Connecting a PV system to the grid and how to earth it

August 26th, 2014, Published in Articles: Energize

 

A key constraint to the implementation of grid-connected small-scale renewable energy activities in South Africa is the lack of pre-approved, generic standards for utility engineers and system promoters to apply in designing and approving the utility interface.

This article covers part two of the recommended wiring procedure for the NRS097-2-1:2010 standard which is available from the South African Bureau of Standards (SABS) and provides guidelines with reference to the way in which a PV system or alternative energy system should be connected and grounded in a TN-S supply environment. This section of NRS 097-2 applies to embedded generators of nominal capacity less than 100 kW, connected to a single-phase, dual-phase, or three-phase low voltage (LV) utility network. The article only looks at a TN-S supply although the available standard covers all other supply configurations.

This section of NRS 097-2 and its accompanying parts address this need. All information provided is subject to revision and the latest versions should be considered prior to any work being done based on information provided in this article.

Information is provided in the interest of promoting standards in the energy industry. All installations should adhere to the Electricity Regulation Act, 2006 (Act No. 4 of 2006) (as amended from time to time) and the regulations promulgated in terms of the Act, as well as any regional by-laws, safety regulations and other applicable manufacturer instructions or recommendations.

Where utilities have allowed embedded renewable energy generation to feed into their networks, this provides a relatively easy way for the private sector to invest in renewable generation in South Africa.

Utilities are receiving an increasing number of requests from customers to allow small-scale embedded generation. As given in the South African Distribution Network Code, the utility is obliged to provide an offer to connect the embedded generator under the conditions as given in “Application for Connection”, referred to in section 3.2 of the Distribution Network Code.

The work on “Grid interconnection of embedded generation: Small-scale embedded generation” is based on the following three key documents:

  • Utility interface (NRS 097 part 2)
  • Embedded generator (EG) requirements, which deal with product type approval, installation requirements and certificate of compliance on the EG customer’s side of the meter (a future document)
  • Utility framework, which deals specifically with the commonly designed unidirectional flow of energy in LV networks, with cumulative impacts of EGs, with substation configuration and metering arrangements (a future document).

In addition, a specification will be developed to provide informative guidelines on the implementation procedures, the application form, the license requirements, the certificate of compliance procedures, commissioning procedures where applicable, and documentation requirements for the embedded generator. The document will address legal issues such as agreements and ownership, and cover metering and revenues from feed-in tariffs. In the case of the utility, it will provide guidelines for the registration process and the record keeping of embedded generators within utility networks and network spurs.

This section of the standard aims to be technology neutral and focuses on the interface between the embedded generator and the utility, although it is expected that the specification will mainly apply to photovoltaic (PV) grid connected systems interfaced through static power converter technology. Static power converters are also utilised to convert for example, wind power, micro-hydro power, pico-hydro power, battery storage energy and fuel cells to grid compatible electricity.

Other conversion technologies are considered where the requirements deviate from those of static power converters. These include induction generators (where the primary resource may for example be wind or hydro) and synchronous generators (where the primary resource may for example be wind, micro-hydro or diesel).

Fig. 1 provides the line diagram of a power system that operates without a backup supply and would therefore not be able to operate as a stand-alone unit during power failures or as a backup supply during loss-of-grid conditions, Backup supply consists of storage (in the form of batteries, fossil fuels or fuel cells) and a synchronous static power converter or a generator which is able to operate in stand-alone mode. UPSs and diesel generators are examples of EGs which operate in stand-alone mode.

Copper Alliance Fig 1

Fig. 1: Earthing of embedded generation systems

There are a number of standards and performance criteria which need to be adhered to:

  • The quality of power provided by the embedded generator in the case of the on-site AC loads and the power delivered to the utility is governed by practices and standards on voltage, flicker, frequency, harmonics and power factor. Deviation from these standards represents out-of-bounds conditions. The embedded generator is required to sense the deviation and might need to disconnect from the utility network.
  • All power quality parameters (voltage, flicker, frequency and harmonics) shall be measured at the point of utility connection, unless otherwise specified (see annex A of the applicable standard). The power quality shall comply with NRS 048-2. This implies that the combined voltage disturbances caused by the specific EG and other customers, added to normal background voltage disturbances, may not exceed levels stipulated by NRS 048-2.
  • The embedded generator’s AC voltage, current and frequency shall be compatible with the utility system in accordance with IEC 61727.
  • The maximum size of the embedded generator is limited to the rating of the supply point on the premises.
  • Where the maximum export potential of the generation device exceeds 10 kW; the device shall be of the three-phase type.
  • In the case of a multiphase connection the embedded generator’s output shall be split over all phases if the EG is larger than 6 kW and balancing of the phases will be deemed desirable.
  • The embedded generator shall synchronise with the utility network before a connection is established and shall not control the voltage, unless agreed to by the utility
  • The static power converter of the embedded generator shall not inject DC current exceeding 1% of the rated AC output current into the utility’s AC interface under any operating condition in accordance with IEC 61727. This relates specifically to embedded generators where the static power converter has no simple separation from the utility network (e.g. inverters which are transformerless).
  • Total harmonic current distortion shall be less than 5% at rated generator output in accordance with IEC 61727. Each individual harmonic shall be limited to the percentages listed in Table 1 of the applicable standard.
  • This section of NRS 097-2 requires that an islanding condition shall cause the embedded generator to cease to energise the utility network within 2 s, irrespective of connected loads or other embedded generators. The embedded generator shall comply with the requirements of IEC 62116.
  • The embedded generator shall provide a means of isolating from the utility interface in order to allow for safe maintenance of the EG. The disconnection device shall be a double pole for a single-phase EG, a three-pole for a three-phase delta-connected EG, and a four-pole for a three-phase star-connected EG. The grid supply side shall be wired as the source.
  • The breaking capacity of the isolation circuit-breaker closest to the point of utility connection shall have a minimum fault current level of 6 kA in accordance with SANS 60947-2.
  • A label on the distribution board of the premises where the embedded generator is connected, shall state: “On-site embedded generation (EG) connected. The EG is fitted with an automatic disconnection switch which disconnects in the case of utility network de-energisation”.

Metering 

All meters utilised by the utility shall be the property of the utility even when the meters are located on the premises of the customer. Meters that are embedded in the customer’s network shall be accessible to the utility on request. Three metering configurations are acceptable in the case of premises where embedded generators are operated. One configuration applies to net metering where price symmetry is given between consumption and generation and two configurations apply to feed-in tariff (FIT) metering.

Generation licenses

The utility has submitted a request to the relevant government department, which recommends that all generators above 1 MW require a license from the authority while generators of less than 1 MW are required to register with the authority. The license requirements may change depending on the outcome of the government department’s review. In terms of the applicable national regulations, all electricity generators, regardless of size, require a generation license. The owner of the embedded generator therefore needs to file a license application in accordance with relevant legislation with the appropriate authority.

Earthing

Each installation shall have a consumer’s earth terminal (see 3.18 of SANS 10142-1:2009) at or near the point where the supply cables enter the building or structure. All conductive parts which are to be earthed (see 3.29.4 and 6.12.3 in SANS 10142-1:2009) shall be connected to the consumer’s main earthing terminal.

The consumer’s earth terminal shall be earthed by connecting it to the supply earth terminal (see 3.78 in SANS 10142-1:2009) or the protective conductor (see 3.15.8 in SANS 10142-1:2009) and, if installed, the earth electrode. The effectiveness of the supply protective conductor shall be determined in accordance with 8.7.5 in SANS 10142-1:2009 (see 6.11.1 in SANS 10142-1:2009).

Where no existing earth electrode exists in the electrical installation, a suitable earth electrode must be installed in accordance with SANS 10199. When installed, the electrode shall be bonded to the consumer’s earth terminal and to the earthing point of the generating set with a conductor of at least half the cross-section of that of the phase conductor, but not less than 6 mm copper or equivalent. This also applies to a single-phase supply.

Rules of thumb established for embedded generation and backup systems

Earthing and wiring guidelines were developed as a result of rigorous analysis. See tables B.2 to B.5 in the NRS 097 standard.

Earth electrode

All backup systems shall have an own earth electrode connected to the consumer’s earth terminal and shall comply with 7.12.3.1.1 in SANS 10142-1:2009. Embedded generators need not have their own earth electrode in accordance with SANS 10142-1, but an own earth electrode is preferred.

N-PE bridge on consumer’s earth terminal

TN-S systems shall be un-bridged (as normal practice), to comply with standard installation requirements for safety.

N-PE bridge on backup supply

TN-S systems shall be bridged.

In the case of backup systems with an internal N-PE bridge, the following is required:

  • For a three-phase system: a four-pole change-over switch including neutral, or a three-pole with overlapping neutral
  • For a single-phase system: a two-pole change-over switch including neutral, or a single pole with overlapping neutral.

Manual change-over switches shall be three position switches, i.e. break-before-make. Change-over switch No. 2 (between AC coupled embedded generator and backup supply) In the case of a three-phase system, there shall be a four-pole change-over switch including neutral, or a three-pole with overlapping neutral. In the case of a single-phase system, there shall be a two-pole change-over switch including neutral, or a single pole with overlapping neutral. The complete standard is available from SABS (sales@sabs.co.za).

Contact Carel Ballack, Copper Alliance, Tel 011 061-5000, carel.ballack@copperalliance.org.za

 

 

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