Characteristics and selection design of DC trunk cable for photovoltaic system

The DC trunk line is the transmission line
of the photovoltaic module system to the inverter after the confluence of the combiner box.
If the inverter is the heart of the entire square array system, then the DC trunk system is a series of aorta
.
Because the DC trunk system adopts a non-grounding scheme, if the cable has a ground fault, it will bring much greater harm to the system and even equipment than AC, so photovoltaic system engineers are more cautious
than electrical engineers in other industries.

Based on the analysis of various cable accidents, we conclude that the ground fault of the cable accounts for 90-95%
of the entire cable fault.

There are three
main causes of ground faults.
First, cable manufacturing defects, for non-conforming products; Second, the operating environment is harsh, natural aging, and external damage; Third, the installation is not standardized, and the wiring is rough
.
The root cause of ground faults is only one --- the insulation of the cable
.
The DC trunk operating environment of photovoltaic power plants is relatively harsh
.
China's large-scale ground power stations are generally in the west, these places are generally deserts, saline-alkali land and daytime temperature difference is large, rat infestation is also more serious, the environment will be very humid
.
The cable is buried in the ground, and the requirements for filling and excavating the cable trench are relatively high; The operating environment of the distributed power station cable is not better than the above ground, the cable will withstand a high temperature, there are technicians to measure and control, the roof temperature can even reach 100-110 °C high temperature, the fire retardant requirements of the cable, and the high temperature has a great
impact on the insulation breakdown voltage of the cable.
Therefore, the selection and design of DC trunk cables for photovoltaic power plants should consider the following points:

1.
Insulation performance of cable

2.
Moisture resistance, cold resistance and weather resistance of the cable

3.
Heat resistance and flame retardant performance of cable

4.
Cable laying method

5.
Conductor material of cable (copper core, aluminum alloy core, aluminum core)

6.
Cross-sectional specifications of cables

At present, China's photovoltaic power station DC trunk cable, most of the use of general low-voltage AC cable to replace, commonly used models are ZR-YJV22 0.
6/1kv, ZRYJY23 0.
6/1kv, most of the cables are copper core cables, but also some power stations gradually began to use aluminum alloy conductor cables, but the insulation material of the cable is basically produced according to the standard of 1kv low-voltage cable
.
In other words, our photovoltaic system engineers have an understanding of the powerful relationship of DC cables, but do not pay much attention to
the technical solutions of cables.

Insulation characteristics of DC mains cables

1.
The field stress distribution of AC cable is balanced, the cable insulation material focuses on the dielectric constant, and the dielectric is not affected by temperature; The stress distribution of DC cable is the largest inner layer of the cable, which is affected by the resistivity of the cable insulation material, and the insulation material has a negative temperature coefficient phenomenon, that is, the temperature increases and the resistance becomes smaller; When the cable is running, the core loss will increase the temperature, and the resistivity of the insulation material of the cable will change accordingly, which will also lead to a change in the electric field stress of the insulation layer, that is, the breakdown voltage of the same thickness of the insulation layer will become smaller
due to the increase in temperature.
For the DC trunk of some distributed power stations, due to the change in ambient temperature, the insulation material of the cable deteriorates much faster than the buried cable, which should be paid special attention
.

2, In the production process of cable insulation layer, it is inevitable to dissolve some impurities, they have relatively small insulation resistivity, along the radial distribution of the insulation layer is uneven, which will also lead to different parts of the volume resistivity is different, under DC voltage, the electric field of the cable insulation layer will also be different, so that the smallest insulation volume resistivity will age faster, becoming the first hidden danger
point to be broken down.
This is not the case
with AC cables.
In layman's terms, the material of AC cable is balanced by stress impact, while the insulation stress of DC trunk cable is always the greatest
impact at the weakest point.
Therefore, AC and DC cables in the cable manufacturing link should have different management and standards
.

3, XLPE insulated cable has been widely used in AC cable, it has very good dielectric properties and physical properties, very cost-effective, but as a DC cable, it has a difficult to solve the space charge problem, which is highly valued
in high-voltage DC cables.
When the polymer is insulated by DC cable, there are a large number of local traps in the insulation layer, resulting in the accumulation of space charge inside the insulation, and the influence of space charge on the insulating material is mainly reflected in the electric field distortion effect and the non-electric field distortion effect, which are very harmful to the insulation material
.
The so-called space charge refers to the part of the charge in a structural unit of macroscopic matter that exceeds the electrical neutrality, and in solids, positive or negative space charges are bound to some local energy level and provide polarization effects
in the form of bound polarizers.
The so-called space charge polarization is a process
in which negative ions accumulate at the interface on the positive electrode side and positive ions at the interface on the negative electrode side due to ion movement when free ions are contained in the dielectric.
In the alternating electric field, the migration of positive and negative charges of the material cannot keep up with the rapid change of the power frequency electric field, so the space charge effect will not occur; In the direct current electric field, the electric field is distributed according to the resistivity, which will form a space charge and affect the electric field distribution, and there are a large number of local states in polyethylene insulation, and the space charge effect is particularly serious
。 Cross-linked polyethylene insulation layer is chemically crosslinked, is a monolithic cross-linked structure, belongs to non-polar polymer, from the entire structure of the cable, the cable itself is like a larger capacitor, DC transmission stops, equivalent to a capacitor has been charged to complete, although the conductor core has grounding treatment, but, failed to effectively discharge, a large amount of DC power still exists in the cable, that is, the so-called space charge, these space charges will not be consumed with the loss of the dielectric like the AC power cable, Instead, it is enriched at the defect of the cable; Cross-linked polyethylene insulated cables, with the extension of use time or frequent interruption and current intensity changes, accumulate more and more space charges, accelerate the aging speed of the insulation layer, thereby affecting the service life
.

Therefore, the insulation performance of DC trunk cable and AC cable are still very
different.