This article looks at some of the factors that should be considered by designers and installers when working with DC systems.
Debbie Shields | Communications Manager
This article looks at some of the factors that should be considered by designers and installers when working with DC systems.
However, with the introduction of new requirements for prosumer electrical installations (PEI) and the increased popularity of solar PV, electrical energy storage systems (EESS), electric vehicle charging, as well as smart integration with building services, greater awareness of the application of DC systems is becoming necessary.
This article aims to provide a brief description of the principles of direct current while considering the different types of DC arrangements detailed in Chapter 31 and Appendix 9 of BS 7671.
A DC supply, however, can be both positive (L+ve) or negative (L-ve) in DC distribution systems.
An electrochemical source such as a battery naturally generates a ripplefree DC output. However, where a direct current is obtained through AC rectification, in which the negative part of the sinusoidal waveform is inverted, the DC output is not necessarily represented as a smooth (ripple-free) waveform.
To be of any use in modern equipment, such as to drive current through a semiconductor light emitting diode (LED) within a lamp or luminaire, significant levels of conditioning must be applied to create a sufficiently smooth DC output to prevent flickering, as shown in Fig 2.
At this point, it should be recognised that the rms value of a sinusoidal current used in normal AC supplies will provide the same heating effect as the equivalent such as that in a ripple-free constant DC supply Irms = Ive
The benefits of using DC in low-voltage DC (LVDC) installations typically include:
Strictly speaking, both a PEL conductor, combining the functions of both a protective earthing conductor and a line conductor and a PEM conductor, combining the functions of both a protective earthing conductor and a mid-point conductor, as shown in Fig 3 a-b, are not live conductors although they carry operating current (see note to 312.1.2). They are typically connected to a means of earth at the source within the DC distribution network with separate protective conductors often distributed throughout the installation (see Fig 3 c to f).
By convention, a PEL two-wire DC circuit (see Fig 3 a) has a single voltage available between, L+ and L- (U0), and may typically employ either a negative or positive earthing arrangement (see Fig 3 c-d).
In addition, where a PEM three-wire arrangement is used (see Fig 3 b), three possible arrangements may be employed:
Appendix 9 of BS 7671 details the earthing systems available for DC supplies.
'An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.'
Similarly, an earth fault current applies to both AC and DC circuits, and is defined as:
'A current resulting from a fault of negligible impedance between a line conductor and an exposed-conductivepart or a protective conductor'.
The following figures based on Appendix 9 highlight the potential current paths for both short-circuit and earth fault conditions in a DC circuit, including both a PEL arrangement and a PEM arrangement.
Figure 4 shows a TN-S DC system with an earthed line conductor (L-) and separate protective conductor throughout the installation. The image highlights a short-circuit between the live conductors although one of those live conductors is also connected to earth.
Figure 5 shows a TN-S DC system with an earthed line conductor (L-) and a separate protective conductor throughout the installation. The earth fault occurs between (L+) and the protective conductor.
In a system having a mid-point (PEM) there are two possible opportunities for an earth fault.
As shown in Fig 6, the earth fault is from (L+) to the separate protective conductor, although connected to the mid-point, a positive DC voltage, to earth and subsequent flow of current is as indicated.
In Fig 7, the earth fault is from (L-) to the separate protective conductor although connected to the mid-point, a negative DC voltage to earth and subsequent flow of current is as indicated.
As can be seen, the nature of DC supplies and the range of possible earthing arrangements creates a more complex range of options when compared to that of AC earthing systems.
DC can provide significant advantages when compared to AC for the supply of a range of final circuits including energy savings and improved integration with solar PV, EES systems, LED lighting and EV systems.
Introduction
The convenience and ease by which an alternating current (AC) can be converted to different voltage levels using transformers for efficient transmission and distribution has made the use of AC the most common form of energy supply in the UK.However, with the introduction of new requirements for prosumer electrical installations (PEI) and the increased popularity of solar PV, electrical energy storage systems (EESS), electric vehicle charging, as well as smart integration with building services, greater awareness of the application of DC systems is becoming necessary.
This article aims to provide a brief description of the principles of direct current while considering the different types of DC arrangements detailed in Chapter 31 and Appendix 9 of BS 7671.
Direct current [DC]
A direct current may be considered as an electrical charge having a unidirectional flow over any given period of time, although the direction of flow of charge will not change or pass through point 0 to the opposite polarity on the x-axis, as would typically be expected from that of a sinusoidal AC waveform (see Fig 1).A DC supply, however, can be both positive (L+ve) or negative (L-ve) in DC distribution systems.
An electrochemical source such as a battery naturally generates a ripplefree DC output. However, where a direct current is obtained through AC rectification, in which the negative part of the sinusoidal waveform is inverted, the DC output is not necessarily represented as a smooth (ripple-free) waveform.
To be of any use in modern equipment, such as to drive current through a semiconductor light emitting diode (LED) within a lamp or luminaire, significant levels of conditioning must be applied to create a sufficiently smooth DC output to prevent flickering, as shown in Fig 2.
At this point, it should be recognised that the rms value of a sinusoidal current used in normal AC supplies will provide the same heating effect as the equivalent such as that in a ripple-free constant DC supply Irms = Ive
The benefits of using DC in low-voltage DC (LVDC) installations typically include:
- Reduction in power consumption.
- Removes the need for switch mode (AC-DC) power supplies for separate items of current-using equipment. Although conversion between different voltage levels will remain necessary for different items of equipment, the losses typically associated with this conversion are lower than for equivalent switch mode power supplies.
- No consideration required for power factor.
- No need to consider reactive power as there is no capacitive or inductive reactance - although there is the potential for energy storage in capacitors and inductors.
- In AC circuits reactance must also be considered to accurately determine voltage drop in larger conductors. In DC circuits only resistance need be considered and since cable resistance is usually small, there is a reduction in volt-drop.
- Fewer components required.
- Improved efficiency and easier integration with DC systems - solar PV, EESS, EV charging etc.
- Controlling arcing - DC circuits can sustain a substantial arc more readily in comparison to that of an equivalent circuit supplied from an AC source. This sustained current output is due to the supply not passing through point 0. In AC circuits, a break can be made by switching action - even if minor arcing occurs this will not be sustained after the zero point has been reached.
- Increased radio frequency (RF) interference - from arcing during switching of DC.
- Lack of familiarity - incorrect selection of switching and overcurrent protective devices. Such types of equipment must be suitably rated for high levels of switching, and fault currents present within DC circuits.
- Difficulty in converting between voltage levels - power electronic converters are necessary.
- Increased risk of galvanic corrosion - to structural metalwork, earthing terminals and conductors - where positive and negative earthed arrangements are used.
Conductor arrangement and system earthing
Although Regulation 312.1.2 details the current-carrying conductors in DC circuits as shown in Fig 3 a-b, reference should also be made to Appendix 9 of BS 7671 when determining the nature of DC earthing systems (see Fig 3).Strictly speaking, both a PEL conductor, combining the functions of both a protective earthing conductor and a line conductor and a PEM conductor, combining the functions of both a protective earthing conductor and a mid-point conductor, as shown in Fig 3 a-b, are not live conductors although they carry operating current (see note to 312.1.2). They are typically connected to a means of earth at the source within the DC distribution network with separate protective conductors often distributed throughout the installation (see Fig 3 c to f).
By convention, a PEL two-wire DC circuit (see Fig 3 a) has a single voltage available between, L+ and L- (U0), and may typically employ either a negative or positive earthing arrangement (see Fig 3 c-d).
In addition, where a PEM three-wire arrangement is used (see Fig 3 b), three possible arrangements may be employed:
- a positive voltage between (L+ and M, or PEM) (Uo),
- a negative voltage between (L- and M, or PEM) (-U0), and
- a combined higher voltage between (L+ andL-) (2Ua).
Appendix 9 of BS 7671 details the earthing systems available for DC supplies.
Short-circuit and earth fault paths
Short-circuit and earth fault conditions are well understood in AC systems. However, the definition of a short-circuit found in Part 2 of BS 7671 applies to both AC and DC circuits; that is:'An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.'
Similarly, an earth fault current applies to both AC and DC circuits, and is defined as:
'A current resulting from a fault of negligible impedance between a line conductor and an exposed-conductivepart or a protective conductor'.
The following figures based on Appendix 9 highlight the potential current paths for both short-circuit and earth fault conditions in a DC circuit, including both a PEL arrangement and a PEM arrangement.
Figure 4 shows a TN-S DC system with an earthed line conductor (L-) and separate protective conductor throughout the installation. The image highlights a short-circuit between the live conductors although one of those live conductors is also connected to earth.
Figure 5 shows a TN-S DC system with an earthed line conductor (L-) and a separate protective conductor throughout the installation. The earth fault occurs between (L+) and the protective conductor.
In a system having a mid-point (PEM) there are two possible opportunities for an earth fault.
As shown in Fig 6, the earth fault is from (L+) to the separate protective conductor, although connected to the mid-point, a positive DC voltage, to earth and subsequent flow of current is as indicated.
In Fig 7, the earth fault is from (L-) to the separate protective conductor although connected to the mid-point, a negative DC voltage to earth and subsequent flow of current is as indicated.
As can be seen, the nature of DC supplies and the range of possible earthing arrangements creates a more complex range of options when compared to that of AC earthing systems.
Summary
The supply of current from a DC source is becoming more commonplace. DC that has been either supplied from a battery source or suitably conditioned AC source is different from AC in that it doesn't change its direction of fl.ow with respect to time, and as such, the effects of frequency, and therefore reactance, can be ignored.DC can provide significant advantages when compared to AC for the supply of a range of final circuits including energy savings and improved integration with solar PV, EES systems, LED lighting and EV systems.
