Explore some of the considerations involved when specifying and installing a DC switch-disconnector, as a means of isolation between the solar photovoltaic array and the inverter.
Where a Solar PV inverter is installed, a means of isolation from both the AC and DC supplies is required (712.537.2.101). This article will focus on DC isolation, which is achieved through a switchdisconnector on the DC side of the inverter (Fig 1).
AC vs DC switching
A DC switch-dis connector faces greater challenges than its AC counterpart. With an AC system operating at 50 Hz, the voltage crosses zero twice per cycle, occurring every 10 milliseconds (Fig 2(a)), which helps suppress electrical arcs. However, DC voltage is constant and lacks this zero-point crossing (Fig 2(b)), making arc extinction more difficult.
In AC systems, the natural zero voltage points in the cycle help extinguish electrical arcs during switching. Conversely, in DC systems, the steadystate voltage means that when contacts are separated, the voltage remains, creating a strong magnetic field within the air gap. This increases the likelihood of air ionisation between the switching contacts allowing current to bridge the air gap creating an arc. To combat this, DC switches may incorporate a spring-assisted mechanism to enable a fast make-and-break action. Some DC switches also employ multiple contacts to extend the arc path and aid in arc extinction.
What is a switch-disconnector?
A switch-disconnector is required for isolation of the inverter, but what exactly is a switch-disconnector?
Part 2 of BS 7671 defines a switch disconnector as: “A switch which, in the open position. satisfies the isolating requirements specified for a disconnector. NOTE: A switch-disconnector is otherwise known as an isolating switch"
Switch-disconnectors, or isolating switches, conform to BS EN IEC 60947-3 and are designed for isolation, functional switching, and emergency switching. They are also capable of switching under load. Table 537.4 of BS 7671 provides guidance on selecting devices for these purposes.
Switch-disconnectors are categorised for utilisation, the categories for solar PV applications are split into DC PV 0, DC PV 1, and DC PV 2. DC PV 0 is unsuitable for switching on-load and therefore not permissible. DC PV 1 is used for single strings, whereas DC PV 2 is necessary when multiple strings are connected in parallel and there is a risk of overload.
DC voltage of a PV string
When selecting a DC switch-disconnector, it is crucial to consider the steady-state voltage of the system. A PV system, unlike a conventional AC electrical system does not have a set nominal voltage; it depends on the number of PV modules that are connected in series.
When specifying a DC isolator, the maximum open circuit voltage (UOCMAX) shall be used (712.512.Ll). This value for a PV string can be determined using Formula 1. Another method for determining UOCMAX is available, although this requires knowledge of the module temperature coefficient and minimum site temperature.
UOCMAX = No. of modules x UOCSTC x 1.2
Where:
- UOCMAX is the maximum string open circuit voltage.
- No. of modules is the total number of modules on the string.
- UOCSTC is the open circuit voltage of the module under standard test conditions as declared by the manufacturer.
- 1.2 is a multiplier to take account of the voltage rise under a temperature lower than that of a standard test conditions.
DC current of a PV system
Knowing the value of DC current that the disconnector will have to interrupt is critical when selecting a switchdisconnector, as higher currents generate more heat in the arc. The magnitude of this current is proportional to the number of PV modules or strings connected in parallel. When selecting equipment for PV arrays, including DC isolators, the short-circuit maximum current (ISCMAX) shall be used (712.512.1.2).
Formula 2 can be used to calculate this value.
Formula 2
ISCMAX = No. of strings x ISCSTC x 1.25
Where:
- ISCMAX is the array maximum short-circuit current
- No. of strings is the total number of strings in parallel
- ISCSTC is the short-circuit current of the module under standard test conditions as declared by the manufacturer.
- 1.25 is a multiplier to take account of the higher irradiance than that of standard test conditions.
Determining the maximum current and voltage
In order to determine the maximum current and voltage for a DC switch-disconnector, the manufacturer's details of the PV module are required. An example of this is shown in Fig 5.
Example
Using the example data given in Fig 5, determine the maximum voltage and current for selecting a switch-disconnector for a single string containing 20 of the PV modules wired in a series configuration, with no other strings connected in parallel. Formulas 3 and 4 can be used to calculate the maximum voltage and current, respectively.
Formula 3
UOCMAX = No. of modules x UOCSTC x 1.2
UOCMAX = 20 x 39V x 1.2
UOCMAX = 936V
Formula 4
ISCMAX = No. of strings in parallel x ISCSTC x 1.25
ISCMAX = x12.5A x 1.25
ISCMAX = 15.6A
Selecting a switch-disconnector
Once the voltage and current of the system have been determined, the appropriate disconnector can be selected. Firstly, the chosen device must conform to BS EN IEC60947-3, ensuring it is suitable for on-load switching. The device will bear markings indicating its function; for a switch-disconnector, these markings are illustrated in Fig 6, indicating its capability for making, breaking, and isolating operations.
Once the maximum current and voltage have been determined, the appropriate switch-disconnector can be selected. The device's capability to safely make and break current depends on the system voltage. Due to the differing voltages in PV systems, manufacturers specify a range of voltages along with corresponding safe operating currents for each voltage, tailored to different switching configurations. Table 1 represents hypothetical manufacturer's data for a DC disconnector.
Using the voltage and current values determined previously, the appropriate wiring configuration can be selected from Table 1.
For switching 15.6 A at 936 V. the disconnector should be wired either in a 4-pole series configuration or a 2-pole series + 2-pole parallel configuration. However, it's important to note that this device is not suitable for making and breaking when wired in a 2-pole series configuration.
The wiring configurations from Table 1 are illustrated in Fig 7. It should be noted however that these examples are not exhaustive in terms of configurations. Full details should be available in data published by the manufacturer of the switchgear.
It is important that the manufacturer's instructions for DC disconnector configuration are followed to avoid failure of the device. An example of a burnt out DC isolator is shown in Fig 8.
Other considerations
The previous sections described the selection process of a DC isolator, there is clearly more to specifying the correct device compared to its AC counterpart.
Consideration should also be given to the termination of the DC conductor in the isolator's terminals (526.9.1). DC cables generally use class 5 (flexible) conductors. Where equipment terminals are unmarked, they should be suitable for all conductor classes without modification (526.2. Note 2). Some equipment terminals are only suitable for certain classes of conductors without further treatment. Where this is the case the terminals, or if space is insufficient on the product, the immediate unit packaging or technical data sheet should be identified with appropriate markings. Where terminals are only suitable for Class 5 flexible conductors they will be identified with the symbol “f”.
Where treatment of the conductors at the terminations is necessary reference should be made to manufacturer's data, which may state that a fine wire conductor requires a sleeve or ferrule.
Where the DC disconnector is located outdoors, it shall be rated for the possible external influences (712.512.102). It is good practice to have the cables entering the bottom of the enclosure through suitably rated stuffing glands.
Summary
Selecting a DC switch-disconnector for a Solar PV system presents unique challenges. The maximum voltage and current must be determined before selection of the appropriate device. The configuration at the switch terminals must also be determined to ensure the device can safely make and break the system's maximum voltage and current.
When using fine stranded wire, the terminal of the disconnector shall be suitable, or the cable shall be suitably treated.
