Feature Switches & Relays
High voltage: Cutting the power
Richard Thornton at Panasonic Electric Works UK explores how electro- mechanical solutions for cutoff tasks are being increasingly implemented within the latest industrial systems, responding to a variety of needs
igh voltage systems are becom- ing increasingly widespread in building and home automation due to the dramatic increase in the use of photovoltaic solar panels. The use of solar panels is encouraged as a result of a number of issues including rising fuel prices, green Eco policies and the general increase in power use within the environment. Even extending the licenses of atomic power plants will not slow down the expansion of this technology over the long-run. The trend is clearly turning from panels installed on the roofs of houses to large systems cover- ing massive industrial buildings, and to solar parks. This development is increasing the productivity of solar power systems dramatically. With the serial connection of solar panels to individual strings common today, large solar systems can deliver voltages of up to 1000V DC and a few hundred amperes of current. When you consider that 50mA of current at 120VDC can kill a person, it becomes clear why so many standards for solar installation prescribe isolation when power of this degree is involved. Besides the danger to people, the system should protect itself from excessive short-circuit currents. A short circuit is usually stopped by a fuse in the branch. However, the cutoff switch in this path must be able to conduct this high current until the fuse breaks. Hence the DC cutoff switching device must exhibit a high short-circuit withstand.
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In currently available systems this cutoff is usually carried out by manual switches in the solar inverter. This is where large cutoff switches are used, which must be operated manually to isolate the DC side of the AC/DC inverter. Such manual cutoff switches are also integrated in DC generator junction boxes to cut off individual
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lines in case of an emergency or when repairs are needed. However manual cutoff switches pose definite disad- vantages outlined below: Fire Prevention: If the solar installation catches fire, the fire department cannot always get to the cutoff switches and operate them manually. In such a case, due to the high DC voltages, extin- guishing the fire is too dangerous. The fire department simply lets the building burn and keeps the fire under control. Insurance providers are already pushing for mandatory, remotely controlled cutoff solutions. This can be achieved by using the HE- V and EP relays from the Panasonic range, these can be used to either isolate individual panel clusters or the entire array.
Downtimes at large power plants: Especially in large solar parks, solar panels are often repaired or replaced. A long downtime of certain solar lines can seriously affect the productivity of the system. Being able to actuate the DC cutoff switch centrally is being pro- moted because time is money here, too. To eliminate these disadvantages, electromechanical solutions for cutoff tasks are being implemented more fre- quently with the latest systems. As a result the major relay manufacturers are developing a range of devices util- ising the ubiquitous electromechanical relay to allow system developers to meet the needs of the new systems and comply with the European and Global standards being introduced. Panasonic has recently introduced a number of products in this sector such as the EP relay. Interestingly there are several points of commonality with the Electric Vehicle market and this has been useful in taking proven relay designs and modifying them to suit the emerging Solar Panel market. However there are several innovations that have been realised in solar panel relays
Figure 1:
The EP relay from Panasonic offers an electromechanical solution for cutoff tasks
when compared to other similar relay designs for example: Extinguishing gas: The switching chamber is enclosed in a ceramic capsule. This prevents the hydrogen gas mixture from escaping. Due to its exceptional thermal capacity, this hydrogen gas mixture cools the resulting arc, causing it to be extinguished rapidly. Blow out magnet: In the upper plastic casing of the relay are two blow out mag- nets. These are placed close to the switching chamber so that the arc, which is simply ionised plasma, is ‘blown out’, away from the contact points. Hence the arc, which can reach 10,000°C at its peak, barely affects the contacts during normal or surge operation. This innovation allows the contact gap, which is normally quite large for high DC loads, to be reduced to a mini- mum. As a result, the relay could be designed to be compact, making it appealing to the increasingly compact solar AC/DC inverters.
In automotive applications, loads of up to 400V DC are switched, due to the predominant use of lithium-ion rechargeable batteries. Photovoltaic applications are more demanding because solar panel lines can generate loads of up to 1,000V DC.
As mentioned, the contacts of the DC cutoff switch used must be able to conduct short-circuit currents briefly without welding. The problem is transferring the heat via the metal terminal and then via the cable to the outside. The other problem is that the powerful magnetic fields can cause the contacts to open briefly in the event of a short circuit, a phenomenon known as levitation, which can lead to contact welding. For example 80A version of the EP relay can withstand a short circuit up of to 2,400A for 6ms. Plans are already being forged to withstand even greater short-circuit currents. Replacing a manual switch with a DC cutoff switch necessarily means that more power is needed to energise the coil. This point was given great consideration during the development of the new relays because high energy consumption by the DC cutoff switch could drastically reduce the efficiency of a solar system.
Richard Thornton is Managing Director of Panasonic Electric Works UK
Many users also take advantage of reducing the coil voltage, either directly or via PWM. The retain voltage should be at least 60 percent of the nominal voltage in this case. Electromechanical relays are being increasingly used in solar inverters and DC generator junc- tion boxes with both current capacities and voltage potentials being raised with each new range introduced to allow the maximum potential to be achieved from the circuit designers efforts. Panasonic Electric Works UK Enter 212
www.panasonic-electric-works.co.uk
SEPTEMBER 2013 Electronics
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