (RES) design and exploitation. The list is provided in the table below.
|IEC 61727-2004||PV systems—characteristics of the utility interface||International Electrotechnical Commission|
|IEEE1547||IEEE Standard for interconnecting DRs with electric power systems||The Institute of Electrical and Electronics Engineers|
|IEEE 929||Recommended practice for utility interface of PV systems||The Institute of Electrical and Electronics Engineers|
|Rule 21-2014||Generating facility interconnections||California of USA|
|CSA C22.3 No. 9-08 (R2015)||Interconnection of DRs and electricity supply systems||Canada|
|CAN/CSA-C22.2 No. 257-06 (R2015)||Interconnecting inverter-based micro-DRs to distribution systems||Canada|
|ENA Engineering Recommendation G83 Issue 2-2012 (Erec G83)||Recommendations for the connection of type tested SSEG (up to 16 A per phase) in parallel with LV distribution systems||UK|
|DIN V VDE V 0126-1-1 (VDE V 0126-1-1)-2013-08||Automatic disconnection device between a generator and the public LV grid||German|
|VDE-AR-N 4105 -2011-08||Power generation systems connected to the LV distribution network—Technical minimum requirements for the connection to and parallel operation with LV distribution networks||German|
|RD 1663/200||Interconnection of PV installations to the LV grid||Spain|
|AS 4777.1-2005||Grid connection of energy systems via inverters||Australia|
|AS 4777.2-2005||Part 1: Installation requirements|
|AS 4777.3-2005||Part 2: Inverter requirements|
|(including the new version of AS 4777)||Part 3: Grid protection requirements|
|GB/T 19939-2005||Technical requirements for grid connection of PV system||China|
|GB/T 19964-2012||Technical requirements for connecting PV power station to power system||China|
|GB/T 20046-2006||PV systems—characteristics of the utility interface||China|
|CNS 15382||PV systems—characteristic of the utility interface||Taiwan|
|TPC Technical Guideline||Taiwan Power Company Technical Guideline for integrating renewable energy with the grid||Taiwan|
|KEPCO Technical Guideline||Korea Electric Power Corporation Technical Guideline for integrating DRs with the grid||Korea|
|JEAC 9701-2012||Grid-interconnection Code||Japan|
By summarizing information provided in these standards and our practical experience we will list the most common effects of RES to electric network:
1) Voltage fluctuation and voltage regulation problem.
2) Changes in feeder voltage profiles, including voltage rise and unbalance.
3) Changes in feeder loading, including overloading of the system components.
4) Malfunctioning of the voltage regulation equipment, including load tap changers, line voltage regulators, and capacitor banks.
5) Variation of reactive power flow upon the operation of switched capacitor banks.
6) Variation of the power factor in a grid.
7) Current and voltage harmonics.
8) Injection of dc current.
9) Malfunction of overcurrent and overvoltage protection devices.
10) Islanding detection and islanding operation.
11) Reverse power flow.
12) Reliability and security of distribution systems.
13) Planning and dispatch of distribution systems.
The severity of these effects varies with the power plant penetration, the location of the RES generation and other electrical characteristics of the distribution system. Of these, overvoltage on feeders is the most generally problematic effect of RES integration. Impact of all these effects to the utility or industrial electric networks can be analyzed with a power system modeling software EA-PSM. Further on we will summarize these effects in greater detail.
Consistent with the IEC 61727 and IEEE 1547 (2003) standards, the voltage of a RES is generally not regulated. Nonetheless, the system voltage following RES integration must not exceed the relevant limits defined in EN 50160. Also, most other standards require that voltage variations at the electricity grid following the integration of RES must not exceed a certain value, which is typically between 3% to 5%. This voltage variation is related not only to power quality problems but also to active power losses in distribution/transmission lines.
Most standards have requirements to maintain >0.9 power factor. However, modern inverters, based on voltage source conversion technologies (IGBT) are capable to control power factor within a range of 0.95 leading and 0.95 lagging or greater. Therefore, power factor regulation becomes less of a problem nowadays.
International standards, such as IEC 61727, IEEE 929, RD 1663, and Chinese GB PV standards impose no requirements on synchronization.
IEC 61727 requires that utility interface equipment is grounded in accordance with IEC 60364-7-712, which in turn requires the installation of protective equipotential bonding conductors that are parallel to, and in as close contact as possible with dc cables, ac cables, and accessories.
IEC 61727 requires that the operation of a RES system should not cause voltage flicker in excess of limits that are specified in the relevant sections of IEC 61000-3-3 (system current 16 A) and IEC 61000-3-5 (system current > 16 A).
Most RES like photovoltaic power plants are connected to power systems via power converters, causing harmonic distortion. Harmonics are limited by EN50160 and IEC61000-2-2 also IEC61000-3-6.
Interconnecting a RES with a grid through an inverter may cause the injection of dc current into the grid. DC injection may adversely influence transformers and other network devices, causing the asymmetrical magnetization and consequent overheating of the distribution transformers.
DC injection of a RES is invariably limited to within 1%, 0.5%, or 0.25% of the rated output current.
Voltage unbalance is described in EN 50160. Standard limits voltage unbalance to the level of 2%.
Islanding Protection and Reconnection
Most international standards or regulations require that a RES should automatically detect islanding and ceases to energize the utility system within a specified period. Furthermore, following a system disturbance, the RES should not energize the utility line for some time after the utility service voltage and frequency have recovered to within the specified ranges. IEC 61727 standard requires to disconnect RES in case of islanding operation within 2 s and reconnect within 20 to 300 s after voltage and grid restoration.