MENU

Best-in-class grid-protection for next-generation PV inverters

Best-in-class grid-protection for next-generation PV inverters

Technology News |
By eeNews Europe



As consumers’ dependence on electricity continues to increase, and governments seek to meet carbon reduction targets by increasing reliance on renewable energy sources, an increasing number of small solar plants are expected to supply energy to the grid. These will comprise various classes of de-centralised generators, from sub-3 kW micro generators to ‘mini’ and ‘midi’ installations spanning 3 kW to 18 kW and ‘maxi’ installations up to around 40 kW, and will be located on residential and commercial properties including farms. Central power generation is also expected to rely increasingly on solar energy, which will be collected using large multi-string and central photovoltaic (PV) generators of around 45 kW and above.

By 2015, installed capacity is expected to exceed 35,000 MW. Of this, residential systems of up to 10 kW and small commercial systems in the 10 kW to 10 kW range are expected to account for around 15,000 MW.

Increasing reliance on distributed grid-tied systems in the micro to maxi ranges, as well as central inverters, calls for greater intelligence and improved performance in order to maintain human safety, equipment reliability and power quality. In particular, the design of the inverter grid interface connection is of critical importance. In addition, more and more grid-tied PV systems are being equipped with battery storage, as on-site consumption assumes increasing importance.

As a result, the inverter of a typical consumer or commercial solar generating station is evolving to become a sophisticated energy-management system capable of balancing the flow of energy in real time. Its responsibilities can include throttling the PV system in response to demand-reduction signals from the distribution network operator, which may be issued to prevent grid overload, or to temporarily disconnect from the grid to protect equipment and ensure human safety. Figure 1 illustrates the main functions of a typical transformerless inverter for a grid-tied application.

 

Figure 1. Functional block diagram of a modern grid-tied inverter

Events that may require disconnection of the inverter can include short-circuit failures on the generator side, temporary transients on the grid side, or other defects such as an excessively discharged local-storage battery, which can place unacceptable drain on the grid. As network operators develop so-called grid codes to help manage the growing number of small distributed generators connected to the grid, inverter control software is required to ensure the inverter always conforms to the required level.

Grid-Protection Relays
To enable the inverter to connect and re-connect under software control, efficient, durable and long-life electromagnetic relays are essential and mandated by the applicable grid monitoring standards. These include the German standard VDE 0126-1-1, which is also used in many territories throughout Europe and worldwide, RD1663/ 2000 in Spain, ER G83/1 in the UK, DK5940 2.2 and CEI 0.21 for Italy, and the international standard IEC 62109-2. Electromagnetic relays are essential for AC grid protection duties in transformerless inverters. A transformerless architecture allows for cost and space savings but requires additional components providing galvanic isolation. This is not possible using a semiconductor switch such as a solid-state relay or power MOSFET.

The internal design of the electromagnetic relay is of critical importance to ensure adequate safety isolation, failsafe operation, low power consumption, and reliable hot-start performance.

Grid-protection relays are used in the line, neutral or both connections as shown in figure 2. Compliance with standards like IEC 62109-2 and VDE0126-1-1 calls for two switches to be used in series, to provide redundancy in the event of a fault. Hence up to four relays may be required in a single-phase inverter, while six or eight may be needed in a 3-phase system. In applications where achieving the smallest possible overall size is a key requirement, designers may use a 4-pole relay such as the Omron G7J or G7Z, or a 2-pole device such as the G7L-PV. On the other hand, some manufacturers favour using two or more single-pole devices such as the G8P to simplify testing. In any case the relays used should be of the monostable normally-open (NO) type to ensure failsafe operation when no power is applied. This is another requirement that semiconductor switches are unable to meet without additional supervisory circuitry; the possibility of failure in the closed position cannot be tolerated.


 

Figure 2. Grid-protection relays are used in series for safety redundancy, and inserted in the line or line and neutral connections.

Coil-Hold Power
One possible drawback of using NO relays is that power must be applied continuously to the relay drive circuit, to keep the contacts closed under normal operating conditions. This increases the energy consumption of the system, which is not desirable in the context of a solar-energy application. Moreover, the combined effects of the power losses in as many as eight relay coils in a three-phase system can be appreciable.

To minimise these losses, some inverter manufacturers have used latching-type relays. However, intrinsic safety cannot be assured, and extra circuitry is required to ensure fail-open operation. In practice, this approach is not widely used. Alternatively, the control signal applied to the NO relay coil may be optimised to save power consumption. For example, with a 12 V DC coil relay, to achieve this, a high initial pulse of around 18 V to 22 V may be applied for a short duration up to around 500 milliseconds to close the relay contacts when the inverter is required to be connected to the grid. This initial pulse can then be reduced to a level of around 4.5 V to maintain an effective coil-holding force. The signal may also be pulse-width modulated to further reduce the total power dissipated in the drive circuit.

Hot-Start Performance
Care should be taken in the design of the drive circuit, however, to ensure reliable hot-start operation to reconnect the inverter when required. The inverter can be exposed to high ambient temperatures, particularly when deployed outdoors in hot climates. In addition I2R heating can contribute to raising the relay operating temperature. Under such conditions, thermal expansion in the components of the contact mechanism can combine with increased copper and iron losses at elevated temperatures, leading to a requirement for increased drive strength to close the relay contacts. It is important that designers should ensure adequate drive strength to provide the necessary power for reliable hot starting.

On the other hand, selecting a relay that is properly designed for AC grid protection – rather than choosing a general-purpose relay that appears to have suitable voltage and current ratings – is recommended in order to ensure reliable hot-start behaviour with minimum power dissipation. Drawing on long experience of optimising relay coil-magnetisation characteristics, Omron has achieved a highly efficient magnetisation circuit resulting in best-in-class coil holding power and switching performance in the G7L grid-protection relay. This double-pole device is currently the benchmark for PV inverter applications, requiring only 320 mW (160 mW per contact) coil holding power.

Lifetime and Current Rating
In addition, designers should take the durability of the relay into consideration. Throughout the typical lifespan of a grid-tied transformerless PV inverter, which is normally an absolute minimum of seven years and typical expectation of up to fifteen years, the relays may be required to disconnect the inverter from the grid on around 500 occasions due to exceptional circumstances. The G7L has demonstrated significant superiority in terms of life load testing, achieving 30,000 operations. This is well above the number of operations expected in a PV application.

It is also important for designers to consider the net value of daily current passing through the relay. In a small residential system typically comprising around seven or eight panels PV panels operating at a total DC voltage of 350 V or 400 V and DC current of 3 A to 4 A, the inverter may be carrying an AC load current of say 3.5 A and the relay may only need to interrupt this full load current flow during the middle of the day. If the inverter employs a soft start and stop feature that breaks the load electronically and then drives the relay to open or close its contacts, very significant increase in electrical life is realised as the relay’s only duty cycle is its carry current and breaking and breaking a few hundred milliamps residual circuit current. On the other hand, in a midi or maxi-size generator where multiple strings of panels feed a single inverter, the DC voltage can be more than double and the AC current can be typically in the region of 10 A to 12 A or much more demanding a relay of higher rating. Various combinations of strings in both parallel and serial connection can lead to large direct current flows needing to be inverted. Relays deployed within the AC grid protection interface must be suitably rated to handle the Generators typical and peak power conversion cycles.

Minimum Air-Gap
While system builders are under pressure to minimise the dimensions of the inverter and associated PV control circuitry, miniaturising components such as the grid protection relay should be approached with care. The applicable IEC 62109-2 and VDE0126-1-1 standards specify a minimum contact air gap when open. The isolation thus provided is necessary to ensure user safety and to prevent unwanted frequencies or excessive short-circuit current being put onto the grid, while also protecting the inverter against potentially damaging conditions on the grid side.

The air gap has been specified at 1.5 mm, but the latest IEC standards have increased this to 1.8 mm. All newly designed inverters destined for the European market and increasing some world-wide markets must meet this minimum standard. In the future, further legislative revisions may call for air gaps to be enlarged still further. As component suppliers seek to deliver more highly miniaturised components to the market, it is worth considering that enlarging the contact air gap calls for corresponding increases in other internal dimensions such as the contact follow distance. Hence, increasing the air gap while reducing overall size demands ingenuity that may not be reflected in the specifications.

Table 1. A 3.0mm relay contact gap can allow the inverter to be used safely at high altitudes above sea level.

In addition, when designing larger inverters for central generation usage, designers should consider the effect of altitude on the air gap size required to maintain the minimum isolation. At higher altitude, a larger air gap is required. Table 1 describes the air gap needed to maintain minimum isolation requirements as the height above sea level increases, according to the correction factors published in IEC60664-2 2011. To ensure reliable and safe operation at all practical altitudes, Omron offers relays with a minimum contact air gap of 3.0 mm for PV inverter grid-protection applications.

High-Temperature Reliability
Finally, it is recommended to consider the effects of sustained high temperature on the reliability and fire safety of the components in the inverter system. Tests for fire-retardant properties, such as the Glow-Wire tests (GWT) applied to components used in white goods have not yet been defined for PV generation equipment. However, growing adoption of consumer-class micro and midi generators may bring demands for more stringent testing and fire-safety standards. For now, equipment designers may expect component vendors to offer devices with increased heat tolerance compared with general purpose products such as Class F insulation in addition to UL94 V-0 flame-retardant plastic cases. In the future, component bases and other constituent parts may be subject to mandatory fire-retardant performance regulations.

Some Inverter systems manufacturers utilise a fan to help keep components at a constant temperature under widely varying ambient operating conditions. The use of a variable-speed fan can help minimise the effect on system power consumption; however, building-in longevity at the component level is generally a more satisfactory solution. While choosing high-quality relays can provide some protection against unknown degradation and corrosion effects that may affect lower-cost alternatives, equipment designers should also consider the ageing effects on other components in the system if operated at high temperatures for sustained periods.

Conclusion
Relatively small grid-connected solar generators are expected to provide an increasing proportion of installed capacity in the future. With large numbers of such plants connected to the distribution network, the inverters that provide connection to the grid must evolve from being simple conversion devices to become sophisticated energy-management systems. A reliable means of disconnecting and reconnecting the system to the grid is needed, to protect users and equipment against potential hazards. To operate reliably under all conditions, the new generation of inverters require electromagnetic grid-protection relays that are designed specifically for the task, offering suitable hot-start performance, low power consumption with normally-open operation, high electrical isolation and long-term durability.

About the author

Steve Drumm is the Market Development Manager of Omron Europe.

If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :    eeNews on Google News

Share:

Linked Articles
10s