Power quality grid code compliance for renewable power plants

 

The introduction of renewable power plants has introduced challenges to the management and regulation of power quality in South Africa. These challenges have been addressed by establishing power quality compliance criteria in line with local circumstances and by the introduction of a power quality guideline for renewable power plants (RPPs).

South Africa has seen significant growth in renewable energy generation in recent years. The rapid addition of solar PV and wind generation to the South African grid presents challenges to the management and regulation of power quality due to the potential power quality (PQ) impact of these generation technologies.

South Africa has a long record of managing and regulating power quality, with the local NRS 048-2 standard being one of the first national power quality standards adopted by any country. The standards locally have been primarily focused on the responsibilities of utilities and customers consuming electricity. The power quality regulatory framework in South Africa includes standards guiding utilities on compatibility levels and limits for PQ parameters – NRS 048-2, technical practices for network services providers (NSPs) – NRS 048-4, power quality instrument specifications – SABS 1816, as well as application practices for customers to deal with power quality on their plant – NRS 048-7.

None of these standards, however, were developed with renewable generation in mind, additionally the South African Grid Code for Renewable Power Plants (SAGCRPP) [1] stipulates PQ requirements that generators should meet, however, with the connection of the first round of RPPs it became evident that there were challenges relating to the process for proving compliance to PQ requirements and agreement on principles pertaining to calculating and assessing the emissions of these plants. This has resulted in some of these standards being revisited to be inclusive of the role and impact of RPPs on the grid e.g. the revision of NRS 048-4: Application guideline for network service providers was done specifically with distributed RPPs in mind and suitable changes made to the document.

Table 1: Monitoring and compliance requirement [1].
Installation level Monitoring Inverter conformance certificate
Category A & B (<5MVA) X
Category B (>5 MVA) & C X X X

Local circumstances

The development of power quality rules and compliance requirements is ideally done with local circumstances in mind. Relevant to connecting RPPs in a developing country, such as South Africa, some of these factors include limited skills, long rural radial feeders with low fault levels and aging and under-maintained network infrastructure. Any compliance requirement that is set should keep in mind that it should be practical and feasible to evaluate against these requirements.

A second factor informing the development of power quality compliance rules is the need to keep the cost burden to a minimum, so as not to unnecessarily drive up generation costs while simultaneously ensuring that NSP power quality staff are not inundated with a larger number of complex applications which would require significant time and resources to process, ultimately clogging the system and prohibiting generators from connecting to the network or receiving grid code compliance.

Meeting these local challenges entailed a process comprising defining suitable PQ grid code requirements for generators, the development of a guideline to assist RPPs to meet their PQ obligations and adaptations to the way network service providers (NSPs) manage PQ in general.

Some of the unique challenges for NSPs in managing the contribution to PQ of RPPs in South Africa are:

  • The bidding process used in the REIPPP requires that grid code compliance be granted for a 20 year window up front. This means that the NSP carries the risk and mitigation costs should a PQ problem manifest itself during this 20 year window.
  • Determining whether RPPs meet their emission limits. The measurement of the emissions contributed by a single source of distortion is a challenge in a real-life power system. With different sources of distortion located all over a network and a continuous changing network configuration, single-point measurements of harmonic active power (for example) cannot be used in the assessment of harmonic emissions contributed by the distorting source under investigation.
  • Managing the expected influx of smaller renewable generators on the South African grid.

Establishing power quality requirements

The specific power quality parameters monitored and regulated may be quite wide ranging and it is common practice internationally for customers and generators to be required to monitor and report on voltage fluctuations: flicker and rapid voltage changes; harmonic currents and voltages including inter-harmonics and disturbances greater than 2 kHz and unbalanced voltages and currents. The original requirements of the SAGCRPP included all of these parameters to be monitored and compliance requirements established.

The approach taken, based on the experience with the first few rounds of RPP bids, was to simplify the compliance requirements for generators in line with the requirements that were in place in the existing South African power quality regulations i.e. NRS 048 parts 2 and 4.

A number of the abovementioned parameters are not regulated within this framework and applying these to generators without established local benchmarks and regulation was deemed impractical and potentially unfair. In line with this, the following parameters have been removed from the grid code compliance requirements:

  • Rapid voltage changes: Standards for rapid voltage change measurement were recently introduced via the IEC 61000-4-30 ed. 3. It will take a few years before this standard is fully implemented in measurement instruments.
  • Inter-harmonics: There are no planning levels given for inter-harmonics in the South African NRS 048-2 regulations, making it impossible allocate emissions for inter-harmonics as per the apportioning methods defined in other parts of the standard.
  • Disturbances greater than 2 kHz: The assessment criteria have not been defined in South African regulation and as such there is no existing to assess against.

The simplified power quality parameters that a generator is required to monitor and report on to demonstrate PQ grid code compliance in South Africa are:

  • Flicker
  • Unbalanced voltages
  • Harmonics

The RPP power quality guideline

The RPP power quality was drafted to provide guidelines and clarity to RPPs to enable them to meet grid code compliance and contractual requirements for power quality. The guideline forms part of the SAGCRPP as an appendix to the main code.

It clarifies a number of issues that were raised with the introduction of RPPs, these include:

  • Providing a step-by-step process for PQ compliance for RPPs
  • Setting compliance requirements based on generator size
  • Establishing monitoring and evaluation requirements
  • Establishing harmonic currents as the compliance measure for harmonics
  • Introducing a group harmonic distortion requirement, this eases the burden of RPPs to meet individual harmonic limits.
  • Clarifying power quality immunity requirements for RPPs

Requirements based on generator size

It is expected that a large number of applications for connecting solar PV to the South African grid will need to be processed by both the national utility, Eskom, as well as the municipalities. This presents a challenge in terms of the regulations that smaller generators need to meet. Requirements to protect the grid and other connected load customers have to be balanced against not creating unnecessarily long and complex application procedures for RPPs. These could result in raised costs and inconvenience for small generator connection and an overload of applications to the NSP staff responsible for assessing connections.

Existing industry practice in South Africa is to apportion power quality emissions to all customers with loads of 5 MVA and up. The requirements for generators are treated similarly, so that all category A generators and category B (<5 MVA) have a simplified compliance requirement.

The simplified process for these smaller plants entails that they are not required to monitor the entire installation and have emissions apportioned. This reduces the burden of proof and the associated costs of these applications. This approach also ensures that there is consistency in the approach taken between loads and generators connected to the grid.

Monitoring and evaluation requirements

The requirement to measure and monitor power quality at the point of connection (PoC) is also based on the generator size. Along with the simplified compliance process for category A and B (<5 MVA), these plants are also not required to prove compliance at installation level or install PQ monitoring instruments Where monitoring is required, all power quality monitoring instruments shall be IEC 61000-4-30 class A compliant.

Table 1 indicates the difference between smaller generators (<5 MVA) which only require conformance certificates at equipment level as opposed to larger generators (>5 MVA) which need to monitor power quality as well as demonstrate compliance at installation level.

Harmonics

Network impedance

Harmonics are the power quality parameter that has presented the most significant challenges for grid code compliance to date. This is due to the fact that both solar and wind generators connect to the grid via harmonic producing inverters. Furthermore, while local and international standards are quite clear on the emission limits for loads and generators connecting to the plant, in practice, except for the most clear cut cases, it is often unclear how to determine whether a RPP is meeting the set limits.

A second challenge related to harmonics is the issue of harmonic network impedances. Harmonic network impedances are critical parameters for assessing emission levels of installations as the impedance is required to assess emission levels and convert current emissions to voltage emissions or vice versa. Effectively the harmonic network impedance plays a significant role in determining the highest value of harmonics that will be experienced at the PoC.

An approach defined in IEC 61000-3-6 is to use “worst case impedance curves” or “impedance envelopes” to manage the risk related to connecting a new plant to the network. This approach is relevant to South Africa and has been adopted as once a RPP has been granted grid code compliance the plant is deemed compliant for the next 20 years.

This poses a challenge to the NSPs as they carry the risk and mitigation cost should a PQ problem manifest itself due to the operation of the RPP in the future. The challenge with selecting an appropriate impedance envelope is that it is a balancing act between managing the risk to the NSP of future required network changes against possible unnecessary costs being placed on RPPs to manage the emission or network impedance of their installation to avoid non-existing problems.

The impedance envelope adopted to date in the SAGCRPP has been to utilise an impedance envelope equal to three times the linear impedance at the PoC. The purpose of such an envelope approach is to provide the RPP with certainty regarding the impedance impact for which it is accountable for; and to provide the NSP with the capacity to cater for the impact of any future connections onto a network.

The choice of using an impedance envelope of three times the linear impedance has been flagged on a regular basis by the RPP developers and their consulting engineers as problematic for their plants. The argument raised was that in some cases it would result in RPPs expending significant money install filters unnecessarily as the plant did not exceed based on the actual simulated impedance for a particular.

Their concerns are balanced against the risk to the NSP of future changes and developments to the network which need to be catered for at the outset as the rules surrounding grid code compliance in South Africa stipulate a 20 year window.

The status quo within the SAGCRPP is that the impedance envelope is required to be three times the linear impedance.

Introducing group harmonic emissions assessment

The management of harmonics is historically based on the fundamental source of harmonics being 6- and 12-pulse rectifiers and prevalent resonances on the power system. Local and international standards have been developed in response to typically limit harmonics up to the 25th harmonic.

Harmonic emissions by distributed generation is, however, typically smaller than the emissions of modern consuming equipment. Therefore the increase in voltage distortion will be small and rarely a problem, especially for those frequency components that have traditionally been dominant in power systems i.e. 5, 7, 11, 13, 17 and 19 [2].

While the levels of harmonic distortion at the dominant power system frequencies are not significant, distributed generation, using power electronic interfaces area source of higher order harmonics. This will lead to higher distortion as these will inevitably coincide with resonances at these frequencies. This effect is countered by the increased damping effect at higher frequencies.

With the above mentioned in mind it was deemed prudent to regulate and limit harmonic emissions from RPPs up to the 50th harmonic, increasing it up from the 25th as has been stipulated in historically in NRS 048-2. However this increase in harmonic order requirements means that it can be quite challenging for RPPs to meet the compliance limits across such a wide harmonic bandwidth.

Introducing a passive filter to address a particular harmonic may, as an unintended consequence, create a resonance point which then causes new harmonic exceedance at the higher harmonic orders. While this issue may be addressed by the application of active filters, this may lead to RPPs having to spend significant amounts of money for the design and installation of minor exceedances whose impact on the network would not necessarily justify the incurred expense.

To address this issue, the group harmonic distortion (group HD) limit was introduced. The group HD breaks the harmonic orders into four harmonic order bands i.e. 2≤h≤13; 14≤h≤25; 26≤h≤39; 40≤h≤50. With respect to the requirements for meeting harmonic emission limits provided by the NSP, the RPP shall be allowed to exceed individual current harmonic emission limits by up to 50% (e.g. if the 5th harmonic limit is 1 A, the RPP may emit up to 1,5 A) provided that the harmonic distortion (HD) band limit is met for the following specified bands.

Under this approach RPPs are given more freedom to exceed by 50% on an individual harmonic order level, provided that the group HD limit within which a harmonic meets its group limit. This approach both addresses the concern of the NSPs with regards to excessive harmonic levels while preventing unnecessary expenditure by RPPs.

Conclusion

The introduction of RPPs to the South African grid has led to a number of challenges pertaining to grid code compliance for power quality. These challenges have led to changes in the power quality requirements for RPPs as well as the introduction of the RPP power quality guideline laying out the process for RPPs to achieve the required compliance. These steps have led to the development of an approach that is specific to the local industry needs in South Africa, while taking cognisance of international best practice.

References

[1]  NERSA: “South African Grid Connection Code for Renewable Power Plants Connected to the Electricity Transmission System or the Distribution System in South Africa – Version 2.9”, 2016.
[2]  M Bollen, F Hassan: “Integration of Distributed Generation in the Power System”, Wiley & Sons, 2011.

Contact Ulrich Minnaar, Eskom, Tel 021 941-5729, minnaauj@eskom.co.za

 

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