Testing of Real Time Power System Control Software Transpower s Pole 3 Project Presentation to Special Interest Group of ANZTB 7 October 2013 Richard Sherry 1
Summary 1. What does the HVDC Control System control 2. The Control System Hardware and Software how it achieves the required control 3. User Requirements of the Control System 4. The Software Testing that we did (and are still doing) Stages of testing Test coverage and the test environments Validation of the environments Some software issues On site tests 5. Questions 2
What does the HVDC Control System Control The main function is to control the electrical power delivered by the HVDC Link which comprises: A bipole in effect two co-ordinated HVDC links three DC cables (350 kv) under the Cook Strait (about 45km) 535km of overhead DC power lines on the South Island, and 28km on the North Island two converter stations one at Haywards (near Upper Hutt) and one at Benmore. Thyristor Valves to do the DC conversion the largest AC power transformers in New Zealand Benmore Haywards The HVDC link can transfer up to 1200 MW of power between the islands (1200 MW is approximately twice the peak power demand of the Greater Wellington region) 3
Converter Station - Benmore 4
5
AC to DC conversion 6
DC to AC conversion 7
What does the HVDC Control System Control Control and Protection of the HVDC Main Plant Deliver the required DC power on the bipole Monitor and Control the DC currents and voltages in each pole Control the HVDC system so it recovers from faults Control the firing angle of the four converter valves Control of the 220 kv AC Network Voltage at Haywards and Benmore Control of various items of non-hvdc plant Switching of the AC Filters at both Haywards (7) and Benmore (5) The DC converters create current and voltage distortions (harmonics) on the AC system that must be controlled - the filters do this 8
What does the HVDC Control System Control The DC thyristor valves are fired in sequence at a known time delay to the incoming voltage waveform (known as a firing angle) 9
What does the HVDC Control System Control HAY 1 stack + 3 Y2 Y6 Y4 r2 y2 b2 Y5 Y3 Y1 D2 D6 D4 r2 y2 b2 D5 D3 D1 10
The control system software The control system software has a lot of things to do The functions are organised in the software into modules (separate software systems) Each module has its own Inputs and Outputs both to the other modules and to other CPUs within itself (more later!) Heirarchical structure but also sequential - any control action that needs to change the HVDC conditions has to propagate down to the firing angle controls 11
The software is modular The control system software Measurement (AC and DC systems) Control Protection HVDC Functions Station Control HVDC AC Filters Bipole Control Pole Control Valve Control AC Voltage AC Filters Machines & Statcom Green = functional software module, Blue = description / detail 12
Valve Control Valve Firing control and protection Firing Angle Control Pole Control VDCL + Protection AC System Fault recovery Bipole Control Station Control HVDC Power Dispatch DC System Fault Recovery AC System Over voltage control TOV AC Filter control Typical DC fault clearance time Typical AC fault clearance time Machine Voltage control AC System Voltage control 10-7 10 Microseconds 1 Milli - second 1 cycle (20ms) 100 ms 1 sec 10 10 3 10 5 seconds 1 min 1 hr 1 day 13
The control system software The software (Pole, Bipole and Station control) Proprietary Siemens systems (SIMATIC-TDC) SIMATIC-TDC Graphical Interface Edited under Windows XP Compiled and loaded to Control Cubicles on Flash Cards On-Line Software can also be accessed and edited The hardware (Control Cubicles) Standard cubicles Contain embedded PCs The number of CPUs required depends on the software needs 14
The control system software Proprietary Siemens systems (SIMATIC-TDC) 15
The control system hardware Pole Control : 16
How the control software works Digital software, normal CPU constraints Analog measurements the measurement software can provide a very fast varying signal into the rest of the control system The measurement inputs are smoothed to avoid noise - and sampled fastest rate is about 0.1 ms (10,000 samples / sec) Control software runs in 5 timed execution cycles - from 1 ms to 32 ms for most of the TDC software (but up to 512ms) Control software also uses 5 interrupts (<1 ms) for immediate processing 17
How the control software works Real Time execution for the cyclic code S1 (1ms) S2 (4ms) S3 (8ms) Interrupts will run whenever triggered The state of the blocks can change e.g. some S2 blocks may have already been executed so the interrupt change will be delayed longer until they are run again. If a cycle time ends before all the code for that cycle has been executed = CPU overload S1block01 S1block02 S2block01 S2block02 S2block03 S2block04 S3block01 S3block02 S3block03 S3block04 S3block05 S1block01 S1block02 S2block01 S2block02 S1block01 S1block02 S2block03 S2block04 S1block01 S1block02 S3block01 S3block02 S1block01 S1block02 S3block03 S1block01 S1block02 S2block01 S2block02 T+1ms T+2ms T+3ms T+4ms 18
How the control software works All code cannot be executed in the smallest cycle time All the components within the software (e.g. standard blocks) have known execution times for their functions Multiple CPUs are usually needed, and CPU loading is a critical issue for the design overload is not an option as this would mean real time control is not being achieved. The cyclic processing can lead to different responses to the same interrupt signal - depending on the point reached in all the slower cycles when the interrupt appears 19
User Requirements Replace Pole 2 controls, new controls for Pole 3, Bipole and Station levels Control of HVDC power transfer Control of the AC network voltages at each end of the link, including harmonic control with the AC filters Protective action for internal HVDC faults Fast recovery of DC power after AC faults, 90% in < 200 ms And after DC line faults, 90% in <100 ms AC system stabilisation after faults AC system frequency control by modifying DC power 20
User Requirements All HVDC vendors have years of HVDC experience The New Zealand DC link requirements still had to be specified in detail Integration of the existing Pole 2 Speed of fault recovery, pole to pole power transfer Overload requirements AC system stabilisation functions The longer term Station Control functions Roundpower function (DC power direction reversal without stopping) All of these are specific to New Zealand in some aspects and we knew would require some design work by the vendors These are important requirements that help to minimise the total power costs in New Zealand 21
Tests Software testing 22
Stages of the Testing (commissioning) Installation and Plant Commissioning Tests Station Testing System Testing AC Yard Equipment AC Filter Banks AC Yard Subsystem AC Yard Energised Auxillary Power Systems Building Systems Valve Cooling Converter Transformers Converter Subsystem Converter AC Energised DC Yard Energised DC Yard Open Line Test System End to End Control Tests Steady State Operation Dynamic Performance Trial Operation Acceptance Thyristor Valves Control & Protection Systems DC Yard Equipment DC Line, Cable & Electrode DC Yard & Transmission Installation tests Station Testing DC sequence control, protection functions System Testing DC and AC system functions, protection and performance 23
User Requirements Test conditions The control system has to work with a wide range of variable conditions : The HVDC link can operate : North or South power direction Monopolar, Bipolar, or Roundpower operation At 350 kv or at 250 kv At any power level from 30 MW up to 1200MW And different DC faults can occur - line flashover, fire, salt pollution, fault may be anywhere on the DC line or cable, faults inside the valves etc The AC systems can have : Varying load connected day/night, seasonal variation Varying generation connected Outages of equipment And different types of AC faults can occur 1 phase, 3 phase, different impedances, different protection operation / speed, etc The controls operate differently and the behaviour during disturbances is different for all these variable factors so it all should be tested (at least to some extent) 24
Test Environments Study Based Simulation Real Time Simulation On Site Individual Tests 600 1000 400 Time period Feb 2012 to Dec 2012 Feb 2012 to Dec 2012 P3 : 16 Feb to 28 May 2013 P2 : 11 Sept to 30 Nov 2013 (planned) Testing on the real system is very expensive and time consuming. The most severe tests if done on the live system would result in major disturbances for consumers So the software has to be tested off line using simulation environments, various test setups are needed for this. 25
How do we test the Control Software So what are the Simulation environments? How do we simulate the real world in order to test the software? Power system models these calculate the time-varying voltages and power flows around the power grid, during system faults and other events (e.g. switching) the testing used 3 types of models: Transient : for normal AC system faults transient analysis, one phase representation, can not simulate events faster than about 1 ms PSCAD : for very fast events HVDC firing controls, Lightning strikes, switching, insulation flashover, electromagnetic analysis, three phase representation RTDS : for very fast events (same component models as PSCAD) but calculated in real time, can be connected to the real time controls 26
Test Environments On site Off site Study Based 1 Study Based 2 Test time Real Time Real Time Not real time (up to 10 mins / sec) Not real time (~ 10 secs / sec) HVDC Controls Actual Actual Model Simplified Model System Model n/a RTDS PSCAD TRANSIENT Pole control Actual Redundant Actual Redundant Complete Model (includes time cycles and interrupts) Simplified Model Bipole control Actual Redundant Actual Redundant Complete Model (includes time cycles) Simplified Model Station control Actual Redundant Actual Redundant Limited Model (includes time cycles) Complete Model (not time cycles) DC network System Full model Full model Full model AC network System 10 bus equivalent 10 bus equivalent (Full model possible) Time step n/a 55 µs 40 µs 0.5 ms Full model (10 bus equivalent) Timescale All All 1ms to 10s 10ms to 1 day + Used for : All All AC and DC fault recovery Station Control 27
Test Environments The AC systems have to be modelled within the processing constraints of the environments available. For the real time tests equivalent (reduced) models are needed in place of the complete network models These 10 bus models have to be validated to confirm that they are representative of real power system behaviour otherwise our off site tests are not very useful Important Validation : Equivalent network -> Larger network model (for AC system response) Transient Model -> Real control system (for Station Control) PSCAD Model -> Real control system (for Pole and Bipole Control) And after on site testing : On site test -> Real time simulation results 28
Validation results - Full vs Equivalent Network Equivalent Networks need to provide the same conditions at the HVDC terminals as we see for a Full Network model during any AC system disturbance A range of studies with different AC system disturbances were used to prove this : Fault Fault Description North A 120 ms 3ph fault at HAY B C D E F G H South K L M N O 300 ms 3ph fault at HAY Trip Condenser C10 at HAY 120 ms 3ph fault at HAY, loss of HAY-LTN-BPE line 120 ms 3ph fault at BPE 120 ms 3ph fault at HAY, loss of HAY-LTN-BPE line and HAY-BPE #1 line 120 ms 3ph fault at BPE, loss of HAY-LTN-BPE line and HAY-BPE #1 line Trip of HVDC bipole, recloses at t=4 sec 120 ms 3ph fault at BEN 300 ms 3ph fault at BEN Trip Generator 1 at BEN 120 ms 3ph fault at BEN, loss of BEN-OHC line Trip of HVDC bipole, recloses at t=4 sec 29
Validation results - Full vs Equivalent Network ; AC fault This is the power flow in one of the 220kV circuits at Haywards Final power is higher as a circuit was tripped for this fault This is known as the Transient response - and is a good match 30
Validation results PSCAD model vs Real Controls ; AC fault This is a phase to phase fault at Benmore at 950 MW South : DC bipole power and AC system voltages 31
Validation Confirm that the reduced network models represent the full network models for the HVDC terminal conditions Confirm that the various models of the control system respond the same as the actual control system for the relevant timescales Confirm that the off line simulation studies match the real-time control system software Allows the amount of testing on the actual power system to be minimised without too much risk 32
Software Issues Off Site Testing uncovered errors in the software which is expected From simple logical errors e.g. software does the wrong thing To more complex design issues e.g. the software design means that the controls are not acting in the expected way And some performance issues e.g. software does the right thing but not fast enough One example of a performance issue : 33
Example of Software Issues Test : Power take over when one pole trips (with a disturbance) The initial test (blue) was too slow to pick up the DC power A software modification led to the response in red 34
Example of Software Issues Test : Power take over when one pole trips (with a disturbance) This was an execution cycle time conflict A slower control of a current limit was preventing a faster power recovery The slow release of the limit (purple) was changed to an immediate release The initial response (blue) then became the much faster response (in red) 35
On Site Tests a video! An example of a software test (a DC line fault in this case) Tests are more interesting on the real system 36
On Site Testing - Issues On site the main issues have tended to be caused by : Measurements from the real equipment signal quality? Measurements from models are usually quite good Run time and Race conditions in the control software Status changes need time to propagate in the controls Undefined states may occur Engineering / Design issues in the control software Still find some of these (low % test coverage) Finding some of these on the Pole 2 control 37
On Site Testing On site tests can match very well to the off site test results The exact AC system conditions on the day of a test are rarely predicted in advance so some differences are expected - e.g. off site we use worst case fault clearing times for AC faults When control action or the speed of the controls is not as expected it needs further investigation similar to the off site analysis 38
Thanks for your attention! Any Questions? 39