3.3 Existing infrastructures
4.1.1 Network structures
The basic DC network alternatives, two-wire and three-wire networks, are defined in (IEC, 2005). In previous studies and publications, such as (Salonen et al., 2008b), also the terms
‘unipolar’ and ‘bipolar’ networks have been used; in this context, they refer to two-wire and three-wire systems. In a two-wire system, the network consists of two circuits; a positive pole (L+) and a return conductor (L-/PEL). In a three-wire system there are two poles, positive (L+) and negative (L-) ones, and a middle conductor (M/PEM). The systems can be earthed or earth isolated, which is discussed later in Section 4.2.2. The maximum voltage in the DC system is1500 V in the two-wire and ±750V in the three-wire system, if the Low Voltage Directive (LVD) is followed (Low Voltage Directive 2014/35/EU). It is also possible to use multiple voltage levels in the DC network, but there should be specific needs to rationalize the feasibility. This option may be worth considering if DC is to be used directly in customer-end loads. This is common in applications where certain voltages are needed directly in loads, such as in data centers. The discussed alternative network structures are depicted in Figure 4.2.
MV
L+
M
L-AC/DC DC/AC
DC/AC MV
L+
L-AC/DC
DC/AC
DC/AC
Figure 4.2:Unipolar (above) and bipolar (below) network structures.
In the two-wire system, the inverters have to be rated for the full DC voltage that is used. In the three-wire system, the connection can be provided between the pole and middle conductors, requiring the inverter input to be rated for half of the maximum input voltage (in normal oper-ation), or between the poles with or without a middle connection. In the three-wire system, the current flowing in the middle conductor in each of the network branches equals the difference between the load at the positive and negative poles in that network section. Therefore, losses are produced also in the middle conductor, and the voltages differ in different network sections.
4.1 LVDC network 49
4.1.1 Network structures
The basic DC network alternatives, two-wire and three-wire networks, are defined in (IEC, 2005). In previous studies and publications, such as (Salonen et al., 2008b), also the terms
‘unipolar’ and ‘bipolar’ networks have been used; in this context, they refer to two-wire and three-wire systems. In a two-wire system, the network consists of two circuits; a positive pole (L+) and a return conductor (L-/PEL). In a three-wire system there are two poles, positive (L+) and negative (L-) ones, and a middle conductor (M/PEM). The systems can be earthed or earth isolated, which is discussed later in Section 4.2.2. The maximum voltage in the DC system is1500 V in the two-wire and ±750V in the three-wire system, if the Low Voltage Directive (LVD) is followed (Low Voltage Directive 2014/35/EU). It is also possible to use multiple voltage levels in the DC network, but there should be specific needs to rationalize the feasibility. This option may be worth considering if DC is to be used directly in customer-end loads. This is common in applications where certain voltages are needed directly in loads, such as in data centers. The discussed alternative network structures are depicted in Figure 4.2.
MV
L+ M
L-AC/DC DC/AC
DC/AC MV
L+ L-AC/DC
DC/AC
DC/AC
Figure 4.2:Unipolar (above) and bipolar (below) network structures.
In the two-wire system, the inverters have to be rated for the full DC voltage that is used. In the three-wire system, the connection can be provided between the pole and middle conductors, requiring the inverter input to be rated for half of the maximum input voltage (in normal oper-ation), or between the poles with or without a middle connection. In the three-wire system, the current flowing in the middle conductor in each of the network branches equals the difference between the load at the positive and negative poles in that network section. Therefore, losses are produced also in the middle conductor, and the voltages differ in different network sections.
50 4 Technical solutions and regulations
The transmission losses for a unipolar network can be expressed as
P=R·I2, (4.1)
whereRis the resistance of the conductor andIis the current flowing in the conductor. For a bipolar network
P =R12·(I12+I22) +RN·(I1−I2)2, (4.2) whereR12is the resistance of the pole conductors (assuming the same for both poles),I1is the current flowing in the positive pole,I2is the current flowing in the negative pole, andRNis the resistance of the middle pole. The voltage drop of a unipolar network can be calculated as
Ui−Ui+1=Ri·Ii, (4.3)
and for a bipolar network positive pole as
U1,i−U1,i+1=R12,i·I1,i+RN,i·(I1,i−I2,i), (4.4) whereU1,iis the voltage drop,R12,iis the resistance of the pole conductors (assuming the same for both poles),RN,iis the resistance of the middle conductor,I1,iis the current in the positive pole, andI2,iis the current in the negative pole, in nodeiand for a bipolar network negative pole as
U2,i−U2,i+1=R12,i·I2,i+RN,i·(I1,i−I2,i), (4.5) whereU2,iis the voltage drop in the negative pole, in node i(Kaipia et al., 2008), (Partanen et al., 2010). An example of the transmission capacities of the unipolar and bipolar networks in relation to AC distribution is depicted in Figure 4.3.
50 4 Technical solutions and regulations
The transmission losses for a unipolar network can be expressed as
P =R·I2, (4.1)
whereRis the resistance of the conductor andIis the current flowing in the conductor. For a bipolar network
P =R12·(I12+I22) +RN·(I1−I2)2, (4.2) whereR12is the resistance of the pole conductors (assuming the same for both poles),I1is the current flowing in the positive pole,I2is the current flowing in the negative pole, andRNis the resistance of the middle pole. The voltage drop of a unipolar network can be calculated as
Ui−Ui+1=Ri·Ii, (4.3)
and for a bipolar network positive pole as
U1,i−U1,i+1=R12,i·I1,i+RN,i·(I1,i−I2,i), (4.4) whereU1,iis the voltage drop,R12,iis the resistance of the pole conductors (assuming the same for both poles),RN,iis the resistance of the middle conductor,I1,iis the current in the positive pole, andI2,iis the current in the negative pole, in nodeiand for a bipolar network negative pole as
U2,i−U2,i+1=R12,i·I2,i+RN,i·(I1,i−I2,i), (4.5) whereU2,iis the voltage drop in the negative pole, in nodei (Kaipia et al., 2008), (Partanen et al., 2010). An example of the transmission capacities of the unipolar and bipolar networks in relation to AC distribution is depicted in Figure 4.3.
4.1 LVDC network 51
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0 1 000 2 000 3 000 4 000 5 000
Maximum transmission power [pu]
Length of line [m]
10 % max. voltage drop
25 % max. voltage drop
4 x 35 mm2XLPE cable
Figure 4.3:Transmission capacity (Kaipia, 2014).
In the two-wire system, the customer division between the poles is one of the tasks that have to be solved in the network planning phase. The objective is to achieve the minimum difference between the two poles so that the losses are minimized. In the customer division it is necessary to consider the long-term operation (energy losses and costs) and maximum power demand situations, such as feeding of the short-circuit to a customer.
The connection of existing conductors can be done in different ways, as described in (Karppa-nen et al., 2015b). The two-wire system has the advantage of decreasing the resistance in case there are multiple conductors that can be utilized per pole if a single cable is used. AMKA cables (PVC-insulated aerial bundled self-supporting cables) are also used and available in four conductor variants in some distribution areas and could be connected similarly to underground cables. In the Finnish standard SFS 6000-8-801 there are guidelines for the use and marking of the existing conductors (SFS, 2017b). Examples of conductor usage in unipolar and bipolar underground cables and aerial bundled cables are given in (Karppanen et al., 2015b).
4.1 LVDC network 51
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0 1 000 2 000 3 000 4 000 5 000
Maximum transmission power [pu]
Length of line [m] 10 % max. voltage drop
25 % max. voltage drop
4 x 35 mm2XLPE cable
Figure 4.3:Transmission capacity (Kaipia, 2014).
In the two-wire system, the customer division between the poles is one of the tasks that have to be solved in the network planning phase. The objective is to achieve the minimum difference between the two poles so that the losses are minimized. In the customer division it is necessary to consider the long-term operation (energy losses and costs) and maximum power demand situations, such as feeding of the short-circuit to a customer.
The connection of existing conductors can be done in different ways, as described in (Karppa-nen et al., 2015b). The two-wire system has the advantage of decreasing the resistance in case there are multiple conductors that can be utilized per pole if a single cable is used. AMKA cables (PVC-insulated aerial bundled self-supporting cables) are also used and available in four conductor variants in some distribution areas and could be connected similarly to underground cables. In the Finnish standard SFS 6000-8-801 there are guidelines for the use and marking of the existing conductors (SFS, 2017b). Examples of conductor usage in unipolar and bipolar underground cables and aerial bundled cables are given in (Karppanen et al., 2015b).
52 4 Technical solutions and regulations