Research and application on multi-terminal and DC grids based on VSC-HVDC technology in China

: Voltage source converter (VSC)-based high-voltage direct current (HVDC) and multi-terminal (MT)/DC grid technologies are the new HVDC transmission technologies after ultra-high voltage alternative current (UHVAC) and UHVDC transmission technologies which have been widely used in China. The application of the new technologies has resulted in a rapid increase in the number of schemes in construction and planning worldwide. This has been stimulated by the greater level of functionality available from the VSC technology, which makes it suitable for a wide variety of applications. These include the integration of off-shore wind farms, embedded links within AC networks and interconnectors, especially where the AC networks are relatively ‘ weak ’ . VSC technology has renewed interest in MT DC systems, which may ultimately lead to wide area DC grids. This study outlines the research and application on MT and DC grids in China with respect to VSC-HVDC key technologies and DC grid key technologies based on the presentation given in the International Workshop on Next Generation Power Equipment held on 23 September 2016 in Xian, China. The briefing details of the VSC-HVDC projects constructed and to be constructed in China are summarised in this study.


Introduction
Global power interconnection (GPI) currently is an ambitious plan to interconnect all the power systems around the world to form a complex giant global power system. Its goal is to promote extensive development, deployment and utilisation of clean energy across the world to ensure stable operation of the complex system and sustainable long-term energy supplies. GPI can be actually considered as smart grids plus ultra-high voltage (UHV) grids plus clean energy, where the smart grids are the foundations, the UHV grids are the key backbones and the clean energy is at the top priority for the PGI. A preliminary indicative plan for the GPI is as shown in Fig. 1 [1].
To realise the GPI and achieve its goal, the way of energy utilisation and the development of the current power systems need to be changed and more efficient, flexible and reliable power transmission technologies and solutions are required. The system studies carried out by researchers around the world indicate that voltage source converter-based high-voltage direct current (VSC-HVDC) and DC grids have many advantages and are considered to be the most effective and promising technical solutions for realising the GPI.
VSC-HVDC is a new type of high-voltage direct current (HVDC) transmission technology based on insulated-gate bipolar transistor (IGBT) converters. The amount of power to be transmitted from one point to the other depends on the magnitude and the phase angle difference of the voltages at the both points, and the equivalent reactance between the two points. VSC-HVDC has the capability to control the amplitude and the phase angle of the voltages independently so as to control both the active and reactive power outputs at both ends of the VSC-HVDC independently.
Due to its many prominent advantages such as flexible and independent active and reactive power control, capable of providing dynamic reactive power compensation, no distance limitation, no commutation failure issues, VSC-HVDC has many applications which cannot be achieved with the conventional line commutated converter (LCC)-based HVDC, such as renewable integration, construction of DC grids, connection to weak power systems and large city power supply and so on [2][3][4][5][6][7][8][9].

System design technologies
System design technologies are mainly divided into three categories: studies, design and engineering. The studies include optimisation studies for topologies and main parameters, overvoltage and insulation coordination studies, loss and harmonic studies, grid code compliance studies and noise studies. The design consists of the voltage source converter (VSC) valve design, control and protection (C&P) system design, cable design, converter station layout design, auxiliary system and cooling system design. Engineering normally deals with civil works, VSC valve unit installations, cable installations, bulk transportation, system tests, commissioning, recommended maintenance and planed & unplanned outages.
Symmetrical monopole configuration is an electrical bipole. In this configuration, the converters are connected between two high-voltage conductor terminals of the same magnitude but of opposite polarity as shown in Fig. 2a. The earth reference can be provided with various methods including the connection of the DC capacitors' mid-point as shown in the figure. Although in this configuration two conductors of opposite polarity carry power, they cannot operate independently, i.e. if one conductor is out of service the healthy conductor cannot continue transmitting power using earth as the return path. This is the main difference between the symmetrical monopole and the full bipole configurations. The majority of VSC-HVDC systems to date utilise the symmetrical monopole configuration.
Asymmetric monopole configuration connects the converters between the high-voltage conductor terminal and the return (either earth or metallic conductor), as shown in Fig. 2b. The earth electrodes at the stations or metallic wires must be designed for flowing continuous full current. Compared to the system with earth return, the metallic return system has higher transmission losses due to the higher resistance of the metallic return path compared to the earth return.
A bipolar arrangement can greatly improve system reliability by using two converters at each HVDC terminal, one connected between the earth and the positive pole and the other one between  the earth and the negative pole, as shown in Fig. 2c. In this configuration, if any of the converters or high-voltage conductors is out of service power transmission can continue (perhaps at a lower level) through the healthy pole using earth or a dedicated low voltage conductor as the return path. In a normal bipolar system, the two poles are symmetrical with equal DC voltage and current ratings. During normal operation, the DC current in the two poles is controlled to be equal so that the neutral current remains near zero.

Overvoltages and insulation coordination
Overvoltages can be classified into two groups, i.e. switching overvoltages and lightning overvoltages, for HVDC transmission systems. The first group is internally generated due to switching, thus called as switching overvoltages. They can be caused by switching of capacitive and electromagnetic loads, load rejection, faults and fault clearance. The second group is externally generated overvoltages from lightning strikes directly to the lines or substations and strikes to the towers or overhead earth wires resulting in back flash overvoltages to the conductors. These overvoltages can occur at both the AC and DC sides and are normally limited by surge arresters (SAs) to protect the system equipment. Insulation is provided by materials which upon application of a voltage across them resulting in no or insignificant flow of current. Insulation breakdown can occur when the voltage across the insulation is greater than that the insulation can withstand. Insulation coordination is the procedure of selection of equipment in relation to the voltages which can appear in the system where the equipment is installed, taking into account the service environment and the characteristics of the available protective devices, i.e. SAs. One of the typical outputs of an insulation coordination study for an HVDC scheme is to determine the ratings and positions of SAs to be installed. Fig. 3 presents a typical SA arrangement for an HVDC station and Table 1 indicates the typical stresses imposed on these SAs under different possible events [8].

C&P system
C&P system is designed having levelling configuration and is divided into three levels. Level 1 contains supervisory control and data acquisition (SCADA) system, level 2 includes control system, VBC (valve base controller) and protection system, and level 3 composes the local inputs and outputs (I/Os).

Level 1:
The SCADA system has the functions of monitoring data and status, issuing commands, receiving and uploading data, implementing event sequence record and event alarm. It is normally arranged as dual-system for high reliability.

Level 2:
The control system is divided into station control, pole control and valve control as shown in Fig. 4. The station control performs the start/stop control, operation mode control and status monitoring & switching of redundancy. Its outputs are fed into the pole control to achieve constant voltage/constant power control, data acquisition and processing of analogue signals. The valve control receives the orders and signals from the pole control to control and to monitor the conducting of power electronic devices within the valves.
The control system described above has the features of good communication compatibility and fast communication speed due to use of IEC 61850 protocol, improved anti-interference performance due to use of optic communication, and high reliability and efficiency due to embedded operating system and compact RISC (reduced instruction set computing).
The VBC as shown in Fig. 5 has the functions to control the circulating current and capacitor voltage, to protect converter valves and to monitor converter valves and VBC. Using the VBC can achieve less than 100 μs of control period, dual redundancy for all the devices, optic-fibre communication, within 5% of unbalanced voltage and less than 5% circulating current.
The protection system is divided into three protection zones, AC switch yard zone, valve zone and DC switch yard zone. The AC zone is to protect the AC equipment installed in the AC switch yard. The valve zone is to achieve the protection of AC bus faults within station, protection of grounding faults, protection of failures of current control and protection of faults resulting in over-current. The DC zone is to protect the DC equipment installed in the DC switch yard.

Converter station layout design
A typical VSC-HVDC converter station composes AC yard, valve hall, DC yard, C&P room, cooling system and other facilitates, as shown in Fig. 6. The valves in the valve hall can be arranged as three-layer valve tower, four-layer valve tower, suspending structure or two-storey building (i.e. a tower sitting on the floor and suspending on the ceiling).

VSC-HVDC equipment
VSC-HVDC equipment can be grouped into three catalogues as shown in Table 2 based on their locations to be installed. Some key equipments are detailed below.
2.6.1 VSC-HVDC converters: VSC converters are the key components in a converter station and the analysis and design of the converters requires the knowledge of electricity, magnetic, machinery, heat combined stresses and so on.
VSC-HVDC converters normally use the six-pulse connection and there are several different types of converter configurations [3]. Compared to traditional two-level and three-level converters, a    Consequently the harmonic performance is excellent and usually no harmonic filters are needed. Currently, the1000 MW/±320 kV MMC VSC-HVDC valves have been developed and have been applied for Xiamen 1000 MW/±320 kV bipole HVDC project and an underground cable interconnection between France and Spain consisting of two 1000 MW links in parallel at a voltage of ±320 kV. At present, the ±500 kV/3000 MW MMC VSC converters are under development and will be used to, e.g. Zhangbei HVDC grid project.
The VSC-HVDC converters developed by Global Energy Interconnection Research Institute (GEIRI) are easily scalable with modular design, easy assembly and maintenance, have superior inflammability conforming to UL94 and excellent seismic performance (0.2 g typical), and use environmental friendly materials (halogen free).
2.6.2 C&P system device: The hierarchical structure of a C&P system device developed by GEIRI is designed as vertical containing operator control layer, control layer and interface layer, and horizontal performing DC control, DC protection, auxiliary power and equipment control. The C&P system device as shown in Fig. 8 has universal rack and modular design, employs ETHERCAT bus and HDLC fibre-optic for inter-rack communications and has high reliability with multi-redundancy.
2.6.3 DC cables: Different cable technologies are commercially available and can be classified, depending on the insulation system, as lapped oil filled cables, lapped mass impregnated (MI) cables and extruded (XLPE) cables. In the present situation, the traditional HVDC oil filled and MI cables can transmit 1200 MW under 600 kV, i.e. 2400 MW for a bipole ±600 kV. The oil filled cables are limited to a length of 80 km to avoid any cavitations due to the filling oil pressure drop during the load cycles. They can be used in HVAC as well as HVDC conditions. The circuit length of MI cables is unlimited. In the present situation, MI cables can transmit 900 MW under 550 kV, i.e. 1800 MW for a bipole ±550 kV. Transportation limits the unit length, the longest HVDC MI cable is 580 km for NorNed HVDC project (between UK and Holland).
The extruded XLPE cables are preferred to be used for underground HVDC transmission projects due to the efficiency of jointing and reduced weight compared with the MI cables. The XLPE cables up to 320 kV are presently installed and in operation, and the technology is still evolving. Cables up to 525 kV have been announced as under qualification, which could bring the maximum transfer capacity up to about 2600 MW. The manufacturing length is restricted to about 10 km depending on the voltage level, which increases the number of factory joints. One of the major challenges for development of higher voltage XLPE cables is the development of special HVDC accessories.
Superconducting HVDC cables are currently receiving significant attention due to their potential for transmitting large amounts of power without the restriction of current. This technology is not yet commercially available for HVDC systems and there are operational challenges to be dealt with. The superconducting power cable consists of a cable core that is housed inside a cable cryostat. Liquid nitrogen circulates inside the cable cryostat, providing cooling for the cable, as it needs to be operated at cryogenic temperatures (commonly between 65 and 72 K (−208 to -201°C). With typical flow speeds of the liquid nitrogen and cooling channel size in the high temperature superconducting cable designs a cable segment length between the cooling stations is in the order of magnitude of 5-10 km. Superconducting power cables commonly employ a lapped paper insulation made from polypropylene laminated paper.

DC grid key technologies and equipment
A DC grid is a network that contains multiple AC/DC converter terminals that are interconnected with DC lines and DC/DC converters in meshes and radials [5]. The advantages of a DC grid are that it increases system flexibility and reliability, and provides redundancy by sharing resources that result in lower power losses and interference [5]. Therefore, DC grids are considered to be the most effective and promising technical solutions for the collection and integration of renewable onshore and offshore wind generation, collection and transmission of remote renewable energy resources to load centres, ocean archipelago power supplies, the construction of new types of urban/distribution power networks, and interconnections of AC power systems [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Thus, the development and application of DC grids have become an important direction for the development of smart grids and the energy internet.

DC grid key technologies
Besides the VSC-HVDC technologies as described in Section 2, the specific key technologies for DC grids can be categorised into three aspects: simulation, C&P and standardisation.

Simulation technology:
Simulation consists of computer simulation and physical simulation. Compared to the physical simulation, the computer simulation is one of the economic and efficient tools for planning, designing and evaluating power systems and can be used to explore and gain new insights into new technologies and to study the performance of the systems. For a DC grid, the computer simulation is normally used for analysing and verifying the system performance, MMC-valve designs and the C&P functions [24,25].
Nowadays, computer simulation technology is classified as off-line simulation, real-time simulation and real-time digital-analogy hybrid simulation. Fig. 9 shows a photo of State Key Laboratory owned by GEIRI within which there is a real-time simulation system. The system can be used to simulate up to ±500 kV VSC-HVDC schemes with RTLAB and is suitable for the simulation of VSC-HVDC systems/grids with up to 20 VSC terminals.

Control and protection:
Apart from the control system for VSC-HVDC systems as described in Section 2.4, the control system especially for a DC grid is very important for controlling the power flowing into and out the grid, DC voltages at busbars and the operation of the grid in steady and transient states. It should also have the functions of coordination control and power mutual support between the DC grid and the AC system where the DC grid is embedded in.
Although there are many different control technologies at various levels which have been invented and studied around the world [14][15][16][17][18][19][20][21][22][23], the voltage margin control, the voltage droop control, the voltage droop control with a voltage margin, a voltage droop control as a backup and so on are the basic control methods as described in [5].
DC grid protection strategy is required normally to be reliable, sensitive, selectable, rapid and controllable. A DC grid protection can be divided into four protecting zones: AC switch yard, converter station, DC switch yard and DC lines. The protection strategies for the first three zones are similar to those for point-to-point (PtP) HVDC systems as mentioned in Section 2.4. AC circuit breakers (CBs) at both ends of a PtP HVDC system are used to clear any DC faults and no DC CBs are installed. But for a DC grid, if using AC CBs to clear the DC faults, AC CBs related to all the connected converters have to trip resulting in whole DC grid being disturbed. Therefore, DC CBs are essential for clearing the DC faults which are the major differences for the protection between the PtP schemes and DC grids. As DC fault currents are less damped and faster rises, DC fault currents are required to be rapidly detected and cleared within few milliseconds. Studies show that travelling-wave protection can detect a DC fault within 1-2, plus 2-3 ms of breaking time, resulting 3-5 ms to clear a DC fault [5].

Standardisation:
Standardisation mainly includes the standardisation of DC voltage levels and key equipment, and benchmark models for DC grid system studies. There are many advantages of standardised DC voltage levels [26]: † Promoting standardisation of power equipment. † Reducing the number of DC/DC converters required. † Enhancing the interoperability and compatibility of power equipment. † Simplifying maintenance and operation of power equipment. † Bringing significant economic benefits.
Considering that DC grids will be gradually developed by the newly proposed DC transmission systems as a precondition, a DC voltage level selection methodology for DC grids has been proposed by considering various technical and economic factors including current limit, operational losses and equipment costs on the premise of defined DC transmission capacity and distance [26].
Key equipment for a DC grid, which can be standardised is DC breakers, DC/DC converters and AC/DC converters to gain the last two advantages as listed above as well as to rationalise their spare parts, gain optimisation of DC design resulting in capital and operating costs and so on.
A DC grid benchmark model can provide a common reference and study platform for researchers to undertake HVDC grid studies and compare the study results and performance and characteristics of different DC control functions and protection strategies. It can also provide reference cases for testing simulators and digital programs. Researchers and organisations from different countries and organisations can use these models for sharing and comparing research results as well as for formulating standards for DC grid equipment and operations. Seven preliminary DC grid benchmark models covering most different HVDC grid applications for different types of studies and CIGRE B4 DC grid test system have been developed by GEIRI and CIGRE SC B4.57 WG, respectively, as detailed in [27][28][29][30].

DC grid key equipment
Apart from the key equipments described in Section 2.6 for VSC-HVDC systems, there are three key equipments specifically for a DC grid: DC CBs, DC/DC converters and DC power flow controllers (PFCs).  Fig. 10 indicate that the AC fault current has zero crossings with lower magnitude due to higher line impedance and its amplitude decays over the time, whereas the DC fault current has no zero crossings with higher magnitude due to lower line impedance and its amplitude rises quickly over the time. Based on the characteristics, three challenges faced by developing HVDC CBs (HVDC-CBs) can be identified and summarised as follows: † How to interrupt a DC fault current without zero crossings? † How to ensure the DC fault current interrupted within very short period (2-3 ms) to protect power electronic devices & to prevent the spread over of the faults? † How to limit the overvoltages generated due to the interruption and to dispose of the energy stored in the system?  a Developed by ABB [31] b Developed by GE (former ALSTOM) [37] c Developed by GEIRI [32] High Volt., 2017, Vol. Several international HVDC manufacturers such as ABB, GE (former ALSTOM) and GEIRI have developed HVDC-CB prototypes [31][32][33][34][35][36][37][38][39]. Different HVDC-CB types are being considered as shown in Fig. 11. Though the manufacturers used different types of power electronic devices or different topologies for their HVDC-CBs the methods to solve the above three challenges are the same, i.e. to use: † Power electronic devices (IGBT used by ABB, thyristors by ALSTOM and IGBT-based MMC by GEIRI) to interrupt DC fault current without zero crossings. † Ultra-fast disconnector to ensure the DC fault current interrupted within very short period. † SAs to limit the overvoltages and dispose the energy stored in the system.
Based on the published and conference publications, the parameters of the HV-DCCB prototypes developed by the three manufacturers are listed in Table 3. Please note that more advanced HV-DCCB prototypes might have been developed by these or other manufacturers which might not be included in this paper due to the limitation of the information sources or not announced publicly.
3.2.2 DC-DC converter: DC/DC converters are also called DC transformers. They are used to realise the interconnection between different voltage levels, i.e. to achieve voltage conversion, and are the essential equipment for a DC grid having more than one voltage levels. They might also have power flow control functions and be capable of isolating faults, i.e. can replace PFCs and fault current limiters. At the moment, DC/DC converters rated in MW to several tens MW are available commercially and will be extended to hundreds of MW in the near future.
There are different kinds of configurations of DC/DC converters [40,41]   circuit theory the branch currents in a DC grid could be controlled as many as the number of available nodes minus one. For example, there are four nodes and six branches in the DC grid shown in Fig. 13a, thus the number of branch currents which can be controlled is three (4 nodes −1 = 3), resulting in three branch currents which cannot be controlled. Under certain circumstances, e.g. one branch is switched out due to a fault on it the other branches connected to the same node might be overloaded. To ensure no branches are overloaded at any operating modes, PFCs are required to be installed. There are mainly two types of PFCs, one is the variable resistor type to insert a variable resistor into a branch to reduce the current through the branch and the other is a variable active voltage source installed in series with a branch. The former one can only reduce the current in the branch but the latter can not only both increase and decrease the current through that branch but also change the direction of the current in the branch. Different topologies of PFCs have been developed and Figs. 13d and e show two PFCs with different topologies [47].

VSC-HVDC projects in China
Several VSC-HVDC projects have been constructed in China and their details are summarised in Table 4, each of them has its own features.

Conclusion
GEI is the fundamental solution to address the contradiction between sustainable development and energy demand. VSC-HVDC technology providing more effective support for the regional power grid construction, and China is accelerating the exploration Fig. 13 DC PFCs a Schematic diagram of a DC grid [5] b Variable resistor type [45] c Variable voltage source type d Transformer-based DC PFC e Dual H-bridge DCPFC [46]  and practice in the field. The experience gained from the all the VSC-HVDC power transmission projects, especially Zhangbei DC grids project, will provide valuable reference to develop and apply this technology worldwide.

Acknowledgments
The authors recognise the work of the many researchers working in the fields of VSC-HVDC and DC grids and thank those who have given permission for the use of their figures in this paper.
This work was sponsored by State Grid Corporation of China, through the 1000-plan Project ([2014]264).