The growing demand, popularity and hence markets in the field of renewable energy, such as solar power plants and battery storage systems evoke an ever stronger interest in high voltage and high current switching devices that meet the contemporary requirements in terms of safety, productivity – and efficiency.
With the HE-V series, Panasonic Industry has developed a compact and low cost but highly efficient relay capable of switching a load of up to 1,000VDC 20A and 40A inrush current, focusing on arc parameters to minimize size and cost with a very effective blow-out mechanism and an optimized extinction gap.
An epic leap in the structure of power supply systems
During the past two decades, energy supply systems have undergone substantial changes. More and more systems with DC power generation like photovoltaic plants have entered the market. Global solar power production has increased exorbitantly – and is expected to further grow. Solar cells produce direct current (DC) power which fluctuates with the sunlight's intensity. For practical use this usually requires conversion to certain desired voltages or alternating current (AC) by using solar inverters. Solar cells are connected in series or parallel to modules and then wired together to increase the output voltage. These arrays are connected in so called string combiner boxes. The typical load of one solar string lies in the range of 10-12A / 600-1,000VDC, is then tied to an inverter which produces AC power at the desired voltage, frequency and phase.
By far the highest power is generated in residential systems: In Europe alone, there are already millions of DC/AC inverters installed with a typical power between 2 and 10kW. They are directly connected to the grid as single-phase or as three-phase system. For the entire safety of the grid systems, these inverters must fulfill several international and local standards and regulations. To achieve full galvanic isolation, a DC main switch as a protective device is required in photovoltaic (PV) systems between the DC side of the inverter and the solar generator. Details are regulated in the IEC 60364-7-712:2017 standard. Most existing inverters use a manual switch to fulfill this requirement and protect humans during installation and maintenance.
Most installations are in residential buildings, containing only one or two inverters, and manual switches are sufficient to fulfill basic safety requirements. With the recent trend in larger PV installations in solar plants or on top of roofs of industrial buildings, there are new regulations from the utilities to control power generation. Due to overcapacity on sunny days, power plants with more than 100KW need a shutdown function to reduce the production capacity. Therefore a high number of inverters requires a remote control function to be connected or disconnected from the grid.
A closer look on HE-V relay specs in PV installations
With its HE-V relay series, Panasonic Industry offers a dedicated DC breaking relay designed for solar power installations. Several DC breakers are already available on the market, but they are quite often developed for hybrid and electric vehicles’ battery disconnect modules. However, there is a big difference in the output power of a solar cell and a battery system. While relays for battery systems must withstand high short circuit currents, the solar cells’ relationship to their operating environment and the maximum power they can produce is more complex. For any given set of operational conditions, cells have a single operating point where the values of the current (I) and voltage (V) of the cell result in a maximum power output. This is known as the maximum power point (MPP). A solar inverter with a maximum rating of 1,000V and 30A has a typical operating point in the range of 500V and 20A.
The primary purpose has been to develop a failsafe relay that is well suited for use in solar inverters and string boxes as well as a general purpose relay for a wide range of DC applications.
Characteristic | Performance |
Contact rating | 20A 1,000 VDC |
| 1,000V DC |
| 40A |
Surge breakdown voltage | 12kV |
Nominal operating power |
|
Contact gap | >3.8 mm |
Holding power | 210mW |
Dimensions (L×W×H) |
|
Ambient temperature | -40 to +85o C |
To fit into an inverter or a DC junction box and replace manual switches, size is an important criterion. Up to six relays are used to cut both polarities in the DC input strings. Another important factor is power consumption. Inverter manufacturers are interested in a high yield rate and using a relay instead of a manual switch causes additional loss because the relay is always in an on-state. 210mW continuous holding power is enough to keep the HE-V relay switched. Only during the short switching act the power is 9 times higher.
Manual switches use screw terminals for wire connection. For currents up to 40A, it is more convenient to use soldering terminals and plug the cables directly to the PC board, which is an additional cost saving factor and also a space saving feature for the end user.
Breaking capability of relay contacts for DC loads
There is no generally valid definition of the term switching or breaking capability for a relay contact. So we use the highest value of current which an output circuit is capable of making and breaking successively under specified conditions. The maximum power to be interrupted by an opening contact mainly depends on the clearance distance (distance between the open contacts) and the contact material. In a DC circuit, the minimum voltage to maintain an arc normally rises above the source voltage and the arc is extinguished. If, however, the supply voltage is sufficiently high to maintain a stable arc across the open contacts, the relay will be destroyed as it cannot withstand the prolonged high temperatures generated by the arc. In the worst case, fire can result or the switching devices explodes. A typical method to define the required arc extinction length is the graphical method using empirical arc characteristics voltage plotted against the current with the arc length as parameter. For higher voltage and current scenarios, empirical data are only available for copper contacts. Hence we use the AYRTON equation to describe this voltage dependency of a steady arc as a function of current and arc length [3].
Uarc = a + (b×g) + ((c+d×g)/I)
with the coefficients a, b, c and d, the arc length g and the arc current I. The coefficients are obtained by experimental data and vary with contact material. For copper electrodes in air we find following values in the literature [4]:
a [V] = 17; b [V/cm] = 22; c [VA] = 20; d [VA/cm] = 180.
Basic design considerations
It is obvious that a contact gap of more than 200mm will not fit in a 40x50x40mm³ relay. One of the most common methods to overcome this would be the use of several opening contacts in series. For each additional opening contact, the breaking voltage is divided by the number of opening contacts. This will also split the total arc energy into smaller units and therefore reduce stress to the contact surface. A further advantage is the increased opening speed by a factor of four compared to a single contact. When interrupting a direct current, the relay must be able to dissipate the total stored energy in the circuit.
Another prominent way to improve the breaking capacity of DC relays is a transverse magnetic field by making use of the Lorentz force of the current, to blow the arc out of the contact region. The redirecting of the electrical arc is carried out by permanent magnets. The permanent magnets are reinforced by pole plates to enable the magnetic field to function throughout the complete contact and extinguishing area.
Hydrogen plasma is known to cool down electric arcs during interruption due to its high thermal conductivity. Special DC breakers use a sealed vacuum chamber filled with hydrogen to cool the arc, like Panasonic EP relays (zum Produkt verlinken). This technique is very efficient but lavishly, even in mass production. A simpler approach is to use the water of the plastic housing in the arc environment. Thanks to the high arc temperature we have a high outgassing rate of the water from the surrounding plastic, which provides an additional cooling effect.
Basic structure and function
The sectional view and the basic structure of the developed relay would look as follows:
For the sake of cost efficiency it is based on a coil and the armature system of a conventional 2 Form A power relay with two double bridge contacts. The outer dimensions are 41.0 × 50.0 × 39.4mm (L×W×H).
The main difference between the body block of the standard relay and the HE-V relay would be the reinforced contact area. Four separate arc chambers provide space for four permanent magnets. To reach the targets for energy saving, the coil bobbin and the magnetic circuit have been modified. This results in a coil holding voltage reduced to 33% of the nominal operating voltage. This amounts to a permanent power of only 210mW. The armature block is slightly changed to fulfill the requirements for reinforced isolation up to 10kVDC. With this construction, a minimum clearance distance of more than 10mm between coil and contact and a surge breakdown voltage of 12kV is achieved.
The movable contacts are directly connected to the armature by insertion molding via the contact spring. The armature is connected to the coil block via the release spring.
Relay Cover
The main difference to a standard relay can be found in the construction of the cover. To increase the arc length, four magnets with a strength of around 60mT are placed in normal direction, 3mm from the contact gap opening. The magnetic circuit is closed by an iron yoke to reach a homogenous magnetic field to achieve permanent arc movement.
Contact system requirement
In typical solar inverter applications, normal switching cycles are without load. However, a continuous current up to 20A has to be conducted. Therefore the voltage drop on the closed position has to be extremely low to keep losses low. This means a contact resistance of not higher than a few mΩ, which mostly depends on two characteristics:
- Contact material: HE-V relay uses AgNi as contact material is, an ideal choice in regards of low contact resistance and high resistance against arcing.
- Contact force: The contact resistance is in direct relation to the contact force and therefore related to the relay power consumption. It is the designer’s task to find the right balance between power loss on the contacts and coil power consumption.
A look back on the experimental setup
The prototypes have been tested with two Regatron TopCon Quadros in series as power supply. To keep the current constant, 800V 60mF capacitors are connected in parallel to a Fritzlen resistor type BW81. For the contact opening test, the relay contact is firstly closed, then current and voltage are adjusted to the requested ratio by adjusting the Fritzlen resistor to the whole circuit. Breaking current and voltage behavior are monitored with an oscilloscope. For all tests shown in this chapter only one bridge contact is used to break the load. After measuring the limit curve for the bridge contact without cover and therefore without magnet, the time limit for the burning arc has been set at 5ms. Each measuring point is the mean value of minimum 3 breaking operations. The arcing time shows a variation of more than 20% so the relay is to be changed for each measuring point to get a stable result, as illustrated in the following diagram:
This load limit curve corresponds to the characteristic curves of an arc for a contact gap between 3 to 5 mm. When measuring the breaking behavior with relay cover and magnet, there has been an increase in breaking power by a factor of 10 for higher currents with more than 20A and a factor of 15 times for the region between 5 and 10A. The breaking capacity of the relay is 40A at 800VDC voltage for series connection. To achieve 1,000VDC, the arcing time limit had to be
extended to 10ms. The result:
Arc duration versus switching cycles
The distance between arc and cover is very critical. The outgassing of hydrogen has a good cooling effect but only for a few switching cycles. The fast erosion limit’s a guaranteed safety breaking for higher number of operations. The following graphs show the increasing arcing time. In a) we see the arcing time for the first breaking operation for a load of 500VDC and 30A and in b) the arcing time after 25 breaking operations.
Arc voltage and current at break; at a)1st operation and b) after 25 operations
Conclusion
The HE-V test results demonstrate that even extremely miniaturized, low power consumption relays can handle high DC loads. The AgNi contact material permits both, a high endurance at medium load making and braking and low contact resistance to avoid losses in a solar inverter. The idea to use a double bridge contact, with arc-driving magnets and a highly resistive plastic cover provides a compact, reliable, quiet and cost-effective air break method for interrupting high DC voltage loads. Especially in solar applications with typical dry load switching the relay can replace manual breakers and bring further performance by remote control function. By using a 2 Form A type construction the contacts can be used either in series or in parallel connection.
For mass production types, the load characteristic is to be reduced compared to previous experimental data. With two contacts in series, a reliable on and off switching of 1,000VDC 10A with a self-extinguishing arc (<5ms) can be reached throughout 1,000 switching cycles. The recommended range of switching cycles is plotted in the left-hand section of the following figure for various combinations of voltage and current with a number of guaranteed switching or breaking cycles. The goal of 1,000V 40A can be reached when we expand the arcing time to 20ms.
For further information please contact the author:
Dr. Dieter Volm, New Business Development Components & Devices, Panasonic Electric Works Europe AG
In a nutshell: the HE-V relays series
Panasonic Industry HE-V relays feature a compact design (L: 41 × W: 50 × H: 39.4mm), maximum rated load of 20A at 1000VDC, and contribute to energy savings in equipment thanks to reduced voltage in the self-holding coils. The HE-V relays are specifically designed for the solar market, for DC applications with high loads including photovoltaic power generation systems, battery charging and discharging systems, inverters and DC load controls.
Find the HE-V relays at Farnell: