Stability Improvement of The Iraqi Super Grid (400kV) using High Voltage Direct Current (HVDC) Transmission

This research analyzes the level of the short circuit effect of the Iraqi super network and decides the suitable location for the High Voltage Direct Current (HVDC) connections in order to obtain the best short circuit reduction of the total currents of the buses in the network. The proposed method depends on choosing the transmission lines for Alternating current (AC) system that suffers from high Short Circuit Levels (SCLs) in order to reduce its impact on the transmission system and on the lines adjacent to it and this after replacing the alternating current (AC) line by direct current (DC) line. In this paper, Power System Simulator for Engineering (PSS/E) is used to model two types of HVDC lines in an effective region of Iraqi networks and to perform comparative studies to test the location of Short Circuit Levels (SCLs) between an actual AC and AC/DC case study in a portion of the Iraqi national network. The results proved the effectiveness of this method in eliminating severe faults and unwanted short currents, and the results showed that the bipolar type is better in reducing Short Circuit Levels of the Iraqi network.

The power transmission capacity of a two-pole HVDC line is the same as that of a single-circuit three-phase HVAC line. Still, there are two conductors in an HVDC, while a three-phase HVAC needs three conductors, so the number of insulators to support the conductors on the tower will also decrease by 1/3. Therefore, HVDC towers are cheaper than HVAC. Direct connection between two AC systems with the same frequency or a new connection within the mesh grid may be impossible due to system instability, two high short circuit levels of undesirable power flow. An HVDC link controls the flow of power to an AC grid. Thus the load flow can be optimized by means of the HVDC connection to increase transmitted power, reduce losses and improve stability A three-phase fault usually causes the highest current. Thus, it is suitable for carrying out short circuit activity of the 400 kV Iraqi network with a three-phase fault that detects the highest circuit currents. This network can be modeled in power flow programs and transient stability programs such as PSS / E. In this paper, solutions are presented to reduce the level of faults by using an HVDC line that prevents the transmission of short circuit currents to the 400 kV network in the Baghdad region, which decreases the short circuit currents at the 132 kV network lower. Section 2 gives a detailed mathematical model for the HVDC system. The HVDC link implementation does not lead to an increase in SCL is covered in Section 3. Section 4 presents details of the Iraqi grid and its problems. Section 5 presents the simulation results for two case studies, while Section 6 concludes the offered results

MATHEMATICAL EQUATIONS
Electronic converters are essential, so the details are brief and are explained in the following relevant excerpts depending on In Rectifier side: In Inverter side: In Transmission line Journal of Engineering Volume 27 November 2021 Number 11

= (8)
The power at the Rectifier end The power at the Inverter end The power at the middle The power Losses are The voltages V di and V dr are regulated by controlling the following:  Tap changer on transformers of the rectifier for alternations of 8-10 s.  The delay angle α of the thyristor for rapid changes of a few milliseconds. When designing an HVDC system, the power of the AC system is considered by the effective short circuit level of the HVDC system. If the HVDC is planned to replace the present AC line, then flow change in the line will be detected using the present data of the system because the location data of the newly planned HVDC is the same as the replaced AC line ( The case study included two cases, monopole and bipolar types of HVDC transmission lines, and is applied to the Iraqi international network of 400 kV. In this work, it is assumed that a threephase symmetrical fault occurs with a high fault level for that network which is done by using the PSSE package to identify the high fault levels and their corresponding bus bars.

SHORT CIRCUIT CAPACITY (SCC)
Short Circuit Capacitance (SCC) or short circuit level (SCL) in the bus is defined as the product of the magnitude of pre-fault bus voltages and the post-fault current. Another definition of SCC can be formulated as follows: While AC systems rise in size, short circuit levels go to increase with the associated troubles. Due to the current control essential in DC transmission, the HVDC jumper can be used to join with two power systems without raising the short circuit capacity. And also, the HVDC link does not lead to an increase in short circuit level at the connection buses. This means that circuit breakers will not be necessary to change in existing networks (Donghui, Z. et al., 2015).,The HVDC systems themself do not add significantly to the short-circuit power of the AC power system.

HVDC MODELING FOR THE IRAQI 400 KV NETWORK
The Iraqi electrical national grid consists of a 400KV super grid and 132 kV Ultra High Voltage (UHV) electrical power transmission networks, and it consists of 33 kV and 11 kV system distribution networks. Iraqi electrical power system is divided, from an operation and control point Journal of Engineering Volume 27 November 2021 Number 11 68 of view (Planning and Studies Office, 2020), into four operational subsystems, namely the North region, the Middle region, the Euphrates region, and the South region. Basically, it is noted that: Iraqi national Extra High Voltage grid (EHV) system comprises (46) bus bars. The presented work shows that the peak of generated electrical power in 2020 is 17048.7 MW while the peak load demand is 16646.16 MW. The Baiji PS and Baiji GPS network is nearly isolated in the northern region due to ISIS violence, while the Erbil GPS, Mosul Dam, and Kirkuk GPS stations are connected. The Middle region contains Haditha Dam; Isolated due to the destruction of ISIS, Diyala GPS, Basmia GPS, Quids GPS, and Kut PS. The Euphrates region has Kherat GPS, Dewaniya GPS, Musayab GPS, Muthanna GPS, and Musayab PS. Finally, the southern region contains Amara GPS, Nassirya PS, Rumela GPS, Hartha PS, Shat Al Basra GPS, Najibia GPS, and Khor Al-Zubair GPS, as shown in Fig. 2. In the same Figure, The Baghdad area is marked by rounded red dotted lines, and the red bus bars represent the position of DC lines which will be explained in the next section. In the Iraqi national network, the Baghdad governorate suffers from excessive loads due to population density. The level of faults is high due to the presence of three stations close to the area, which are Basmaia Gas Power station feed 3000MW in the 400kV grid, Musayab GPS, and Kut Thermal Station, which may cause major faults to the Baghdad region of 400 kV.

RESULTS AND DISCUSSION
In this work, a three-phase symmetric fault is assumed to occur near bus bars of the Iraqi electrical network (400 kV); the voltage magnitude should not drop below 0.9 per unit (within acceptable limits) at high voltage and extra high voltage networks. In each test system, PSSE has been used to identify the high fault levels and their corresponding connection bus bars with and without the DC line, and then the minimum total losses have been used to find the optimum location. Table  1. shows the short circuit level for some stations in the Iraqi 400 kV network. The monopolar and bipolar are planned and optimized, according to the fault level results, to coordinate generation standards and loads on the AC network with an emphasis on the boundaries of the AC and DC network. It is suitable for comparing fault levels before and after high voltage AC/DC coupling to demonstrate the feasibility of HVDC for reducing short circuit levels in the transient sub-period. For Monopole DC Network: AC grid is joined to converters, i.e., bus bar Basmaia G (BSMG) connected to a converter1 (in Rectifier side), and bus bar AL-Amine (AMN4) connected to a converte2 (in Inverter side), and DC line connecting between the two AC bus bars and of length 41km with a limit of 1400 MW. It is shown in Fig. 3, where the direct current line connecting the Baghdad region and Basmaia region and the AC transmission lines mean disconnection. Table 2.
shows the SCL for AL-Amine (AMN4) is (33511.4A) and is minimized to (25327.7A) with variance value (8,183.70A) and the BSMG SCL is 30812.5A and is reduced to (18478.5A) with variance value (12,334.0A) when the AC line is replaced by the DC single line; Between AMN4 and BSMG. Also, Fig. 4 showed how the short circuit levels decreased in bus bars for the Baghdad region and its adjacent stations after applying DC line implementation.   For Bi-pole DC Network: Bi-polar DC converter (two Monopolar) is joined to AC systems, i.e., bus bar (AMN4) connected to a converter1 (in Rectifier side), and bus bar Bagdad East G (BGE4) connected to a converter2 (in Inverter side), and connecting by DC line with a length 40Km of the limit of 1400 MW. It is shown in Fig. 5, where the direct current line connecting BGE4 and AMN4 and the AC transmission lines mean disconnection. Table 3 shows the SCL for BGE4 is (30401.6A) and is decreased to (17405.6A) with variance value (12,996.0A), and the AMN4 SCL is reduced to (22942A) with variance value (10,569.4A) when the AC line is replaced by the double DC line; Between AMN4 and BGE. Also, Fig. 6 showed how the short circuit levels decreased in bus bars for the Baghdad region and its adjacent stations after applying DC line implementation.

CONCLUSIONS
The use of the PSSE Tools application facilitates an engineer to realistically engage with a wide range of investigations into electrical power system design and operation. We can summarize this research as follows:  The use of an HVDC system is necessary in the case of large faults over small distances, where the other benefits are particularly helpful in addressing a problem. Most of the benefits are the elimination of severe faults and the reduction of unwanted short currents.  It is evident from the previous results that the DC line between 4AMN and BSMG is more effective for reducing SCL in Baghdad region because the high generating capacity of BSMG causes the highest SCL contributions in Baghdad/Rusafa buses; of Iraqi super grid then connecting this bus bar to monopole dc network is more efficient than other bus bars.  Before HVDC implementation, the highest SCL was in the Baghdad area in AL-Amine (AMN) because the high capacity KUT PS station causes the highest SCL in its transmission line. Then coupling (AMN) with BGE4 by DC bipolar network is more effective than other bus bars.  Two types of HVDC, monopolar and Bipolar, are applied and studied in the Iraqi national grid, specifically in the Bagdad region. The results are cleared that a bipolar type is better in reducing SCL. This issue is related to the problem from which there is a problem of short currents inflation in stations near Baghdad.  Finally, it is found that the bipolar affects the SCL of the Iraqi network connecting the bus bar greater than the monopole as shown in the average reduction in SCL of Fig. 7 for monopole type and Fig.8 for bipolar type.   R r = Converter transformer in rectifier dc side winding commutating resistances, /phase. R i = Converter transformer in inverter dc side winding commutating resistances, /phase. X r = Converter transformer in rectifier dc side winding commutating reactance, /phase. X i = Converter transformer in inverter dc side winding commutating reactance, /phase. R d = Resistance of DC line, /phase.