TF-77 VSC HVDC Converters as Virtual Synchronous Machine?
1. Introduction
This article will highlight the motivation and scope of a TF recently completed on the analysis of additional inertia to be provided by Power Electronic Inverter sources, within Study Committee B4, refer to as TF-B4-77.
Today the power system is changing, moving away from the traditional fossil fuel based thermal generation and moving towards an AC system with a large amount of renewable generation along with an increasing number of DC transmission connections. Renewable energy is commonly connected to the grid via a Power Electronic (PE) interface which does not naturally exhibit the same properties as synchronous generation. Eventually, as the trend to decommission older fossil fuel-based generation in favour of renewable sources continues, there will be less synchronous generation and hence less inertia and less dynamic over-current, unless these can be realized by alternative means.
The increase in PE’s connecting generation to the grid raises the question of whether these PE interfaces can be made to emulate the same behaviour as synchronous machines and thereby, from the AC power systems designer’s perspective, reducing or even eliminating the impact of having less synchronous generation. Some power system utilities have already tried to address this by proposing new grid codes, requiring PE converters, connecting generation (or energy storage) to the grid to behave as if they were synchronous generators. However, vendors of PE equipment have raised concerns, highlighting the cost impact on the PE equipment of these possible requirements.
2. Generation or Transmission
Generation, with respect to power systems, refers to the conversion of some form of energy; fossil, wind, solar, into electrical energy. DC Transmission is a means of ‘transporting’ electrical real power, that has been generated at one location to a load at another location, in an efficient and controllable manner. The controllability of DC transmission is important as the DC transmission link can act as a barrier, often referred to as a ‘firewall’ between two independent AC systems; allowing power flow exchanges to take place so long as they do not impose undue stress on one of the interconnected power systems.
Since Voltage Sourced Converter (VSC) technology has been introduced to High Voltage Direct Current (HVDC) there has been a tendency to think of an inverter (the receiving terminal) as a source of generation within the AC power system to which it is connected. This is leading to proposals that future VSC converters should respond to AC system events in a way that emulates a synchronous machine; providing inertia and fault current in the event of an AC system disturbance.
A DC transmission link has little inherent energy storage [1] and hence, a response to a sudden demand in power flow at one end of a DC link will be rapidly transferred to the other terminal and hence to the other AC system. If the other AC system is not able to accommodate the change in power demand the DC link cannot respond.
Should future grid need demand that DC transmission links themselves should be able to supply real power, beyond today’s capability, then this will necessitate a substantial change in converter design to include the additional energy storage elements which will lead to extra cost and, in some circumstances, to commercial conflicts between whether the DC link is a transmission asset or a generation asset. Provision of such additional ancillary services by DC links may some time even erode their commercial viability, if these are not properly accounted for and evaluated.
3. Grid-Connected and Grid-Forming
Most PE converters, today, operate on the principle that there is, already, an existing AC system and the converter controller ‘locks’ to the phase of the AC voltage as its reference, referred to as ‘Grid-Following’. Real and reactive power flow is then a function of the magnitude and angle of the voltage produced by the converter with respect to the magnitude and angle of the voltage reference, from where the controller is taking its reference measurement. In this type of control, there is an interaction between the converter output current and the grid voltage at the point of interconnection. This requires a minimum Short Circuit Ratio (SCR) [2] at which the controller can stably operate. Inherently, therefore, this type of control requires an existing AC system, with a certain minimum Short Circuit Level (SCL), for the controller to lock on to.
A second basic type of converter control is referred to as ‘Grid-Forming’. The basic idea of a Grid Forming converter is that it can create an AC voltage with a controlled magnitude and frequency at its AC terminals in the absence of another source of AC voltage; hence, it can supply a passive load.
The terminologies for VSC operation modes are widely used but lack any strict definition. The Task Force adopted the following definitions:
- A Grid-Following (or Grid-Connected) converter is one that matches the AC grid voltage and frequency and can provide reactive current equal to the steady-state rated current during AC faults.
- A Grid-Forming converter is one that can regulate both instantaneous AC frequency and AC voltage.Such a converter is also able to provide reactive current equal to the steady-state rated current during AC faults.
- A Synchronous Grid-Forming converter is a Grid-Forming converter that is also able to operate in parallel with other AC frequency regulating equipment and converters.
- A Virtual Synchronous Machine (VSM) is a (Synchronous) Grid-Forming converter with energy storage capable to deliver additional energy, for a short period of time, from the converter rather than the DC link and rated to provide a current greater than the steady-state rated current during a fault.
The Task Force identified that while Grid-Following and Grid-Forming VSC converter systems are widely available, the next step to Synchronous Grid-Forming converters is a challenging topic for future development.
4. Short Circuit Current
When an AC system fault occurs, a synchronous generator will inherently produce a large fault current, typically around 6 pu, which can be accommodated by the large thermal mass of the machine without incurring damage to the machine. Whilst this fault current can be problematic for the grid, in terms of the maximum fault current to be interrupted by AC switchgear, the fault current can also provide system benefits.
PE’s have negligible thermal mass and are, therefore, unable to carry large temporary overcurrents. The short-term and steady-state current carrying capability of a PE converter is, very nearly, the same value. Hence, a converter that has been designed and optimised for a particular voltage and current will not be able to respond to a disturbance by providing more than rated current, unless increasing their effective rating, which would lead to increased capital cost, losses and footprint.
5. Fast Fault Current Response
If there is a sudden loss of generation within the AC grid the remaining generation must compensate by providing additional real power. In a synchronous generator dominated AC system the additional power, initially, is provided from the inertia of the synchronous generators and therefore responds ‘instantaneously’ to the AC disturbance. Taking energy from the inertia of the synchronous generators will result in these generators slowing down, that is, the AC frequency will start to fall. Within the AC system there will be load-shedding protection schemes co-ordinated with the maximum permissible Rate-of-Change-of-Frequency (RoCoF). However, with reduced synchronous generation, hence, inherent inertia, the AC frequency can fall faster at some parts of the grid, possibly transgressing the minimum frequency before the RoCoF load-shedding protection has had time to operate. To minimise the impact of this, PE converters should not only rapidly inject fault current into the AC system to both; help AC protection systems to identify the fault and to reduce AC grid voltage depression but should also immediately follow fault clearing by injecting real power to avoid high RoCoF and excessively low values of system frequency.
From an AC system perspective, the ‘ideal’ response to an AC system fault would be to mimic the behaviour of a voltage source behind an impedance as this will be an instantaneous response with the consequential current flow as a function of the voltage difference across the equivalent converter impedance and the impedance itself. As PE’s have practically no thermal mass and are very sensitive to excess current, most converter controllers operate on a principle of primarily controlling the current flowing through the PE’s. In the event of a system disturbance the first response of such a controller would be to hold the current to its present value and then, based on measurement of the change in AC voltage, change the current demand of the converter. This response, therefore, has an inherent delay.
6. Conclusion
The TF set out to provide an understanding of how the AC grid is changing through the transition from mostly fossil fuel driven synchronous generation to renewable resources connected through PE. In addition, but separate, is the question of how converters associated with DC transmission can contribute to the stability of the AC grid.
As a result of the on-going changes in AC grids some proposals have been made to impose new requirements onto future PE converters including those associated with DC transmission. In this paper the TF has attempted to identify how PE technology differs from synchronous generation and consequently, how trying to make PE converters replicate synchronous machine behaviour can have an impact on converter cost, losses and footprint. The impact of these changes should be fully analysed before imposing such requirements . This will be the scope of a new study committee B4 working group
7. Reference
- J Fradley, R Preece, M Barnes, “Assessment of the Impact of MMC-VSC Intrinsic Energy on Power System Stability”, IEEE, The Journal of Engineering, Issue 17, 2019
- B Franken, G Andersson, “Analysis of HVDC converters connected to weak AC systems”, Power Systems, IEEE Transactions on, vol 5, no 1, pp.235,242, Feb 1990